NWS Mobile, AL wrote its last area forecast discussion before the first event at Mar 25, 5:55 PM CST, 18 minutes earlier.
Area forecast discussion, Mar 25, 5:55 PM CST, 18 minutes before
Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
655 PM CDT Thu Mar 25 2021
.MESOSCALE UPDATE...Current radar shows light, scattered showers
over the western Florida Panhandle and portions of south-central
Alabama. A cluster of thunderstorms is also moving over eastern
Clarke County, Mississippi and Choctaw County, Alabama. Thus, the
main area of concern for the next couple of hours is focused over
interior southeast Mississippi and southwest Alabama. Any storms
that do develop over these areas are moving into a region of
sufficient shear (around 65-70 knots bulk shear) and instability
(1700-2000 J/kg MLCAPE, 1300-1500 SBCAPE). There is also an
incoming cold front from the west (currently moving over central
Mississippi). However, the main upper dynamics look to be shifting
off to the north/northeast early this evening. Therefore, the
question remains if any further convection will initiate,
particularly over the area of concern. Hi-Res guidance is not very
optimistic in this regard and merely shows some scattered showers
(potentially a few storms) over the local area through the
remainder of the evening. This will continue to be monitored, but
overall the trend for severe weather seems to be decreasing this
evening.
The Tornado Watch (#50) remains in effect until 8 PM CDT for
Wilcox, Choctaw, and Clarke counties (AL). The Watch has been
cancelled for Wayne County (MS) and Washington County (AL). /26
&&
.PREV DISCUSSION... /issued 620 PM CDT Thu Mar 25 2021/
NEAR TERM UPDATE /Now Through Friday/...PDS Tornado Watch #50
remains in effect for counties west of the Alabama River until
8pm this evening. As of 5:45 PM CDT, Perry and Greene Counties
have been removed from the watch. Conditions across the forecast
area have begun to ease, though remain at levels supportive of
severe weather. MUCapes around 2000j/kg, effective Bulk Shear wind
values are still around 75-80 knots over the remaining counties
in the watch, and 0- 1km helicities have fallen to around 200
m^2/s^2. The better upper dynamics have shifted to the northeast,
though. With a cap present over areas along and southeast of I-65,
stronger storms much less thunderstorms period, have been
limited. It is still way too early to say the threat of severe
weather has ended, though, especially over the watch area.
/16
AVIATION...
00Z issuance...At 23z, general mid to upper MVFR conditions were
noted across most of the forecast area, with low end MVFR over the
coastal counties of Alabama and the western Florida Panhandle, along
with southeast Mississippi. CIGS are expected to drop to general low
end MVFR by 06z, then MVFR to IFR levels overnight as a slowing cold
front crosses the area beginning later tonight and stalling near the
coast by the end of the day Friday. CIGS will remain at MVFR levels
close to the surface front through Friday, rising to VFR north.
South to southwest winds 10 to 15 knots will ease to 5 to 10 knots
ahead of the approaching front, then shift to northwesterly 5 to 10
knots after the front passes.
/16
PREV DISCUSSION... /issued 414 PM CDT Thu Mar 25 2021/
NEAR TERM UPDATE /Now Through Friday/...PDS Tornado Watch #50
remains in effect for counties west of the Alabama River until
8pm this evening. The forecast area and nearby remains conducive
for severe development. The latest SPC analysis is advertising
MUCapes of 2500-3000J/kg, effective bulk shear values of 50kts
southeast to 80 kts over the watch area, and 0-1km values of 250
m^2/s^s over the watch area.
Outside of today's developing severe weather, an upper level
ridge stretching north along the East Coast from high pressure
centered over the western Caribbean will help to deflect
northeastward today's upper level weather maker, currently near
the ArkLaTex. A cold front associated with this system will sag
southeast across the forecast area beginning later tonight before
stalling along the northern Gulf coast by Friday afternoon.
Rain will last through the night along and ahead of the approaching
front. Even with the upper level dynamics moving quickly off this
afternoon into tonight, enough instability remains for some
thunderstorms to mix in this evening into the early overnight
hours. A few strong to marginally severe storms are possible with
MLCapes around 1500 J/kg, as the front sags south across the
forecast area tonight. With winds settling tonight, fog
development is likely southeast of I-65 as the front approaches
the coast. This fog will be slow to mix out Friday morning with
the boundary in the area along the coast. The stalled boundary
will begin to move inland late Friday into Friday night as a
surface ridge stretching west over the northern Gulf coast re-
establishes southerly flow over the Southeast. Drier air will move
south across the northern half of the forecast area Friday, but
with southerly flow quickly becoming re-established, moisture
levels quickly recover into Friday night.
Looking at today's WWAs....
Peak wind reports around 30kts/35mph have been received at area
airports across the forecast area. Will be continuing with the
Wind Advisory for the rest of the afternoon. Reports coming in
from area beaches are showing double red flags flying at a few
beaches, with surf heights of 4-5 feet. Tidal gauges on Mobile are
reading about 2 feet above astronomical levels, so am continuing
the High Risk of Rip Currents and Coastal Flood Advisory. Am
continuing the High Surf Advisory for now, with onshore flow
expected to strengthen a bit this afternoon.
With the upper ridge over the eastern Conus, temperatures are
expected to remain well above seasonal norms. Am still expecting
today's high temperatures to top out in the upper 70s to lower
80s. Tonight, low temperatures ranging from the upper 60s
southeast of I-65 to around 60 over the northwestern border of the
forecast area are expected. Friday will see temperature stopping
out in the mid 70s well north of the Highway 84 corridor to around
80 along the I-10 corridor. /16
* Previous Mesoscale Update issued 1207 PM CDT...
The latest 16z visible satellite imagery suggests that strong heating
underway across southeast Mississippi into interior southwest
Alabama with several breaks in the cloud cover. In addition, there
are clearly visible cloud streets (HCRs) with agitated cumulus
developing which are a sign of increasing instability and strong shear.
In addition, latest 16z surface observations show an enhancement
of low level convergence across interior southeast Mississippi and
southwest Alabama, with southeast surface winds along the coast in
southwest Alabama going more south-southwest into south central
into southeastern Louisiana.
Latest near term guidance, supported by the latest VAD profilers
from both KLIX (Slidell, LA) and KMOB (Mobile, AL), show a low
level jet max based between 4-5kft that extends southwest from the
Louisiana coast into the northern Gulf waters. Near term models
suggest that this low level jet will evolve east and nose into
southeast Mississippi within the next few hours. This forcing
combined with the near surface wind convergence should be enough
to initiate convection over interior southeast Mississippi between
12pm-2pm along the existing cloud streets. These storms would
reside in a highly favorable environment for severe weather.
Latest VAD profiler from KMOB (1641z) shows excellent veering low
level profile with increasing speeds from the surface up to 8kft.
This would support a sickle type low level hodograph with long
clockwise turning hodographs and the ingestion of nearly
streamwise vorticity into developing updrafts. The latest 15z
observed sounding from KJAN already suggests an uncapped sounding
with SBCAPES of 3314j/kg. This volatile environment suggests the
potential that any convection that can grow upscale and become
sustained would quickly develop supercell characteristics and the
potential to produce potentially long tracked strong to violent
tornadoes, large hail up to golfballs, and damaging straight line
winds up to at least 70 mph. The only uncertainties that still
remain are the degree of overall forcing and actual storm
initiation. So if storms can form, then the environmental
parameters will support high end severe weather. This may be a
potentially dangerous severe weather setup. This is most likely to
evolve over portions of interior southeast Mississippi into
southwest Alabama where a Potentially Dangerous Tornado Watch
remains in effect until 8 pm CDT.
