Area Forecast Discussion
Issued by NWS Huntsville, AL

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831
FXUS64 KHUN 210825
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
325 AM CDT Mon Oct 21 2019

.NEAR TERM...(Today)
Issued at 324 AM CDT Mon Oct 21 2019

Patchy dense valley fog persists in northeast AL into portions of
southern middle TN (mainly in Franklin County). The fog is a bit more
widespread in northwest GA and east central AL. With southeast flow
developing, some of this fog and low cloudiness will likely advect
northwest into our area. The question will be if wind speeds increase
mixing enough to limit dense fog. For now, will maintain an SPS
through 11Z for this fog, but may need to upgrade to an advisory if
fog becomes more widespread.

For the remainder of the day, attention turns to an evolving QLCS
from the western Ozarks into central TX. CAMS and short range models
suggest a distinct weakening/disorganization of this line as it
moves east of the MS River late this morning. Weak instability looks
to be the major factor here, with drier air and extensive cloud cover
and possible rain showers/drizzle developing ahead of the line. So,
despite increasing deep layer and low level bulk shear/helicity,
agree with SPC on the removal of the Slight Risk from our counties.
The models, especially the NAM and HRRR indicate a meso-low feature
developing in south central MS that moves northeast toward central AL
near the BHM area by mid to late afternoon, but would focus better
potential for severe weather to our south. Will adjust our
HWO/outlook graphic for a marginal threat of damaging winds today.
The good news in all of this is that there is a decent shot of
widespread 0.75-1.25 inches of rainfall, with locally higher
amounts. Temperatures will likely be held in the u60s-l70s for most
of the day, peaking in the m70s just ahead of the incoming area of
showers and thunderstorms.

.SHORT TERM...(Tonight through Wednesday)
Issued at 324 AM CDT Mon Oct 21 2019

Stratiform rain on the back end of the QLCS will end abruptly from
west to east as the cold front passes this evening. Sharply cooler
air will arrive late tonight with lows in the u40s-l50s. On Tuesday,
brisk west-northwest flow will keep good cold advection going with
highs only in the 60s. Gusts of 15-25 mph will make it feel chillier
given dew points dropping into the u30s-l40s. The surface high over
TX on Tuesday will shift east into MS and AL by Wednesday morning.
Lows will dip into the u30s-l40s. Wednesday should be a much more
pleasant weather day with light winds and full sunshine. Highs in the
u60s-a70 will not feel as chilly as a result.

.LONG TERM...(Wednesday night through Sunday)
Issued at 324 AM CDT Mon Oct 21 2019

By Wednesday night, the center of the large area of high pressure
over the southeast shifts into North Carolina and Virginia. Expect
light winds through the overnight hours with little to no cloud
cover. This should provide a good setup for radiational cooling and
allow for temperatures to drop into the 40 to 45 degree range in most
locations.

During the day on Thursday, a weak frontal boundary stretching
southward from a surface low over eastern Canada tries to push
southeastward into the Tennessee Valley. However, the very strong
ridging over Virginia and the southeast should keep the Tennessee
Valley dry and cloud free. Most guidance stalls the front northwest
of Kentucky and Tennessee through Thursday evening. However, later
Thursday night, some guidance does forecast a warm front moving
northward from the Gulf of Mexico into southern or central Alabama
and Mississippi. Convergence along and deeper moisture just north of
this boundary could produce some isolated to scattered showers over
northern Alabama.

Model guidance continues to handle the evolution of additional
energy aloft upstream of the stalled front differently during the day
on Thursday. GFS continues to phase this energy with the main jet
stream, amplifying the longwave trough axis. This would provide for
a more progressive movement eastward and intensification of the cold
front. This would in turn bring more widespread precipitation into
the Tennessee Valley on Friday/Friday night ahead of the front. It
would also return dry conditions to the area more quickly by Saturday
morning.

ECMWF moves the energy aloft more to the south and doesn`t phase it
with the longwave trough axis associated with the stalled front
northwest of Kentucky Thursday night into Friday. This would provide
for a cut-off low scenario over Texas instead of a deepening
longwave trough axis and take longer for precipitation to develop
over the Tennessee Valley. For now, keeping close to blended guidance
in between the two solutions. Therefore, kept isolated to scattered
precipitation chances in the forecast late Thursday night through
Sunday. If the ECMWF solution is correct, better instability develops
on Saturday possibly producing some additional thunderstorm
activity, but for not will leave storms out of the forecast. Either
way, high temperatures should be cooler Friday into Saturday, only
reaching the lower to mid 60s.

ECMWF stalls the front in further north over central Alabama or
Georgia early next week. GFS again differs and pushes the front and
better moisture into the northern Gulf of Mexico. This again provides
a significant difference in forecast output. For this forecast
package remaining closer to the blended guidance, which trends
toward the more progressive movement southward of the front.
Temperatures should also warm back into the lower to mid 70s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1130 PM CDT Sun Oct 20 2019

No changes from previous aviation forecast reasoning, as VFR
conditions currently exist at the HSV/MSL terminals. However, a
strengthening low-level jet should result in an increase in initially
high-based stratocu and perhaps a few SHRA (especially invof MSL)
overnight. A plume of richer tropical moisture will spread northward
into the region prior to sunrise as SSE flow increases to 12G20
knots, and this should lead to development of an overcast MVFR
stratus deck, which will likely persist throughout the morning and
into the early aftn hours. The coverage of SHRA in advance of an
approaching cold front will increase early Monday aftn across
northwest AL, with a weakening frontal squall expected to reach
MSL/20Z and HSV/22Z. Although strong wind gusts and lightning may
prompt AWW issuance with this line, it does appear as if it will
weaken as it encounters a more stable environment and becomes
embedded in a vast amount of prefrontal SHRA activity. In addition to
the precipitation, we have also included prevailing MVFR vsbys/IFR
ceilings in the forecast. Winds will veer to SSW at MSL prior to the
end of the TAF period, with lgt-mod rain expected to continue for
approximately 3 hours in the wake of this windshift axis.


&&

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

$$

NEAR TERM...17
SHORT TERM...17
LONG TERM...KTW
AVIATION...70/DD


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at weather.gov/huntsville.



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