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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
000
FXUS64 KHUN 211130
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
630 AM CDT Sun Oct 21 2018

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 500 AM CDT Sun Oct 21 2018

An amplifying mid-level longwave trough will continue to spread 
eastward across New England and the mid-Atlantic region this morning,
before lifting northeastward and offshore later today. Strong deep- 
layer north-northwesterly flow across the local forecast area will 
begin to subside and back to the west-northwest by late this 
afternoon as this occurs, and a low amplitude ridge translates 
eastward across the southern Plains.

At the surface, a rather strong cold front continues to accelerate 
southward and should be approaching the north central Gulf coast by 
sunrise. The pressure gradient to the south of a Canadian high 
settling into the mid-MS Valley has been sufficient to maintain gusty
north-northeasterly low-level flow for much of the night, with 
temperatures expected to fall into the m-u 30s for most of the region
by 12Z, given the magnitude of CAA. In fact temps may still reach 
32F for a couple of hours on either side of sunrise in portions of 
northern Lincoln/Moore and Franklin Counties in TN, but will not last
long enough to warrant a Freeze Warning. Although the surface ridge 
is expected to shift eastward across KY/TN today and reduce the local
pressure gradient, at least weak CAA will persist this morning, and 
with a model consensus of 850-mb temps in the 5-10C range most 
locations will struggle to achieve max temps in the m-u 50s.

.SHORT TERM...(Tonight through Tuesday night)
Issued at 500 AM CDT Sun Oct 21 2018

It appears as if mid and high-level flow will continue to transition
to more of a zonal orientation tonight and tomorrow, to the north of
a subtropical ridge retrograding across the southern Gulf of Mexico.
Models still suggest that the passage of at least a couple of weak 
disturbances embedded in this regime will yield a gradual increase in
cirroform cloud cover, which could begin as early as this evening 
and continue through Monday. At this time, the high-level cloudiness 
is not expected to be widespread or dense enough to have a 
significant impact on low temperatures tonight, in an ordinarily 
favorable pattern for radiational cooling. However, we have increased
temps just a bit above previous forecasts to account for this 
possibility. With this in mind, we no longer have any locations with 
temps reaching 32F, and the probabilities for a widespread frost have
been reduced, but are still greatest across portions of northwest AL
and southern middle TN. Conditions will support the initial 
development of frost in the 4-6Z timeframe, with frost becoming most 
widespread between 9-13Z. Light low-level southerly flow will return 
to the region on Monday, as the surface ridge shifts further 
eastward, and this will promote afternoon temps in the mid 60s for 
most of the area.

Another strong norther stream vort max is predicted to dig 
southeastward into the Great Lakes region on Monday night/Tuesday 
morning, before lifting northeastward into coastal New England 
Tuesday afternoon/night. This will force winds aloft to veer to the 
northwest once again, with a weaker/secondary cold front in the wake 
of the vort max expected to enter the TN Valley late Tuesday 
afternoon or Tuesday evening. With the exception of a potential 
interruption from mid and high-level clouds, conditions will be 
favorable for radiational cooling once again on Monday night, with 
lows expected to drop into the lower 40s. In spite of the passage of 
the reinforcing cold front, stronger mixing and weak thermal 
advection should result in even warmer lows on Tuesday morning. Highs
will rebound into the m60s-around 70 for most of the area on 
Tuesday.

.LONG TERM...(Wednesday through Saturday)
Issued at 500 AM CDT Sun Oct 21 2018

On Wednesday, most models continue to show general upper level 
ridging building over the southeastern CONUS (centered over Missouri 
and Arkansas). Two systems will impact locations to the south and 
west of the Tennessee Valley into the afternoon hours. One is a 
surface low in the northwestern Gulf of Mexico and a warm front that 
extends eastward from this feature. At the same time, a longwave 
trough axis is shown pushing into the western edge of the ridge axis.
There won't be many sensible weather impacts from either of these 
systems through Wednesday night, except maybe some increasing cloud 
cover. Afternoon highs on Wednesday should be near or below normal, 
given forecast 925 mb temperatures, likely in the mid to upper 60s at
least. Developing mostly cloudy conditions toward midnight into the 
daybreak hours on Thursday should keep overnight lows warmer (in the 
mid to upper 40s) than the previous few days. But no precipitation is
expected, as the surface low pushes due east and keeps this activity
along the Gulf Coast and into southern Alabama primarily.

During the day on Tuesday, models start to diverge. Some models move
this surface low further northeast, while models such as the GFS 
keep it moving almost due east still. If models showing the further 
northeast movement of this low verify, isolated to scattered shower 
chances are possible. At this point SuperBlend is sticking closer to 
this solution. Therefore, kept isolated to widely scattered 
precipitation chances in the forecast on Thursday. Abundant cloud 
cover definitely looks like the more certain outcome. This will 
likely lower afternoon highs into the upper 50s to lower 60s (maybe 
even cooler than that). The discrepancies with the surface low track 
continue and are even more significant Thursday night. ECMWF would be
much wetter in locations east of I-65. At this point, given such 
discrepancies, kept to middle ground with scattered chances of 
precipitation.

Additional upper level energy and cold air aloft swings southeast 
via the main jetstream into the Midwestern states on Friday. Although
the bulk of the precipitation from the Gulf of Mexico system should 
move east of northern Alabama by Friday, lingering clouds and 
possibly some drizzle or very light rain looks possible east of I-65.
With zonal flow, high temperatures will likely remain in the upper 
50s to lower 60s. As we move into next weekend, models diverge again 
concerning how quickly and where this upper level energy and better 
moisture convergence actually goes. However, kept isolated to 
scattered showers in the forecast given the strong upper level 
forcing shown by models pushing into the area. It should be slightly 
warmer though over the weekend with highs back in the lower to mid 
60s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 630 AM CDT Sun Oct 21 2018

VFR/SKC conds will exist at both terminals today, with NE flow on 
the order of 10G18 knots expected to subside throughout the afternoon
hours as a surface ridge migrates eastward across KY/TN. The local 
pressure gradient will relax early this evening, with winds becoming 
lgt/vrbl overnight, as scattered high-level clouds begin to 
overspread the region from the west.

&&

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

$$

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


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at weather.gov/huntsville.