Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 160220

Area Forecast Discussion
National Weather Service Huntsville AL
820 PM CST Fri Dec 15 2017

.NEAR TERM...(Tonight)
Issued at 820 PM CST Fri Dec 15 2017

Current temp trends have already lowered to their forecasted lows in
some locations thx in part to clr skies/optimal radiational cooling
conditions and light winds. Temps are mainly in the mid/upper 20s
while dew pts crater into the lower 20s. Temps may still fall a
couple of more degrees heading into the overnight hrs, given the
predom 2-3F temp/dew pt spread, under mostly clr skies. As such, early
morning lows Sat have been lowered a few degrees based on the latest
trends, with unseasonably cold conditions xpcted for the remainder
of tonight. No other changes were needed to the ongoing forecast at
this time.

.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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