Area Forecast Discussion

FXUS64 KHUN 152325
Area Forecast Discussion
National Weather Service Huntsville AL
525 PM CST Fri Dec 15 2017
For 00Z TAFS.
.NEAR TERM...(Tonight)
Issued at 257 PM CST Fri Dec 15 2017
Latest water vapor imagery shows an upper trough over the northeast 
CONUS, with ridging over much of the Intermountain West and a strong 
cut off low over the Baja Peninsula. At the surface, high pressure 
continues to build through the Gulf Coast states, with winds across 
the local area generally from the west-northwest. Cloud cover has 
been slow to erode today, and has prevented temps from rising too 
much, with highs topping out in the upper 30s. These clouds are 
slowly starting to erode, however, with mostly clear skies expected 
by the early evening hours. This will make for another cold night 
across the TN Valley, with overnight lows dropping into the mid to 
upper 20s once again. 
.SHORT TERM...(Saturday through Sunday)
Issued at 257 PM CST Fri Dec 15 2017
The TN Valley will begin to see quite a pattern change beginning at 
the start of the short term. The axis of an upper trough will 
quickly exit the area as an upper ridge builds northward in response 
to a fairly strong cut off low ejecting into southwest Texas. 
Meanwhile, high pressure build drifts southward and southerly 
surface flow will return across the region. As south to southwesterly
flow strengthens through the vertical column, warm and moist air 
will quickly filter in across the area. This, along with mostly clear
skies will make for a mild day across the TN Valley, with 
temperatures rising into the lower to mid 50s. 
The upper ridge will strengthen during the overnight hours on 
Saturday, as the upper low shifts northeastward and into the Mid 
Mississippi Valley. Moisture will continue to advect into the the 
local area and keep overnight lows from falling below the mid 30s. 
The upper low will weaken fairly rapidly as it continues its 
northeastward progression. Despite the weakening forcing associated 
with this feature, the low level jet will strengthen over the Lower 
Mississippi Valley early Sunday morning and place the TN Valley in a 
favorable location for strong lift. PW values will surge to around 
1.2 inches, which exceeds the 90th percentile for mid-December. 
Given the strength of the forcing associated with the low level jet, 
along with the abundance of moisture, numerous to widespread showers 
will move into the TN Valley Sunday morning and persist through the 
afternoon hours. At this point, a fairly strong inversion in the 
lower levels, along with poor lapse rates, will keep any thunder out
of the area. 
.LONG TERM...(Sunday night through Thursday)
Issued at 257 PM CST Fri Dec 15 2017
Models begin to diverge quite a bit with their solutions behind 
Sunday. The low level jet will lift northward and away from the 
region, however several weak waves will continue to rotate around 
the northern periphery of the ridge. As southerly flow persists, 
moisture will continue to stream northward, with light showers 
continuing through the overnight hours. By Monday, a strong wave 
will push northward, with isolated to scattered showers possible 
through the day. The GFS is indicating mid-level lapse rates 
strengthening and this may provide for isolated thunderstorms, though
everything would be elevated given fairly warm temperatures just 
above the surface. A cold front is then forecast to move through the 
forecast at some point between Tuesday and Wednesday, as an upper low
shifts eastward towards the area. Again, models differ on their 
timing and evolution of this front, with the GFS quite a bit faster. 
Thus, blended guidance was followed closely for Tuesday and beyond. 
Temperatures through the long term period will be unseasonably warm 
as the southerly flow pumps in the warmer Gulf air. Highs will rise 
into the lower 60s by Tuesday, with overnight lows in the 50s. 
Depending on when the cold front moves through, we'll see a brief 
cool down toward seasonal norms, though temps will still remain mild 
for December. 
.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 525 PM CST Fri Dec 15 2017
With a weak area of high pressure building ewd into the region, VFR
conds are xpcted thru the TAF period. Light/var sfc winds will turn
toward the SSW near 7KT Sat morning.
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