Sunday, November 9, 2014

Arctic intrusions could be the norm for the second half of November

As you've no doubt heard, the first major Arctic outbreak of the "winter" is closing in on the Mid-South (and please don't call it the polar vortex or I'll have to sic Guido on you!). Enjoy the next 36 hours or so because we won't see temperatures this warm again for some time. Model data is very consistent in the general pattern indicating cold air will move in by mid-week and stick around a while.

Very cold air has moved into the northern plains this afternoon and will continue to move south over the next 3 days or so. By week's end, freezing temps will be found nearly to the Gulf Coast. Temps near 70 in the central Plains will be long gone by tomorrow.
In fact, multiple rounds of cold air could move over the area, reinforcing the general pattern of well-below normal temperatures for several days. Right now, there is evidence to indicate that cold air (of varying magnitude) could possibly last up to 2 weeks, or right up until around Thanksgiving!

Below are the temperature anomalies (departure from normal) for 5-day windows through 2 weeks from now. Shown are two versions of the same model - the Global Forecast System (left) and GFS Ensemble (right). The European model supports this general cold idea as well through at least 10 days. You'll see that the models are similar in the days 1-5 and 6-10 periods while the GFS stays colder longer (into the day 11-15 period).

Departure from normal temperatures from the GFS and GEFS models for 5-day periods starting today. Description of these graphics is provided in the blog text, but suffice it to say, very cold temperatures will move in and hang around for more than just a couple of days. The GEFS model data indicates a slow retreat of the cold air after Nov 19, while the GFS reinforces it. Graphics courtesy WeatherBell.
The GFS model averages about 5 degrees C below normal in the metro from Nov 9-14 (top left panel), even considering that one of those days - tomorrow - will be above average. The GFS then cools even more to about 8-9 degrees C below average from Nov 14-19 (middle left panel) and about 11 degrees C below average between Nov 19-24 (bottom left panel).  What do those departures translate to in temperatures we experience?

Nov 9-14 daily avg temp: 54 F - dept from normal: -9 F = GFS forecast avg temp: 45 F
Nov 14-19 daily avg temp: 52 F - dept from normal: -15 F = GFS forecast avg temp: 37 F
Nov 19-24 daily avg temp: 50 F -dept from normal: -20 F = GFS forecast avg temp: 30 F

Wow! Remember that the daily average temperature is the average of the high and the low temps. If that daily average of 30F were to occur, it means that the low temps would be well down into the 20s with highs likely only in the 30s to near 40! Below are the forecast highs and lows from the GEFS model shown above (right hand panels) for the next 2 weeks. Note that these temps tend to run 2-4 degrees cooler than actual and are useful only for trend analysis once you get beyond the first week or so, NOT for actual temperature forecasting. It just gives us an idea of what to expect.

GEFS forecast highs and lows for the next 2 weeks at Memphis. After the first 5-7 days, this information is only useful for trend analysis, not actual temperature forecasting.
As far as precipitation, when it stays this cold for this long, with average daily temps in the 30s, any system with precipitation that gets close will have to be monitored carefully for the possibility of some type of freezing. The first one to watch will be next weekend. It's still way too early to know exactly what to expect and when, but it bears monitoring, which we'll be doing all week long. We will keep you updated!

One thing is for certain - if you haven't gotten out the cold weather clothes or fired up the heater yet, this week will be when you need them! Tomorrow, we'll release our 2014-2015 Winter Outlook which may provide an idea if this is a trend for the upcoming winter or not. Stay tuned!

Erik Proseus,
MWN Meteorologist

Follow MWN on Facebook, Twitter, and Google+
Visit on the web or on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

No comments: