Active weather for the first week of December

This weekend's recap

In Friday's blog, the main topic was the wet weekend weather. We got that, though it could've been much worse for the St. Jude Memphis Marathon, or even more likely, the Christmas parades later in the day. In fact, though several entrants and (by my eye test) hundreds of spectators skipped the Bartlett parade Saturday afternoon, it wasn't too bad with "heavy sprinkles" at times but no downpours. Most of the rain fell after dark. Today felt like mid-December with cool temperatures and low overcast hanging around, but very little rain.

In case you were wondering about the scale of the @StJude Memphis Marathon... pic.twitter.com/j77M194EfO

— Kyle Veazey (@kyleveazey) December 3, 2016

Monday low pressure

Another topic in Friday's blog was the upcoming active wether pattern this week. I'll provide a bit more detail on that tonight, starting with tomorrow's low pressure passage. Low pressure will develop along the western Gulf coast and begin to move northeast during the day, increasing our rain chances as it draws closer. Rain chances will ramp up from the south by early afternoon, becoming likely for the entire area by late afternoon and continuing overnight as the low will pass by just to our east. The map below shows the newest hi-res North American model (NAM) surface forecast at 3am Tuesday. I've annotated the low (red L) cold fronts (blue lines) and a warm front (red line).

The newest high-resolution NAM model shows surface low pressure just southeast of Memphis at 3am Tuesday with a cold front extending south to the Gulf and another cold front approaching from the west. The position of the low means few thunderstorms, if any, and no severe weather, but a good dose of rain. (WxBell)

With the low to our east, we won't get a good southerly wind and will avoid the "warm sector" of the system east of the low. That means thunderstorm chances will remain fairly low, though a few are possible as the low approaches, and severe weather chances should remain well to our south, along the central Gulf coast. We'll get more rain to help our drought situation, but even the rain shouldn't be heavy for prolonged periods. Most model data suggests we'll top out at no more than an inch. By the time you wake up Tuesday morning, the rain should be nearly done.

Mid-week lull

Tuesday and Wednesday will be "tweener" days - between major weather systems with weak high pressure in control. Clouds are likely to stick around, though a few breaks could bring peeks of sunshine. Temperatures will generally be seasonal with lows in the 40s and highs in the 50s. Wednesday night is when the "Big Kahuna" Arctic front arrives, as posted in Friday's blog. Scattered showers are expected with the front, but precipitation should be mostly an overnight event on Wednesday night.

Late week freeze

MUCH colder air, originating in Alaska, will dive southeast into the western U.S. Monday, cross the Rockies Tuesday, and surge across the central U.S. Wednesday, arriving in the Mid-South by Thursday morning. The coldest air of the season for the U.S. will arrive on gusty wind with temperatures falling towards freezing Thursday morning and remaining in the 30s all day with wind chills in the 20s. By Friday morning, lows will reach the lower 20s with wind chills in the teens and highs again only be in the 30s, or about 15-20° below normal.

The American GFS model temperature anomaly map (how much warmer/colder than normal temperatures will be) for Friday morning shows a large area of below normal temperatures east of the Rockies. We'll be about 15° below normal late this week. (WxBell)

Snow references, and what I think

I've seen several references, including in national, model-driven "crap apps," a national weather network forecast, and even in local weather broadcasts, to the possibility of snow Thursday morning. Yes, it will be cold enough to snow at some point Thursday morning. But the two main players in the mid-range model world - the American and European model - differ on how quickly we get to "cold enough."

In my 20 years of experience forecasting Memphis weather, these types of systems (in which a strong cold front arrives from the northwest, dropping temperatures as moisture departs) are not prolific (or even marginal) snow producers. Almost always, sufficient moisture for precipitation is gone by the time the low level thermal (temperature) profiles are cold enough to support snow. If it's going to snow in these situations (at least in this part of the country), the most common scenario is to have the sudden burst of cold air produce snow flurries or a brief snow "shower" as the last of the moisture moves out. I can't rule this out early Thursday morning. I also am not including it in the forecast at this time. If I saw that possibility occurring, I likely wouldn't include it in the forecast until about 36 hours ahead of time. There's too much uncertainty in predicting the precise temperature profile to include it in this type of atmospheric setup unless it's a virtual lock. That won't happen until we're fairly close to the event in question.

Below is a piece of evidence from the colder of the two models - the American GFS - to not include snow. It indicates the higher elevations of the Ozarks is the closest place to us that would get anything worth mentioning.

The American GFS model total snowfall through Friday evening indicates that no accumulating snow will fall in the metro. A couple inches could be possible in the Ozark Plateau however. (WxBell)

After a drop into the deep freeze Thursday and Friday, a slow moderation begins next weekend. In addition, the next weather system capable of producing precipitation arrives next Sunday. At this time, it appears temperatures will be warm enough to support rain, so that's my forecast. We'll keep an eye on timing to see oif any precipitation can occur prior to the arrival of sufficiently warm air. THIS is the type of scenario that, historically, has provided a slightly better chance of wintry precipitation. Again, let me be clear, that's not in the forecast either. And it's still technically fall...

As always, get the complete forecast, with regular updates made by an actual person, not a computer model, via our website or mobile apps. Links provided below. Pay the $0.99 for human-powered weather and avoid the hype and the crap!

Erik Proseus
MWN Meteorologist

p.s. 2017 MWN wall calendars featuring pictures of Mid-South weather taken by local photographers (and myself), as well as Mid-South weather events and records throughout, are now available. These are made primarily for your enjoyment. We price them to break even. Check them out here.

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