MWN Blog

Wednesday, February 10, 2021

Latest details on our approaching ice storm [as of Wednesday 6:45pm]

[ UPDATE 6:45pm: While temperatures dropped a bit more quickly than expected late this afternoon and now sit below freezing across all of Shelby County, this does not change the earlier forecast for precipitation amounts or impacts. Most, if not all, precipitation was going to fall once temperatures fell below freezing anyway. ]

Current Warnings

Most of us are now under an Ice Storm Warning (counties in purple below). This includes Memphis and Shelby County and most of the greater metro. This warning is issued when there is reasonable confidence that at least one-quarter of an inch of ice (freezing rain) will fall, resulting in significant disruption to travel and commerce and threatening to life and property.

Warning map as of 1:00pm. Ice Storm Warning in purple, Winter Weather Advisory in blue.

The blue shaded counties (Tate and Marshall in the greater metro) are under a Winter Weather Advisory with lesser amounts of ice forecast, specifically up to 0.20".

What type of precipitation and when?

Temperatures have started falling off their highs in the upper 30s to near 40 degrees and will continue to slowly regress. I expect northwestern areas of the metro (Crittenden/Tipton) to be at freezing around sunset. For Shelby County and the rest of the metro, it will likely be down to 32 degrees by 7-10pm this evening. Any precipitation that falls after that time will be in a wintry form.

Because temperatures above about 2000 feet will be well above freezing, the precipitation will fall as rain. Because surface temperatures will be below freezing, once it hits the ground precipitation will freeze on whatever it touches that is at or below 32 degrees. This is freezing rain (or ice), and it is the most dangerous type of winter precipitation. Freezing rain accumulates ice on exposed objects, tree branches, power lines, vehicles, elevated surfaces like overpasses and bridges, and eventually regular road surfaces. The most likely time for freezing rain to occur is between 10pm tonight and 1pm Thursday. The heaviest will occur between about 4am-10am.

How much?

Confidence has increased in the warned counties to expect that more than 1/4" of freezing rain will accumulate. Some areas to the north of Memphis (Tipton County and points north and west) could see a 1/2" of ice during this event. Some models are even showing up to 3/4"-1" of ice. We feel that that is a very low chance of occurrence.

Forecast ice accumulation for the Mid-South from NWS-Memphis.

One of the saving graces of moderate to heavy freezing rain is that it tends to runoff well and not accumulate. In other words, moving water doesn't freeze as quickly as standing water, especially with a temperature just below freezing. If it is coming down fast, it simply runs into the gutters and storm drains. This is one reason I expect that actual ice accumulation on objects could be less than the amount that actually falls. I hope I am right!

What are the impacts and for how long?

The impacts from a moderate ice storm are many, but not as great as say the Ice Storm of 1994 some of you remember (which actually occurred 27 years ago today), which produced well over an inch of ice and left 300,000 MLGW customers without power, some for weeks. The main concerns are:

Falling tree branches due to the weight of accumulating ice
Snapped power lines, from either the ice or branches falling info them, resulting in power outages
Bridges and overpasses frozen like popsicles - very dangerous travel
Surface streets icing over
Improper use of generators if power is lost, resulting in avoidable illness or death
Kroger running out milk and bread (OK maybe not, but once in a while we need a good laugh!)

The Experimental Winter Storm Severity Index from the NWS depicts a level of impact due to an upcoming winter storm based on a variety of factors. For this storm, the NWS predicts "Moderate Impacts" (level 3 of 5) as described in the legend.

All of these are entirely possible given the potential of the storm. In particular, I highly discourage unnecessary travel after midnight tonight through most of Thursday. If you have to cross a bridge or overpass, it could be a real challenge, particularly Thursday morning. Power outages are likely; we just have to hope they are minimal and able to be rectified quickly.

Starting tonight, except for perhaps a couple hours tomorrow afternoon, we will not see temperatures back above freezing until likely the middle of next week, as an Arctic blast moves in. Consider this when preparing for the storm. There also won't be a lot of sunshine to help melt ice, particularly in normally shady spots. Even with reduced travel, I expect primary roadways to improve by Friday, if not before. Secondary roads may need another day.

So to recap:

Ice Storm Warning is in effect for most of the metro. Winter Weather Advisory for Tate and Marshall Counties.
Best chance of freezing rain is 10pm tonight through 1pm Thursday
Heaviest precipitation will occur between 4-10am Thursday
Areas in the Ice Storm Warning will see 0.25"-0.50" of ice. Tate and Marshal Co's will receive up to 0.20" of ice.
Significant impacts are expected starting early Thursday and perhaps continuing into Friday.
Power outages are likely and travel is strongly discouraged unless absolutely necessary.

Once this event moves into the rearview mirror, we'll look ahead to early next week when another potentially potent system moves through the Arctic airmass entrenched over the Mid-South!

