Category: General Weather

After the weekend, the next storm that the Northeast will be watching comes in the form of an Alberta Clipper late Tuesday into Wednesday. These storms are called Alberta Clippers because they come southeast from Alberta, Canada to the Northeastern United States. This means they have quite the cold punch with them. They also have a strong high pressure system closely trailing the low pressure system so that the cold air usually comes rushing in with strong winds. However, the other part of the name, clipper, gives reference to how the system generally doesn’t drop much precipitation as a result of the area of low pressure being quite dry and moving quite fast through an area.

In many cases however, the Alberta Clipper can drop plenty of snow in a given region. The Alberta Clipper is known for containing plenty of cold air and an appreciable amount of energy, but a lack of precipitation. So if moisture from a low pressure system to the south is able to phase, or combine into the system, which usually occurs in the Eastern United States, the clipper can redevelop off the East Coast into a strong nor’easter. This occurred January 25-28, 2015 and resulted in snowfall over 30 inches in portions of eastern and central Massachusetts. The other way Alberta Clippers can pack a punch, which is usually weaker than when clippers phase to strengthen the system, is to have the trough that the low pressure system resides in turn negative. This means that the direction from the northern part of the trough to southern part of the trough goes from northwest to southeast. This causes a strong draw of moisture into the storm on favorable winds and can also cause a redevelopment low into a strong low pressure system in the Northeast, usually east of Massachusetts. The proximity of the redevelopment low pressure system to land can determine if the resultant storm skirts by the region with minimal snowfall or a closer track leading to higher snowfall. One other part of forecasting an Alberta Clipper is to forecast its track from Northwestern Canada to Northeastern United States. There have been plenty of times where the models have the system way to far north and end up trending it farther south over time. This can trend any negatively tilted troughs further west and have a new, stronger low pressure system develop closer to land than out to sea.

The Alberta Clipper system taking aim at the Northeast does have a negatively tilted trough and also has quite a bit of instability in the low levels of the atmosphere. Snow squalls could occur as a result of the low level instability and a stronger low pressure system does look to develop somewhere east of Massachusetts. Temperatures may be just borderline enough that precipitation could start as rain instead of snow as well. While, it is NOT anticipated that this system will become one like January 25-28, 2015 and produce high amounts of snow, it is possible that current models aren’t depicting the whole picture of what this system could become especially with possible trends south with the system and possible adjustments with the model solutions after the weekend rainstorm moves through. Currently, snow totals are expected to be light.

Have you ever wondered how lake effect snow develops? It works as a result of buoyancy and wind direction and is easier for it to take place in the earlier part of winter. One of the biggest pieces of the puzzle, aside from a totally below freezing atmospheric column, is the fact that lakes need to be warmer than the cooler air coming in across the lakes. In fact, it needs to be 13 degrees colder at the surface than at around 1.5 kilometers into the atmosphere. This is because there needs to be enough lift to generate the storm itself.

It is easier for this setup to take place in the transition from fall to winter because the area’s average air temperatures are falling while the lake’s average sea surface temperatures are falling slower, or if not, remaining the same. This is as a result of a few of water’s properties (heat capacity and thermal conductivity) which allow it to change from warm to cold or cold to warm temperatures slower than the air. Also later in the winter, the lake has a tendency to have a frozen surface, which shuts off the possibility of lake effect snow. One of the most common ways this setup works is to have a cold front come in from Canada with a vigorous piece of energy associated with it and move right over the lake. This often happens as a strong low pressure system is leaving the Northeast and cold air streams into the Northeast behind it.

So, in our setup, the lake is warm, the air is cold, now buoyancy comes into play because the cooler air aloft is more dense and falls while the warmer air rises, condenses and creates our snow storm. The other piece that is necessary with lake effect snowfall is to have a favorable wind. This is very important because often, if the wind direction stays constant for a long period of time, the snowfall amounts can pile up within the favorable area for snowfall while the area just outside the favorable area can have minimal amounts. For example, from November 17-19 of 2014, around 5 feet of snow fell in Cheektowaga, New York off of Lake Erie while just a few miles away only received a few inches. So wind direction is important because, in some cases it can mean the difference between a few inches and many feet.

This can also work with the ocean in the same way that it does with the lake. A warm ocean compared to very cold temperatures 1.5 kilometers into the atmosphere can lead to snowfall along the eastern side of the Northeast and especially Cape Cod. Such an event is forecast to take place on Monday as a strong arctic cold front approaches from the northwest. Some of the coldest air forecast to come into the region with around -10 to -12 Celsius temperatures at 1.5 km while the sea surface temperatures just east of Massachusetts are still around 9 Celsius. That’s a temperature difference of around 20 degrees Celsius around 1PM on Monday! Of course, enough lift has to be in place and moisture within the area where growth of dendrites, the type of snowflake that allows for the highest accumulations, occurs in the atmosphere. Some dendritic growth zone moisture is available at the time of ocean effect snowfall, but lift is really lacking. So snowfall accumulations should be minimal. Sometimes Boston gets into the snowfall as well, but the wind needs to have a slight easterly component. If it does, then some snowfall could occur.

Much of the talk over the past few days has been about the record breaking temperatures around Christmas time. And rightly so, because many temperatures reached the mid to upper 60s in the East and Northeastern US breaking old records by as much as 5 to 10 degrees while many hoped for a White Christmas. This all happened at the same time as intense storms ripped through the Midwest and Southern US, leaving damage behind. However, a  quick cold shot will bring the Northeast quickly right back to reality come Monday as a piece of energy dives into the Northeast that can trace its origins to Northwestern Canada and will bring with it, very cold temperatures. Some of the coldest temperatures that the Northeast has had all winter are expected. Monday, as high pressure sits in Southeastern Canada, cold air will be able to drain south from Canada on northerly winds. Temperatures may not get out of the 30s in many spots on Monday and dew point temperatures (a measure of moisture) will be below freezing, indicating the strength of the cold and dry air.

The strength of this cold and dry air is very important because right after this cold shot passes through, moisture will be coming up from the south in a very common setup, for what is called overrunning.  What happens is that warm, less dense, moisture from the south rides over cooler, denser air at the surface and results in precipitation. It is common for this setup to have precipitation hold off as the moisture aloft fights with the drier air at the surface to be able to reach the surface. As the moist air evaporates before it reaches the ground, it actually acts to cool the air, thus keeping temperatures near or just above the freezing mark during the day, much like sweating keeps us cool during the summertime. With surface temperatures remaining around freezing, the initial precipitation that does fall will have an opportunity to fall as wintry precipitation. Forecast atmospheric profiles, known as soundings, show that temperatures at the surface and aloft will remain below freezing as the warm moist air enters the region so the precipitation type should start as snow. However, as the warm, moist air continues to come in, especially since it will be helped along with any additional daytime warmth, midlevel temperatures will be able to warm and the atmospheric profile will begin to look like one that will produce sleet and freezing rain before eventually changing to rain by days end. The strength of the winds aloft behind the warm front (signaling the entrance of the warm, moist air) is important because this will tell how quickly the warm air mass can come in from the south and change everything to sleet, freezing rain and rain and it will tell how far inland the warm air is able to come from the ocean as the winds come in from the southeast in the form of a coastal front. If this coastal front is able to set up, snow fall amounts could be increased on the western cold side of the coastal front. In addition snowfall amounts may be bumped up just to the north of the warm front because it will have decent mid to lower level energy along it, but at the same time, most of the energy appears to be outside of the zone for best snowfall growth so total snowfall won’t be as high as it could be. The most likely time frame for the snow to occur would be the morning into the midday hours ahead of a possible change over to sleet and freezing rain, which would occur from the midday hours on and a changeover to all rain could occur in the afternoon to evening hours. Of course, if any snow accumulation does occur, rain on top of that would spell trouble for driving as slush may form on the street, further exacerbated by any sleet and freezing rain that could fall on Tuesday. Of course, there’s always the wrinkle of if a coastal low pressure system does form because this would also act to keep cooler air around longer extending the longevity that the wintry precipitation resides in New England on Tuesday instead of rainfall. The area of low pressure should then push northeast and be out of the area overnight Tuesday night.

It has been interesting to watch the development of the low pressure system over the Midwest toward the end of the past workweek and into this weekend. Basically what happened with this storm was that an energetic disturbance was able to dive south out of Canada and create a setup conducive for storm intensification just east of the Rockies.

But it all started out back in East Asia as an area of low pressure exited there with a strong area of high pressure just south of it. It then moved toward Western Alaska, still with the high pressure area just to its south. Then the high pressure area was able to intensify just south of Alaska and this was able to help change the previously horizontal flow in the Eastern Pacific to become wavier or meridional, which helps storm development. This is indicated in an index called the Eastern Pacific Oscillation that basically identifies the high or low pressure departures from normal with a negative index meaning a high pressure area near Alaska and a low pressure area for a positive index. This index dropped from very positive to slightly negative indicating a change to a higher pressure in that area and a more meridional flow. As a result, a stronger northwest jet developed in Western Canada helping the energetic disturbance dive further south from Canada to just east of the Rockies.

Another part of the equation was getting the cold air into the Northern Plains in the first place and developing an area for the storm to develop at all. This was a put together as the low pressure system that developed starting last Monday and intensified through Thursday sent cold air rushing into the Plains allowing the energetic disturbance from before to dive south in already colder air.

This storm is the first snow storm of the winte, dropping plenty of snow from Iowa into the Great Lakes. As of Saturday morning, some areas saw over a foot of snow with snow continuing in Illinois and moving into Michigan. As the storm continues to move into Michigan, it will develop a negative tilt, which allows it to grab warm, moist air and, within cold air, turn it into snow easier. Snow accumulations are forecast to total 5 to 10 inches, but there could be more than that where the instability is the strongest and if it resides in one area for longer periods of time.  Furthermore, where the winds are blowing in from the right direction, additional snow could be had as lake effect snow forms, especially as the ground surface is able to warm after the storm and cooling winds stay around.

In addition to the snow, strong winds are also forecast along and near the Great Lakes where the water allows for less friction and strong winds. For example, in Western New York, winds will be from 20 to 40 miles per hour and may gust to 50 or 60 miles per hour from this evening into the overnight. With winds this strong, power outages cannot be ruled out.