Please continue to monitor the evolution of storms over the next
few hours and be prepared to act quickly if warnings are issued
for your location. /JLH
SHORT TERM /Friday night Through Sunday night/...Friday night
through Saturday night, the upper ridge stretching north along the
Eastern seaboard remains intact, though by Sunday, an upper level
shortwave trough swinging east across the Northern Mississippi River
Valley to Mid-Atlantic region shifts the upper ridge east, whilst
leaving the upper high over the Caribbean/Gulf of Mexico. A low
level ridge stretching west over the northern Gulf coast re-asserts
itself, with the stalled surface boundary moving inland as a warm
front. For Sunday into Sunday night, guidance has begun to diverge
again with its handling of an upper level shortwave trough that has
moved over the Desert Southwest. This trough is connected through an
upper trough to the northeastern Conus system. The GFS is
advertising the southwestern system remaining connected, whilst the
ECMWF is advertising the system becoming detached and organizing
into an upper low. The ECMWF is also advertising a deep
northeastern Conus upper trough. Both are advertising a cold
front moving across the Southeast in response to the passing
northeastern Conus trough, with the ECMWF advertising by Monday a
weaker and faster frontal passage as compared to the GFS. By
Monday morning, the front has moved south of the coast.
For the forecast,have went with a blended approach. Guidance
maintains a small chance of precipitation near the warm front as
it moves inland Friday night through Saturday, then rain coverage
begins to increase as the next cold front approaches and crosses
the forecast area Sunday/Sunday night. Temperatures remain well
above seasonal norms through Saturday night, with the cold front
bringing more seasonal temperatures to areas where it crosses.
Sunday and area wide Sunday night. Low temperatures in the low to
mid 60s are expected Friday and Saturday nights, with high
temperatures Saturday ranging from around 80 along the coast to
low to mid 80s inland. Sunday, upper 70s to low 80s are expected
across the majority of the forecast area, with mid 70s across
northwestern- most portions of the forecast area. Sunday night
will see a return of cooler air to the forecast area, with low
temperatures in the 50s expected. /16
EXTENDED TERM /Monday Through Thursday/...The differences in the
medium range guidance continue, with the ECMWF advertising a
closed upper low over the Tx/Ok panhandle moving east-northeast
towards the Mid-Atlantic Region through Tuesday, and the GFS
advertising a weakening upper trough over the Southeast through
the same period. The ECWMF brings a return of southerly flow with
the passing upper system, whilst the GFS maintains the surface
boundary over the northern Gulf of Mexico. By Wednesday, the GFS
has returned the Southeast to southerly low level flow. Both have
an upper trough diving southeast over the Plains, eventually
moving over the Mississippi River late Thursday. A cold front
crosses the forecast area Wednesday night into Thursday. Timing in
the medium range guidance is pretty consistent for the forecast
area, though the GFS is advertising the front stalling south of
the forecast area Thursday, and the more aggressive ECMWF
advertising the front pushing well south of the Gulf coast.
Temperatures see a rise to well above seasonal temperatures into
Wednesday, then drop to around seasonal norms for Thursday.
/16
MARINE...Strong onshore flow will ease later tonight into Friday as
a cold front approaches area waters. The front will stall along the
coast early Friday night then retreat north, bringing a short period
of variable winds along with fog development. Moderate to strong
onshore flow returns by Saturday, but another front moving over area
waters Sunday into Sunday night will bring moderate to strong
offshore flow for the beginning of the week.
/16
&&
.MOB WATCHES/WARNINGS/ADVISORIES...
AL...Wind Advisory until 10 PM CDT this evening for ALZ051>060-
261>266.
Coastal Flood Advisory until 1 AM CDT Friday for ALZ263>266.
High Rip Current Risk through Sunday afternoon for ALZ265-266.
High Surf Advisory until 7 AM CDT Friday for ALZ265-266.
FL...Wind Advisory until 10 PM CDT this evening for FLZ201>206.
Coastal Flood Advisory until 1 AM CDT Friday for FLZ202-204.
High Rip Current Risk through Sunday afternoon for FLZ202-204-
206.
High Surf Advisory until 7 AM CDT Friday for FLZ202-204-206.
MS...Wind Advisory until 10 PM CDT this evening for MSZ067-075-076-
078-079.
GM...Small Craft Advisory until 7 AM CDT Friday for GMZ630>636-650-
655-670-675.
&&
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