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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 140803

Area Forecast Discussion
National Weather Service Huntsville AL
203 AM CST Thu Dec 14 2017

.NEAR TERM...(Today through Friday night)
Issued at 202 AM CST Thu Dec 14 2017

At 07Z, a cold front was situated from just northwest of London KY
through Nashville to between Memphis TN and Jonesboro AR. Despite a
decent surface pressure gradient, there is quite a large range in
temperature across our forecast area this morning due to a few cold
valleys decoupled from the boundary layer. Temperatures range from 
25 at Valley Head up to 47 at Muscle Shoals. Satellite imagery
indicates a clear sky over the TN valley at this hour. Behind the
cold front, clouds are dropping southward through northern KY,
southern IL into eastern and central MO. These are progged to make a
bit more progress before isentropic downglide dissipates these before
reaching southern middle TN or north AL as seen in NAM RH fields and
HREF ceiling probability. So will go with a sunny forecast today 
with highs a few degrees cooler than Wednesday, with middle 40s to 
lower 50s. It will be a chilly night areawide with lows in the 20s
except over the larger water bodies where readings stay near 30-32F.
Flow should be just enough to prevent any fog development.

A positively tilted trough/vorticity lobe will drop southeast from
the eastern TN valley into TX on Friday. A relatively dry atmosphere
should keep the sky mostly cloud free. A surface ridge will be
building east across the southern U.S. into Friday night. After highs
in the 40s, lows Friday night should once again drop into the 20s.

.SHORT TERM...(Saturday and Saturday night)

Issued at 202 AM CST Thu Dec 14 2017

Saturday should be a nice day as southerly flow advects 850 mb
temperatures back into the 6-7C range. Clouds are unlikely to return
before dusk. Highs should reach the lower 50s at most valley

As has been advertised in recent days, a quick moving shortwave in 
mid to upper levels will be ejecting northeastward out of TX Saturday
night, reaching the MS river around 12Z Sunday. Good upper level 
divergence at 250mb across the mid South and TN valley will support 
good UVVs and induce a strong south-southwesterly low level jet of 
50-60kt. Resulting isentropic ascent will be strong with good 
pressure advection indicated on the 295-305K levels. PWs will 
increase above 1.0 inches with very good MTVs also progged. So,
despite low levels beginning rather dry with dew points in the 30s,
will bring rainfall into most of the area after midnight Sunday
morning. An elevated MCS (mostly showers) will develop and expand 
out of MS into north AL and southern TN at the nose of the strong 
low level jet and best isentropic ascent during the early morning 
hours of Sunday. Sounding profiles may support a few ice pellets 
mixing in over southern middle TN or northeast AL, but confidence is 
not high enough to include just yet. Have also left thunderstorms out
for now due to positive Showalter Indices (SI), but we can't rule 
out some higher based thunderstorms rooted at or above 750 mb based 
on the steepening lapse rate profile. Will monitor future model runs 
as we move closer to the event. 

.LONG TERM...(Sunday through Wednesday)
Issued at 202 AM CST Thu Dec 14 2017

In the beginning of this forecast period widespread rainfall 
associated with a shortwave trough axis moving within the 
subtropical jet. This rainfall will be occurring over the MS River 
Valley and gradually moving east into the western portions of the TN 
River Valley. In addition, a warm front should be moving north from 
the Gulf of Mexico towards the TN Valley on Sunday. This will result 
in temperatures rising after midnight and during the day on Sunday. 
Lows may actually have occurred around midnight on Sunday morning.
Meanwhile, the kinematics accompanying the trough axis include a 
substantial increase in wind shear with low-level SRH values between 
200-300 m2/s2 and curved hodographs/veering profile from the sfc to 
850 mb. What this means is that despite a generally stable moist 
adiabatic environment, broad rotating and bowing segments will be 
possible especially as the warm front stalls over the TN Valley. In 
addition, daytime highs will be in the low 50s but there may be a 
north-south gradient in later forecasts as greater confidence is 
realized on warm front movement. 

As this trough moves off to the NE, the TN Valley (and much of the 
Southeast) will be under the influence of subtropical ridge centered 
just southeast of Florida over the Bahamas with strong SW flow aloft 
and southerly surface flow. The resulting warm air advection will 
bring some relief to the cold temperatures with daytime highs near 
60 on Monday and overnight lows in the 40s. However, as an elongated 
trough moves within the subtropical jet over S Plains/MS River 
Valley, will likely see additional shower development within the SW 
flow (weak undulations within the SW flow).  This may occur before 
the main trough and accompanying cold front arrives on 
Tuesday/Wednesday. There is typical model timing differences but 
given the trajectory of this front, temperatures should be only 
modestly affected and mostly the overnight lows once the front 
passes. Furthermore, will keep showers in the forecast for both 
Monday and Tuesday given the timing differences and potential 
pattern change. Speaking of the pattern change, it does appear that 
mean troughing/blocking pattern that had been in place for a while 
over the eastern half of the country will be replaced with ridge and 
trough. Left Wednesday dry and slightly cooler after the front 
passes sometime on Tuesday.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1100 PM CST Wed Dec 13 2017

No changes to previous aviation forecast reasoning, with VFR       
conditions expected to persist at both terminals overnight. 
Occasionally scattered mid and high-level clouds will continue in the
prevailing WNW flow aloft, with sfc winds diminishing/veering in 
advance of an approaching cold front. Frontal wind shift to NNW 
should occur around 09-10Z, and although no precipitation is 
associated with the front there is a bit more concern for low stratus
clouds in it's wake. At this point, we will introduce SCT020 btwn 
10-16Z, since it appears as if the more solid deck of overcast 
stratus clouds should be displaced to the N/E of both terminals. Any 
lingering stratus will begin to lift/scatter by late morning, with 
mid/high-lvl clouds returning late in the TAF period.





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