RSS Mesoscale Discussions from Storm Prediction Center
No watches are valid as of Wed Nov 25 19:35:02 UTC 2020.
SPC 1630Z Day 1 Outlook
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 2000Z Day 1 Outlook
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.
Read more
SPC 1730Z Day 2 Outlook
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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SPC Day 1 Fire Weather Outlook
Day 1 Fire Weather Outlook
NWS Storm Prediction Center Norman OK
0934 AM CST Wed Nov 25 2020
Valid 251700Z - 261200Z
Minor adjustments were made to the ongoing outlook to include more
of south-central New Mexico and portions of far west Texas into the
elevated area. Morning guidance suggests that wind gusts in this
area will exceed 20-25 mph at times amidst a dry airmass with
near-critical RH values.
The remainder of the forecast is on track and no changes are needed.
See the previous discussion below for more information.
..Cook.. 11/25/2020
.PREV DISCUSSION... /ISSUED 0132 AM CST Wed Nov 25 2020/
...Synopsis...
A mid-level trough will deepen across the Inter-mountain West as a
trough aloft de-amplifies across the eastern CONUS. The eastward
advance of the western upper trough will induce surface
lee-troughing across the southern High Plains during the day and
building surface High Pressure across the Pacific Northwest and
western Great Basin by the end of the period. Dry and breezy
conditions are expected to accompany the lee-troughing across the
southern High Plains, with a surface pressure gradient becoming
established across California, marking the beginning of a multi-day
offshore flow event conducive to wildfire spread.
...Southern High Plains...
The latest model guidance consensus depicts 15-20 mph sustained
westerly surface winds and 15-25% RH by afternoon peak heating along
the lee of the southern Rockies towards the Trans Pecos range.
However, these conditions are expected to be relatively localized in
most places given upper ridging and associated lack of upper
support. Guidance consensus does depict a larger region of Elevated
conditions across east-central New Mexico, where confidence is high
enough to maintain an Elevated highlight.
...Portions of southern California...
After 06Z Thursday, the aforementioned pressure gradient is expected
to intensify across portions of southern California, encouraging at
least widespread 15-25 mph sustained offshore flow. However, these
stronger winds are expected to arrive in advance of the drier
airmass, with surface RH values in the 25-35% range expected before
12Z (end of the period). Given the delay of the arrival of drier
air, an Elevated area has been withheld this outlook. Shortly after
12Z, drier air and even stronger winds are expected to promote
considerable wildfire spread potential. Please see the latest Day 2
Fire Weather Outlook for more information regarding Thursday's
wildfire-spread potential.
...Please see www.spc.noaa.gov/fire for graphic product...
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SPC Day 2 Fire Weather Outlook
Day 2 Fire Weather Outlook
NWS Storm Prediction Center Norman OK
1102 AM CST Wed Nov 25 2020
Valid 261200Z - 271200Z
...CRITICAL FIRE WEATHER AREA FOR COASTAL RANGES OF SOUTHERN
CALIFORNIA...
The ongoing forecast is on track. Morning model guidance continues
to indicate areas of elevated to critical fire-weather conditions,
with 40-60 mph wind gusts in terrain-favored areas and very low RH
(around 12-15%) likely from mid-morning onward.
See the previous discussion below for more details.
..Cook.. 11/25/2020
.PREV DISCUSSION... /ISSUED 0206 AM CST Wed Nov 25 2020/
...Synopsis...
A positively tilted mid-level shortwave trough is expected to dig
into the southwest CONUS, with strong northwesterly flow aloft
overspreading parts of southern California during the Day 2/Thursday
period. At the surface, a pressure gradient is expected to peak in
intensity. The strong pressure gradient and reinforcing upper
support will both contribute to very strong/dry offshore flow across
the southern California Transverse Ranges, promoting considerable
wildfire-spread potential. Meanwhile, weak lee troughing is expected
across the southern High Plains prior to the passage of a cold
front, with very modest dry/breezy conditions accompanying downslope
flow along the lee of the southern Rockies.
...Portions of southern California...
Between 15-18Z, offshore surface winds are expected to increase to
25+ mph, with widespread gusts easily exceeding 40 mph as the
surface pressure gradient strengthens to -3 to -5 mb (LAX to DAG
gradient) per latest guidance consensus. These stronger winds are
expected to coincide with 10-20% RH, atop fuels that are at least
modestly receptive to fire spread (having been devoid of at least
two weeks of meaningful precipitation accumulations). Through
Thursday Night/Friday Morning, guidance consensus depicts a
critically strong pressure gradient persisting to the end of the
period, with poor RH recovery expected. As such, a Critical area has
been introduced where confidence is highest in a high-end Critical
event occurring, where stronger winds overlapping the lower RH and
more receptive fuels is expected during the day through overnight.
...Parts of the southern High Plains...
Weak lee-troughing is expected to persist across the southern High
Plains, but with mediocre upper support. The latest guidance shows
regions of localized 15-20 mph sustained westerly winds overlapping
15-25% RH across eastern New Mexico into southwest Texas. However,
given the isolated, brief nature of the overlapping favorable
surface winds/RH, fire weather highlights have been withheld this
outlook.
...Please see www.spc.noaa.gov/fire for graphic product...
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