Severe Weather in the South Today, Bigger Outbreak Friday?

Severe weather is likely across portions of the Gulf Coast and Mississippi Valley today, but another, perhaps more widespread, outbreak is possible at the end of the week.

Surface analysis as of 7am CDT Wednesday April 26. Image provided by NOAA.
Surface analysis as of 7am CDT Wednesday April 26. Image provided by NOAA.

 

A strong cold front is moving into the Mississippi Valley and Texas this afternoon, and it is helping to trigger strong to severe thunderstorms across portions of the region. Ahead of the front, temperatures are into the 70s and lower 80s, with dewpoints in the upper 60s and 70s, so there’s plenty of warm, moist air in place. Behind the front, temperatures quickly drop into the 50s and 60s. Thunderstorms will continue to develop in the unstable airmass ahead of the front, with some of the storms containing large hail, heavy downpours, damaging winds, and possibly some tornadoes. Earlier this morning, some storms produced baseball-sized hail and wind gusts in excess of 70 mph in portions of Missouri, Arkansas, and Oklahoma. The threat should start to diminish across the region as we head into the overnight hours.

A more significant severe weather outbreak is possible later Friday into Saturday from the Southern Plains and Texas into portions of the Mississippi and Tennessee Valleys. Low pressure will move out of Texas and head northward, drawing warm moist air northward from the Gulf of Mexico once again. North and west of the system, much cooler air will be in place (more on that in a moment).

Models are showing that a very unstable airmass will be in place across Texas and the Lower Mississippi Valley Friday afternoon. Image provided by Pivotal Weather.
Models are showing that a very unstable airmass will be in place across Texas and the Lower Mississippi Valley Friday afternoon. Image provided by Pivotal Weather.
Models are showing that a very unstable airmass will be in place across Texas and the Lower Mississippi Valley Friday afternoon. Image provided by Pivotal Weather.
Models are showing that a very unstable airmass will be in place across Texas and the Lower Mississippi Valley Friday afternoon. Image provided by Pivotal Weather.

 

As low pressure rides along the boundary between the two airmasses, it will help to trigger strong to severe thunderstorms across the region. The threat will continue into the overnight hours Friday night, shifting into the Mississippi and Tennessee Valleys on Saturday as the system continues to progress northeastward. Some of the storms may produce torrential downpours that could trigger flash flooding, large hail, damaging winds, and tornadoes.

Portions of the Mississippi Valley could received 5-10 inches of rain between  Wednesday and Sunday. Image provided by WeatherBell.
Portions of the Mississippi Valley could received 5-10 inches of rain between Wednesday and Sunday. Image provided by WeatherBell.

 

Behind the storm, as colder air settles into the region, a different threat is evolving – heavy snow. While it’s getting late in the season, heavy snow is not uncommon in the Central and Southern Rockies at this time of year. Some of the higher elevations in Colorado and New Mexico could receive 1-2 feet of snow Friday into Saturday. East of the Continental Divide, especially in the High Plains, snow is also possible, especially from eastern Colorado and western Kansas into portions of the Oklahoma and Texas Panhandles. In Denver, there is still a big question as to whether to precipitation falls mainly as rain, snow, or a wintry mix. Some snow accumulation seems likely at this point, but it’s still a little too early to tell whether there will be heavy snow in the city itself.

GFS models forecast for snowfall through Sunday morning across the Plains and the Rockies. Image provided by WeatherBell.
GFS model forecast for snowfall through Sunday morning across the Plains and the Rockies. Image provided by WeatherBell.

 

Severe weather outbreaks are not uncommon at this time of year. In fact, from late March into early May is when they are most likely. The largest tornado outbreak on record occurred 6 years ago this week. Between April 25 and 28, 2011, a total of 362 tornadoes were observed from Texas to New York and portions of southern Canada, resulting in 324 fatalities, 317 of them on April 27, the most active day.

Map showing tracks of all 362 tornadoes from the April 2011 "Super Outbreak". Image provided by Encyclopedia Brittanica.
Map showing tracks of all 362 tornadoes from the April 2011 “Super Outbreak”. Image provided by Encyclopedia Brittanica.

From Record Highs to Blizzard Conditions

Much of the nation’s midsection has been enjoying temperatures more typical of April than February for the past week, with a few hundred record high temperatures broken. That is about to change, as Mother Nature will remind the region that is still February.

Numerous record highs are expected across the Plains and Midwest for one more day. Image provided by WeatherBell.
Numerous record highs are expected across the Plains and Midwest for one more day. Image provided by WeatherBell.

