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Severe Severe Threat 2/16-19

3km NAM showing quite a bit of warm air in the mid-levels.

Looking through the 3km NAM soundings and the WRFs, I think I’m gonna go with northern MS with the highest threat area.
 
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Looking a little through the HRRR and 3km NAM this morning and one thing I do notice and both models really want to increase directional shear across central and north AL well into the evening. Don’t be surprised if this allows a more discreet storm mode to take shape even if for a short period.
 
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Area Forecast Discussion
National Weather Service Birmingham AL
255 AM CST Thu Feb 16 2023

...New SHORT TERM, LONG TERM...

.SHORT TERM...
(Today through Friday)
Issued at 255 AM CST THU FEB 16 2023

Through Tonight.

//Severe storms likely today and tonight//

After 7 days of advertising today`s severe threat, now it`s go time
to put your preparations and severe weather plan into practice. An
upper low over western Kansas will move northeastward and shear out
as it moves into Iowa later today, as its associated southern stream
trough phases with the northern stream. This positively tilted
trough will extend southwestward from Wisconsin to New Mexico by
this afternoon. A broad swath of 50 to 60kt southwesterly mid-level
flow will extend well ahead of this trough over a moist and unstable
warm sector, with various weak subtropical perturbations evident in
water vapor imagery extending southwestward back to Mexico. A
surface low has developed along a frontal boundary over northwest
Arkansas, and will remain fairly steady state as it lifts
northeastward today, before beginning to deepen over the Ohio Valley
tonight. The associated cold front will reach eastern Mississippi
late this afternoon and move through Alabama tonight.

The 6z special sounding from Jackson MS in support of PERiLS
indicates an impressive elevated mixed layer with a 700-500mb lapse
rate of 8 C/km as well as a strong cap. This cap should hold
overnight south of northern Mississippi and will remove the
overnight marginal risk for our northwest counties. Despite the lack
of greater synoptic forcing, models including CAMs are consistent in
a weak impulse in southwest flow aloft breaking the cap and
initiating supercells across central and southern Mississippi late
this morning, then moving northeastward into our southwest counties
early this afternoon. These discrete to semi-discrete supercells
will then continue northeastward across the forecast area through
the afternoon hours with additional development occurring along the
cold front over Mississippi and in between. You don`t often see the
number of identifiable supercells apparent in CAM output in this
area that recent runs have shown. A moist (PWATs around 1.75") and
volatile air mass will be in place especially along and west of I-65
with SBCAPE values of 1000 to 2000 J/kg, steep mid-level lapse
rates, and 0-6 km bulk shear values around 60 kts.

Afternoon 0-1km SRH values are somewhat on the lower end for
tornadoes in our area at 150 to 200 m2/s2. This could potentially be
under-done if models are overdoing mixing, though the 850mb and
925mb winds are also somewhat modest. These values are still
sufficient for tornadoes given the available instability, lapse
rates, and supercell mode. These values ramp up to 200 to 300 m2/s2
closer to sunset. Even if there are not a lot of tornadoes initially
in the afternoon, don`t let your guard down as the tornado threat
often quickly ramps up around sunset when SRH increases and LCLs
drop. Overall the parameters support a mention of strong tornadoes
in the enhanced risk area. Have also bumped up the hail size mention
to golf ball size given the steep mid-level lapse rates and
supercell mode. Based on some of the hail analogs in soundings,
would not be surprised to get some >2" diameter reports especially
with initial activity. Also, with models trending a bit slower with
the front and back edge of the severe threat and also quicker with
the initial discrete development, have widened the threat time
ranges when warnings will be possible in a given location. Multiple
rounds will be possible through the afternoon and evening hours.
While area-average rainfall will be in the 0.75" to 1.75", some
heavier swaths of 2 to 4 inches are indicated by some CAMs where
storms train over the same area. This will result in the potential
for isolated flash flooding as mentioned in the HWO. This could
become more widespread if the bands set up over the urban areas, but
confidence in placement of these higher totals is too low for a
flood watch at this time.

32/Davi
 
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Very interesting wording on the NWS BMX updated Mesoscale Discussion

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BIRMINGHAM AL
858 AM CST THU FEB 16 2023

...NEW MESOSCALE UPDATE...

.MESOSCALE UPDATE FOR SEVERE WEATHER...
ISSUED AT 856 AM CST THU FEB 16 2023

PREVIOUS FORECAST REMAINS ON-TRACK. OVERNIGHT MODELING ALONG WITH
06Z AND 12Z LOCAL AND REGIONAL SOUNDINGS INDICATE STRONGER
CAPPING HAS MATERIALIZED AND IS HELPING TO SUPPRESS SHOWER
DEVELOPMENT ACROSS THE AREA EARLY THIS MORNING.

