RSS Mesoscale Discussions from Storm Prediction Center

SPC Nov 25, 2020 1630 UTC Day 1 Convective Outlook

SPC 1630Z Day 1 Outlook
      
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1027 AM CST Wed Nov 25 2020

Valid 251630Z - 261200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
THROUGH LATE EVENING FROM SOUTHEAST LA TO PARTS OF AL/NORTHWEST
GA...AND ACROSS THE LOWER OH VALLEY...

...SUMMARY...
Isolated damaging winds and a tornado or two will be possible this
afternoon trough late evening from southeast Louisiana to central
Alabama and northwest Georgia, and across the lower Ohio Valley.

...Lower OH Valley this afternoon/evening...
A midlevel shortwave trough over MO will continue east-northeastward
to the upper OH Valley overnight.  A surface cyclone near Saint
Louis will likewise move east-northeastward toward Lake Erie, and
both the midlevel trough and cyclone are expected to weaken
gradually by tonight.  A narrow zone of surface
heating/destabilization and boundary-layer dewpoints in the upper
50s is expected this afternoon from southern IL into southwestern IN
and western KY, southeast of the surface cyclone.  Though
surface-based buoyancy will remain weak, vertical shear will be
favorable for a low-end threat of low-topped supercells or short
line segments capable of producing isolated damaging winds and
perhaps a tornado.

...Southeast LA to central AL/northwest GA through late evening...
Farther south, a pre-frontal squall line is moving eastward across
southeast LA, central MS, and northwestern AL.  The influence of
this ongoing convection will likely limit the potential for
destabilization this afternoon, immediately to its north into TN. 
The ongoing convective band will shift slowly eastward, with some
low-level moistening/destabilization possible into central AL. 
Deep-layer and low-level vertical shear/SRH will tend to weaken
through the day from southwest-to-northeast as the primary synoptic
wave passes to the north over the OH Valley.  Still, some low-end
potential will remain for line segments and embedded supercells
capable of producing isolated damaging winds and a tornado or two.

..Thompson/Wendt.. 11/25/2020

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SPC Nov 25, 2020 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook
      
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0127 PM CST Wed Nov 25 2020

Valid 252000Z - 261200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THROUGH LATE
EVENING FROM SOUTHEAST LA INTO PARTS OF AL AND NORTHWEST GA...AND
ACROSS THE LOWER OH VALLEY...

...SUMMARY...
Isolated damaging winds and a tornado or two will be possible
through late evening from southeast Louisiana to central Alabama and
northwest Georgia, and across the lower Ohio Valley.

...20z Update...

The ongoing forecast remains on track. Only minor changes have been
made on the western edges of the OH Valley and southeast
LA/AL/northwest GA Marginal risk areas. These adjustments have been
made based on the current location of the cold front and surface
low. A few strong gusts and possibly a brief spin-up remain possible
through this evening.

..Leitman.. 11/25/2020

.PREV DISCUSSION... /ISSUED 1027 AM CST Wed Nov 25 2020/

...Lower OH Valley this afternoon/evening...
A midlevel shortwave trough over MO will continue east-northeastward
to the upper OH Valley overnight.  A surface cyclone near Saint
Louis will likewise move east-northeastward toward Lake Erie, and
both the midlevel trough and cyclone are expected to weaken
gradually by tonight.  A narrow zone of surface
heating/destabilization and boundary-layer dewpoints in the upper
50s is expected this afternoon from southern IL into southwestern IN
and western KY, southeast of the surface cyclone.  Though
surface-based buoyancy will remain weak, vertical shear will be
favorable for a low-end threat of low-topped supercells or short
line segments capable of producing isolated damaging winds and
perhaps a tornado.

...Southeast LA to central AL/northwest GA through late evening...
Farther south, a pre-frontal squall line is moving eastward across
southeast LA, central MS, and northwestern AL.  The influence of
this ongoing convection will likely limit the potential for
destabilization this afternoon, immediately to its north into TN. 
The ongoing convective band will shift slowly eastward, with some
low-level moistening/destabilization possible into central AL. 
Deep-layer and low-level vertical shear/SRH will tend to weaken
through the day from southwest-to-northeast as the primary synoptic
wave passes to the north over the OH Valley.  Still, some low-end
potential will remain for line segments and embedded supercells
capable of producing isolated damaging winds and a tornado or two.

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SPC Nov 25, 2020 1730 UTC Day 2 Convective Outlook

SPC 1730Z Day 2 Outlook
      
Day 2 Outlook Image
Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1107 AM CST Wed Nov 25 2020

Valid 261200Z - 271200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
SOUTHEAST TX INTO SOUTHWEST LA EARLY FRIDAY MORNING...

...SUMMARY...
Isolated large hail may occur with thunderstorms that develop across
parts of southeast Texas and southwest Louisiana, mainly from
Thursday night into early Friday morning.

...Southeast States into New England...

A mid/upper shortwave trough over the central Appalachians/lower
Great Lakes vicinity Thursday morning will develop east/northeast
across the Mid-Atlantic and Northeast through Thursday evening. At
the surface, a cold front will extend from western NY/PA into
central AL and southwestward near the LA/TX Gulf Coast vicinity
Thursday morning. The front will not advance much across the
Southeast as the upper trough and weakening surface low track well
north of the region. In fact, a weak shortwave upper ridge will
develop over the south-central states in the wake of the ejecting
northeastern upper trough. This will allow for the cold front to
become diffuse, and in some places retreat northward as a warm front
as a southerly low level flow warm advection regime develops across
TX and into the lower MS Valley after 00z. The northern segment of
the cold front will shift eastward across New England and the
Mid-Atlantic through the afternoon, moving offshore by evening.
Showers and isolated thunderstorms are expected near/just ahead of
the front from the southern U.S. through much of the Atlantic Coast.
However, weak instability and modest shear will limit organized
severe potential. 

...Southeast TX into Southwest LA...

An upper trough will deepen over the western states Thursday, and
become increasingly cut-off over the Four Corners by Friday morning.
As this occurs, a surface trough is forecast to develop from
southwest TX into OK, resulting in increasing southerly return flow
across southern/eastern TX and the lower MS Valley by Thursday
afternoon. This will allow 60s F surface dewpoints to advance as far
north as roughly the I-20 corridor from eastern TX into MS by late
in the forecast period ahead of a cold front surging southward
across the southern Plains. Forcing for ascent will remain weak
across the region, and deterministic as well as
ensemble/probabilistic guidance varies, but isolated thunderstorms
could develop in this warm advection regime late in the forecast
period. If this occurs, increasing southwesterly shear and steep
lapse rates atop a near-surface cool layer could support elevated
supercells. While this threat is highly conditional on storm
development, elevated supercells could produce large hail, mainly
across southeast TX into southwest LA early Friday morning.

..Leitman.. 11/25/2020

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