Saturday, December 16, 2017

Weather patterns shift as Christmas nears

Cooler and dry weather has dominated the Mid-South weather pattern for more than a week, with no rain since the 5th and daily average temperatures below normal for 9 consecutive days through Friday. That pattern will be interrupted by a more progressive one the next week. The polar jet stream remains to our north, but a series of upper level low pressure systems in the southern, subtropical jet stream kick out of the desert southwest across the southern U.S. This will mean a couple of shots at rain in the next several days, but overall milder weather.

The first upper-level system moves through tonight, bringing welcome rain to dry ground. Rain amounts have trended up a bit and I expect most of us could get at least a half inch. Light rain could begin as early as 10pm or so, with the heaviest after about 3am and lasting through 8am. By late morning, rain will be gone, leaving a dry rest of your Sunday. Following overnight lows in the lower 40s, we should see afternoon highs rebound to the upper 50s with partial sun.

Total precipitation from tonight/Sunday morning's system should be in the neighborhood of 0.50". (WxBell)

The Mid-South will be between systems Monday, though with increasing low level moisture, morning drizzle and low clouds are expected. The afternoon looks better with highs probably into the 60s. By Tuesday, the next low pressure system will begin ejecting out of the southwest flow from the desert southwest and toward the region. This will bring a chance of showers Tuesday and higher chances Tuesday night into Wednesday. This system is where medium-range models start to diverge just a bit. The track of the low will be key to placement of heaviest rain. The European model brings a stronger system right over the region with heavy rain likely. The GFS (American) model takes the low to our south, and thus the heavy rain misses us. Both indicate that by Wednesday night the rain is gone however.

The second low pressure system in the forecast brings rain Tuesday into Wednesday. The GFS model tracks the low to our south, while the European model (not shown) takes it directly over the Mid-South. (WxBell)

Thursday will be another "tweener" day with mild conditions, but to our west, a harbinger of potential trouble lurks. Much colder air surges into the Plains behind a cold front as upper level low pressure develops in the southern stream once again. That cold air moves towards the Mid-South Friday as the low brings additional precipitation chances to the region. Temperatures do appear to stay warm enough on Friday for all precip to be liquid, but it could start to get colder as the day goes on.

The GFS model for Friday morning shows cold air surging east behind a cold front, while low pressure in the southern stream (see south TX) moves northeast, drawing moisture north into the region. Rain is expected Friday, but the pattern could hold into the weekend as the cold air continues to shift east. (WxBell)
For Christmas weekend, all bets are off and all options appear to be on the table. The model data to this point has not been consistent either run-to-run (remember, models run multiple times a day) or with each other. Even this morning, the GFS and European are nowhere close on potential outcomes beyond Friday. The overall pattern though does appear to favor cold and potentially wet. We'll be watching carefully throughout the week for additional clues and some consistency in the model data.

In the meantime, it's not unwise to start giving some thought to potential hazardous scenarios. In this case, that could mean difficult travel due to snow, or perhaps more likely ice, throughout Christmas weekend. Now is not the time to hype, or even to just give credence to the worst case scenarios without acknowledging the equal likelihood of nothing at all. If you see that in your social media feeds, it's time to stop following that source. Below is some great advice from NWS-Fort Worth as winter weather scenarios start to pop up, especially more than a few days out.

And here's another good one that reminds us how foolish it is to trust detail of a long-range forecast, courtesy of NWS-Kansas City.

Also a reminder that most of the weather apps on your smartphones are computer model driven. That means they are like playing the Plinko game above, and they change (sometimes wildly) multiple times a day when new data comes out. We highly recommend you download, and use, the mobile app for your local weather needs, as the forecast is written by me, not a model. (Link to download below.) If traveling out of town, get the free StormWatch+ mobile app for iOS. You can get the National Weather Service forecast for anyplace in the U.S. on that app. (Android version coming in early 2018, but not by Christmas, sorry! You can visit on Android for the same forecast info though.)

We'll have further updates throughout the week on our social media channels, this blog, and in the official MWN Forecast. Stay tuned - and remember a day-old forecast is worse than a day-old donut!

Erik Proseus
MWN Meteorologist

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