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  • Current Tropical Systems
    Melissa

Severe WX Severe Weather Event 3/28/17-3/31/17

EXCELLENT discussion by Davis out of BMX on all of the caveats to tomorrow's forecast.

"A complex forecast is setting up for Thursday`s potential severe
weather event. Synoptically, a longwave neutrally tilted trough
will be advancing into the area, taking on a negative tilt by
Thursday evening. Within this trough, an upper low will be moving
through the Ozarks while a pronounced southern stream shortwave
will be moving along the Gulf Coast/northern Gulf, extending well
south into the Gulf in association with a strong subtropical jet.
Strong upper-level forcing will overspread the Deep South and Gulf
Coast due to vorticity advection and diffluent flow aloft. A
surface low will be located near St. Louis with a cold
front/surface trough extending southward through the Mississippi
Valley.

Favorable dynamics, shear, lapse rates, and moisture will
be present for severe weather, but there are still two main
uncertain mesoscale details that will determine whether
significant destabilization occurs. The first question is if the
line of storms expected to develop late this afternoon/early this
evening over the Arklatex region will hold together and move into
the area Thursday morning in association with a lead shortwave and
low level jet. If it were to hold together then a scenario
similar to last Saturday could evolve where later activity is
inhibited. However, the majority of the model guidance including
the CAMs and the typically reliable WRF-ARW and NSSL WRF currently
indicates this activity will either dissipate or become scattered
enough to be a non-factor. Therefore, current thinking is that
morning convection will not be a limiting factor. The second
question is whether convection developing along the LA/MS Gulf
Coast associated with the southern stream wave/subtropical jet
will cause convection further north to be "cut off"/suppressed.
This will be something to watch in future CAM runs. Currently,
however, CAMs seem to suggest that this activity may develop too
late, be too far south, or be too scattered to prevent strong
destabilization/supercell development ahead of the front over
northern Mississippi. Additionally, forcing with the upper low and
higher dew points already being in place may prevent coastal
convection from being an inhibiting factor.

In that case, a favorable environment for supercell development
will be in place over northern Mississippi with 1000-2000 J/kg of
CAPE and 50+ kts of southwesterly shear helping to keep storms
ahead of the slow-moving front. These supercells would then have
the potential to move into West Alabama during the late
afternoon/early evening, with sufficient helicity/backed surface
winds for a couple tornadoes and large hail. Keeping in mind the
above uncertainties, the enhanced risk area from SPC in our far
western/northwestern counties depicts the potential threat well,
with a threat gradually decreasing off to the east. In addition,
southwesterly Corfidi vectors indicate that some of the
convection associated with the southern stream wave will move into
mainly our southern counties Thursday afternoon and pose a threat
for mainly damaging winds and hail. Please stay tuned to future
updates and remain weather aware on Thursday, as the forecast will
be sensitive to small scale details."
 
For today, HRRR is indicating the potential for a long tracking supercell or two later today. It has varied in location, but the idea of deep layer shear supporting organized, long track supercells is what to take away....

P4vGg0Z.png
 
After looking at the 12z NAM I really think the eastward extent of that line is main player. NAM has it developing toward the coast and cutting off north Alabama development. I wouldn't be surpised if it is overforecasting the line organization and how far south it extends. If the line falls apart I could see the leftover boundary enhancing the low level backing winds for parts of north Alabama. If the NAM is correct, most of the action will be north and west of Tuscaloosa county.
 
Morning everyone,
During this severe weather event we are going to be beta testing a new plug-in that will directly post SPC MD's, Watches and Convective Outlooks directly to this thread during the duration of this event. Please note this experimental and there is a delay associated with how often we get updates (working on that issue). With the experimental nature of this test we may deem it necessary to stop the automatic posting at anytime and without warning. Later on we may also add additional NWS office feeds to display warnings as well.

All posts of this nature will come from our beloved @TWBot. If you have any questions or complaints send them my way and keep them out of this thread for now.

Thanks,
Wes
 
Last edited:
SPC 1300Z Day 1 Outlook
​


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0748 AM CDT Wed Mar 29 2017

Valid 291300Z - 301200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
PORTIONS OF THE SOUTHERN PLAINS INTO THE MIDDLE AND LOWER
MISSISSIPPI VALLEY...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM EASTERN
PORTIONS OF THE SOUTHERN PLAINS THROUGH THE MIDDLE AND LOWER
MISSISSIPPI VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AREA...

