Area Forecast Discussion

FXUS64 KMOB 180455
Area Forecast Discussion
National Weather Service Mobile AL
1155 PM CDT Sat Mar 17 2018
.DISCUSSION...Updated for latest aviation discussion below.
06Z issuance...Fairly widespread area of IFR/borderline LIFR 
ceilings in place across the southern half of the forecast area 
late this evening. Expect these conditions to continue through the
overnight and into the first part of the daylight hours. In 
addition, there is the potential for areas of fog to develop 
after 18.10z at all TAF sites. Chance for showers and 
thunderstorms will increase after mid morning into the afternoon. 
Southerly winds persist through the period. 05/RR
.PREV DISCUSSION... /issued 1002 PM CDT Sat Mar 17 2018/ 
DISCUSSION...See updated information for land areas below.
UPDATE...The earlier scattered convection over our region has
come to an end, with our local radars now back in clear air VCPs
as of 945 PM. We have updated the forecast to remove POPs for the
rest of the evening. The next upstream shortwave trough will be
approaching the Lower Mississippi Valley overnight. High resolution 
guidance and most of the deterministic model guidance are in 
agreement that most of the ascent with this next feature will remain 
to our west through 12Z, though there is some indication that 
isolated light showers could develop over our western zones late, so 
will leave only 20 percent POPs in place across this portion of the 
area during the overnight hours. The main impact overnight looks to 
be development of patchy to areas of fog, especially south of the 
U.S. Highway 84 corridor, where the NAM-12, RAP, and HRRR are all 
indicating development overnight, not to mention >60% SREF 
probabilities of visibility less than 1 mile. We have included areas 
of fog vs. patchy over the central and southern zones in the latest 
update (keeping patchy mention for now in the north). The overnight 
shift will monitor observations closely for dense fog development. 
All other forecast elements look on track. /21 
PREV DISCUSSION... /issued 420 PM CDT Sat Mar 17 2018/ 
NEAR TERM /Now Through Sunday/...A cold front will move into the 
Tennessee Valley this evening then slowly sink southward and stall 
across south central Mississippi and Alabama on Sunday. South of the 
this front, a very moist airmass will remain in the place with 
dewpoints in the mid 60s. This afternoon daytime heating is 
generating scattered showers and isolated thunderstorms across 
portions of the area. These will continue through early evening 
before dissipating tonight as the boundary layer cools. Although an 
isolated shower is possible overnight, most areas will remain dry. 
The increased moisture and light winds will allow for at least 
patchy dense fog to develop late tonight into Sunday morning. Lows 
tonight fall into the low 60s inland to mid 60s along the coast.
A shortwave is expected to generate showers and storms across 
eastern Texas this evening which may congeal into an MCS and move 
eastward along the stalled front overnight. Depending on how well 
developed the convection becomes, it could act to enhance 
thunderstorm development on Sunday. Meanwhile, the airmass along and 
south of the stalled front is expected to become moderately unstable 
by tomorrow afternoon with MLCAPE values around 1000 J/Kg. In 
addition strong deep layer shear of 50 to 60 knots is expected. As a 
result, any cells that develop near or just south of the boundary 
will be capable of becoming severe with damaging winds and large 
hail. An isolated tornado is also possible due to the enhanced shear 
near the front. This is where the Storm Prediction Center has 
outlined a slight risk of severe storms, roughly north and west of a 
line from Leakesville to Evergreen to Greenville. /13
SHORT TERM /Sunday night Through Tuesday night/...An upper level
shortwave over the Great Plains pushes east into the Mississippi 
River Valley Sunday night into Monday morning. Ahead of this
system, onshore flow will continue to pump Gulf moisture into 
local area and convection will likely be ongoing along and south 
of a quasi-stationary front draped across the Mid South. Despite
the loss of diabatic heating after sunset, an EML overspreading 
the region from the west will maintain at least a modest amount of
elevated instability throughout the night, with MUCAPE remaining 
around 1500 J/kg per latest guidance. Thus, expect storms to 
continue during the overnight hours Sunday night into Monday. 
Effective bulk shear around 50 knots and mid-level lapse rates 
increasing to around 7.0 C/km (courtesy the aforementioned EML) 
will support the chance for some strong to possibly severe 
thunderstorms capable of producing large hail and gusty straight- 
line winds.
The ingredients for strong to severe storms linger throughout the
day Monday as the upper trough and attendant surface low continue
to push east into the Tennessee Valley. The return of daytime
heating results in further destabilization of the warm sector,
with guidance continuing to suggest upwards of 2500 J/kg SBCAPE 
developing during afternoon hours. Strengthening 850 mb flow 
supports effective bulk shear values around 50 knots, meaning 
storms that are able to form will have the chance to organize and 
become strong to severe. However, there is some uncertainty 
regarding convective initiation Monday afternoon, as some high-
res guidance suggests warm 850 mb temperatures maintaining a 
capping inversion over our area through much of the day, and the 
best forcing to overcome this cap would stay primarily to our 
north in closer proximity to the surface low and upper shortwave. 
Due to this uncertainty, have kept the best chances for strong to 
severe thunderstorms on Monday over the northeastern portion of 
the local area, where the best forcing will likely be realized 
before the cold front eventually sweeps through. There is a 
marginal risk across the remainder of the area, as any storms that
are able to form will be capable of organizing and becoming 
strong to severe as well. The primary threats will be large hail 
and damaging straight-line winds. Please see the Storm Prediction 
Center website ( or our homepage 
( for the latest information regarding severe 
weather potential Sunday night and Monday.
The cold front passes through Tuesday morning, effectively ending
rain chances and ushering in cool and dry air to finish off the 
short term period. Tuesday night lows will be much cooler as a 
result, dipping into the low 40s across the area. /49
LONG TERM /Wednesday Through Saturday/...A deepening upper
longwave trough over the eastern CONUS pushes east over the 
Atlantic Ocean through the long term, with dry deep-layer 
northwest flow setting up over the local area as an upper ridge 
builds to our west. Will see cool and dry conditions beneath 
mostly clear skies Wednesday through the remainder of the week as
a result. Highs reach the mid and upper 60s each day and lows dip
into the upper 30s to low 40s each night. /49
MARINE...A predominantly light to occasionally moderate south to 
southwest flow is expected to continue over the coastal waters 
through Monday as Atlantic high pressure continues to ridge west 
across the eastern Gulf and a frontal boundary remains nearly 
stationary well to the north of the marine area. A stronger cold 
front approaches the marine area Monday night and is expected to 
move east across the coastal waters Tuesday with winds shifting to 
the northwest and increasing, along with building seas. /13
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