One more warm day is expected today, with highs well into the 60s and 70s likely setting more records. However, a cold front will sweep across the region, bringing an end to the record heat, and setting the stage for a snowstorm.

The low pressure system that brought more rain to California over the past couple of days will head eastward, bringing some snow into the Rockies today. As that system moves into the Plains on Thursday it will start to strengthen, drawing moisture northward from the Gulf of Mexico while cold air flows southward on the backside of the storm into the Northern Plains. Where these airmasses meet, snow will develop across the Central Plains states. The snow will be accompanied by winds of 20-30 mph, gusting to 40 mph or more at times, resulting in near-blizzard conditions across portions of South Dakota, Nebraska, southeastern Wyoming, and eastern Colorado on Thursday.

More than a foot of snow may fall in a swath from the Plains into the Upper Midwest. Image provided by Pivotal Weather.
More than a foot of snow may fall in a swath from the Plains into the Upper Midwest. Image provided by Pivotal Weather.

As the system heads eastward, snow will move into portions of the Mississippi Valley and the Upper Midwest on Friday. The heaviest snow looks to stay just south of the Twin Cities, but even there, moderate to heavy snow is likely. By the time the storm moves out on Saturday, a foot or more of snow is possible in a swath from the Central Plains into the Great Lakes.

Snow isn’t the only threat from this system. As the storm moves eastward, record warmth will remain in place across the Midwest. With warm, moist air in place and a strong cold front approaching from the West, strong to severe thunderstorms are possible across portions of the Lower Great Lakes and Ohio Valley on Friday. Some of the stronger storms that form may contain damaging winds, hail, and possibly tornadoes.

The Storm Prediction Center has already highlighted the Midwest as an area to watch for severe weather on Friday. Image provided by NOAA.
The Storm Prediction Center has already highlighted the Midwest as an area to watch for severe weather on Friday. Image provided by NOAA.

The system will continue to move eastward, bringing some rain to the East Coast on Saturday, but amounts should be fairly light, and additional severe weather is not anticipated.

Change is in the Air

As we head into the end of August, some familiar things start to happen. Children will start to head back to school. Baseball’s pennant races heat up while football at all levels gets ready for the start of the season. Halloween candy starts to appear in stores and it will be followed shortly by pumpkin-flavored everything. In terms of the weather, familiar things happen there too. The tropics start getting more active and the cold fronts dropping southward from Canada pack a little more punch than they usually do during the summer.

One of those cold fronts will be moving across the Plains states and into the Midwest over the next few days. Ahead of the front, summertime heat and humidity remains in place, with temperatures well into the 80s and 90s common. Behind the front is much cooler and drier air. By Friday morning, temperatures will be 10-20 degrees below normal across much of the Plains states and Rocky Mountains as a large area of high pressure builds in from Canada. Low temperatures will drop into the 40s and 50s across the Northern Plains, with 30s and even some upper 20s from the Rocky Mountains into interior portions of the Pacific Northwest.

Temperature anomaly map based on the GFS model for Thursday morning. Image provided by Pivotal Weather,
Temperature anomaly map based on the GFS model for Thursday morning. Image provided by Pivotal Weather.

As that cooler air settles into the region, some snow is possible across the higher elevations of the Northern and Central Rockies. Snow levels will remain fairly high, but a few inches of snow may accumulate on top of some of the higher peaks in Wyoming and Colorado.

Snowfall forecast based on the GFS model through Friday morning. Image provided by WeatherBell.
Snowfall forecast based on the GFS model through Friday morning. Image provided by WeatherBell.

Ahead of the front, a warm and humid airmass will remain in place, with high temperatures well into the 80s and 90s for the next few days. As the front approaches, it will trigger showers and thunderstorms. Some of the storms could be strong to severe, with hail and gusty winds possible, along with a few tornadoes. The biggest threat looks to be heavy rain and flash flooding. Because the front will be moving fairly slowly, some of the heavier storms will linger over the same areas. Rainfall totals of 1-3 inches will be common across parts of the Eastern Plains and Mississippi Valley, with some heavier amounts possible. This will likely lead to flooding in some areas.

Expected rainfall totals across the Midwest through Friday morning. Image provided by WeatherBell.
Expected rainfall totals across the Midwest through Friday morning. Image provided by WeatherBell.

Meanwhile, in the Atlantic, we’re keeping an eye on a disturbance east of the Lesser Antilles and Tropical Storm Gaston out in the Central Atlantic. There was a third system, Tropical Depression Fiona, but it fizzled southeast of Bermuda earlier today.

Satellite loop showing Tropical Storm Gaston (far right) and a disturbance approaching the Eastern Caribbean. Loop provided by NOAA.
Satellite photo showing Tropical Storm Gaston (far right) and a disturbance approaching the Eastern Caribbean. (Click for loop) Provided by NOAA.

Tropical Storm Gaston is centered about 700 miles west of the Cape Verde Islands this afternoon, heading towards the west-northwest at 21 mph. Maximum sustained winds are near 65 mph, and additional strengthening is expected. Gaston should become a hurricane by Wednesday afternoon. The forecast for Gaston is to turn more towards the northwest, heading out in the Central Atlantic Ocean before starting to weaken in a few days. Gaston will likely bot be a threat to any land areas.

Meanwhile, a disturbance located a few hundred miles east of the Lesser Antilles is being carefully monitored for signs of development. United States Air Force Reserve Hurricane Hunter aircraft investigated the system earlier today and found it to be a weak system with an ill-defined center. Conditions are still somewhat favorable for further development, and anyone with interests in the northeastern Caribbean should continue to monitor the progress of this system. The future of this system is still a big question mark. Most of the forecast models bring the system towards the Bahamas over the next several days, though a few have the system dissipate completely before then. Once it gets to the Bahamas, there is considerable spread among the models as to where it will go, assuming it even survives that long. Some have it turn northward and head towards the Carolinas or Georgia. Some bring it into Florida, then up into the Southeast, and others bring it across Florida and into the Gulf of Mexico. As for how strong it could be, that’s an even bigger question mark. As mentioned previously, some of the models have it dissipate completely. There are other models that keep the system as a tropical depression or weak tropical storm into the Bahamas. There are others that have it as strong as a Category 2 hurricane. Once the storm actually forms (assuming it actually does), the computer models should start to get a better handle on its future.

Computer model forecasts for the track of a disturbance approaching the Eastern Caribbean. Image provided by Tropical Tidbits.
Computer model forecasts for the track of a disturbance approaching the Eastern Caribbean. Image provided by Tropical Tidbits.
Computer model forecasts for the intensity of a disturbance approaching the Eastern Caribbean. Image provided by Tropical Tidbits.
Computer model forecasts for the intensity of a disturbance approaching the Eastern Caribbean. Image provided by Tropical Tidbits.

High Heat Gives Way to Afternoon Thunder across the Northern US

Strong high pressure has allowed for above normal temperatures in much of the central and eastern United States. That high pressure system will slowly move from the Central Plains on Saturday to the Southeastern United States by the early part of next week. This will take the above normal temperatures and above 100 degree heat index, which takes into consideration humidity for a truer feels-like temperature, into the Southeastern United States. Temperatures have been and will continue to be over 90 degrees for much of the central and eastern United States. Heat waves will most likely be reached across many areas including Lawrence, Massachusetts which reached 91 on Thursday, 99 on Friday and already reached 90 before midday on Saturday. Minneapolis, Minnesota reached 90+ degrees as high temperatures from Wednesday to Friday and could reach 90+ on Saturday too. In addition, Ann Arbor,, Michigan reached 90+ degrees for high temperatures on Thursday and Friday and could reach 90+ on Saturday for a third day in a row, the criteria for a heat wave.

Strong high pressure to end this past week and continue into early next week. Above normal heights often indicate above normal temperatures.
Strong high pressure to end this past week and continue into early next week. Above normal heights often indicate above normal temperatures. Image provided by Tropical Tidbits.

 

All this heat is usually followed by thunderstorms during or by the end of the day. As such, by sunset Saturday, a low pressure system will be moving northeast out of the Rockies into Southern Canada and will drag its cold front through the Northern Plains by Sunday morning. This will turn above normal temperatures into more normal temperatures for this time of year. However, this temperature change will mean strong to severe thunderstorms will be forced along by this cold front. A stronger than normal low level jet stream is in the Central and Northern Plains on Saturday and then it will be in the Great Lakes on Sunday before being in the Northeast on Monday. This low level jet stream will help to keep thunderstorms sustained as they pop during the day. During this time, the low pressure system in Southern Canada will be deepening allow the better forcing for showers and thunderstorms to be closer to the deepening upper level low pressure system in Southern Canada. Convective energy will also support thunderstorms along the northern periphery of the United States as the cold front moves east with 3000+ J/kg of CAPE on Saturday and Sunday. On Monday, after the front crosses the Great Lakes and with a little more marine influence stabilizing the airmass, the convective energy will be a little less, but will still reach into the 1000s of J/kg.

 

Showalter index showing the location of best severe weather potential with most negative numbers moving across the northern US.
Showalter index showing the location of best severe weather potential with most negative numbers moving across the northern US. Images provided by Accuweather.
Convective energy represented by Convective Available Potential Energy (CAPE) traversing the northern US.
Convective energy represented by Convective Available Potential Energy (CAPE) traversing the northern US. Images provided by College of DuPage.
Saturday, Sunday and Monday analogs for severe weather where colorations indicate where 5 severe reports have occur with an area nearby.
Saturday, Sunday and Monday analogs for severe weather where colorations indicate where 5 severe reports have occur with an area nearby.

 

On Tuesday, another low pressure system is forecast to eject off the Rockies and allow for more severe weather in the Northern Plains with more warm air around the region. Warm air will once again traverse east. While there won’t be a strong high pressure system around the region indicating warm air, 90 degree temperatures are still possible with above normal warm air in the low levels of the atmosphere in the northern US. The Northeastern US will have a chance to be cooled off Wednesday into late week with temperatures below 90 degrees should onshore winds occur.

 

 

 

Two Rounds of Thunderstorms in the Northeast

Overnight Saturday night, a disturbance will be moving from the southwest to the northeast providing a chance for showers and thunderstorms. Coupled with the disturbance moving through, upper level divergence will be overhead in Southern New England, especially near Cape Cod, but also in interior Massachusetts and Southern New Hampshire where a low level jet is also located. The upper level divergence provides large scale lift, which can aide in the development of heavy rain. Also a rise to 1.5 to 1.75 inches of precipitable water occurs during that time showing the increase in moisture in the atmosphere at that time. Further adding to the event, an increase in upward motion signified by convective energy (CAPE) begins just after sunset on Saturday and continues into Sunday afternoon. Another ingredient present for this time frame is shear, which rises to between 30 and 40 kts Saturday night and into the first half of Sunday, but mostly resides in the mid layers of the atmosphere. This means that convection may be elevated allowing for heavy rain, wind and lightning as the main threats. All of these combine to allow for a chance for thunder across southern portions of New England Saturday night and into Sunday. The passage of the disturbance around midday means that most of the thunder should occur before midday, but some instability and low level moisture may allow for a few more storms before sunset Sunday, especially in the interior.

The disturbance as depicted by the GFS model that allows for the possible thunder on Saturday night in the Northeast.
The disturbance as depicted by the GFS model that allows for the possible thunder on Saturday night in the Northeast. Courtesy of Accuweather Pro.
NAM short range model depiction of the heavy rain on the Cape and showers and possible storms in interior Massachusetts and Southern New Hampshire.
NAM short range model depiction of the heavy rain on Cape Cod and showers and possible thunderstorms in interior Massachusetts and Southern New Hampshire. Courtesy of Pivotal Weather
CIPS analogs showing the possibiltiy of severe weather in interior Southern New England and eastern New York.
CIPS analogs showing the possibiltiy of severe weather in interior Southern New England and eastern New York. Courtesy of the Cooperative Institute for Precipitation Systems (CIPS)

The next chance for thunder comes on Monday night into early Tuesday in the Northeast when a cold front moves through. The air mass comes in from Canada and changes the humid warm weather with high temperatures in the 90s and dew points in the mid to upper 60s to temperatures in the mid 80s and dew points in the upper 50s. This change in low level warmth and moisture coupled with a center for storms (the front) often translates into storms. New York is forecast to have the highest convective energy before sunset on Monday. After sunset, the convective energy decreases, but mid-level lapse rates in the 6 to 7 degree range may be able to compensate while combined with mid-level shear carrying the left over pieces of thunderstorms from New York into the Northeast. Once again, large scale lift and plenty of precipitable water means heavy rain may be a threat, especially since storms may be in the dissipating stage after sunset. Often, this dissipating stage brings heavy wind and frequent lightning as the main threats as well, especially as the stronger low level winds move in.

Overnight mid-level lapse rates Monday night helping to aide convection.
Overnight mid-level lapse rates Monday night helping to aide convection. Courtesy of the College of DuPage and Victor Gensini
CIPS analogs showing the possibility of severe weather in interior Southern New England and New York on Monday night and into early Tuesday.
CIPS analogs showing the possibility of severe weather in interior Southern New England and New York on Monday night and into early Tuesday. Courtesy of the Cooperative Institute for Precipitation Systems (CIPS)
Below normal temperatures at 850 mb allowing for cooler morning temperatures Wednesday.
Below normal temperatures at 850 mb allowing for cooler morning temperatures Wednesday.

