Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 210446 AAD

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1146 PM CDT Fri Apr 20 2018

.DISCUSSION...Updated for latest aviation discussion below.


06Z issuance...Guidance continues to advertise a return of
easterly flow overnight, with a slug of moisture above the surface
moving over the area beginning after sunrise. The level of the
moisture has risen, with guidance advertising the base of the
moisture around 4k`, and with a greater dewpoint depression. Have
bumped cloud bases upwards and delayed the low level clouds
moving over area TAF sites. VFR conditions expected the rest of
the night into Saturday as a result. Cloud cover increases
Saturday, but CIGS remain at or above VFR levels through the day
into the evening. The increasing moisture levels bring CIGs/VISBYs
dropping into MVFR levels Saturday night, and may see low end
MVFR/IFR conditions after the current package.



.PREV DISCUSSION... /issued 1023 PM CDT Fri Apr 20 2018/

DISCUSSION...See updated information for land areas below.

UPDATE...An update was sent to the forecast to lower overnight
low temperatures just a bit in a few spots as surface observations
indicate readings already near forecasted minimums as of 10 PM. We
are still generally expecting lows to range from the upper 40s to
lower 50s over the interior, with mid 50s to around 60 near the
immediate coast. Skies generally remain clear overnight, with the
exception of a few cirrus passing over mainly southern portions
of the area. /21

PREV DISCUSSION... /issued 626 PM CDT Fri Apr 20 2018/

DISCUSSION...Updated for latest aviation discussion below.

00Z issuance...Surface high pressure well north of of the forecast
area will shift to over the East Coast overnight, bringing a shift
in low level flow from northeasterly to easterly overnight, then
southeast by the end of the forecast. With the shift in winds to
easterly, guidance is advertising a slug of low level moisture
(centered around 950mb) moving over the area beginning around
sunrise. This will bring a drop in CIGs from general VFR to mid
MVFR beginning around sunrise, the a rise back to VFR by 18z as
the boundary layer deepens and the moisture layer mixes out. Am
not anticipating any low end MVFR/IFR CIGs/VISBYs affecting
operations with this package.


PREV DISCUSSION... /issued 353 PM CDT Fri Apr 20 2018/

NEAR TERM /Now Through Saturday/...A shortwave upper ridge will
move eastward across the central Gulf Coast through Saturday. This
will allow for a slight increase in high temps Saturday, while
maintaining dry conditions. Meanwhile, southeasterly low level flow
will increase through the day as a cold front begins to approach
from the west. Moisture levels are expected to increase enough near
the coast for the possibility of a few light showers by the
afternoon. Lows tonight will range from the upper 40s and low 50s
inland to upper 50s along the coast. Highs on Saturday will climb
into the low to mid 70s. /13

SHORT TERM /Saturday night Through Monday night/...An upper level
low over the south Plains pushes east into the lower Mississippi
River Valley and Mid South Saturday night into Sunday. Upper
level height falls ahead of this system, in conjunction with upper
diffluence associated with the left exit region of a jet max
rotating through the base of the trough, will serve to increase
deep-layer ascent along and ahead of an attendant surface front. A
quickly moistening warm sector ahead of this front should see
enough destabilization (~500 J/kg MLCAPE) to support showers and
thunderstorms, with the best potential for thunder coincident with
the greatest forcing and instability along the front and near the
coast. Veering deep layer wind profiles and effective bulk shear
magnitudes at or above 40 knots could allow any stronger cells to
become better organized and capable of producing severe weather.
Given the uncertainty in available instability, however, only a
marginal threat is in place at this time. Generally expecting
rainfall amounts of up to 2 inches with this system, with local
amounts up to 3 to 4 inches not out of the question. These totals
may not appear heavy at first glance, but it should be noted that
those who received significant rainfall with the previous system
(i.e., interior southeast Mississippi and southwest Alabama) are
likely to be more vulnerable to flooding than normal, as this
additional rainfall only exacerbates the already elevated rivers
and streams.

The cold front pushes through Sunday night into early Monday as
the upper low weakens and pushes off to the east, eventually
settling over the southeastern states Monday and Monday night.
The passage of this front should mark the end of precip for most,
particularly near the coast. Farther inland, could see some light
showers continue throughout the day Monday in the wraparound
region of the surface low. /49

LONG TERM /Tuesday Through Friday/...The upper low stalls over
the southeastern states as another quick-hitting shortwave rotates
through the region to start the long term period. Lingering
wraparound moisture over inland and western portions of the area
should be enough to squeeze out some showers as this shortwave
passes through Tuesday.

The first in a series of troughs digs from central Canada
southeast into the upper Mississippi River Valley and Midwest on
Wednesday, helping to finally push that stalled upper system over
the southeastern states east and away from the area. As the former
trough continues to dig into the eastern CONUS, it pushes another
cold front southeast towards the Gulf coast. We see a bump in rain
chances along and ahead of that front Wednesday night before it
passes through and brings dry conditions Thursday. Dry weather
will be short lived, as the next in the series of upper troughs
digs into the upper Midwest Thursday night. This pushes yet
another front towards the area, with moisture quickly returning
and rain chances increasing Friday ahead of the front. /49

MARINE...A moderate easterly flow tonight will increase on Saturday
as a cold front approaches from the west. A moderate to sometimes
strong southeast to south flow develops Saturday night ahead of the
front. Winds become westerly in the wake of the front by Monday,
with a light to moderate west to northwest flow then continuing into
the middle part of mid next week. /13




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