Milder for Christmas while longer term cold looms

It will be pretty uneventful weather wise for Christmas around the area, expect sunshine to make a return with highs approaching 50.  It will be a “brown” Christmas this year with no snow on the ground, but that’s a distinction shared with a majority of the lower 48.  This NWS graphic shows 71% of the country will not have a white Christmas this year.  That’s the lowest percentage in the last 8 years.

2014-12-24-1412-WHTEXMAS-KEAX-SFC-PRCT-H000

Oddly enough, the higher peaks of Hawaii are under a Blizzard Warning:

2014-12-24-1412-HAWAIIBZ-PHFO-WWA-MAPS-H000

8+ inches are expected to fall on the tops of the volcanoes there.

Elsewhere in the lower 48, Winter storm watches/warnings/advisories cover the central and northern Rockies, Wind advisories are up for the Mojave Desert, SE AZ & SW NM as well as for the eastern Great Lakes, upper Ohio Valley and upstate NY.  Flood watches are up or New England.

2014-12-24-1412-CONUSWWA-EDDX-WWA-MAPS-H000

The remainder of the country is rather quiet.

Around here we’ll have a shot of seasonably cooler air to start off the weekend.  There may be some light rain/light snow as this cool front moves in and we’ll have more on that Friday.  To the distant north, cold air is now building in NW Canada and the Arctic.

2014-12-24-1400-12ZGFSLR-HSFC-2MT-ARCT-H000

The chill is building further south than in recent weeks with subzero readings now reaching into northern Alberta.

We’re starting to see a realignment of the jet stream pattern.  Take a look at the pattern back on 12/9 (left) and the pattern expected on 12/29 (right).  The fast cross-Pacific jet has retreated from as far east as near 140W, back to near 160E.  The flow has also slowed down and become more amplified (click to make the image more readable).

2014-12-05-1651-12ZECMNHI 2014-12-24-1427-12ZECMWF-H200-WND-NOHM-H144

The flow which was formerly west to east and kept the Arctic air at bay is now forecast to become more north-south allowing cross polar flow.

The upper air chart for next Monday shows a ridge extending north to Alaska and a trough downstream over west central North America.

2014-12-24-1427-12ZECMWF-H500-GHA-NOAM-H144

This type of flow promotes the buildup of a strong Arctic high:

2014-12-24-1427-12ZECMWF-HSFC-MSL-NOAM-H144

The 5K foot temperature chart shows the flow of cold into the central and western U.S.

2014-12-24-1427-12ZECMWF-H850-TMP-NOAM-H144

In term of snowstorm potential, there are some signals for increased snow in the longer range, but there is no coherent signal for a big snow within the next 10 days at this time. That could well change however and with the incoming cold, certainly chances will be better than they’ve been for the month so far.  A lot will have to do with just how strong and deep the Arctic chill gets.

This example map shows a moderately strong cold Arctic surge with an active southern storm track (like we are likely to have this winter).  When the cold is not too deep, it allows systems coming out of the southern storm track to be able to lift more north and west as they track across the U.S.  Just how far N & W can vary with each storm, but the opportunity exists anywhere from the Southern/Central Plains to the Mid Mississippi Valley, Ohio Valley and Great Lakes to get heavy snow.  Ice is more likely from the Red River Valley to the Mid South. Northern stream clipper lows provide light snows to the Northern Plains and northern Great Lakes.

weak arctic-full

Here’s an issue we can run into is if the Arctic surge is too strong and too sustained.  The colder, denser higher pressure air will tend to suppress southern storm track systems southward across the southern states.  Heavy snows are more likely from the Southern Plains through the Mid South and Lower Ohio Valley.  Ice can occur from the Texas Plains into the Southeast.  Clipper lows give lighter snows to the Upper Midwest, Great Lakes and Northern Plains.

strong arctic-full

Last winter, we didn’t have an active southern storm track and the Arctic air masses were strong and deep.  This kept strong winter storm systems less frequent here but more active in the eastern U.S.

strong arctic-blocking

 

So the question to be answered as we move into this colder pattern will be just how strong and how persistent this incoming chill will get.

 

 

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