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April likely to begin with a shot of cold air

The irony of this post, as we write it, is the fact that temperatures are currently surging into the upper 60’s and lower 70’s. A warm airmass is in place, and even behind some rain which fell this morning, highs will reach well above normal. Playoff hockey is just a few weeks away, as is opening day in Major League Baseball. Mother nature, however, has other plans — and she doesn’t seem ready to cooperate with the calendar just yet, as it flips to April.

A major transition in the atmospheric pattern throughout the northern hemisphere will begin next week. On Monday a fairly weak and fast moving storm system will move through the area, with some rainfall anticipated. Behind this storm system will be a Canadian airmass with temperatures perhaps slightly below normal for Tuesday. Model and ensemble guidance indicate a -EPO ridge developing over the Northeast Pacific and Alaska region for middle on next week. This ridge will cause energy to dig and cut over the Southwest United States. Heights will build over Central and Eastern United States with fair weather in local region Wednesday and Thursday. Temperatures will likely rise to above normal levels again during the later part of next week.

GEFS showing -EPO ridge developing out Northeast Pacific next week

GFS ensembles showing -EPO ridge developing out Northeast Pacific next week

Over time, however, the -EPO ridge shifts further east into the West Coast of North America and carves out a larger anomalous trough over much of the Central and Eastern United States. The GFS ensembles force Polar vortex south into Hudson Bay and Southeast Canada with above normal heights in the North Pole and Greenland. The ECMWF ensemble guidance has the polar vortex as far south as James Bay around April 4th. If these general ideas are correct, a significant cold shot with well below normal temperatures will arrive during the week of April 4th. That means high temperatures possibly in the 40s, with low temperatures in the 20’s and 30’s.

GEFS showing 2m temperatures averaging below normal over Central and Eastern United between April 3rd and April 8th

GFS ensembles showing 2m temperatures averaging below normal over Central and Eastern United between April 3rd and April 8th

Support for this cold shot on ensemble guidance comes comes from tropical forcing currently over Western Pacific shifting into Western Hemisphere & Africa during this period. Additional support is provided by the MJO, expected to be in phases 8 and 1 that are typically supportive of colder weather pattern over Central and Eastern United States. We have also seen a complete breakdown of the stratospheric polar vortex, which will for allow for more ridging to develop near the north pole.

200mb velocity potential anomalies showing tropical forcing shifting from Western pacific to over Western Hemisphere

200mb velocity potential anomalies showing tropical forcing shifting from Western pacific to over Western Hemisphere (Image credit: Micheal J. Ventrice, PhD)

At this point, we are aren’t anticipating any major winter storm threats. Temperatures below normal in April are much different than temperatures below normal in January. However, with favorable tropical forcing and cold air available, the active subtropical jet could allow for above normal precipitation and the threat for winter weather across the interior during this time period.

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