141
FXUS64 KMOB 042344
AFDMOB

Area Forecast Discussion
National Weather Service Mobile AL
544 PM CST Wed Dec 4 2024

...New AVIATION...

.AVIATION...
(00Z TAFS)
Issued at 544 PM CST Wed Dec 4 2024

VFR conditions will prevail through midnight as mid and high
level clouds spread eastward over the region. IFR ceilings will
then spread from north to south over the area, along with
increasing rain chances from northwest to southeast, after
midnight into Thursday morning as a cold front approaches and
begins passing through the area. The front is expected to pass
through the forecast terminals between 14-18z Thursday before
moving offshore. Winds will be generally be 10 kt or less through
the period, starting out as southerly this evening, veering to
the southwest tonight, and becoming northerly with the frontal
passage. /22

&&

.PREV DISCUSSION... /issued 407 PM CST Wed Dec 4 2024/

..New NEAR TERM, LONG TERM, MARINE...

NEAR TERM...
(Now through Thursday)
Issued at 407 PM CST Wed Dec 4 2024

A highly amplified trough will continue to dig south into the
eastern half of the country as a strong upper low moves into the
Great Lakes and northeast U.S. This will send a strong cold front
south into our region by tomorrow morning, pushing offshore by
early afternoon. Ahead of the front, deep layer moisture will
increase from the southwest with precipitable water values
climbing to close to 1.5 inches. In addition, a subtle shortwave
over north TX will move into the base of the longwave trough,
helping to aid upper level divergence across the area. The
combination of this lift with low level warm air advection will
set the stage for light to moderate rain to increase from west to
east, especially after midnight and into Thursday morning. There
will be some light rain that spreads into the area this evening,
but it will take awhile to moisten the atmospheric profile as the
12z Slidell sounding indicated a very dry deep layer airmass. Rain
will clear from northwest to southeast through the day Thursday
with most areas dry by early to mid afternoon.

Lows tonight will only drop in the mid 40s over far north and
eastern areas to low and mid 50s elsewhere. After frontal passage
tomorrow, temps will level off or slowly begin to fall in the
afternoon with many areas away from the coast staying in the 50s.
Current guidance indicates the immediate coast could still reach
the mid to upper 60s, but that might be generous.

Strong cold air advection sets in by late tomorrow afternoon into
tomorrow night. Widespread freezing conditions are likely, except
for at the immediate coast. Lows are expected to be in the mid to
upper 20s well inland with low to mid 30s along the I-10 corridor.
Adding in breezy northerly winds and we will see wind chills in
the teens and 20s by early Friday morning. We may very well reach
our Cold Weather Advisory criteria (apparent temperatures 11-20
degrees over the inland counties and 16-25 over the coastal
counties). Cold Weather Advisories are a new product this season
and replace Wind Chill Advisories. There is also an Extreme Cold
Warning (replacing the Wind Chill Warning) for apparent
temperatures lower than the criteria stated above. 34/JFB

LONG TERM...
(Friday through Wednesday)
Issued at 407 PM CST Wed Dec 4 2024

For the start of the extended period, northwesterly flow aloft,
along with deep layer ridging, will allow for a cold, dry airmass
to filter into the region. Friday will be very cold, with
temperatures falling to around 10 to 15 degrees below the long-
term average for this time of year. Highs on Friday will only be
in the upper 40s to low 50s across the area, with a few spots over
the western Florida panhandle topping out in the mid 50s. Lows
once again drop into the upper 20s to low 30s, with mid 30s along
the coast. Saturday will be a touch milder, with highs ranging
from the mid 50s northwest to the low 60s southeast and lows
ranging from the mid 30s inland to the mid 40s along the coast.