...SUMMARY...
Severe thunderstorms are expected later today into tonight from
parts of east Texas, eastern Oklahoma, and southeast Kansas into the
lower and middle Mississippi Valley. Very large hail, tornadoes (a
couple of which could be strong) and damaging wind are all possible.

...East Texas and Oklahoma, southeast Kansas into the middle and
lower Mississippi Valley areas...

Ongoing line of storms moving through eastern OK, extreme western AR
and southeast TX should continue a gradual diminishing trend as it
advances east this morning, though an isolated strong wind gust or
two cannot be ruled out over southeast TX. The expansive convective
debris will delay destabilization until at least early afternoon
when pockets of diabatic warming and northward advection of modified
continental polar air should contribute to some boundary-layer
recovery and a corridor of 500-1500 J/kg MLCAPE from eastern
portions of the Southern Plains to the lower to middle MS valley.
Initial surface low has consolidated over eastern OK this morning in
association with a lead northeastward-ejecting shortwave trough.
This feature will move very slowly northeast into southeast KS
during the day while weaker secondary cyclogenesis is possible
farther south across AR during the evening as another lobe of
vorticity rotates through the base of the upper trough accompanied
by a strengthening secondary branch of the low-level jet.

Current indication is that additional storms will likely develop
from southeast KS through eastern OK and eastern TX this afternoon
in corridor of stronger destabilization and within the evolving
pre-frontal confluent flow regime. This activity will subsequently
move east into southern MO, AR and LA. Mid-level jet rotating
through the base of the upper trough will contribute to favorable
vertical shear profiles for supercells, and low-level hodographs
will support potential for a few low-level mesocyclones and
tornadoes (a couple of which could be strong) especially as storms
move east into AR and southern MO where strengthening low-level jet
will be maximized during the late afternoon into the evening.
Initially discrete modes will promote a risk for very large hail and
a few tornadoes, but tendency will be for storms to consolidate into
lines by mid-late evening which suggest primary threat should
eventually transition to damaging wind overnight. Due to the
stabilizing effects of morning convection which will delay and
potentially limit boundary layer recovery, will not introduce a
moderate risk at this time but continue to evaluate during the next
update.

..Dial/Grams.. 03/29/2017


Read more

For additional SPC information click here
 
The 3km NAM also quickly decays the overnight squall line by 15z. It also shows a few significant updraft helicity swaths, mostly within the MDT risk. The NSSL-WRF holds the overnight line together, and does a decent job of minimizing severity/coverage in Northern MS. A lot of uncertainty still.

Northern MS/NW AL/Southern TN is still the most favored area.
 
Last edited:
SPC 1630Z Day 1 Outlook
​


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1147 AM CDT Wed Mar 29 2017

Valid 291630Z - 301200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE
OZARKS/LOWER MO VALLEY SOUTHWARD TO LA/LOWER MS RIVER VALLEY...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE
OZARKS/LOWER MO VALLEY SOUTHWARD TO LA/LOWER MS RIVER VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EAST
TX/SOUTH-CENTRAL PLAINS TO MS RIVER VALLEY...

...SUMMARY...
Severe thunderstorms are expected later today into tonight from
parts of east Texas, eastern Oklahoma, and southeast Kansas into the
lower and middle Mississippi Valley. Large hail, tornadoes, and
damaging winds will be possible especially late this afternoon and
tonight.

...Ozarks/lower MO Valley to ArkLaTex/lower MS River Valley...
The closed upper trough currently over the south-central Plains at
late morning will continue a slow-general-northeastward advance
toward the lower MO River Valley through late tonight and early
Thursday. Ahead of this system, 12Z regional observed soundings
reflected considerable lapse-rate-related impacts from the
overnight/early morning squall line that continues to move generally
east-northeastward and decay across the Ozarks and Ark-La-Tex
vicinity. This may have ramifications on the
extensiveness/likelihood of significant hail potential, for
instance, although the air mass will recover/gradually moisten and
destabilize to the south of a northward-moving warm front across the
Ozarks/lower MO Valley, and ahead (east) of a slow-eastward-moving
cold front.