THE FORECAST AREA REMAINS POSITIONED BETWEEN A BROAD MID-LEVEL
RIDGE OVER THE BAHAMAS AND CUBS WHILE A CLOSED LOW WAS LOCATED
OVER NORTH-CENTRAL KANSAS EMBEDDED WITHIN A POSITIVE TILT TROUGH
OVER THE PLAINS. SURFACE HIGH PRESSURE WAS POSITIONED FURTHER EAST
ACROSS THE CENTRAL ATLANTIC WHILE SURFACE LOW PRESSURE WAS
POSITIONED ACROSS THE OZARK MOUNTAINS IN ARKANSAS. A STATIONARY
FRONT EXTENDS NORTHEAST OF THE LOW ACROSS THE OHIO RIVER VALLEY
WHILE A COLD FRONT EXTENDS FROM THE SURFACE LOW SOUTH INTO COASTAL
TEXAS.

LOCAL ENHANCED RAP 13 KM MESOANALYSIS DEPICTS A BROAD WARM SECTOR
ENCOMPASSING ALL OF MISSISSIPPI INTO AT LEAST THE WESTERN HALF OF
CENTRAL ALABAMA, CHARACTERIZED BY TEMPERATURES ALREADY IN THE
UPPER 60S TO AROUND 70 DEGREES. DEW POINTS, LIKEWISE, ARE IN THE
LOW TO MID 60S ACROSS MISSISSIPPI INTO MUCH OF ALABAMA.

WITH THE MORE ROBUST CAPPING AND SOME CLOUD BREAKS, WHILE OUR
FORECAST HIGH TEMPERATURES HAVE BEEN TOWARD THE UPPER END OF
GUIDANCE, IT REMAINS POTENTIALLY ATTAINABLE IN THIS SCENARIO. MORE
THAT SUFFICIENT INSTABILITY WILL EXIST WITH UNCHARACTERISTICALLY
HIGH NUMBERS OF BUOYANCY THAT EXCEEDS THE MORE TYPICAL VALUES
EXPERIENCED FOR THIS TIME OF YEAR IN OUR PART OF THE COUNTRY.

IN THE SHEAR-INSTABILITY PARAMETER SPACE, WE MORE TYPICALLY FOCUS
ON THE LOWER CAPE-HIGHER SHEAR PORTION OF THE SPECTRUM, HOWEVER,
SETUPS LIKE THIS RESEMBLE THE MORE OPPOSITE END OF THE SPECTRUM
FOR OUR AREA WITH HIGHER CAPE VALUES THAT DO NOT REQUIRE AS STRONG
OF MAGNITUDE OF SHEAR THAN WE TYPICALLY EXPECT. WIND SHEAR IS
MORE THAN SUFFICIENT FOR SEVERE THUNDERSTORMS AND TORNADOES.

THE CAPPING OFF THE SURFACE, AS DEPICTED ON THE OBSERVED
SOUNDINGS, ARE ASSOCIATED WITH A SLUG OF DRIER AIR THAT COMPOSES
THE ELEVATED MIXED LAYER. SOME OF THIS DRIER AIR WILL WORK
DOWNWARD TOWARD THE SURFACE AND CAN RESULT IN AT LEAST A TEMPORARY
REDUCTION IN LOW LEVEL MOISTURE. AS WE PROGRESS THROUGH LATER
THIS AFTERNOON AND INTO THE EVENING HOURS, BOUNDARY LAYER VERTICAL
MIXING WILL DECREASE AND THIS WILL ALLOW FOR LOW-LEVEL MOISTURE
VALUES TO INCREASE. DUE TO THIS POTENTIAL, WE MAY SEE SLIGHTLY
HIGHER-BASED CONVECTION THROUGH THE AFTERNOON HOURS WITH
POTENTIALLY INCREASED VISIBILITY OF SUPERCELL STRUCTURES WITH THE
MORE CLASSIC SUPERCELL STORM MODE PREFERRED. AS WE MOVE INTO THE
EVENING AND OVERNIGHT, THE LOW-LEVEL MOISTURE VALUES WILL AGAIN
INCREASE AND RESULT IN LOWER CLOUD BASES, LOWER LIFTED
CONDENSATION LEVEL HEIGHTS, AND WILL CONTINUE TO SUPPORT NOCTURNAL
SEVERE WEATHER AND TORNADO POTENTIAL. ADDITIONALLY, AS VERTICAL
MIXING DECREASES INTO THE EVENING, THE LOW-LEVEL JET IS EXPECTED
TO INTENSIFY AS WELL, FURTHER PROVIDING SUFFICIENT WIND FIELDS FOR
SEVERE STORMS AND SOME TORNADOES.

