Posted by: splillo | April 9, 2012

Stormy week ahead for the Plains

We are facing another pattern shift coming up this week. Two significant changes can be expected:

1) A break down of the negative NAO block in the Davis Straight (see my last post). This block is anchoring an upper level low over the Northeast which is responsible for the cool and windy weather in the last few days, and will be the cause of some showery weather this week.

2) A significant trough dropping into the western U.S. by the middle of the week. Fairly strong ridging over western North America right now will be breaking down and shifting northeast into the Midwest as multiple disturbances move onshore from the eastern Pacific.

The most immediate effect from these changes will appear in the Plains this week. As these storm systems move across the Rockies, they will set off multiple rounds of severe weather. Below is the forecast surface chart from the Hydrometeorological Prediction Center for Thursday. Notice the frontal boundary across Oklahoma and the dryline in western Texas. These act as triggers for severe thunderstorm development. There's also a tight pressure gradient up through the Plains states supporting strong south/southeasterly flow carrying warm moist air from the Gulf of Mexico.

The next change that will occur with the pattern shift will be warmer temperatures returning to the Northeast. The upper low over us now will be allowed to kick out, and the ridging in the western U.S.  will shift into the Midwest and eventually spill into the East by the weekend. Now, this warmth will not even come close to that of the March heat wave, but it will feel a little more spring-like. Disturbances riding up and over the ridge could still deliver rounds of clouds and showers.

Outside of the contiguous U.S., Anchorage, Alaska officially has their snowiest winter on record after a storm dropped 3.5" yesterday. The image to the right is courtesy of the NWS in Anchorage. Winter is far from over there also. Troughs digging in from the North Pacific will continue to threaten Alaska with rain, wind and snow next week, especially as we see this pattern shift.