 

Thereafter, as mentioned earlier, the Northeast has cooler temperatures and drier air come in for Wednesday as a Canadian air mass moves in. Low level temperatures will be below normal and will allow for morning temperatures around a refreshing 60 degrees.

Severe Weather Likely in the Northern Plains This Weekend

Severe weather will be the story in the Northern Plains for the weekend and into Monday as a low pressure system ejects off the Rockies and an upper level low pressure system moves across the Northwestern portion of the United States. The storms through the weekend and into Monday, forced along by this boundary will have ample amounts of convective energy in the form of CAPE (Convective Available Potential Energy) and turning of the winds in the form of wind shear. Wind shear will be helped along each day by a strong low level jet stream that will be increasing in strength daily as the upper level low pressure system approaches from the West. Also, low level moisture will be plentiful allowing for strong moist inflow into the storms. Warmth will be strongest Sunday with surface temperatures reaching well into the 90s.

Forecast position of low pressure systems around the time of severe weather on Saturday and Sunday.
Forecast position of low pressure systems around the time of severe weather on Saturday and Sunday. Image provided by NOAA

 

So, the warmth, moisture, and winds are all there, the last piece of the puzzle for severe weather is the need for a boundary which will break the cap on the atmosphere and cause convection. This is similar to the reaction of breaking the full soda bottle’s cap after the bottle has been shaken. Various models show the general evolution of the Saturday evening storms starting in the northern North Dakota and evolving southeast into southeastern North Dakota as a low level boundary moves southeast across the Northern Plains allowing the cap to be broken. The upper level low pressure system enters the western United States so stronger disturbances are closer the region on Sunday. Storms will be moving across the region from southwest to northeast in the average low to mid-level flow as a result. The stronger boundary on Monday will allow for more thunderstorms along with stronger wind. Temperatures won’t be as high though on Monday, but the added forcing of the cold front will add to the convective energy.

 

Forecast ind shear in the Northern Plains on Saturday and Sunday. 30 kts of shear is generally good enough with ample CAPE. Notice the 30 to 50 kts over the weekend.
Forecast shear in the Northern Plains on Saturday and Sunday. 30 kts of shear is generally good enough with ample CAPE. Notice the 30 to 50 kts over the weekend. Image provided by TwisterData.com
SPC's SREF model indicating the chances for tornadoes during the weekend in the Northern Plains.
Short Range Ensemble Forecast model indicating the chances for tornadoes during the weekend in the Northern Plains. Image provided by NOAA.

 

To get an idea of the possible outcomes of the daily convection, analogs of the event can be perused. The Northern Plains are within an area where at least 30% of the analogs to this event have at least 1+ severe report during the weekend and into Monday. According to the analogs, the severe weather on Saturday will be across western and southern North Dakota and into central and northern portions of South Dakota. Sunday’s analogs have the severe weather in the western Dakotas, but still a strong signal moving northeast into the eastern portion of the Dakotas. Monday’s severe weather threat according to the analogs appears within eastern North Dakota. Eastern North Dakota and into western Minnesota has the highest threat for tornadoes during this period according to the analogs.

 

Forecast lightning probability within an area indicates severe weather in higher probability areas. Notice chances in Northern Plains after Sunday evening.
Forecast lightning probability within an area indicates severe weather in higher probability areas. Notice chances in Northern Plains. Image provided by NCAR.
Analog's probability of severe weather. Notice the possibility in the Plains Saturday and then Sunday. Courtesy of CIPS.
Analog’s probability of severe weather. Notice possibility in the Plains. Image provided by CIPS.

Often, to approximate the threat of severe weather, the number of lightning strikes within an area can be used. In this case, the ensemble put out by the National Center for Atmospheric Research (NCAR) shows a low amount of lightning strikes Saturday night, but at the end of the run, by late evening, early overnight on Sunday a higher amount of lightning strikes approaches the Missouri River so that appears to be the higher severe threat. In addition, the Storm Prediction Center’s ensemble model has a tornado ingredient parameter showing the possibility of a tornado based on the right ingredients being around indicates a 30%+ chance for a tornado in the Northern Plains Saturday and Sunday nights. In addition to the threat of a tornado, hail, wind and heavy rain, especially within any thunderstorm is possible.

