Saturday, October 22, 2011

Outlook for Next Week: First Major Snow in Rockies; Freeze Likely Deep South...

The Highlights:
The weather roller coaster ride that is typical for late October and early November will be prevalent again next week, starting mild to downright warm in many areas before turning cold once again.  Locally heavy snows will fall across parts of the Rockies by midweek (including the first snowfall of the season for the lower elevations of the same area), and the first widespread frost and/or freeze of the season appears likely for much of the South by late in the week...

Strong Cold Front to Bring First Widespread Freeze to South:
A strong cold front will progress Southeastward across the central and eastern two-thirds of the country during the week, bringing another significant cooldown in its wake.  Below are the latest GFS model projections of the location of the front (in blue) at 7pm CDT on Tuesday, Wednesday, Thursday and Friday:

Forecast valid 7pm CDT Tuesday, 10-25-11

 Forecast valid 7pm CDT Wednesday, 10-26-11

 Forecast valid 7pm CDT Thursday, 10-27-11

Forecast valid 7pm CDT Friday, 10-28-11

Temperatures will plummet to levels some 20 degrees below normal in many areas behind the front during the week.  Below are the temperature comparisons to normal (anomalies) as forecast by the GFS model, valid at 7pm CDT Wednesday, Thursday, Friday and Saturday, respectively.  The scale on the right is the departure from normal in degrees F.  Watch as the widespread greens, blues and purples (which indicate an increasing departure from normal toward the cool side) overtake the nation during the period:

High pressure and calmer winds building into the region behind the front will result in the first widespread frost and/or freeze of the season from Texas across the Deep South by the mornings of the 30th and/or the 31st.

The image below shows the GFS model's forecast of the temperature anomaly valid 7am CDT on Sunday, October 30th:

The darker purple shaded area across central and southcentral Texas shows where the model is forecasting a temperature of 20 degrees below normal at that time.  The normal low on that date for that region is an average of 55 degrees.  A little simple math indicates it will be a chilly morning indeed (55-20 = 35 degrees on average).

It will be very windy during the initial 24 hours or so after the front passes through a given area, with sustained winds of 25-35 mph and gusts to 45 mph possible in many areas from the central & southern Plains into the Northeast.

Locally heavy rainfall will break out along and ahead of the cold front from the mid-Mississippi Valley into the Mid Atlantic and New England from mid through late week.  Unfortunately, at this time it appears that no widespread rain will take place in association with the front in the drought stricken areas of the southcentral Plains...

The image below is the GFS model's forecast of total precipitation during the period 7am CDT today and 7am CDT on Sunday, October 30th.  The scale on the right is in inches:

1st Heavy Snow in the Rockies; 1st Low-Level Snow Adjacent Areas:
Denver will likely see its first snowfall of the season on Wednesday, as cold air behind the strong front combines with widespread moisture and lift via a strong middle and upper level storm system expected to lift across the region.

For you history/weather trivia buffs out there, the average date of the first snowfall in Denver is October 19th.  The earliest was on September 3, 1961 and the latest was on November 21, 1934.  

Snow may be locally heavy at times in the higher elevations of the Rockies, from Montana Southeastward into Colorado.  The GFS model is forecasting accumulation of up to 12 inches in some of the higher elevations of the Rockies in southcentral Wyoming, central and northcentral Colorado by 7pm CDT on Wednesday the 26th, as shown in the image below: 

Keep in mind we are still several days out from this event, but the brighter orange and yellow shaded areas on the above image (which correspond to the higher elevations of the Rockies) stand the best chance of receiving locally heavy snow by midweek.  Stay tuned for updates as the event approaches...

The grey shaded areas on the map over Nebraska and Kansas show where some flurries may mix in with the cold precipitation before it ends late Wednesday, however accumulation is not likely in this area.

Tropical Trouble for Florida, or Strike 3 for the Tropics?
As mentioned in a post yesterday, a disturbance to the South of the Yucatan peninsula bears watching, although the latest model trends indicate that it is less likely to affect Florida than it looked like yesterday.  There is still enough uncertainty to warrant a "keep an eye on the tropics" bubble for the far Southern portions of the state late in the week, which is why I've included it in the outlook.

The models are currently split between moving the system inland over Mexico, or having it picked-up by the cold front and trough before it reaches Florida.  The latest model composites are shown on the image below:

During the last 12 hours, both the HWRF and the GFDL models (major short term hurricane forecast models) have shifted and are showing the system moving inland over Mexico:

Some uncertainty still remains, so we need to keep an eye on this system for at least the next 3 days or so before we can completely write-off a threat for Florida...

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