Blocking Pattern Gives Way to Severe Weather in the Plains on Sunday

The United States will finally be exiting the blocking pattern as a cold front coming into the Northeast from Canada helps to break down the strong to north to south and south to north flow and push it east. This allows high pressure with high pressure and the sunshine that we all enjoy to move in for a large portion of especially early the next work week. During the past week, the Eastern US has been locked in a dull, damp and dreary drizzly pattern. Temperatures were up to 7 degrees below normal in the Mid-Atlantic and Northeast. In Bangor, Maine temperatures were actually 1 degree above normal, but this would seem to be an outlier within all of the other stations during that time frame. Factors such as the cloudiness, precipitation and cooling onshore surface winds all contributed to keeping surface temperatures below normal. In addition, the precipitation was well above normal for that time period in portions of the East . Yes, the region had a low pressure system in place, but there was also a draw of moisture from as far south as the Gulf of Mexico as a result of strong flow coming in from the south. Some places along the Mid-Atlantic received 5 to 10 inches of rainfall from the morning of May 1st to the morning of May 7th. This was as much as around 5 inches above normal in that region. Most of the Northeast remained around normal for this time of year precipitation-wise.

Streamlines during the time period of May 1st to May 5th. Winds rotate counterclockwise around the lowest value so that winds appear to flow in off the water toward the northeast. This acted to keep temperatures down during that week.
Streamlines during the time period of May 1st to May 5th. Winds rotate counterclockwise around the lowest value so that winds appear to flow in off the water in the Mid-Atlantic and Northeast. This acted to keep temperatures down during that time frame.
Temperature departures from normal during the time period of May 1st to May 5th. Below normal temperatures are over the bulk of the Central and Northeastern US.
Temperature departures from normal during the time period of May 1st to May 5th. Below normal temperatures are over the bulk of the Central and Northeastern US.
Precipitable water departures from normal during the time period of May 1st to May 5th.. Notice the strong draw from the south and Gulf of Mexico.
Precipitable water departures from normal during the time period of May 1st to May 5th. Notice the strong draw from the south and Gulf of Mexico.
Precipitation departures from normal during the time period of May 1st to May 5th. Notice the above normal precipitation generally follows the precipitable waters departures from normal.
Precipitation departures from normal during the time period of May 1st to May 5th. Notice the above normal precipitation generally follows the precipitable waters departures from normal.

 

Farther west, as this block breaks down, the upper level low pressure system that was west of the West Coast will move onshore and begin moving northeast toward Canada starting Sunday morning. Since winds move counterclockwise around low pressure systems and the pressure system is far enough south, warm/moist air will be able to surge north from the south allowing for strong thunderstorms to form over the Plains on Sunday. Wind flow will be quite strong in the mid-levels of the atmosphere and mid-level lapse rates are expected to be strong enough to sustain strong to severe thunderstorms capable of producing large hail, damaging winds and some tornadoes are possible as well. As a result, the Western side of the Southern and Central Plains has highlighted with a moderate outlook.

Wind departure from normal for Sunday. Notice the above normal winds at 500 mb, which will help to sustain severe weather.
Wind departure from normal for Sunday. Notice the above normal winds at 500 mb, which will help to sustain severe weather.
A picture of the draw of moisture from the south. Colorations are above normal transport values.
A picture of the draw of moisture from the south. Colorations are above normal moisture transport values, which occur ahead of the triggering disturbance called a dry line.
Sunday's Severe weather outlook with Western Oklahoma and portions of Western Kansas highlighted with a moderate category.
Sunday’s Severe weather outlook with Western Oklahoma and portions of Central Kansas highlighted with a moderate category.
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