The pattern begins to change as we get into Sunday and especially
into the start of the week. Looking aloft, upper-level flow turns
southwesterly on Sunday as a shortwave trough, embedded within the
subtropical jet stream, begins to eject to the northeast.
Additionally, high pressure at the surface pushes to our east,
allowing for winds to shift from northeasterly on Saturday to
southeasterly on Sunday. The onshore component of this flow, both
at the surface and aloft, will allow for a warm air advection
regime to set up across the region. Not only will temperatures
rise by about 10 to 15 degrees on Sunday (in comparison to
Saturday), but with increased moisture, rain chances will also
return to the forecast starting late Sunday afternoon. A surface
boundary/trough appears to develop to our west (in response to the
shortwave aloft) and will move into the area Sunday night. The
latest deterministic and ensemble guidance are pretty much in
agreement, suggesting that the southern stream trough will stretch
out and start weakening Sunday night as it becomes absorbed by a
larger longwave trough digging into the central US. Latest
ensemble trends are also a bit quicker with the shortwave
ejection, suggesting that the best forcing will lift away from the
region during the morning hours. Instability is also rather
questionable at this point, due to the quicker ejection (leading
to the surface trough entering the region earlier, limiting
diurnal heating) and poor lapse rates. So although shear values
would be supportive of storm organization, the limited instability
and the displacement/exiting of the best forcing will keep the
overall severe risk low. However, with PWATs approaching 2 inches,
broad diffluence aloft, and the surface trough slowing down due
to the loss of forcing, heavy rainfall can be expected throughout
the day on Monday.

Forecast details become very murky for the Tuesday and Wednesday
period. The aforementioned surface trough will likely be lingering
somewhere near or over our eastern zones. Looking aloft, the
longwave trough will begin to dig further into the central US,
helping to send a strong cold front to the south. The questions
will be 1) how will the longwave trough evolve over these two
days, and 2) how will its evolution affect the next front, and
thus our weather, for this period. Looking at 00Z ensemble cluster
analysis, most ensembles suggest that the trough should generally
spread southeastwards, remaining broad and positively- tilted the
entire time. This solution would lead to a rather rainy day on
Tuesday due to the cold front approaching from the west and its
interaction with the stalled surface trough to the east.
Instability in this solution continues to remain rather low as
warm air advection is not strong enough to push in the better
instability that resides offshore. The front moves through by
Tuesday night and we start to dry out on Wednesday. The current
forecast package follows this solution due to the better model
support. There is, however, one ensemble cluster (this cluster has
lower ensemble support than other clusters, but interestingly
enough is similar to the 12Z GFS deterministic solution) that
suggests that upper ridging will attempt to build over the eastern
US, potentially blocking the upper trough from moving eastward.
Instead, it digs southwestward resulting in a deeper, tighter
trough over the south central US. This trough then ejects
northeastward on Wednesday, potentially gaining a negative tilt in
the process. This solution also results in cyclogenesis occurring
near our local region. If this were to occur, the warm sector
would have no problem moving onshore. In the end, this solution
would lead to a drier day on Tuesday (being in between waves) and
a much more active day on Wednesday, with the potential for a few
severe storms. We will continue to monitor trends closely over the
coming days. /96

MARINE...
Issued at 407 PM CST Wed Dec 4 2024

Onshore flow will gradually increase tonight ahead of an
approaching cold front. The front will move through the coastal
waters late Thursday morning through Thursday afternoon. A period
of strong offshore flow is expected late Thursday afternoon into
Thursday night and a Small Craft Advisory will be needed for the
majority of the forecast area. Seas will build to 4-6 ft over the
Gulf waters beyond 20nm. Winds will gradually diminish on Friday
and Saturday. Onshore flow will strengthen again Sunday night
ahead of the next approaching front. 34/JFB

&&

.PRELIMINARY POINT TEMPS/POPS...
Mobile      53  62  31  51  33  59  44  70 /  80  80   0   0   0   0   0  10
Pensacola   56  66  35  53  37  60  47  69 /  70  80   0   0   0   0   0  10
Destin      57  68  37  55  40  61  49  69 /  60  70  10   0   0   0   0  10
Evergreen   46  59  29  51  29  57  37  68 /  80  70   0   0   0   0   0  10
Waynesboro  48  57  26  49  29  56  38  66 /  90  40   0   0   0   0  10  30
Camden      46  57  26  48  27  55  36  65 /  90  60   0   0   0   0   0  20
Crestview   45  63  32  54  29  61  37  69 /  60  70   0   0   0   0   0  10

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...None.
FL...None.
MS...None.
GM...None.
&&

$$

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