Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 241735 AAA
AFDMOB

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1235 PM CDT Mon Jul 24 2017

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

&&

.UPDATE...A deeply moist airmass (precipitable water values
locally 2.25"+) is in place across our forecast area underneath a
mid level trough axis that extends southwestward across southwest
AL and southeast MS. Radar has continuously indicated an area of
training convection impacting southeast MS through the morning,
with localized bands of storms producing 4 to 8 inches of rain,
with isolated amounts over 10 inches across portions of Stone
county. With additional training convection possible through the
afternoon, have opted to issue a Flash Flood Watch for our 5
southeast MS counties through 7 PM this evening, where confidence
was highest on heavy rain/flooding potential. We will need to
watch for additional development into southern portions of Mobile
and Baldwin counties, where locally heavy rains occurred earlier
this morning. Otherwise, will continue the high POPs through the
rest of the day across the region. /21

&&

.PREV DISCUSSION... /issued 633 AM CDT Mon Jul 24 2017/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...
12Z issuance...A chaotic look to the sky this morning with cloud
bases at multiple, but higher levels. Some cb/tsra in the
vicinity. Environment supports an increased coverage of
convection this morning. Lowest bases in tsra mainly MVFR levels
with vsby lowering to MVFR, potentially briefly to IFR levels with
the passage of any heavier rainfall. Other threats to
approaches/departures will be brief strong and erratic convective
wind gusts today. /10

PREV DISCUSSION... /issued 409 AM CDT Mon Jul 24 2017/

NEAR TERM /Now Through Monday night/...Upper level map analysis
shows a long-wave trof positioned from the Great Lakes, southward
into the southeast. From the southeast US, a high level shear zone
is oriented west southwest to the upper Texas coast. South of this
feature, a narrow mid level ridge axis extends from the Bahamas,
west into the Gulf. An assessment of deep layer moisture from area
raobs at 24.00Z shows precipitable water values ranging from 1.9 to
2.1 inches over the central Gulf coast. The main driver for
convection today will be the passage of a series of mid level
impulses within the central Gulf coast upper level shear zone,
acting on deep layer moisture and a destabilizing environment. A
surface focus for storms will also come from interaction of
mesoscale boundaries. Considering high levels of deep moisture, lift
and instability thru the course of the day, scattered to numerous
showers and storms is anticipated. This is consistent with solutions
from short range guidance, ensembles, the high resolution HRRR and
WRF-ARW. The stronger storms will be efficient in depositing locally
heavy rains in a short period of time, brief strong wind gusts and
frequent lightning. Could be some mainly nuisance type flooding in
the usual low spots and poor drainage areas. Will carry over a
chance of storms into the evening, then a gradual southward shift
to the coast and offshore late in the night.

With the higher coverage of rains expected today and thickening
cloud cover, highs mostly in the mid to upper 80s. Overnight lows in
the lower to mid 70s inland to mid to upper 70s coast. /10

SHORT TERM /Tuesday Through Wednesday night/...The upper longwave
trough over the eastern CONUS squeezes south and east as a large
area of high pressure builds northeast from the Four Corners
region and into the central Plains. The squeezed trough serves as
a source for a series of impulses that are pinched from the main
flow and move southwest over the local area, around the eastern
periphery of the Plains high circulation. These upper impulses
will serve to enhance our otherwise typical summertime convection
in an environment characterized by PWATs around or just above 2.0
inches and MLCAPEs around 2000 J/kg. Thus, have opted to keep
likely PoPs over much of the area Tuesday afternoon and high-end
chance Wednesday. Despite the help from these impulses, a diurnal
trend in coverage and intensity should remain dominant, with more
numerous and stronger storms occurring during afternoon hours and
convection waning (but not ceasing entirely) during overnight
hours. Stronger storms will be capable of producing frequent
lightning, gusty winds, and locally heavy downpours.

With the more numerous showers and storms (and associated cloud
cover) during the short term period, high temps should remain
somewhat in check, only reaching the upper 80`s to around 90 for
most areas Tuesday and Wednesday afternoons. Overnight lows will
remain muggy, ranging from low to mid 70`s inland to upper 70`s
along the immediate coast. /49

LONG TERM /Thursday Through Sunday/...The long term period is a
battle between the strong upper high over the Great Plains and
another deepening trough over the eastern CONUS. The GFS and Euro
remain in general agreement in depicting a more amplified upper-
level pattern setting up late this week into the weekend, with
the Plains high shifting back towards the southwestern U.S. and a
deep trough digging from the Great Lakes southward into the
northern Gulf. If this solution holds true, we would see a return
of more typical isolated to scattered summertime convection and
hotter temps (low 90`s with heat indices around 100 to 105)
Thursday and Friday as the upper ridge extending eastward from
the Great Plains rotates south over the local area. This would be
followed by another round of cooler (upper 80`s) and wetter
weather over the weekend as the eastern CONUS trough digs south,
pushing a weak front through the local area. /49

MARINE...A ridge of high pressure will gradually build across the
central Gulf of Mexico this week. A light to moderate westerly wind
component generally prevails. Winds, waves and seas locally higher
with the passage of storms. Frequent lightning activity likely in
and near any of the stronger marine storms. A few waterspouts
possible as well. /10

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...None.
FL...None.
MS...Flash Flood Watch until 7 PM CDT this evening for MSZ067-075-076-
     078-079.

GM...None.
&&

$$

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