CALM BEFORE THE RAIN AND SNOW...
The weather around the region looks quiet Wednesday as we get set for weather system that will shed rain (and some snow) Thursday night and early Friday. As mentioned yesterday, details regarding the evolution of the snow were uncertain but widespread precipitation was a given and generous at that. Here's where we stand.
At 500mb, short wave energy is reflected by at closed circulation over California Wednesday morning. Over the next 48 hours that streaks east in the sub-tropical jet setting up what should be a welcome and appreciable precipitation event with liquid equivalent totals of 1/3 to 3/4 of an inch likely.
Ahead of the disturbance, a relatively pleasant December day is expected Wednesday. One issue early may be some pockets of dense fog which are expected to burn off by mid-morning. That leaves the area with partly cloudy skies and mild temperatures in the afternoon. Highs will range from the mid 30s near HWY 20 to the low to mid 40s elsewhere.
Later Wednesday night clouds develop which thicken Thursday as a precipitation field grows ahead of a surface low emerging over SW Kansas. Some of this may get into my southern counties Thursday afternoon but the best lift arrives Thursday night when the deformation band lifts north into the region.
As was the case yesterday, confidence remains low regarding precipitation type, especially near and north of HWY 30 where conflicting thermodynamic profiles exist in guidance. That makes it tough to draw the rain snow line and pin point amounts, especially with the potential for rain to change to snow. Incorrectly determining the speed of that process can really wreck a forecast both on the high and low side.
As it stands now the GFS/GEFS suite of models is warmer suggesting mainly rain or mixed precipitation in the far north. The EURO, its ensembles, and models such as the NAM and HRRR are colder. They appear to be analyzing the strong vertical motion and evaporative cooling that keeps the column cooling during the heavier precipitation. That results in more snow than rain and some heavy wet accumulations up around HWY 20 and points north.
At this point, some mixed rain and snow (or even a change to snow later in the event) could get as far south as HWY 30. South of there it appears a cold rain will fall with little if any snow.
Getting to the models, this is what they are suggesting for snow totals. With the track and thermal profiles still somewhat in doubt, there could be significant changes in future model runs even into Thursday morning. I will also add, what you are looking at is model output. By no means is it a forecast. This is the guidance forecasters like me use to build a forecast. As you will see there is quite a bit of variety on snowfall totals in my northern counties.
I will start with the GFS and similar models which depict little if any snow in my area south of HWY 20.
The national model blend
Now, those models that show accumulations into my northern tier of counties, especially north of HWY 30 and closer to HWY 20.
The 12k NAM
The SREF ensemble
The Canadian GEM
My thinking is hedged towards the slightly further south EURO and some of its counterparts. I think the forcing looks sufficient to get the dynamic cooling necessary to keep precipitation all snow for a longer period of time. If indeed that turns out to be the case, a snow advisory may be issued for the north at some point. This is a very challenging forecast is certainly still in flux. We should have a pretty good idea of where trends are heading Wednesday night. Right now, the EURO shows these odds for an inch or more of snow.
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As for precipitation totals, here's what models are suggesting in that regard.
The Weather Prediction Center
The system ejects off to the east early Friday leaving the region with a chilly weekend. Highs Friday and Saturday should remain in the low to mid 30s north to near 40 south (any area in the north that gets snow cover may be hard pressed to get above freezing). A new twist today is that most models are indicating a period of light snow or rain later Saturday afternoon or evening. This being a new trend confidence is low. We'll see where that stands tomorrow. Sunday looks dry and readings may go up a couple degrees as winds turn to the southeast.
The atmosphere really unloads next week with a major storm coming out of the SW. This is going to bring a wide variety of weather to the central Midwest which could include a full fledged blizzard in the Plains and upper Midwest to thunderstorms in the warm sector Tuesday. Windy conditions ahead of the storm brings rain and mild weather to my area. Behind it, windy and much colder conditions with snow showers appears inevitable around December 15th.
The GFS and to a lesser degree the EURO, indicates the potential for Arctic air to enter the Midwest just before Christmas. At 500mb notice all the red at high latitudes from Alaska through Canada and into Greenland. That indicates blocking and high pressure in the polar regions which displaces cold southward into the U.S. That is the set-up you look for to let the Arctic hounds loose. Something to monitor in the days ahead.
That's all I have for you tonight. Take it easy, but take it! Roll weather...TS