Thursday, December 21, 2017

Winter officially begins wet; Christmas appears dry and cold

The winter solstice, the astronomical time that marks the official start of winter, occurred Thursday at 10:28am. From now through mid June, days progressively get a bit longer. But Friday may seem like the longest day of the year for children just out of school for Christmas break who can't get out due to steady, even heavy, rain!

Flash Flood Watch in effect Friday through early Saturday

Winter will start with above normal precipitation as multiple inches of rain appear likely on Friday and Saturday. Low pressure will move across the Midwest, dragging a cold front into the region. Meanwhile, another front will move through Texas and slowly towards the Mississippi River Valley near Memphis by Friday night, traveling along the stalled front. This will mean a prolonged period of potentially heavy rain as moisture from the Gulf of Mexico pools along the front. It will be mid-day Saturday before the front moves far enough east of the area as the low moves well to our northeast to bring an end to the rain.
Forecast rainfall totals through Saturday evening from the NWS Weather Prediction Center. A swath of very heavy rain will occur along the I-40 corridor Friday and Friday night. (WxBell)
When all is said and done, some places along the I-40 corridor could see 48 hour precipitation totals of as much as 5"! Widespread 3-5" totals are likely throughout the metro, most of that falling in a 24-hour window from Friday morning through Saturday morning. Rainfall of this intensity could easily overwhelm storm drains, particularly since rainfall earlier this week served to moisten up the ground, resulting in more runoff from this storm system. Flash Flood Watches are in effect and areas that normally see ponding or deep water in heavy rain are likely to see it again. Warnings are certainly not out of the question as we see exactly how rain evolves and areas creeks and tributaries could overflow their banks Friday night.  In fact minor flooding is already forecast along the Loosahatchie River in northern Shelby County.

The NWS has placed the I-40 corridor, or from southwest of Little Rock to northeast of Nashville, including the Memphis metro, in a "Moderate Risk" of rainfall capable of producing flash flooding, or a 20-50% chance of exceeding flash flood guidance.

NWS Excessive Rainfall Outlook for Friday and Friday night shows a category 3 of 4 (Moderate) risk of flash flooding. 
A few thunderstorms are also possible in the heavy rain pockets, though severe weather is not currently anticipated, apart from the potential flooding. If you will be out on the roads Friday, or particularly Friday night, you are encouraged to watch for signs of high water and avoid those areas! You know the mantra: "Turn around, don't down!" There is nothing so important that you have to risk your life, or others, for the most direct route. Allow extra time to reach your destination if you must be out and be sure the wiper blades are operating well!

Christmas Weekend

As rain moves out Saturday morning, cloud cover will stick around and it will get colder. After highs in the 60s Friday, Saturday's temperatures will remain in the 40s all day with a cold north wind. Christmas Eve and Christmas Day were be monitored closely earlier this week for the potential for some winter mischief, but it appears the only "mischief" we might get could include some additional cloud cover on Sunday and the very low potential for a few sprinkles or a late evening snow flurry, as an upper level trough moves through. Most moisture will remain north of the region, so I expect a mainly cold, dry Christmas Eve with lows in the 30s and highs in the 40s.

Christmas Day itself will not be white, which is not a major surprise given that our odds any year of a White Christmas stand at less than 2%! Expect a cold day with some sunshine and highs that barely reach out of the 30s as polar high pressure builds in to our north.

Next week - another threat of winter mischief?

Tuesday and Wednesday also looks dry and cold with lows in the 20s and highs in the 40s. A slow moderation in temperatures is expected as we near the end of the week when the next potentially wet system starts to move in from the west behind the retreating polar high. This one we'll need to again monitor closely as an early arrival of precipitation into a retreating cold air mass could mean the potential for a wintry mix. For now, we'll call it a rain, and hope that it is gone by Saturday, December 30, when the Memphis Tigers take on Iowa State in the Liberty Bowl! It's simply too far out with too much model uncertainty to say otherwise!

Thank you so much to all of you who follow us throughout the year! We wish each of you a very Merry Christmas! Stay tuned to the latest updates tomorrow and Saturday, as well as through the holidays, via our social media channels and mobile app! Pertinent links can be found below.

Happy Holidays!

Erik Proseus
MWN Meteorologist

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