Arctic chill is on the way for much of the country– but for the central U.S. this is a “typical” mid winter Arctic surge and not all that uncommon. It will feel a lot colder, especially after a mild December to date. New Year’s Eve still looks quiet but seasonably cold and there are still big questions regarding what happens this weekend (if anything) with the storm system which will bring winter weather to the Southwest U.S. this week.
All-in-all it is a rather typical late December day across the region. As shown below, temperatures are in the upper 30s to low 40s over the district – which is actually a degree or two above average. Remember that average highs in the closing couple of days in December are in the upper 30s.
The cold blue and purple colors over the Central/Northern Plains west to Montana and east to Minnesota show the Arctic air. Subzero weather continues in parts of ND/MN. Colder weather has actually spread southward to the Desert SW with Phoenix/Tucson in the mid 50s..where averages for this time of year should see temperatures in the upper 60s. It’s easy to see in the map, that the main push of the cold has been down the Front Range of the Rockies where there is less of a push eastward.
This cold air will bring snows to unusual locations. The map below shows 3-day snow forecasts for the Southwest. Snows will be visible on the mountains just E-N of Los Angeles, and from coastal locations, snow will be visible on the peaks south to San Diego and the Mexican border. Las Vegas NV is actually expecting a dusting to 1/2″ of snowfall and in an odd twist, there is a greater potential for accumulating snows in Las Vegas NV than there is in Chicago IL in the next 7 days.
Because of this, Winter Storm Watches cover a decent-sized chunk of land over AZ/CA/NV.
For us, the coldest morning of the week will be on December 31, Wednesday morning. Lows will range from the single digits out in the western district to low or mid teens for SEMO/SW IL and STL. Average lows for 12/31 range from 18-20 so temperatures will be 4-6 degrees colder than average for STL/SEMO/SW IL to as much as 10-15 degrees colder than average for the KC area. The strongest and deepest push of cold air is west of the district, with a cold core of -20 to -30F temperatures over western NE/WY/CO. If we had snow on the ground around here we could have been significantly below zero in KC and around zero in STL/SEMO/SW IL.
What’s powering the chill is a very strong high pressure area, forecast to peak early Tuesday morning at a central pressure of 1062 MB, or 31.36″ on your home barometer. The center of the high will build down from Montana into far W NE/E CO and rapidly collapse later in the week over the southern Plains. Pressures here won’t get that high.. they’ll peak at around 1038 MB in SEMO/SW IL to around 1042 MB in KC. That’s still a lot higher than we’re at right now so if you notice aches and pains tomorrow, it might well be the increase in atmospheric pressure.
25 years ago..another strong Arctic high took more of a direct aim on our area. That brought record cold to the region with temperatures around -20F locally in late December, 1989. The difference then was there was a lot of snowcover down over the Midwest/Plains and that high moved from Canada to the central U.S. and not south down along the Rockies.
This high gives us more of a glancing blow to the west and there is plenty of snow-free ground around here. After the cold wave peaks New Year’s Eve Morning, it quickly collapses. These two charts show temperature difference from average 12/31 at 6AM and then Friday 01/02 at Midnight. The lighter blue to white tones on the right image show temperatures just a few degrees below to near average. The darker purple and green tones have retreated westwards and become more scattered. (Click to make the images more readable).
Some sort of system is expected to result from the winter storm in the Southwestern U.S. this week. Unfortunately, as was indicated yesterday, there has been almost no confidence in any one outcome for this system. The situation remains that way today. Solutions for our western district vary from a significant winter storm to an icy mix to light snows or even nothing at all. Solutions for eastern areas of our district range from snow potential to a wintry mix to rain. Computer model guidance continues to flip back and forth from one computer model run to the next and from one computer model to another. There is still no real point in spending a lot of time on this system right now, and it’s just something to keep in mind as we move deeper into the week.