NWS Mobile, AL wrote its last area forecast discussion before the first event at Mar 17, 10:55 AM CST, 8 minutes earlier.
Area forecast discussion, Mar 17, 10:55 AM CST, 8 minutes before
Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1155 AM CDT Wed Mar 17 2021
.UPDATE...Tornado watch #29 has been issued and includes areas
along and north of a line from Camden, Grove Hill, Chatom, AL, to
Leakesville to New Augusta MS through 7 PM CDT. This is a
Particularly Dangerous Situation. Zones will be updated to include
headlines. /10
&&
.PREV DISCUSSION... /issued 1003 AM CDT Wed Mar 17 2021/
MESOSCALE UPDATE...Weather is quiet thus far, but is expected to
rapidly change. A very moist and unstable air-mass exists over
the deep south late this morning. The axis of a powerful southern
stream storm system aligned over Texas was resulting in strongly
diffluent flow from the Mid-South to the central Gulf coast which
will begin to increase large scale ascent over the local area and
increased coverage of storms. At the surface, a complex pattern
is in place. Locally, a surface trof of low pressure was analyzed
east to west along the Highway 84 corridor, embedded within a zone
of rich moisture characterized by dewpoints in excess of 70
degrees F. This feature will be the primary source of surface
focus initially today under the approach of larger scale
diffluent flow. An assessment of 17.12Z upper air soundings just
upstream at New Orleans is very concerning with a volatile
environment in place. Forecasters see substantial steeping of mid
level lapse rates averaging 8.1C/KM in the H50 to H70 layer.
Within this layer, 9.0C/KM was shown in the H60 to H70 layer.
Considering these steep lapse rates, most unstable convective
potential energy already at 2300 J/KG and wet bulb zeroes ~9200'
favors the threat of large damaging hail in severe storms today.
Other concerns are tornadoes. Strengthening shear profiles
resulting in strongly curved hodographs are resulting in 0-3km
shear ~380 m2/s2. Within that layer, 0-1km shear is ~230 m2/s2.
Considering, presence of surface trof near-by and as the
environment undergoes further destabilization/strengthening bulk
shear profiles more intense storm updrafts in any discrete
isolated to scattered supercells going into the afternoon which
is shown in the high resolution convective allowing models radar
reflectivity forecast simulations. These storms will likely
exhibit rotation leading to an enhanced/moderate tornado threat.
There is concern that some of these tornadoes, moving from
southwest to northeast, could be intense and long track. The
greatest threat is aligned along a line from Crestview FL to
Dauphin Island, AL where an enhanced risk of severe weather is
outlooked. Northward, along and north of a Greenville AL to
Wiggins MS line, the risk is even higher at a moderate risk.
A 90 to 100 kt upper level jet structure rounds the base of the
upper trof and imparts a negative tilt across the Lower MS River
Valley and Mid-South by this evening, intensifying large scale
deep layer ascent and organizing storms ahead of an eastward
moving cold front. There may be a brief lull in the convection
this evening, then another round of severe weather with the
passage of the squall line passage overnight which will likely
contain straight line damaging winds in any bowing segments, large
hail and tornadoes. It is urgent to stay weather aware today
ensuring you have multiple ways to receive warnings. Be prepared
to also move to a place of shelter quickly in the case warnings
are issued for your area. /10
PREV DISCUSSION... /issued 644 AM CDT Wed Mar 17 2021/
AVIATION...
12Z issuance...IFR to occasional LIFR/IFR ceilings expected
through mid morning, followed by mainly MVFR to occasional VFR
ceilings. Areas of fog along the Alabama and Florida coasts this
morning will likely reduce visibilities to 2SM or less, also
improving through mid morning. Rain and thunderstorm chances will
continue to increase throughout the day. This is especially true
for inland southeast Mississippi and inland southwest Alabama this
morning, with a gradual uptick in potential intense thunderstorm
activity by early afternoon lasting through the overnight hours.
Thunderstorms could be intense with large hail, damaging winds,
and tornadoes (some strong), with reduction to IFR/LIFR ceilings
and visibility in any convection. Low level wind shear will become
a concern along and ahead of an eastward moving line of convection
tonight, with shear through FL020 approaching 40 to 50kts. /22
PREV DISCUSSION... /issued 509 AM CDT Wed Mar 17 2021/
NEAR TERM UPDATE /Now Through Thursday/...
Severe Weather Outbreak Increasingly Likely Across Portions of
the Deep South...