Saturday's severe weather chances courtesy of SPC. Notice the slight category over the Northern Plains.
Saturday’s severe weather outlook from the Storm Prediction Center. Notice the slight category over the Northern Plains.
Sunday's severe weather chances courtesy of SPC. Notice the enhanced category over the Northern Plains.
Sunday’s severe outlook from the Storm Prediction Center. Notice the enhanced category over the Northern Plains.
Snow is possible in some higher elevations of the Northwest after the low pressure system leaves the area.
Snow is possible in some higher elevations of the Northwest after the low pressure system leaves the area. Image provided by the College of DuPage.

 

To approximate the strength of the system causing the weather, not only is the severe weather ahead of it a good measure, but also the strength of the cold air behind it. Often, a quick change from warmer to cooler weather creates windy weather and strong wind gusts between 40 and 60 mph are possible in the Northern Plains after the system leaves. Furthermore, with the cold air gushing in behind the system, some left over snowfall is possible in the Northwestern US.

Widespread Severe Weather in the East Starting to Wind Down

Severe weather has been widespread across the eastern half of the nation for the past 2 days, but high pressure will bring much quieter conditions this weekend.

A cold front has been slowly dropping southward over the past 2 days, replacing warm and humid air with cooler and drier conditions. Ahead of the front, strong to severe thunderstorms have cut a large swath of damage.

Reports of severe weather from Thursday June 16. Image provided by the Storm Prediction Center.
Reports of severe weather from Thursday June 16. Image provided by the Storm Prediction Center.

 

On Thursday, severe weather was widespread across the Mid-Atlantic states from mid-afternoon through the late evening hours. There were over 250 reports of damage from wind gusts as high as 77 mph, with a majority of the reports concentrated in West Virginia, Virginia, and Maryland, including the Washington, DC metropolitan area, where thunderstorms dropped 2.58″ of rain in just a couple of hours at Dulles Airport. In addition to the wind damage and heavy rain, hail as large as baseballs was observed with some of the strong storms. One tornado was also confirmed in Barnes Gap, Pennsylvania.

While most of activity quieted down in the evening, thunderstorms flared up across North Dakota and Minnesota. A cluster of storms moved across the state early in the morning, producing baseball-sized hail near Bismarck. Wind gusts as high as 80 mph were also reported across the region.

Radar loop for the Southeast from Friday afternoon June 17. Loop provided by the College of DuPage.
Radar loop for the Southeast from Friday afternoon June 17. Loop provided by the College of DuPage.

 

By Friday afternoon, activity started to flare up again across the Southeast ahead of the cold front. A line of strong to severe thunderstorms quickly developed from Mississippi to South Carolina and began to march southward. Widespread wind damage was reported across the region as wind gusted as high as 70 mph in some of the stronger storms. These storms also produced torrential rainfall, with 2.32″ falling in one hour in Columbus, Mississippi.

Reports of severe weather from Friday June 17. Image provided by the Storm Prediction Center.
Reports of severe weather from Friday June 17. Image provided by the Storm Prediction Center.

 

Across the Plains states, under sunny skies temperatures soared well into the 90s and lower 100s on Friday.  Another cold front moving into the Western Plains provided the necessary lift to trigger thunderstorms across portions of Kansas and Nebraska. Once the activity got going, it quickly exploded into a large cluster of strong to severe thunderstorms. In McCook, Nebraska, the temperature reached 99 degrees during the afternoon with a dewpoint in the upper 60s to lower 70s. As a severe thunderstorm moved into the region, it produced a wind gust to 72 mph and dropped over 2 inches of rain on the area. It also sent temperatures tumbling into the middle 60s.

Radar loop for Kansas and Nebraska from Friday afternoon June 17. Loop provided by the College of DuPage.
Radar loop for Kansas and Nebraska from Friday afternoon June 17. Loop provided by the College of DuPage.

 

Much quieter conditions are expected over the weekend as a large area of high pressure builds into the eastern half of the nation. Sunshine and seasonably warm temperatures are expected for most of the region, with thunderstorm activity mostly confined to the Gulf Coast, near the dying frontal boundary that produced severe weather over the past few days, and the Northern Plains, where a strong cold front will slowly move through this weekend.

Heavy Rain in the Plains and Mississippi Valley Tuesday

 

After Sunday’s cold front ushers in cooler and drier air for Monday in the Northern Plains, a warm front will push north Tuesday. This warm front is attached to a low pressure system that will be moving northeast through the Rockies and into the Northern Plains. Areas in between the northward moving warm front and the southeastward moving cold front could see some strong to severe thunderstorms on Tuesday. This would mainly occur in the eastern Central Plains and portions of the Middle Mississippi Valley where convective energy, CAPE, and turning of the winds, shear, overlap to be able to support these storms.