THE OVERALL EVENT STORM MODE WILL SUPPORT SUPERCELL STRUCTURES
WITH AN OVERALL GENERAL TENDENCY TO EVENTUALLY ATTEMPT TO CONGEAL
INTO A LINE OF DEEP CONVECTION. IT IS EXPECTED FOR SUPERCELLS TO
DEVELOP AND MOVE ACROSS PORTIONS OF THE WESTERN AND INTO THE
CENTRAL COUNTIES THIS AFTERNOON AND EVENING. FOR THE LATER PART OF
THE EVENING AND OVERNIGHT, WE COULD RETAIN SOME SUPERCELLS ACROSS
THE EASTERN COUNTIES, THOUGH SOME LINEAR FORMATION WOULD
EVENTUALLY BE EXPECTED.

GIVEN CONTINUED CLOUD BREAKS SOUTH, PERSISTENT LOW-LEVEL WARM
ADVECTION AND A GRADUALLY ERODING CAPPING INVERSION THAT LOOKS TO
GIVE WAY ACROSS THE AREA WITH TIME AS SHORTWAVE IMPULSES WITH SOME
HEIGHT FALLS LATER TODAY WILL SUPPORT ALL MODES OF SEVERE
WEATHER. TORNADOES WILL BE AN AREAWIDE RISK WITH BEST POTENTIAL AT
LEAST INITIALLY ACROSS PORTIONS OF THE WEST AND SOUTHWEST
COUNTIES THROUGH EARLY AFTERNOON. DAMAGING WINDS WILL BE POSSIBLE,
ESPECIALLY WITH ANY LINEAR CONVECTIVE CONFIGURATION THAT
MATERIALIZES LATER TODAY. LARGE HAIL WILL BE POSSIBLE WITH THE
STRONGER STORMS AS MORE INTENSE UPDRAFTS ARE EXPECTED GIVEN THE
STRONGER BUOYANCY. ADDITIONALLY, LOCALIZED FLOODING WILL BE
POSSIBLE WHERE TRAINING OF HEAVY CONVECTION OCCURS, ESPECIALLY
OVER TERRAIN AND URBANIZED AREAS.

THROUGH NOON, EXPECT A FEW LOW-TOPPED LIGHT SHOWERS WILL STILL
OCCUR FROM TIME TO TIME THROUGH THE MORNING HOURS THAT DEVELOP
BENEATH THE CAP LOCATED AROUND 805 MB PER LATEST SOUNDING. THIS
ACTIVITY WILL MOVE QUICKLY NORTHEAST OVER 50 MPH AS THE LOW-LEVEL
JET FROM THE SOUTHWEST CONTINUES TO INCREASE IN THE LOWER AND MID
LEVELS.

WE ARE MONITORING CONVECTION DEVELOPING OVER SOUTHERN MISSISSIPPI
THAT WILL BE MOVING NORTHEAST TOWARD OUR SOUTHWEST COUNTIES
TOWARDS EARLY AFTERNOON.

THE NEXT MESOSCALE UPDATE WILL BE SENT BY NOON.
 
one of the storms in a line of developing storms just west of Mccomb Miss has been showing rotaion off and on, and has started a little bit of a right turn just east of NE. May be about to do something?
 
WEATHERBOYROY said:
one of the storms in a line of developing storms just west of Mccomb Miss has been showing rotaion off and on, and has started a little bit of a right turn just east of NE. May be about to do something?
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I just noticed it, too. These storms are starting up earlier than I expected. The storm west of Wesson looks like it is tightening up.
 
Those strung out storms on the central AL/MS border are telling sign on the current bad low level lapse rates. Another thing to think about, don’t be surprised if this isn’t an all or nothing event. Either some supercells get going and give some problems with tornadoes and hail or on the other hand even the “squall line” is a weak mess.
 
Arcc said:
Those strung out storms on the central AL/MS border are telling sign on the current bad low level lapse rates. Another thing to think about, don’t be surprised if this isn’t an all or nothing event. Either some supercells get going and give some problems with tornadoes and hail or on the other hand even the “squall line” is a weak mess.
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Think the tops are being blown off these storms in linear fashion, for now.
 
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