Latest thinking is that surface-based storms will develop as early
as mid-afternoon near the eastern KS surface low southward along the
front, including far eastern portions of KS/OK into western portions
of MO/AR. Moderate low-level hodograph curvature (beneath strong but
somewhat backed mid-level southwesterly winds) and resultant strong
low-level shear/SRH will support a tornado risk aside from large
hail and evolving damaging wind risk into this evening.

Farther south, storms may diurnally be a bit more isolated across
the remainder of the ArkLaTex vicinity, although an ongoing
quasi-linear cluster across near-coastal southeast TX may persist
east-northeastward into LA as it favors a zone of outflow-related
weak boundary/zone of differential heating. This cluster and other
peripheral development may pose a damaging wind/tornado risk through
the afternoon and evening hours.

Later tonight, storms should increase in coverage/intensity
initially across portions of AR/LA as forcing for ascent/DPVA
increases related to an east/northeastward-ejecting vorticity maxima
within the base of the larger-scale trough. Related mass response
should result in increasingly strong low-level winds/confluence. A
mixed mode of storms should be prevalent, with hail initially
possible prior to a more common damaging wind/embedded tornado risk
as storms toward/across the MS River late tonight.

..Guyer.. 03/29/2017


Read more

For additional SPC information click here
 
SPC 1300Z Day 1 Outlook
​


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0748 AM CDT Wed Mar 29 2017

Valid 291300Z - 301200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
PORTIONS OF THE SOUTHERN PLAINS INTO THE MIDDLE AND LOWER
MISSISSIPPI VALLEY...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM EASTERN
PORTIONS OF THE SOUTHERN PLAINS THROUGH THE MIDDLE AND LOWER
MISSISSIPPI VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AREA...

...SUMMARY...
Severe thunderstorms are expected later today into tonight from
parts of east Texas, eastern Oklahoma, and southeast Kansas into the
lower and middle Mississippi Valley. Very large hail, tornadoes (a
couple of which could be strong) and damaging wind are all possible.

...East Texas and Oklahoma, southeast Kansas into the middle and
lower Mississippi Valley areas...

Ongoing line of storms moving through eastern OK, extreme western AR
and southeast TX should continue a gradual diminishing trend as it
advances east this morning, though an isolated strong wind gust or
two cannot be ruled out over southeast TX. The expansive convective
debris will delay destabilization until at least early afternoon
when pockets of diabatic warming and northward advection of modified
continental polar air should contribute to some boundary-layer
recovery and a corridor of 500-1500 J/kg MLCAPE from eastern
portions of the Southern Plains to the lower to middle MS valley.
Initial surface low has consolidated over eastern OK this morning in
association with a lead northeastward-ejecting shortwave trough.
This feature will move very slowly northeast into southeast KS
during the day while weaker secondary cyclogenesis is possible
farther south across AR during the evening as another lobe of
vorticity rotates through the base of the upper trough accompanied
by a strengthening secondary branch of the low-level jet.

Current indication is that additional storms will likely develop
from southeast KS through eastern OK and eastern TX this afternoon
in corridor of stronger destabilization and within the evolving
pre-frontal confluent flow regime. This activity will subsequently
move east into southern MO, AR and LA. Mid-level jet rotating
through the base of the upper trough will contribute to favorable
vertical shear profiles for supercells, and low-level hodographs
will support potential for a few low-level mesocyclones and
tornadoes (a couple of which could be strong) especially as storms
move east into AR and southern MO where strengthening low-level jet
will be maximized during the late afternoon into the evening.
Initially discrete modes will promote a risk for very large hail and
a few tornadoes, but tendency will be for storms to consolidate into
lines by mid-late evening which suggest primary threat should
eventually transition to damaging wind overnight. Due to the
stabilizing effects of morning convection which will delay and
potentially limit boundary layer recovery, will not introduce a
moderate risk at this time but continue to evaluate during the next
update.

..Dial/Grams.. 03/29/2017


Read more

For additional SPC information click here
 
The 3k nam and 12k nam have trended a little quicker with the upper trough and not as sharp...with the wider base, I am not sure there is much difference except I could see the line further ahead than shown.
 
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