Cooler and drier air (green colors) moving into the Northern Plains behind a cold front.
Cooler and drier air (green colors) moving into the Northern Plains behind a cold front. Courtesy of College of Dupage.
A warm front nestled between North and South Dakota.
A warm front nestled between North and South Dakota. Courtesy of College of DuPage.
Showalter Index showing possible instability in the Northern Plains, but higher values occur in the Eastern Central Plains and Middle Mississippi Valley.
Showalter Index showing possible instability with negative values in the Northern Plains, but even lower negative values occur in the Eastern Central Plains and Middle Mississippi Valley. Courtesy of AccuWeather.
Severe weather categorical outlook for Tuesday showing the highest probability of severe weather in the Central Plains and Middle Mississippi Valley.
Severe weather categorical outlook for Tuesday showing the highest probability of severe weather in the Central Plains and Middle Mississippi Valley. Courtesy of the Storm Prediction Center.

Farther north along the warm front in the Northern Plains, factors like limited sun exposure, saturated atmospheric columns and a lack of a shear/CAPE overlap may preclude strong to severe thunderstorms all together. There is some question as to how far north the warm front will be able to push into North Dakota so if it is able to push farther north and allow more sun, some strong to possibly severe thunderstorms may occur. At the very least, some thunderstorms may be embedded in what does appear to be a heavy rain event as a result of a vigorous upper level disturbance that appears to go negatively tilted in the upper levels of the atmosphere. This negative tilt allows for a strong draw of moisture into the system as a result of a strong low level jet stream. At the same time, very moist low levels and total column precipitable water values between 1.5 and 2 inches get wrapped around the northeastward moving low pressure system bringing the potential for a heavy rain event into the Northern Plains. Furthermore, large scale lift as a result of a collocated upper level jet stream will help to support the event. All told, 1 to 2.5 inches of precipitation in 24 hours from morning Tuesday to morning Wednesday seems reasonable for this event, but could be higher in any embedded thunderstorm.

The vigorous possibly negatively tilted energy moving into the Central and Northern Plains.
The vigorous possibly negatively tilted energy moving into the Central and Northern Plains. Courtesy of College of DuPage
Strong low level jet in the Northern Plains along with the Central Plains and Middle Mississippi Valley helping fuel heavy rain and strong to severe thunderstorms respectively.
Strong low level jet in the Northern Plains along with the Central Plains and Middle Mississippi Valley helping fuel heavy rain and strong to severe thunderstorms respectively. Courtesy of College of DuPage.
Forecast rainfall totaling 1 to 3 inches. Possibly higher in any thunderstorms from Tuesday morning to Wednesday morning.
Forecast rainfall totaling 1 to 2.5 inches. Possibly higher in any thunderstorms from Tuesday morning to Wednesday morning. Courtesy of the Weather Prediction Center of NWS.

 

It is also interesting to note that the ECMWF model contains around an inch less of precipitation than the GFS model, with the NAM model being closer to the mean of the two models despite the ECMWF model reaching the above parameters. With this in mind, it appears any flooding risk would be low, but could still occur if the higher end of the precipitation range is realized.

 

The low pressure system is forecast to move away from the Northern Plains allowing for drier air to come in for Wednesday and Thursday. However, clouds are forecast to return as another low pressure system ejects off the Rockies for Friday allowing for some more rain.

 

 

 

Severe Weather In the Northeast on Saturday?

For the second weekend in a row, there is a threat for severe weather across parts of the Northeast. Unlike last weekend, when the threat was centered on areas from the Delmarva Peninsula southward to the Carolinas, the threat on Saturday is focused on the Northeast.

Strong to severe thunderstorms moved across the Midwest Friday evening. Loop provided by College of DuPage NexLab.
Strong to severe thunderstorms moved across the Midwest Friday evening. Loop provided by College of DuPage NexLab.

Low pressure is moving across Lake Superior this evening, with a warm front extending east and southeast from the system across the eastern Great Lakes and into Virginia. A cold front trails the system into the Dakotas and Wyoming. South of these two fronts a very warm and humid airmass is in place, with temperatures well into the 80s and 90s on Friday. Dewpoints were in the 60s to lower 70s across much of the region as well. As the cold front moved into the Upper Midwest, it helped ignite a line of strong to severe thunderstorms. These storms produce golfball-sized hail and wind gusts as high as 76 mph as they cross Minnesota and Wisconsin Friday afternoon and evening.

Usually, strong to severe thunderstorms diminish during the evening hours as they lose the heating of the sun. However, with the warm and humid air in place across the Midwest, these storms will likely continue to march eastward overnight ahead of the cold front.

Radar loop predicted by the NAM model from Friday night into Sunday night. Loop provided by College of DuPage NexLab
Radar loop predicted by the NAM model from Friday night into Sunday night. Loop provided by College of DuPage NexLab

Forecast models indicate that this line of thunderstorms will move across portions of New York and Pennsylvania Saturday morning and afternoon before moving into portions of New England. How far north and east the warm front moves will help determine where the best chance for severe weather will be located.

High temperature forecast for Saturday afternoon based on the WRF model. Image provided by WeatherBell.
High temperature forecast for Saturday afternoon based on the NAM model. Image provided by WeatherBell.
Dewpoint forecast for Saturday afternoon based on the WRF model. Image provided by WeatherBell.
Dewpoint forecast for Saturday afternoon based on the NAM model. Image provided by WeatherBell.

Right now, it looks like the warm front will only make it into New York and southwestern New England before the cluster of showers and thunderstorms arrives. Some of the stronger storms could produce downpours, strong winds, and hail across portions of New York, Pennsylvania, and New Jersey.

Across New England, the threat for the afternoon seems less, with more typical showers and thunderstorms expected. However, this does not mean that the threat is zero. If the warm front were to make progress farther northward, the threat would increase. The bigger threat in this region could come at night. The warm front should eventually push through during the evening, allowing the warm and humid air to move into Southern New England. As the cold front continues it eastward march, another round of showers and thunderstorms will likely develop along and ahead of it. As these storms move across New England overnight, some of those storms could produce heavy downpours and strong winds.

Forecast for 500mb heights and winds based on the WRF model. Image provided by Pivotal Weather.
Forecast for 500mb heights and winds based on the NAM model. Image provided by Pivotal Weather.

After the cold front moves through Saturday night, skies should become partly to mostly sunny across the Northeast on Sunday with near-to-below normal temperatures expected. An upper-level low will slowly move across the Gulf of Maine early next week, keep temperatures near or below seasonal normals while heat and humidity dominate the remainder of the eastern two-thirds of the nation.

The Heat is On Across the Plains

A large ridge of high pressure will shift from the West coast into the Nation’s midsection over the next few days, bringing with it some of the hottest weather so far this year to the Plains states.

Temperature anomalies for the past 30 days across the nation. Image provided by WeatherBell.
Temperature anomalies for the past 30 days across the nation. Image provided by WeatherBell.

A persistent ridge of high pressure has been anchored across the West for the past few weeks, keeping temperatures well above normal for much of May and early June. Numerous records were set across the region, even in normally hot locations like the Desert Southwest, where temperatures exceeded 110 degrees several times. In Death Valley, California, which is frequently the nation’s hotspot, the first 8 days of June have averaged 10.6 degrees above normal, with high temperatures exceeding 115 degrees each day.

While the West has been baking, temperatures across the Plains states have been 1 to 3 degrees below normal for the past month. That is about to change as the ridge slides eastward. By the end of the week and the weekend, the ridge will be centered across the Plains and doesn’t look to move that much right through next week.

Map showing heights at the 500mb level across the United states on Friday June 8. Image provided by Pivotal Weather.
Map showing heights at the 500mb level across the United states on Sunday June 12. Image provided by Pivotal Weather.
Map showing heights at the 500mb level across the United states on Friday June 15. Image provided by Pivotal Weather.
Map showing heights at the 500mb level across the United states on Friday June 17. Image provided by Pivotal Weather.

With the ridge of high pressure in place, temperatures will soar well into the 90s across the Plains states and adjacent portions of the Mississippi Valley for much of the remainder of the week, with some triple-digit heat possible across parts of the Dakotas, especially Friday and Saturday.

High temperature forecast based off of the GFS model for Friday June 10. Image provided by WeatherBell
High temperature forecast based off of the GFS model for Saturday June 11. Image provided by WeatherBell

Some relief will settle into the Northern Plains in the form of a cold front early next week, but the heat will continue from the Southern Plains and Texas eastward into the Southeast. Across these areas, humidity levels will be higher, with dewpoints rising into the 60s and 70s. The result will be heat index values well over 100 degrees across parts of these areas.

The ridge will also act to suppress thunderstorm activity across the Plains states for much of the remainder of the week. Cluster of thunderstorms may develop across the Northern Rockies and ride over the Ridge and into the Great Lakes and eventually the Northeast later in the week. One of these clusters could produce some severe weather across the Eastern Great Lakes and Mid-Atlantic states on Saturday.

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