Erik Proseus
MWN Meteorologist

----
Follow MWN on Facebook and Twitter for routine updates and the latest info!
Complete MWN Forecast: MemphisWeather.net 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

Sunday, February 7, 2021

Arctic cold air and precip chances this week - what is known and unknown?

I've purposefully not put together a blog on this coming week's weather pattern until now, because sometimes there's just too much guesswork to forecast with much confidence. But as we prepare for the week ahead, it's time to at least address the knowns and unknowns, and perhaps describe why this forecast appears to be so problematic.

Knowns

A few days ago, models, forecasters, and weather apps were debating whether an Arctic cold front would settle into the Mid-South early this coming week. We now know that that front arrived last night, bringing us overcast and cold conditions today, but will likely move back to our north for the first half of the week. It should reach northwest TN by Monday night, bringing us near average temperatures as wind turns southeast, and perhaps a few peeks of sunshine Monday afternoon. Look for low to mid 50s with any luck tomorrow.

The surface weather map forecast for early Monday evening shows a warm front to our north, putting the metro back in "warmer air" by that time. (NWS/WPC)

The front should remain just to our north Tuesday and Wednesday. With moist southwest flow aloft over the area, and a disturbance or two moving through, we'll see minor light rain chances on Tuesday and Tuesday night and slight better precipitation chances Wednesday. A few days ago, we debated a wintry mix on Tuesday. Now, it's pretty clear that it's all liquid with low temperatures in the 40s and highs in the low 50s. (There is a chance that the front will move back towards the metro Tuesday, but not far enough south to bring cold air to freeze any light precipitation. The high Tuesday could be optimistic though.)

I'll come back to Wednesday night through Thursday night in a minute - the details of this time period are definitely NOT known.

Also known (I think): the Arctic front will have finally moved well to our south by Thursday night. That will bring bitterly cold air on gusty north wind and it will likely be dry Friday and Saturday. The magnitude of the cold air (i.e., HOW cold?) is a still a bit of an unknown as well.

Unknowns

Let's re-visit the Thursday timeframe. Forecast confidence is extremely low for this day, and that's precisely when the bulk of our precipitation is expected this week. Model data has been consistent in its inconsistency and each model seems to be bullish on its own solution. It's kind of like a 50-50 split in the Senate. The trick is figuring out which side might blink and in this case, there's no tie-breaking vote to be cast. The ultimate solution likely lies somewhere in between the extremes, and that's where our forecast falls until some consensus can be developed (there's a better chance of that in weather than in politics!).

So what specifically do we see in the models? The European model is consistently warmer during the precipitation event Thursday with all rain until the precip departs, then cold and dry. The American GFS model is consistently colder and brings a wintry mix on Thursday that is more favorable for ice than snow, then dries us out as the cold-enough-for-snow air arrives Thursday night, much like the Europeans.

As an example of how different they are, the Sunday morning run of the Euro model has us at 42° Thursday at noon, while the GFS has Memphis at 27° at the same time. Both have precipitation departing around that time. So what is it going to do? With our luck, it'll be 33° and raining, can I get an Amen?? (I love forecasting in the south!)

The other unknown is how cold we get for Valentine's Day weekend. While the models again differ on the magnitude of the cold air outbreak, the correct answers are "cold enough to snuggle up to your Valentine," "cold enough to get some extra firewood for the fireplace", or just #StupidCold. The more aggressive GFS model drops lows to near ZERO next weekend with highs not far above 10°. That's obnoxious.

The perhaps-optimistic Euro model has lows in the upper teens to lows 20s and the coldest day (Saturday) with a high of 36°. Again, the reality is likely somewhere in between. For now though, I'm undercutting the Euro model just a bit, because I look at the GFS and just can't....

In reality, although a full discussion of the topic is likely good material for a separate blog post, meteorologists have access to "ensembles" of these particular models. In short, ensembles take many different iterations of a particular model (say the GFS) and produce a mean, or average, of all of those iterations to give us a more reasonable estimate of what will actually happen. You may have seen "spaghetti plots" of tropical storm tracks, which show all the potential tracks of a storm. Same thing with ensembles, only instead of storm tracks, it's temperature spread.

In the case of the GFS ensemble, its lows next weekend are close to what the Euro model has (upper teens) and highs in the mid 20s to low 30s. That seems much more plausible, though we can't rule out the extremes, like the main GFS model which would allow us to deep-freeze a COVID vaccine in open air.

Looking beyond this week (which should not be done considering we can't figure out this week!), a general moderation in temps is forecast following Valentine's Day, but probably still below normal for a minute. We'll need to closely monitor precipitation chances for a while, as "below normal" in February can still spell T-R-O-U-B-L-E if you don't want snow (or S-N-O-W-D-A-Y if you do!).

We'll have more on our social media channels and other means this week, especially as we get closer to Thursday and models hopefully become more agreeable! For the specifics of all of this, see our forecast in the MWN app or MWN website.


MWN is a NOAA Weather Ready Nation Ambassador	Meteorologist Erik Proseus is an NWA Digital Seal Holder

Pages