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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
000
FXUS64 KHUN 151240
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
640 AM CST Thu Nov 15 2018

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 530 AM CST Thu Nov 15 2018

A closed mid/upper-level low currently situated across southeastern 
MO will continue to deepen and lift slowly northeastward into the 
lower OH Valley this morning, before accelerating eastward across KY 
later this afternoon ahead of a northern stream trough digging into 
the upper MS Valley. As this occurs, deep layer flow will veer to the
west-northwest during the late morning/early afternoon hours, 
inducing stronger isentropic descent and ending the threat for 
measurable precipitation across our CWFA. However, prior to the onset
of stronger subsidence, large scale ascent related to the upper low 
will be sufficient to warrant a slight-low chance of light 
precipitation, mainly for the northeastern half of the forecast area 
where the depth of the low-level saturated layer will be a bit 
greater. This precipitation will initially come in the form of light 
freezing rain for portions of southern middle TN and Jackson county, 
AL, but should transition to light rain for all but the highest 
elevations during the late morning hours as temperatures rise very 
slowly above freezing. Further to the southwest of the region where 
measurable precipitation will be possible after 12Z, forecast 
soundings depict a fairly shallow saturated layer just above the 
surface, and this implies that only a few flurries of snow will be 
possible, mainly east of a line from Florence to Decatur and 
Guntersville.

The threat for lingering light rain and snow flurries across 
northeast AL and southern TN will slowly diminish this afternoon. 
However, low stratus clouds will be slower to clear the region, and 
should result in a very small diurnal temperature range, with most 
locations only reaching the lower-mid 30s for max temps. A few upper 
30 degree readings are possible across the southwestern counties, if 
sufficient scattering of the low cloud deck can occur later this 
afternoon.

.SHORT TERM...(Tonight through Saturday night)
Issued at 530 AM CST Thu Nov 15 2018

Models suggest that west-northwest flow aloft will continue from 
tonight-Friday morning, in the wake of the departing trough, but 
should back to the west and subside later in the day as a mid-level 
vort max digs southeastward across the northern high Plains. Deep- 
layer flow will remain unamplified, but should back further to the 
west-southwest and increase once again during the period from Friday 
night-Saturday night as another/stronger vort max drops southeastward
through the northern/central Rockies. At the surface, a broad ridge 
of high pressure will remain centered along the Gulf coast, 
providing light/variable flow at night and light west-southwesterly 
flow during the day when boundary layer mixing will be deeper. 
Although periods of high-level clouds will be possible in the flow 
aloft, abundant sunshine both Friday and Saturday will lead to warmer
afternoon temps in the mid 50s Friday and lower 60s Saturday. 
Conditions will be quite favorable for radiational cooling tonight, 
with dewpoints in the l-m 20s, and this should lead to lows in the 
25-30 degree range for most locations. Given very rich soil moisture,
we may also see development of freezing fog across portions of the 
region early tomorrow morning. Depending on the coverage of high- 
level clouds at night, radiational cooling processes could be quite 
efficient on both Friday and Saturday nights as well, along with an 
attendant threat for fog or freezing fog. However, due to gradual 
modification of the airmass, we anticipate slightly warmer low temps 
both mornings.

.LONG TERM...(Sunday through Wednesday)
Issued at 530 AM CST Thu Nov 15 2018

Models continue to show this fairly weak and dry cold front 
northwest of the Tennessee Valley region on Sunday. ECMWF is little 
more aggressive with its movement eastward than the GFS and retains 
deeper moisture and thus some light showers along it. This should 
make for a pleasant day on Sunday, while just a bit cooler (highs in 
the upper 50s to lower 60s). GFS has a hard time retaining this 
moisture as the front pushes further southeast into northern Alabama 
towards daybreak on Monday into the afternoon hours. Given that the 
forcing is not very strong very dry air is in place ahead of the 
front, think GFS might be on the right track. But for now kept a 
compromise between the two by including some isolated showers and 
significant cloud cover across northern Alabama Sunday morning 
(mainly north of a line from Cullman to Scottsboro communities) 
through noon.

Most guidance weakens and moves this front into north central 
Alabama Sunday afternoon through Monday, before stalling it. As a 
result, some mixing in the boundary layer near the surface looks 
possible Sunday night and should keep winds from decoupling (near 5 
or 6 mph) through Monday night. Some lingering scattered cloud cover 
and these winds should keep fog at bay around daybreak on Tuesday. 
Some guidance hints at lows dropping into the upper 20s to lower 30s 
near and north of the TN/AL border by daybreak on Tuesday, but this 
cold air advection may be too aggressive given the stalled nature of 
the front. For now kept a bit warmer and closer to blends with 
temperatures dropping into the lower to mid 30s instead.

Models show very little evidence of the front holding together at 
the surface on Tuesday. However, aloft there is still a weak signal 
of the boundary aloft through central/southern Alabama. This should 
keep at least some scattered cloud cover over most of northern 
Alabama through much of the day. Slightly cooler daytime highs in the
lower to mid 50s will be the main change into Tuesday with a tad 
more cold air advection behind the front.

High pressure builds further south on Tuesday night as a more zonal 
flow aloft develops. Overall, this will mean little difference in 
weather conditions. Another cool night (but not unseasonably so) is 
expected with lows dropping into the mid 30s. Dry and near normal 
temperatures will continue on Wednesday with highs in the mid to 
upper 50s and sunny skies.

By Wednesday evening, stronger shortwave energy aloft develops over 
Texas/Arkansas. This forcing pushes east approaching Mississippi and 
northwestern Alabama by Thursday morning. At this point, models keep 
the atmosphere very dry near this feature. Also, ridging tends to 
keep this energy moving northeast into the lower Great Lakes or Ohio 
Valley due to the blocking area of high pressure over the 
Appalachians. So only introduced isolated rain shower chances just 
into northwestern Alabama as a result.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 640 AM CST Thu Nov 15 2018

Satellite imagery and surface observations confirm that an overcast 
deck of low stratus clouds will persist across the region for much of
the day, as an upper low moves northeastward across KY. MVFR cigs in
the 1500-2500 ft range are expected to continue thru 21Z/MSL and 
23Z/HSV, with a brief period of IFR conditions also possible between 
12-16Z. Although a few pocket of light snow flurries may impact the 
HSV terminal early in the period, they should result in no 
significant impact to airport operations and the probability of 
occurrence is too low to mention in the TAF. Stratus will begin to 
lift/scatter from SW-to-NE late this afternoon, and we have indicated
mostly clear skies after 16/02-04Z. Due to recent rainfall and 
abundant soil moisture, patchy BR will be possible at both airports 
near the end of the TAF period. Sfc winds will veer from WSW to WNW 
and remain at speeds arnd 10 knots today, before becoming lgt/vrbl 
overnight.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...70/DD
SHORT TERM...70/DD
LONG TERM...KTW
AVIATION...70/DD


For more information please visit our website 
at weather.gov/huntsville.