Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 131133

Area Forecast Discussion
National Weather Service Huntsville AL
533 AM CST Wed Dec 13 2017

For 12Z TAFS.


.NEAR TERM...(Today and Tonight)
Issued at 254 AM CST Wed Dec 13 2017

A fairly active mean trough pattern (that should be shifting late in
the forecast) will continue to send mid-level shortwave troughs with
varying levels of cold air advection behind accompanying cold fronts
southeast over the TN Valley over the coming week. The first couple
of fronts will be dry and the forecast mostly challenged by thermal
impacts, but with the pattern change at the end of the forecast
period, the `last` front brings more shower activity.

The latest one arrived yesterday bringing very cold hard freeze
conditions this morning. Another shortwave trough with yet another
reinforcing front will be taking a different trajectory further north
within the prevailing westerlies. This will mean a slower moving cold
front which will actually lag behind the passage of the shortwave
trough over the OH River Valley. With the approach of the shortwave
trough a polar jet streak will arrive over much of the southeast with
higher wind speeds over portions of the TN/OH River Valleys. With
mostly clear sky, increased pressure gradient, a shifting Gulf of
Mexico high to the east, and momentum mixing down today, expect
west/NW winds to back to the southwest/south to 10-15 MPH gusting to
20 MPH especially during the afternoon. This southerly flow will
allow a brief return in low 50s daytime highs mainly over NW AL where
`slightly warmer` air will be able to advect northwest quicker than
the eastern half of the forecast area.

Temperatures should then fall close to freezing tonight underneath a
clear sky and diminished winds as the lagging cold front finally
arrives early on Thursday morning.

.SHORT TERM...(Thursday through Friday night)
Issued at 254 AM CST Wed Dec 13 2017

After the front arrives, cooler air looks to slowly advect southward
during the day. Light northerly surface winds and weaker cold air
advection appear to be the cause for the lag. With daytime heating
possibly interrupted by low clouds, daytime high temperatures on
Thursday are challenging to forecast. Will keep a blend of the
guidance with some slight adjustments upward in the daytime highs to
account for a cold bias in the guidance. In addition, will keep a
north (`cold`) to south (`warm`) temperature gradient.

Meanwhile, an elongated longwave trough formed from the merging of
sheared mid-level troughs within the southern flank of the polar jet
over the Intermountain West, and another one within the subtropical
jet, will move towards the MS River on Thursday. The arrival of yet
another cold front with this sheared trough over the TN Valley on
Friday will push the stalled/slow moving polar front further south
along with the cold air that has been lagging just to the north. As a
result, Friday`s temperatures will be noticeably cooler (daytime
highs in the 40s) along with mostly cloudy sky conditions during the
morning hours. After sunset, temperatures should drop below freezing
on Friday night.

.LONG TERM...(Saturday through Tuesday)
Issued at 254 AM CST Wed Dec 13 2017

A mild and dry day is expected Saturday with dew points staying in
the lower 30s at most. Southerly flow will pick up substantially
Saturday night across the central Gulf Coast into the lower MS valley
in advance of the shortwave depicted by both medium range models.
This wave ejects from the southern Plains and rapidly through the TN
and lower MS valleys late Saturday night into Sunday morning. In
fact, both the GFS and ECMWF are quite similar with their mass fields
in this regard. They also suggest very dry low levels will likely be
overcome by falling precip as early as late Saturday night, so will
stay the course with PoP increasing Saturday night into the likely
range in northwest AL with chances elsewhere. Suggested blended
guidance brings the PoP into the categorical range on Sunday and
cannot disagree with a large area of showers moving across. Showalter
indices still remain quite positive, so will keep all showers for
now. The warm sector should remain bottled up well to the south with
a better thunderstorm threat. Rainfall amounts of one half to one
inch look quite obtainable, if not a bit heavier in spots.

Then differences continue between the GFS and ECMWF Monday into
Tuesday.  Will side closer to the more progressive ECMWF in this
pattern which leads to a dry Monday and Tuesday.  If model consensus
leans closer to the slower GFS, we may need to adjust later
forecasts. In either case, our airmass will remain temperate with
readings fairly close if not just above seasonal normals early next


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 533 AM CST Wed Dec 13 2017

VFR conditions are expected to continue across northern Alabama for
the next 24 hours. Winds will increase from the SW after 15-16Z with
windspeeds of 15-16 kts gusting up to 25 kts until 00Z. After that
winds should diminish to 8-10 kts.





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