An upper level trough will move over the Central and Southern Plains
states today as it begins to take on a negative tilt. A split flow
of the subtropical jet and polar jet is evident over the deep south
rounding the base of the aforementioned trough, supporting ample
upper level difluence moving into the region this afternoon into
tonight. This will favor large scale ascent across the region, with
best upper level difluence located to the northwest of our CWA. At
the surface, a warm front will continue to lift north into north-
central Alabama this morning, leaving a wide open warm sector with
ample southerly surface winds off the Gulf of Mexico over our CWA. A
strengthening surface low pressure system will move from Oklahoma
into southeast Missouri by midnight. A cold front progresses through
the CWA overnight into Thursday morning, clearing the area near
daybreak Thursday.
Multiple hazards are expected over the next 30 hours, and each is
broken down below. A regional outbreak of severe thunderstorms is
possible this afternoon into Thursday morning, with multiple rounds
of severe weather. Details on location, timing, and hazards is also
detailed below.
Severe Weather Outbreak Increasingly Likely This Afternoon Through
Thursday Morning:
This Afternoon and Evening: Isolated, discrete to semi-discrete
supercell thunderstorm development remains possible this afternoon
across the entirety of our CWA. Best timing for convection to
develop appears to be after 1 PM CDT. These thunderstorms will move
from southwest to northeast across the CWA through about 8 PM CDT.
The environment is favorable for any discrete supercells to produce
strong tornadoes, damaging winds to 70+mph, and large hail upwards
of golf ball size. CAPE appears to be plentiful, with surface based
CAPE values ranging between 2,000 to 2,500+ J/kg. Mixed layer CAPE
values will approach 1,500 to 2,000+ J/kg. Steep lapse rates will
also be present, with 7 to 8C/km mid-level lapse rates and near
7C/km low-level lapse rates. Wind shear will also be plentiful, with
around 40 to 50kts of deep layer shear which will favor
super-cellular storm mode. SRH values of 150 to 250 m^2/s^2 appear
likely in the sfc-1km and sfc-3km layer, suggesting the potential
for robust rotating updrafts. The biggest question that currently
exists is convective coverage this afternoon and evening. Forecast
models differ on convective initiation, and it is possible that
storms struggle to develop with better forcing for ascent arriving
later in the evening into overnight. However, it is important to
note that any storms that develop will pose a threat for all severe
hazards including large hail to golf ball size, damaging winds to
70mph+, and tornadoes(some of which could be strong).
Tonight into Thursday Morning: An even more likely threat for severe
weather will evolve during the overnight hours. A line/broken line
of intense thunderstorms will likely be ongoing along of and ahead
of the front, with embedded supercell structures. Best timing for
this feature will likely be between 8PM to 1AM across southeastern
Mississippi, 9PM to 3AM over southwestern Alabama including areas
west of I-65, and 1AM to 6AM for the Florida Panhandle and
southwest/south-central Alabama east of I-65. Adequate CAPE will
still be present over the area, with near 1,000 J/kg mixed layer and
surface based CAPE. Shear increases considerably, with near 50 to
55kts of deep layer shear owing to a strengthening nocturnal low
level jet. This will help to enlarge hodographs, aiding in 300 to
400 m^2/s^2 SRH values in the sfc-1km and sfc-3km layers. There
remains uncertainty in regards to if the line remains broken or if
it is a solid line/qlcs, which will have an impact on severe
hazards. Shear vectors off the front at around 45 degrees suggest
that a more broken line appears possible, which would continue
favoring potential for strong tornadoes, damaging winds to 70mph+,
and large hail to golf ball size. If the line turns out to be more
congealed a larger damaging wind threat would evolve, along with a
continuation for tornadoes and some instances of large hail. The
convective evolution of the line will have to be monitored going
into the day Wednesday and Wednesday evening.
In summary, a multi-episode severe event appears probable, with
afternoon and evening convection still uncertain across our CWA.
Another round of severe weather will likely impact the area
overnight Wednesday into Thursday morning in the form of a
convective line. Afternoon and evening thunderstorms could produce
strong tornadoes, damaging winds to 70mph, and large hail to golf
ball size. Hazards during the overnight are a little more uncertain,
however still expect damaging winds and tornadoes, with some
instances of large hail in any thunderstorms.
Other Hazards:
Flash Flooding Forecast:
The flash flooding threat will again increase across our northwest
CWA this afternoon and tonight as additional convective development
is likely. A Flash Flood Watch has been issued for Choctaw,
northern Clarke, and Wilcox counties in Alabama, and Wayne county in
Mississippi. Much of this rain will move across the same areas that
have already received several inches of rainfall yesterday.
Additional rainfall amounts of 1 to 3 inches will be possible across
the watch area and this will exacerbate ongoing flooding conditions
and possibly initiate additional areas of flash flooding.
