January Thaw Moving In

After a cold couple days our January thaw is ready to move in. Melting snow, unseasonably mild temperatures, and a rain storm or two will leave winter lovers with a stream of tears come next weekend.

While the usual winter weather weenies are doubting the coming torch using arguments like, “Well, January’s temperatures have largest standard deviation of any month so +10 really isn’t that unusual,” the signs are strong for an impressive period of above normal temperatures. Many of us in Connecticut will have no snow on the ground in a week’s time.

The big question is how long does the thaw last? Current indications are that while the next 10 days will be mild, potentially quite mild compared to typical January weather, the thaw may be on borrowed time. I expect a return to winter by January 15th at the earliest but more likely by January 20th.

— Meteorological Discussion Below —

Day 1-5 00z Euro Ensemble 850 T Anomalies

Day 1-5 00z Euro Ensemble 850 T Anomalies / ย Courtesy Alan Huffman

Day 6-10 00z Euro Ensemble 850mb T Anomalies

Day 6-10 00z Euro Ensemble 850mb T Anomalies / Courtesy: Alan Huffman

The synoptic setup is pretty torch-tastic for the east coast. Above you can see the 850mb temperature anomalies with the greatest departures centered over New England. Below is the 00z Euro Ensemble day 6 to day 10 500mb anomalies.

Day 6-10 00z Euro Ensemble 500mb Height Anomalies

Day 6-10 00z Euro Ensemble 500mb Height Anomalies

A strong -PNA signal with a large trough over the west coast will pump up the ridge in the southeast. A southwesterly flow begins to advect milder air into New England and we’re looking at a pretty classic January thaw. Here are the raw 12z GFS MOS temperatures (lows/highs) for BDL. Keep in mind climo is 18/34 so that’s 7 days above average with 3 days of greater than +10 departures next week!

Screen shot 2013-01-04 at 2.17.15 PM

Beyond day 10, however, it’s unclear how much longer our thaw will last. This weather pattern is far different from last year and the warm-up is likely on borrowed time. The polar vortex over the North Pole is getting a big shake up.

The typically roaring zonal flow above the Arctic Circle thanks to the Polar night jet is slamming on the brakes.

ecmwfzm_u_f24

ecmwfzm_u_f144

In fact the night jet essentially reverses around 80N in the next couple days which will effective disrupt the Polar Vortex. The Euro forecasts of the polar vortex disruption can also be seen in through a potential vorticity perspective. Here’s the 600K PV analysis from today and the 10 day Euro forecast.

ecmwfpv600a12

ecmwfpv600f240

This PV split (notice the high PV lobes over the North Pole near D0 and separate at D10) will introduce some solid Arctic air into North America. The Arctic Oscillation will go strongly negative as the PV slides off the North Pole. The question will be where does the storm track set up and whether or not the true Arctic cold is able to slide far enough east for New England. The Euro Ensembles also couple the -AO with a -NAO thanks to ridging over Greenland. Cross your fingers snow lovers!

Models do show the milder pattern breaking down post-day 10 and it seems reasonable to expect winter will make a pretty sizable come back later this month. Stay tuned!

Advertisements

Winter May Finally Be Ready To Move In

After a seemingly never ending stretch of mild and snowless weather there are signs of a major change to the hemispheric weather pattern by next weekend.

The Arctic Oscillation has been strongly positive with a very powerful polar vortex over the North Pole. This has effectively kept cold weather bottled up in northern Canada. The ice cold stratosphere north of the Arctic Circle is beginning to warm and that warming is going to dislodge the polar vortex from the North Pole and bring winter to the United States by 1/16 or so.

Here’s the 500mb height anomalies on the European computer model for next weekend (day 8-10 ending on 1/16).

This is a total change from the pattern we’ve been dealing with since Thanksgiving. The +EPO Alaskan vortex is gone and replaced by a strong ridge over the Bering Strait which is helping to dislodge the PV over the North Pole. In addition a brief +PNA spike with ridging on the west coast helps send cold south.

Besides changes in the troposphere the changes in the stratosphere are impressive with a notable warming over the North Pole and disruption of the polar vortex. Here is the 10 day 30mb Euro temperature forecast. Between D0 and D10 there is a nearly +25C warming at 30mb at the North Pole. Definitely a change.

Beyond Day 10 the pattern remains chilly for the northeast with the +EPO nowhere to be seen and a -AO. The presence of a -AO has a big impact on temperatures across the U.S.

For people who want snow I don’t think this will be a prolific snow pattern with little upstream blocking (i.e. +NAO) but I do see seasonable to at time below normal temperatures for the second half of January. No question this is a major pattern change across the northern hemisphere. The question I have is how long does this change last – will it stick around through February? I don’t know.

If I had to guess I could see two wintry threats one around 1/16 with a clipper-type system and another around 1/21.

Just How Close Will it Get?

This is the storm that refuses to die. For most of Connecticut the storm is basically a non-event with no snow expected at all. In southeastern parts of the state it’s possible that the storm backs in just enough to produce a quick burst of light snow.

Areas further east like the South Shore of Boston and Cape Cod now may actually get hit by a decent thump of snow. The SREF and GEFS have lead the way showing the potential for this storm to come a bit closer than forecasted in the last day or so. Keep in mind this is after or medium range models totally backed away from this storm impacting us at all. What a long and strange trip it’s been with this storm.

Here’s the 00z NAM forecast precipitation. The light blue is >0.1″ with and the dark green is >1″ of precipitation. The NAM is likely too robust with the northwest extent of snow but it has continued to push west.

I’d say the odds of an inch or two of snow in southeastern Connecticut are about 25% but the rest of the state should get by with flurries at the worst. Never say never but right now it would take something close to a miracle to get a more significant (say 4 or 6″ snow) back into the Connecticut River Valley.

This weather pattern has been raising hell with our computer models. A fast flow coupled by an extreme block in the northern Latitudes complete with a retrograding polar vortex make this a very unusual and anomalous pattern that is difficult for the models to resolve.