SPRING SNOWSTORM, SOUTHERN STYLE...

If you are in my northern counties, especially north of HWY 30 the coming storm will be nothing more than a close call without so much as a snowflake. If you're in the south, especially south of I-80 get ready for a potent spring snowstorm that could dump up to 8" of snow along the HWY 34 corridor Thursday night. For some, in far southern Iowa, WC Illinois, and extreme northern Missouri this could be one for the record books as far as late season snowstorms go.

This is the Winter Storm Severity Index issued from the Weather Prediction Center. You can see in the orange an red shades where moderate to major impacts are expected. In far southern Iowa significant tree and power outages are possible from the weight of the heavy wet snow.

Winter storm warnings and advisories are now in effect for Thursday night and Friday morning in the southern half of my area.

Late Wednesday night you can see the energy that will drive the storm getting set to move on the central Midwest.

The Weather Prediction Center shows these odds for at least and inch of snow.

The NWS in the Quad Cities put out this graphic in a situation report regarding storm impacts.

The NWS in Des Moines produced a similar report.

As with any snowstorm, the track, moisture, and intensity are keys to who gets the snow and how much falls. What will make or break my area is the strength of the dry air that's in place over my northern counties. This amazingly cold air mass produced a low in Cedar Rapids of 15 Wednesday morning breaking the old record of 17 set in 1928....the second consecutive morning with a record low. As most of you know the colder the air the drier it is. Proof of that was the available water vapor (or PWATS) which were as low as .15" in in the northeast half of my region Wednesday. Today at 1:00 pm they are still shown holding in that range.

That cold, dense, dry air is going to be tough to budge and in the graphic below you can see the sharp gradient between the moisture fighting north and the dry air (in red) stubbornly resisting. That will create a very tight cut-off to the northern edge of the snow. You are either in or you are out.

That stationary wall of dry air will provide generous lift as relatively warm moist air glides over the top creating the band of snow that runs along its southern flank. All the models show the sharp northern edge of the snow shield. Where they disagree is where that sets up. As was the case yesterday the hi-res CAMS push the snow further north, almost to HWY 20. I can't see any logical reason for that so I have thrown those solutions out. Here you can see what they depicted for snowfall. Notice too how contaminated they are showing 20-21 inches of snow in SC Iowa. I would bet strongly against that!

The 3k NAM

The NAM (12K)

That brings me to the reality of the model world and in a rare show of solidarity the GFS and EURO have been in lock step the past 24 hours. That leads to high confidence in their depictions. In other words, I'm buying what they are selling. Here's the GFS

Finally the EURO.

Boiling it down for my area, it appears the 1-2 inch line should run close to Cedar Rapids to Freeport, Illinois. The 2-3" line from Iowa City to near Clinton and Sterling/Rock Falls. The 4-5" line cuts from Washington, Iowa to near Moline and then on to Princeton, Illinois. Near and south of HWY 34, 5 to 9" totals should prevail with some banding producing amounts that in spots could reach 10". We are still far enough out in this event that a slight shift north or south is possible and in a situation like this that could add or subtract 1-2" in some of the fringe areas. We will need to monitor that closely Thursday.

Most of the snow holds off until after 5 or 6 pm where it develops. It will last the majority of the night and roads will become snow covered and hazardous, especially near and south of I-80. We'll get a fresh look at trends tomorrow morning to firm up the numbers. Hopefully, this is winter's last hurrah and it will be many months before the flakes fly again. Roll weather...TS

ARCHIVED POSTS
RECENT POSTS
© 2019 Terry Swails