WINTER STORM COMES OUT SWINGING..
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WINTER COMES OUT SWINGING
A potent mid-winter storm is in the process of pounding the region with heavy rain that in some places ends with heavy snow. By the time the system departs, precipitation over an inch is possible in many areas. In fact, the GFS shows the odds of an inch or more falling at 80 to as much as 99 percent from the area near and west of the Mississippi.
Odds that high of an inch or more of precipitation in early February are rare indeed. The moisture feed northward out of the Gulf of Mexico has to be extreme. In this unusual case, it's being powered by an 850 mb jet streak (roughly a mile up) that's roaring at 78 knots (90 mph) early Thursday.
That results in PWATS, precipitible water vapor avialable for precipitation that's shown at 1.5 inches near the SE tip of Iowa. That's high even by mid-summer standards.
This graphic measures the water vapor at 425 percent higher than normal.
That's over 6 standard deviations above normal, very rightous!
Throw in this tight closed low over Missouri at 500mb, and you have the energy and lift to wring the moisture out in the form of heavy precipitation.
With all that in mind, it's no wonder this will be a generous precipitation producer across my area. Here's what models are advertising for precipitation totals through the event.
The GFS
The EURO
The Canadian hi-res HRDPS
The national model blend
The Weather Prediction Center QPF outlook.
I took the average of 7 different models of projected rainfall for the Quad Cities Thursday and came up with a total of 1.47 inches. If that happens to verify, it would be more rain in a day than typically falls in the entire month of February at the NWS office on Davenport, which is 1.44 inches. Crazy!
SO WHAT ABOUT SNOW?
One trend I've noticed tonight is that the high-res convective allowing models (CAMS) have continued to shift the track and heavy precipitation band ever so slightly to the southeast. The deterministic runs of the GFS and EURO have too but not to the degree of the CAMS. That trend is important as it would bring heavier snows to and beyond the Mississippi. It could also decrease totals in the NW which earlier looked to be a lock for the heaviest amounts.
Currently the NWS has has all the warnings and advisories posted for the area NW of a line from roughly Washington, Iowa to Galena, Illinois. If this new trend ends up verifying the NWS will need to push the adviories further to the southeast including places like the Quad Cities, Clinton, Muscatine, and Freeport. Alterations could be made to the warnings in the NW as well. This is all being contemplated at the NWS office right now and any changes will be out by Thursday morning
The other challenging aspect of this storm is the transition to snow. The process does not appear to take place until the 850 low passes south of any given location that recieves snow. Notice at 6:00am (with the 850 circulation SW of the area), the 0 isotherm ( the freezing line necessary for snow) runs from north of Dubuque to about Cedar Rapids. That implies the air aloft to the east is still warm enough to produce mainly rain over the majority of my area.
By 9:00am the 850 circulation is approaching Peoria and the freezing line has quickly advanced east of the Mississippi with dynamic cooling taking place in earnest. What I'm saying is the transition from rain to snow should take place around 7:00am west and 10:00am in the east. At that time, snow should commence falling at a good clip in the primary band. As the storm travels northeast, the snow advances with it departing all areas by late in the afternoon. There should be a good 6-8 hour window where accumulations occur.
That brings us to accumulations which are going to be tricky. It's hard enough to call when it's an all snow event, it's really tough when you have to time a transition. If the switch takes place earlier or later that could alter accumulations up or down by a couple of inches. Banding, which is tied to intense forcing in a small coridor, can also produce narrow bands of heavier snow. They are extremely hard to define at this distance and would be found in the core of the heavy snow band, often no more than 20 miles wide. This is the type of system with that capability.
Last but not least, if the trend of a track further southeast continues, amounts could very well end up lower on the fringe of the snowband in my NW counties. As you can probably tell, I'm working through this as new model data comes in. The new GFS, which has just arrived has held staunch, the EURO further southeast. I do think all the models are a bit too high on snow amounts. I'm using a method that accounts for snow ratios lower than 10:1 and I still suspect they are 1-2 inches too high...for what it's worth.
My best estimate on snowfall is based on the deterministic run of the EURO. Frankly, I think it is a good compromise and I'm leaning heavily on it. Subtract an inch or so from what it depicts and you should be in the ballpark. I think the 2 inch line stretches out on a line that runs close to Burlington, Geneseo, and Sterling Rock Falls. Amounts fall off sharply southeast of that line. As you go northwest 3-5 inch amounts are likely over much of eastern Iowa and could exceed that in a 30-40 mile swath that extends northeast between Cedar Rapids and the Quad Cities where banding is possible. In this area, some 6-7" totals are possible in spots. Here's what the EURO suggests for total snowfall
The EURO
The GFS is good on the southeast extent of the snow but likely too far northwest with its high end totals.
Here's what some of the hi-res models are indicating. Notice the placement of the snow band is further southeast.
The 12k NAM
The HRRR
One thing is for sure, what snow falls is going to be wet and stick to everything like glue. Winds could gust 30-35 mph but due to the heavy wet nature of the snow, little if any blowing snow is anticipated on the ground. However, there could be a few hours where heavy snowfall and winds concide to produce low visibilities, especially in the NW half of my area. Temperatures Thursday will generally be in the low to mid 30s, coldest during the snowy period.
After the system departs, temperatures will be colder Friday with highs in the upper 20s to low 30s. Mid 30s are expected over the snowcover Saturday and Sunday with low 40s where limited snowcover exists in the southeast. A dry weekend is expected in all locations.
That's the best I can do for now. Roll weather...TS
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