Tuesday, April 25, 2023

When Will This Cool/Unsettled Pattern Change?

It's not uncommon for April to feature stalled low pressure with long stretches of "cool" air across the Great Lakes/New England.  Our long stretch of warmth a few weeks ago makes this cooler/cloudy/rainy pattern that much harder to deal with.  What does the long range (2-3 week outlook) say?

The southern oscillation index went through some significant changes over the last week and a half. The greater these day-to-day changes the stronger the effect across North America, Here is the peer reviewed research:  PAPER HERE

SOI analog forecast show the Great Lakes/Eastern US trough backing off by mid month.  How quickly will the Southeast ridge build back is the big question.

Another element going forward this summer and fall will be the building El Nino. I'm not going to go into the specifics of that here. Perhaps a blog post on this in the weeks and months ahead. But as La Nina decays, the overall effects will be cascading across the Pacific augmenting the jet stream and the weather across North America.

Watch the decaying La Nina as shown through the weekly sea surface temperature anomalies from December 2022 to mid April 2023:  Watch the blue area disappear slowly.

Sea Surface Temperature Anomalies

The big drivers of the pattern change have been the ridge of high pressure in the Gulf of Alaska and the western corners of North America along, the deep low pressure over the Aleutian islands and the stalled ridge over Greenland (negative North Atlantic Oscillation). The position of these high pressure ridges have allowed low pressure to undercut them through the middle of the US. This has significantly suppressed the Bermuda high which brought us the warmth a couple weeks ago locking in below normal temperatures across the eastern 2/3 of the US.

Based upon all of these factors, I don't foresee any long stretches over warmth across the Great lakes and Ohio valley through at least the middle of May.  

The reinforcing front Sunday/Monday of next week could produce rain/snow mix.  Here was my forecast tweet from April 20 on the early May rain/snow mix potential.

As of this writing (April 25) the new EURO and GFS models show the Greenland Ridge & western North America ridge (red areas) holding strong until the second week of May then gradual weakening. This flattens the trough across the central US bringing an end to the unsettled pattern. These solutions were indicated on the SOI analogs more than a week ago.

Temperatures per the GFS also corroborate what the SOI analogs indicated 2 weeks ago.  That is gradual improvement across the central US and Ohio Valley but still no big signs of any long stretches of warmth.