GET READY, WINTER IS WAKING UP...
Old man winter has been taking a big old nap for the better part of the last 3 weeks here in the central Midwest. Mild temperatures and rain instead of snow has been the rule. Heck, we even had a couple of extremely rare tornadoes for good measure Monday. Here's the temperature departures sinc December 28th. From Hudson Bay through the Gulf of Mexico, North America was cooking by January standards!
For some perspective, here's the high temperatures at the Quad City International Airport since December 28th. For the 3 week period ending January 17th the coldest high temperature has been just 30 degrees. Only 2 of the 21 days have seen highs below freezing. The warmest day of the period was the 29th with a record breaking 67! Every single low has been above normal with the coldest reading 15 degrees. To date, the month is running over 10 degrees above normal per day. Man, that is toasty.
Here's the high and low temperature plots relative to average. That is impressive.
Measurable snowfall over much of my area the past 3 weeks ranges from nothing to 2 inches. Notice how quickly amounts go up though in far NW Iowa, SE South Dakota, and SW Minnesota where a swath of 18 to 24 inches extends from ne Sioux Falls to Minneapolis. You are in or you are out!
TIME FOR SOME PAY-BACK
As everybody knows, all good things come to an end and that's definately going to be the case with the mild snowfree environment of January to date. Before we get to snow chances, just a quick mention of something going on in the stratosphere, where a well defined area of low pressure resides known as the "polar vortex". It's at its strongest in the winter and is focused near the North Pole, 10-30 miles above the Earth's surface in a layer known as the stratosphere. It contains the coldest air in the Northern Hemisphere. It can slosh around, stretch, and even break into two pieces.
Generally the plar vortex is stable and keeps the worst of it's cold confined to northern latitudes well removed from the U.S. Every so often (but not every winter), the strong winds that circulate around the vortex weaken or are disrupted. That allows the vortex to stretch and send an Arctic air mass into the lower latitudes of the Midwest. You can see in the graphic the typical stable vortex on the left and the disrupted one on the right.
Now, look at what the EURO is indicating January 19th at 10mb in the stratosphere. Instead of one stable core polar vortex, it has stretched into two distinct cores. That means the bitter cold air under the warming in the middle is capable of escaping and penetrating the U.S.
The 12Z EURO was all in on the idea. Around the 30th of January, it depicts and off-shoot of the polar vortex that has shifted south and is located north of Lake Superior. A strong northwest flow is driving Arctic air into the central U.S.
The morning of the 31st, the EURO is depiciting widesread temperature anomalies of 28-34 degrees below normal.
That translate to lows below zero in all locations, perhaps as cold as 15 below in my northern counties.
The GFS does not buy into the idea showing a warmer solution with no sign of that vortex north of the Great Lakes and that is a consideration. Of course, we are a way out and perhaps things modify or realign differently on the EURO but I think the cold is on the table, especially now with the MJO going through cold phases and the EPO (eastern Pacific Oscillation) and AO (Arctic Oscillation) turning negative. These are all strong teleconnections for a progressivly colder pattern. It will take a few days for the tide to turn but the change appears to be underway "if" the EURO is correct. Time will tell.
Here's the phase 1 temperature and precipitation analogs
SNOW FOR SOME...
That leads me to Wednesday night's storm which remains on track to deliver a healthy snow to the upper Midwest. But outside of my NW counties, avoids much of the rest of my region with minor amounts. In fact, the initial stages of the storm will be rain for all but the north where a mix eventually changes to snow. Despite the event being on our doorstep, Tuesday nights data is not as clear as I would like in my NW counties. Models like the EURO, GFS, HRRR, and Canadian have shown a slight shift southeast on heavier totals. They seem to be initialzing a tad cooler which produces less mix and an inch or two more of snow. It is hard to say if this is a solid trend or not. However, going with it there could be a band of snow 2-4 (possibly 5) inches near and NW of a line from Cedar Rapids to east of Galena. Southeast of a line extending from Iowa City to Maquoketa and Rockford amounts drastically diminish and it's mostly a rain event with a brief change to snow before ending. Accumulations would be an inch or less in SE Iowa and WC Illinois if trends hold. Here's what models are suggesting. These are not forecasts, just the raw data that allow forecasters like me to make interpretations like I just did.
The Canadian GEM
The SREF mean
The 3K NAM
Here is the official NWS forecast which based on the latest data looks pretty good. You can see there are large ranges in the northwest due to the lower level of confidence. That's probably the way to play it until we get more data in the morning on thermal profiles and track. However, most of the latest guidance has amounts on the higher side of the range in places like Cedar Rapids and Dubuque.
As things stand now, no advisories are out for any part of my area but I would say it's possible that the NWS will go with winter weather advisories for a few counties in the NW especially near and NW of HWY 151 including Cedar Rapids, Dubuque, and locations further NW. Most of the heavier snow is over late Wednesday night but some snow showers will linger into Thursday morning. After highs in the mid to upper 30s Wednesday readings will cool into the low to mid 30s Thursday. If there is a good snow cover in my NW counties highs there may hold in the upper 20s to near 30.
That's all I have time for now. Have a solid day and Roll weather...TS