SNOW FOR A FEW, RAIN FOR MOST...
The first winter like storm of the season is on it's way to the central Midwest. For some in the north, several inches of wet snow is possible. For others in the south, especially south of HWY 30, temperatures will be just warm enough for what looks to be a cold rain. The main thrust of the precipitation occurs Thursday night and early Friday.
Confidence has grown enough over the past 24 hours for winter weather advisories to be issued in the far north. They extend from there northward into southern Minnesota and SW Wisconsin.
Late Wednesday the short wave responsible for the storm is ejecting out of the Southwest where it will journey northeast and cross the Midwest Thursday night. The GOES 16 water vapor loop shows the moisture plume coming north on the sub-tropical jet out of the Southwest. The upper air energy is also visible further to the NW entering Utah.
Healthy dynamics and a moist air mass will combine to bring widespread precipitation, something the regions needs. Tuesday starts as the 22nd consecutive day with no precipitation in Dubuque. That is dang hard to do. Before this system departs, most area will see 1/4 to 3/4 inch of liquid equivalent. Here's what models are indicating for total precipitation.
The 3k NAM
The 12K NAM
The SREF ensemble
The national model blend.
The Weather Prediction Center.
The challenging part about this forecast is the track and placement of the rain snow line. Many models, especially the U.S based GFS/NAM have moved more northerly depicting a warmer with little if any snow from HWY 20 south, the majority of my area. The EURO remains on a more southerly track with at least some minor snow accumulations down to about HWY 30. My guess is the EURO is seeing dynamic cooling and even some convective potential with with a bit of small CAPE (instability).
In my area most of the precipitation holds off until Thursday evening. For the most part precipitation begins as rain and generally stays in that form the balance of the event. However, near HWY 20 evaporative cooling Thursday night might allow a mix and then a full changeover to snow. It's even possible that some parts of this area could see a period of freezing rain or sleet.
This area north of HWY 30, especially near the HWY 20 corridor is where the uncertainty with track leaves a low confidence forecast. Just a very slight northward shift and even this area sees mainly rain and little snow accumulation. Much like what the GFS is indicating. On the other hand, a small southward track (or one like the EURO indicates) increases the chances for snow. Here's what raw model output is indicating for snow totals late Wednesday night. I'll start with the more aggressive models for snow in my northern counties.
Other guidance is further north keeping the majority of the snow in NE Iowa and SW Wisconsin.
The SREF ensemble mean
The 3K NAM
The 12K NAM
The Canadian GEM
The national model blend.
Personally, aside from the EURO and HRRR, I'm concerned that most models are now trending slightly north keeping the brunt of the accumulating snow just north of HWY 20 in NE Iowa and SW Wisconsin. It looks to be a very close call. In fact, so close this may not be resolved up north until the event kicks in. For now the winter weather advisory continues for the HWY 20 corridor. However, it would not be surprising to see a slight northward adjustment to the advisories as new data comes in Thursday.
The storm bodily pulls out by afternoon Friday but clouds and cool weather will prevail. Highs in the mid 30s are expected north with low to mid 40s elsewhere. Another round of forcing is expected Saturday. It's not a well organized system and models have been backing off on its precipitation potential. It may kick up some scattered light rain or snow showers in the afternoon and early evening but the threat has really diminished in recent model guidance . Highs of 35-40 should prevail. Sunday we'll be in-between storms and were looking at a dry and seasonally cool day with highs around 37 north to 42 south.
Early next week a sprawling high amplitude low pressure takes shape over the western Plains. As it deepens, southeast winds begin to tug warm moist air back into the region. Showers arrive late Monday night and may even blossom into thunderstorms Tuesday as the storm cranks up. This promises to be another significant rain producer as we stay in the warm sector of the storm during its peak precipitation period. Here's the storm at its peak Tuesday evening.
Eventually, a powerful cold front breaches the Midwest sending temperatures spiraling downward Wednesday. Once in the cold air, vorticity rotates around the core of the upper air low generating occasional show showers and flurries Wednesday night and Thursday. Little accumulation is likely.
I do think this storm finally opens the door to some sustained cold the week before Christmas. Here's the 5 day temperature departures December 18-23rd. The cold is pushing.
That's a ways into the future, and meantime there's plenty of energy and weather in the pattern to keep me focused on the present. Just wish we could get some snow for Christmas. I'm working on it! Roll weather...TS