What Is Driving Cool, Wet Spring? Frost/Freeze Possible Next WeekendApril 27th, 2013 at 12:04 am by Chad Evans under Chad's WLFI Weather Blog
WHERE’S ALL THE WARMTH BEEN THIS SPRING?
The Greenland Block (AO & NAO):
It is the same pattern that has only occasionally broken down this spring: the Greenland Block. This pattern became established in February & has been a semi-permanent feature since with cold, cloudy, wet weather for nearly 3 months (only short breaks in between). It has really affected the entire pattern in the Northern Hemisphere, acting as a large rock in a stream, just making everyone’s pattern from Russia to the U.S. stagnant.
However, it has brought the warmest spring on record to parts of northeastern, eastern Canada & over Greenland. Additionally, very warm weather brought blossoming cherries to Washington State in mid-March; very early for that area. On the flip side, cold, wet spring has been persistent to the central & eastern U.S. & over central & western Europe.
Germany is now having their coldest spring since 1883 after the coldest March since 1883. Moscow had their coldest March since the 1950s & 2/3 of all weather stations in Czech Republic broke March records for cold.
Meanwhile, a large part of China has had its warmest spring on record
A measure of this blocking is the AO & NAO. This block creates negative AO & NAO. Notice how there has been negative AO for nearly all of of late winter & spring:
The Greenland Block puts a huge upper ridge over eastern Canada & Greenland, producing & deep trough in eastern & central U.S. & ridge in western U.S. It also blocks systems from moving in the central U.S. It also puts the borderline between winter & spring in the Plains & Midwest with pooling Gulf moisture.
This, with very active MJO every 30-40 days since January, brought a wet winter & especially wet April. MJO refers to clusters of intense rain/storms in the west Pacific that eventually get pushed into the U.S. They usually affect the West Coast, as such was the case in the winter. In April, the moisture has generally ejected into the Plains, bringing drought-busting rain & snow with FLOODING rainfall from Kansas & Iowa to Wisconsin, Missouri, Illinois, Indiana & Michigan. This entire set-up created the very wet regime in tandem with the overall cool weather.
This Greenland blocking pattern forces a trough into Europe with cold weather. It is a block that sits & can largely be tied to a warming stratosphere over Greenland related to changes in thickness of the ozone layer & can be tied to the lack of sea ice right now there. That ice has melted a lot in the past few years, but very strong, persistent winds of tightly-wound Polar Vortex last spring (brought us mild winter & record warm March & overall spring by keeping cold air tightly wound & confined) has scoured out so much Arctic sea ice.
The PNA has also been negative (which is actually an earmark for La Nina, but everything is currently neutral in equatorial Pacific), which produces cool weather in eastern U.S. & warmth & upper ridging in the West. The negative phase enhances upper troughing (cold) in Europe & over Siberia.
PNA is forecast to go positive as we get into mid-May & NAO/AO go neutral as Greenland Block sees a bit longer of a break down. This may cut the extreme wetness & warm it up for longer than 1- to 3-day spurts.
WORLD TEMPERATURE DEPARTURES IN MARCH (BRIGHT RED DELINEATES RECORD WARM MARCH……..ORANGE/RED WARMER THAN NORMAL MARCH………BLUES ARE COLDER THAN NORMAL MARCH):
ANOTHER COLD SNAP COMING…….BRIEF SUMMARY……..
After a significant warm-up this week, another cold snap may bring frost to the area next Saturday & Sunday mornings. A light freeze may occur in Newton, Jasper, Pulaski, Fulton, White, Cass & Miami counties with lows of 30-35 across the viewing area.
We need to watch to see if that freezing line moves southward. Given the warmth early to mid week, the fruit trees will greatly advance, making them a bit more susceptible to frost/freezing.
NEXT WEEK OVERALL SUMMARY……..
After showers & few t’storms late Saturday-Saturday night, tapering Sunday (highs generally 65-70), look dry with near 81 early week with showers/storms by Wednesday-Wednesday night, tapering Thursday.
After the long, long winter, 80s are possible to Minnesota by early week!
I wouldn’t rule out a few severe stormsWednesday-Wednesday night, though the severe threat does not look significant or widespread here (wind/hail).
After that, that cool weather comes in with 59 Thursday, 55 Friday & Saturday with those overnight lows dropping to 30-35.
Rain doesn’t look particularly heavy from these two events with 0.18-0.55″ from northwest to southeast from first one & 0.40-0.75″ from second one.