Dense Fog: Some patchy to areas of dense fog, primarily confined to
coastal counties of Alabama and Florida, spreading into the marine
area where dense fog advisories are in place. Fog may reduce
visibility to near 1/4 mile at times before dissipating by mid
morning.
Wind Advisory: A Wind Advisory is in effect from 10AM CDT today
through 4AM CDT tonight. Strong frequent wind gusts associated with
an increasing pressure gradient, daytime heating, and strong low
level winds above the surface will be likely, ranging from 30 to
35mph.
Rip Currents: A high risk for rip currents remains in place through
Friday morning. Strong southerly winds over a broad swath of the
Gulf of Mexico will aid in generation of large period swell packets
that will continue to affect the coastline through Friday morning.
It is possible this risk could expand further in time if high period
swell continues.
High Surf: A High Surf Advisory has been issued for tonight due to
large breaking waves of 5 to 6 feet expected in the surf zones of
Alabama and Florida.
Rest of the Near Term:
Thursday Morning into Evening: Once the line of convection has
shifted east of the CWA, strong westerly winds of 10 to 20mph can be
expected behind the cold front. Temperatures can be expected to warm
into the upper 60's, with lower to middle 70's possible over the
Florida panhandle and south-central Alabama. Dry weather prevails
behind the front, with ample clearing by early afternoon. MM/25 /22
SHORT TERM /Thursday night Through Friday night/...The closed upper
level shortwave system over the Tennessee River Valley moves off,
leaving a mean upper trough stretching southwest over the region.
Westerly low level flow at the beginning of Thursday night shifts to
northerly Friday with cooler air moving over the Southeast.
Temperatures below seasonal norms are indicated in the Short Term,
with low temperatures in the 40s expected Thursday and Friday
nights. High temperatures ranging from around 60 to the mid 60s
expected Friday.
/16
EXTENDED TERM /Saturday Through Tuesday/...Dry and cool weather
through Monday night. Ridging aloft slowly builds over the region
and then continues to persist through Monday. The ridging begins to
gradually weaken going into Tuesday as the next shortwave moves east
over the southern Plains during this time, moving to a location over
the central CONUS by the end of the period. An expansive area of
surface high pressure over the entire eastern CONUS at the beginning
of the period gradually gets squeezed by the aforementioned
encroaching longwave trough from the west and an unsettled upper
pattern on its SE periphery by early Monday, however surface ridging
still wedges into our region as late as Monday night before finally
being broken down by the advancing trough altogether. The next rain
chances are on Tuesday with the approach of the Plains trough.
Confidence is low for Tuesday rainfall (especially east of I-65)
because the model solutions vary due to phase speed differences in
the approaching progressive mid- and upper trough. A warming trend
sets up during the extended starting off with highs on Saturday in
the low to mid 60s across the area, increasing to the low to upper
70s by Tuesday. Low temps follow suit with generally low to upper
30s to lower 40s across the area on Saturday night increasing to the
low to the lower to mid 50s by Monday night. JEH/88 and /23 JMM
MARINE...A Dense Fog Advisory remains in effect for all bays, the
MIssissippi Sound, and the Alabama Gulf waters out 20 NM through 10
AM this morning. Low visibility to one nautical mile or less will
make navigation difficult. If you must navigate, proceed with
caution, use proper fog signals, and make sure all running lights
are on. A cold front is expected to move across area waters late
tonight into Thursday, with winds and seas increasing and
seas building through this evening. In addition, strong to severe
storms are becoming increasingly likely tonight into Thursday
morning. Moderate to strong westerly then northwesterly flow will
then follow for Thursday and Friday, becoming northeasterly over the
weekend. /22
&&
.MOB WATCHES/WARNINGS/ADVISORIES...
AL...Wind Advisory until 4 AM CDT Thursday for ALZ051>060-261>266.
Flash Flood Watch through late tonight for ALZ051-053-054.
High Rip Current Risk through late Thursday night for ALZ265-266.
High Surf Advisory from 8 PM this evening to 6 AM CDT Thursday
for ALZ265-266.
FL...Wind Advisory until 4 AM CDT Thursday for FLZ201>206.
High Rip Current Risk through late Thursday night for FLZ202-204-
206.
High Surf Advisory from 8 PM this evening to 6 AM CDT Thursday
for FLZ202-204-206.
MS...Wind Advisory until 4 AM CDT Thursday for MSZ067-075-076-078-079.
Flash Flood Watch through late tonight for MSZ067.
GM...Small Craft Advisory from 4 PM this afternoon to 4 AM CDT
Saturday for GMZ630>632-650-655-670-675.
Small Craft Advisory from 4 PM this afternoon to 6 AM CDT Friday
for GMZ633>636.
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