Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 070450 AAA

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1050 PM CST Wed Dec 6 2023

...New AVIATION...

Issued at 850 PM CST Wed Dec 6 2023

Current temperatures are fairly chilly across the Tennessee
Valley, mainly in the mid to upper 30s. Although, a few spots have
already dipped into the lower 30s. Skies are mostly clear and
winds have decreased to be light and variable this evening, aiding
in radiational cooling. This has led to the chilly temperatures
thus far and will continue through the overnight. Lows are still
forecast to drop into the upper 20s to lower 30s. Additionally,
for the more sheltered and fog-prone areas, patchy freezing fog is
possible later tonight into the early morning hours. Overall, the
forecast remains on track with little to no changes needed.


(Thursday through Saturday)
Issued at 302 PM CST Wed Dec 6 2023

The 500-mb ridge (noted above) will continue to translate
eastward tomorrow, providing a mostly sunny and dry day, with
highs rebounding back into the u50s-l60s after a cold morning. As
the axis of the ridge crosses the region late Thursday afternoon-
Thursday evening, mid/high-level flow will back to WSW,
introducing a greater coverage of high clouds that should work in
conjunction with strengthening southerly flow in the low-levels to
keep minimum temps several degrees warmer Friday morning (m-u
30s). Cloud layers aloft will continue to thicken and descend
during the day on Friday, but with the departing surface high
allowing for an increase in southerly winds in the low-levels,
afternoon temps will jump back into the l-m 60s. Deep-layer
warm/moist advection is predicted to increase considerably
beginning Friday night, as west-southwesterly flow strengthens
into the 35-45 knot range to the east of an amplifying trough
across the Rockies, and this along with a weak southern stream
disturbance lifting northeastward from the northwestern Gulf Coast
will bring an increasing coverage of showers to the region by
early Saturday morning.


(Saturday through Tuesday)
Issued at 250 AM CST Wed Dec 6 2023

Update: Overall, there have been no significant changes (with 12Z
model guidance) to thoughts outlined below regarding the storm
system this weekend. If anything, it does appear that chances for
rain and a few elevated/embedded thunderstorms may begin a bit
earlier in the day (especially across northwest AL), but the most
widespread coverage of precipitation should occur from Saturday
evening into early Sunday morning as a strong cold front shifts
southeastward through the CWFA. Recent QPF data still suggests
that most of the region will receive between 1.5-2 inches of rain
during this period (along with locally higher amounts), and this
could result in some flash flooding. A limited risk for strong-
severe thunderstorms may also materialize during the period from
late Saturday afternoon-Saturday evening, generally across the
southwestern portion of the CWFA, where boundary layer dewpoints
may reach the lower 60s prior to frontal passage. As stated below,
strong gradient winds may eventually warrant issuance of a Wind
Advisory for much of the day Saturday extending into Saturday

A dynamic storm system will impact the Tennessee Valley this
weekend, bringing high chances for showers and thunderstorms, the
high probability of wetting rains of at least 1-2", and breezy
gradient winds. A rapidly deepening mid/upper trough is progged to
shift from the Southern Plains Saturday into the Mid South and
Ohio/Tennessee valleys by Saturday night into Sunday. Strong
dynamic lift will allow rain showers to develop Saturday morning
and overspread the area by the afternoon and evening, with PoPs
increasing from 20-40% in the morning to 40-60% by the afternoon.
The greatest window for rainfall will be Saturday evening/night
into early Sunday morning along and just ahead of an approaching
cold front.

With Pwats increasing to 1.2" to 1.4" (90th to 95th percentile
per BMX sounding climatology) and mean flow parallel to this
approaching frontal boundary, the setup for very efficient rain-
producing convection will be in place. 80-90% of grand ensemble
members indicate 24-h rainfall totals of at least 1" with 40-60%
of members exceeding 2". Our official storm total rainfall
forecast from Saturday morning to Sunday evening indicates between
1.5" to 2.25" over most of the Tennessee Valley, with locally
higher amounts probable. This rain will undoubtedly provide some
relief to our drought stricken area -- but given that much of it
may fall in a 12 hour window, localized excessive rainfall and
minor flooding issues may occur.

The strong dynamics of this system will also be something we`ll
have to continue to watch. At the very least, feel that strong
gusty winds and a potential Wind Advisory day could be in the
cards for us Saturday afternoon/evening through Saturday night and
Sunday morning. Given the strong wind shear (bulk shear values of
60 kts and 0-1 km shear of 20-30 kts), the potential for
organized convection will exist -- the magnitude of which will be
highly dependent on the thermodynamic environment. These type of
wind fields at the very least could produce a few strong to
marginally severe storms -- with the primary uncertainty being the
thermodynamic environment. Ensembles indicate a narrow window
from the late afternoon through late evening of some surface-based
instability across mainly western portions of the HUN CWA.
However, the ensemble mean continues to favor the warm sector
establishing itself more to the south and west and much higher
probs of surface-based instability across the Pine Belt and
Wiregrass of central/south Mississippi and south Alabama. We will
continue to monitor any severe potential and fluctuations in the
timing/evolution of this system and key mesoscale features as the
event draws closer.

Medium to high chances (50-70%) for rain will continue Sunday
morning as the front slowly progresses eastward across the
Tennessee Valley, before gradually tapering off during the
afternoon/evening. In it`s wake a cooler, drier air mass will
settle into the region early next week, with highs dropping back
into the 50s and lows into the mid/upper 20s to lower 30s.


(06Z TAFS)
Issued at 1050 PM CST Wed Dec 6 2023

VFR conditions will persist through Thursday evening. There is
the potential for patchy fog development tonight, but this is not
expected to affect the TAF sites. Light and variable winds
overnight will become southwesterly and increase to between 5-10
knots by early Thursday afternoon.




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