Saturday, November 9, 2013

Holy cold snap, Batman! Major Canadian air intrusion poised to dive south

As we approach mid-November, we've had a fair number of cold fronts that have dropped overnight lows near or below 40, some places in outlying areas (especially to the northeast of the metro) have touched freezing, and we've had some days with highs only in the 50s. However, the first official "winter is getting closer" cold front is due in on Tuesday, so get out those heavy coats if you haven't yet!

A very cold airmass residing over north-central Canada is poised to make a run into the central portion of the U.S. then continue to dive south into the southeast U.S. as a major trough sets up over the eastern half of the nation.  This will bring our coldest temperatures yet this fall and likely drop everyone - including those in the warmest parts of the Bluff City - below freezing Wednesday and Thursday mornings.

Let's take a look at a couple of representations of the cold air infiltration. First, from the European model, showing the temperature anomalies (departure from normal) at ~5,000' up (below).  Keep your eye on the purple area (coldest temps relative to normal) in Canada as it drops south into the Mid-South. The loop begins Saturday morning and ends Thursday morning.  (All graphics below courtesy of WeatherBell.com.)

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Next, we look at the European model's U.S. counterpart, the GFS, for a little closer look.  Below we see near-surface temperatures (actual, not departure from normal) beginning Sunday evening and running through Thursday morning. Pretty easy to spot the leading edge of cold air as it dives south.

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A few snapshots of near-surface temperatures from the European model show it's projections for Tuesday afternoon maximum temperatures and Thursday morning low temperatures.  Note that actual daily high temperatures on Tuesday will probably occur at midnight (maybe near 50), with temps falling throughout the day.

European model forecast maximum temperatures between noon-6pm Tuesday (the high for the day will likely occur at midnight, with temperatures falling throughout the day).
European model forecast minimum temperatures for Thursday morning. 
One big question that people always ask (and it's never too early in the year to ask it, of course) when major cold airmasses move into the region is the chance of precipitation, especially hoping for some good news (or bad depending on your point of view) about "non-liquid" precipitation.  I kept my mouth shut on this a couple of days ago -- though trust me, I was looking -- when the European model seemed to think some "non-liquid" precipitation might fall.  I knew it was too early in the season and models tend to moderate as they get closer to an event.  Alas, I was right, they have.

Below is the expected RAINFALL amounts from the cold front during the morning hours on Tuesday.  You'll notice by the amounts (trace to a couple of hundredths) that the amounts are minimal.  This will NOT be a major rain maker.  I can also tell you that it is ALL LIQUID.  You're going to have to wait until deeper in the season for something of the non-liquid variety.

Expected rainfall amounts on Tuesday morning with this cold front are very light.
The bigger story, besides the falling temps, will be the wind. In the wake of the front, Tuesday's wind will be out of the north in the 10-20 mph range at least, with gusts possibly as high as 30+ mph. Below is the forecast wind chill values at 6pm Tuesday from the GFS -- in the 20s area-wide!

Wind chill readings in the 20s are forecast early Tuesday evening as north wind reaches as high as 20-30 mph.
Bottom line:  Plan for a significant outbreak of Canadian air mid-week behind a Tuesday morning cold front that will drop temperatures through the 40s Tuesday, bring light rainfall during the morning, as well as 20-30 mph wind throughout the day. Lows Wednesday morning will be in the 20s to near 30, highs Wednesday (despite sunshine) will likely remain in the 40s, even cooler temperatures are possible Thursday morning, then conditions moderate back to near normal as we head towards the weekend.  The complete forecast can be found on MemphisWeather.net and our mobile apps.

So, are you ready for some cold weather (even for a couple days) or are you already longing for summer warmth?

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