Friday, February 12, 2021

First take on President's Day winter storm

For all of you who have been begging for winter - whether you wanted really cold temperatures, ice (why would you want that?), or a "real snow" - I hope a week from now you're satisfied. Winter has arrived and will be locked in for a good week. 

Before I get into our President's Day storm, I can't skip over the fact that it is going to get #StupidCold. Cold like we haven't seen in a couple of years and a duration  that hasn't visited the Memphis area in 20 years.  We've been below freezing since Wednesday evening and may not see 33 degrees again until mid to late next week. The last time we had five consecutive days below freezing was around New Year's 2001. 

A listing of all streaks of 5 or more calendar days in which the temperature did not rise above freezing since 1875. 2001 was the last time we reached 5. We may reach 7-9 days in this Arctic outbreak.

The current forecast low on Tuesday morning is 8 (I'm not missing a digit) with a wind chill below zero. That hasn't happened in 3 years (January 2018). Please take care of your pets, other people, and your pipes. Pipes on exterior walls can be a problem when the temperature approaches or drops below 20 degrees.

Moving on to the fun stuff... when will we get a REAL SNOW??  Monday

The general pattern

It's still a bit early, and once again we have model inconsistencies, but at 3 days out a definite pattern has emerged and now it's about nailing down the details as the storm starts coming into the range of our higher resolution models. In sum, we'll have cold air in place, low pressure tracking by well to our south, an upper level trough moving through the Mid-Mississippi Valley, and strong southwest flow bringing in the moisture aloft.

The Weather Prediction Center's thinking on the general pattern is shown below. That NWS graphic indicates the potential that more than 0.25" of liquid precipitation will fall in snow or sleet form between 6am Monday-6am Tuesday. The general ratio used for snow is 10:1 - meaning that 0.25" of liquid falling would be 2.5" of snow. For sleet, it's 3:1, which would equate to about 3/4" of sleet. We are currently right around a 50% chance of that happening, but I will tell you the model data would indicate that chance should probably be higher.

Probability of 2.5" of snow or 0.75" of sleet between 6am Monday and 6am Tuesday (NWS/WPC)

The details we can offer 3 days out

Cold air at the surface is a given - mid 20s for highs on Sunday, mid teens for lows on Monday morning, and near 20 Monday afternoon. Above us, it appears there will be a little bit of above-freezing air a few thousand feet up until Monday morning. Models indicate we could start to see some precipitation spitting down on us Sunday afternoon but slightly better chances are Sunday night. With that warm air above, I foresee sleet or a sleet/snow mix to start. By about sunrise Monday, as the stronger dynamics approach, that air aloft will cool to freezing and the stage will be set for snow. The even won't wind down until late afternoon Monday with some flurries likely lingering into Monday evening. 

The upward motion generated by the low pressure trough moving through will be quite impressive. The cold air is equally impressive. And the total atmospheric moisture will be more than sufficient as well. All good news for snow-lovers. The caveats are the track of the low which affects the temperatures aloft. If we were to be slightly warmer aloft, or for a longer period of time, we would get more sleet. If it starts colder than anticipated Sunday night, then we might start as snow earlier and the totals would be higher. 

For now though, I'm siding towards the cooler (and more reliable with the recent ice storm) GFS and NAM models. Thus, I believe we could see a thin layer of sleet covering the landscape Sunday overnight, then snow most of the day Monday. Heaviest snow is most likely from mid-morning to mid-afternoon. Snow amounts will be higher west of the river and lower east of the metro, but my early thinking is 3-5" of snow in the metro is realistic. It would not shock me if some areas see more than that. Believe it or not, the upper end of that range would put us in the top 25 snowiest days on record in Memphis.

Adjustments are likely as we get closer to the event and there will be model runs that forecast as much as a foot. If it seems too good to be true, it probably is! I am not one to knee-jerk changes to the forecast because of a single model run, but if I see trends in one direction or another, I'll adjust. Remember that wind chills during this event will be in the single digits - it will be #StupidCold.

Looking ahead, we'll probably just be recovering from the President's Day system when the next one arrives late Wednesday into Thursday. This one could be even more tricky as we'll be starting to emerge from the Arctic Outbreak, so temperatures will be crucial. Early trends are towards more freezing rain and sleet than snow, but it's a week out and we might just get some freezing rain and rain! 

More updates to come on social media. Follow our latest forecast in the MWN app or our mobile website.

Erik Proseus
MWN Meteorologist

Follow MWN on Facebook and Twitter for routine updates and the latest info!
Complete MWN Forecast: on the mobile web or via the MWN mobile app
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!
MWN is a NOAA Weather Ready Nation Ambassador Meteorologist Erik Proseus is an NWA Digital Seal Holder

No comments: