SPC MD 1761


MD 1761 Image

Mesoscale Discussion 1761
NWS Storm Prediction Center Norman OK
1101 AM CDT Tue Oct 27 2020

Areas affected...Eastern Kansas and far western Missouri

Concerning...Freezing rain 

Valid 271601Z - 271830Z

SUMMARY...Light to moderate freezing rain will be possible through
the late morning hours for east/southeast Kansas and into far
western Missouri.

DISCUSSION...Regional radar trends show a broad region of
precipitation moving from northern OK into southeast KS and western
MO. Surface temperatures across this area are largely in the upper
20s and low 30s. Aloft, morning regional soundings as well as recent
RAP forecast soundings show a 3-5 C warm nose between 800-700 mb
that is fostering a mixture of freezing rain along with some sleet -
as supported by recent surface observations and mPING reports across
the area. Although most forecast guidance shows temperatures
climbing above freezing by early afternoon, the onset of
precipitation prior to this warm up will allow for at least a couple
hours of light to perhaps moderate freezing rain and sleet.
Temperature trends will be monitored through the day for the
expected warm up, which, if delayed, could result in a prolonged
period of freezing rain.

..Moore.. 10/27/2020

...Please see www.spc.noaa.gov for graphic product...


LAT...LON   39139593 39559556 39739487 39449444 38969425 38609428
            38019436 37369467 37039517 37059559 37089607 37349629
            37939615 38489603 39139593 

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SPC MD 1760


MD 1760 Image

Mesoscale Discussion 1760
NWS Storm Prediction Center Norman OK
0840 AM CDT Tue Oct 27 2020

Areas affected...Northwest Texas...central and western
Oklahoma...and far southern Kansas

Concerning...Freezing rain 

Valid 271340Z - 271745Z

SUMMARY...Freezing rain and sleet will continue from central
Oklahoma into the eastern Texas Panhandle through the remainder of
the morning.

DISCUSSION...Recent mPING reports from central OK and northwest
Texas indicate that freezing rain with embedded pockets of sleet
continue to overspread the region. The recent 12 UTC Norman, OK
sounding reveals a 7-8 C warm nose between 800-700 mb on top of a ~1
km deep sub-freezing level (with a minimum temperature near -6 C).
This thermodynamic profile supports the observed precipitation
trends, and with little change expected in the next few hours,
should continue to favor freezing rain with embedded sleet. The
strong 850-700 mb isentropic ascent that is the primary forcing of
this morning's precipitation is expected to persist into the early
afternoon hours, especially across central OK per recent guidance.
This will also act to reinforce the warm nose aloft and continue to
support freezing rain. Although models do show a signal for surface
temperatures to warm just above freezing by around 18 UTC for
central OK, there is some uncertainty in the temperature forecast
given widespread cloud cover, persistent northerly winds/weak cold
advection at the surface, and only modest warm advection at around
925 mb. As such, confidence in ice accumulations will remain highest
along and west of the I-44 corridor where precipitation along the
mid-level warm front is most likely to overlap with sub-freezing
surface temperatures through the early afternoon. Due to the early
timing of this winter storm, fall foliage remains on most trees,
which when combined with ice accumulation and 10-15 mph surface
winds will increase the potential for widespread power outages
through the day.

..Moore.. 10/27/2020

...Please see www.spc.noaa.gov for graphic product...


LAT...LON   35860061 36839954 37339883 37819762 37999674 37679610
            37019568 36269616 35329695 34959710 34199742 33699837
            33339891 33309939 33510018 33840067 34530084 35370079

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SPC Oct 27, 2020 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0748 AM CDT Tue Oct 27 2020

Valid 271300Z - 281200Z


Severe thunderstorms are not expected across the contiguous United
States through tonight.

The main upper-air feature influencing this period's forecast is a
well defined synoptic-scale cyclone, currently centered over
southern AZ per moisture-channel imagery.  The associated 500-mb low
is forecast to pivot roughly eastward and reach southwestern NM by
00Z, then move to near ROW by 12Z tomorrow.  An extensive fetch of
cyclonic flow aloft will accompany this feature from the
southeastern Great Basin and Colorado Plateau across northern MX,
the southern Rockies, and the southern Great Plains.

At the surface, 11Z analysis showed a wavy, quasistationary frontal
zone across northern FL, extreme southeastern LA, and the TX shelf
waters of the Gulf.  A slow-moving but strong cold front was drawn
from the southern Appalachians across western MS, western LA, the
middle TX coastal waters, and deep south TX, merging with part of
the southern baroclinic zone over the northwestern Gulf.  The
western Gulf part of the combined boundary will move little for most
of the period, while the eastern part remains stationary to
northward drifting.

...Southern Plains...
Large-scale forcing for ascent (elevated low-level warm advection
already in place at first, then DCVA aloft shifting in from the
west), juxtaposed with a favorable corridor of moisture transport,
will spread eastward over the southern Plains through the period. 
These will contribute to multiple episodes of thunderstorms rooted
atop the elevated frontal surface, beginning with the ongoing widely
scattered thunderstorms embedded in a large area of convective
precip offering winter mixed precip across parts of OK and northwest
TX.  Another major precip episode in the form of a coarsely north-
south convective plume is likely late overnight across northwest,
central and north-central TX into OK, also containing sporadic
thunder. By that time, isolated thunderstorms may also begin across
east TX and the Arklatex region, in a moistening regime of marginal
buoyancy between the southern Plains plume and the Gulf activity
discussed below.

...Gulf Coast States, Southeast...
Tropical Storm Zeta (as of this writing) is forecast by NHC to
reinvigorate to hurricane strength over the Gulf north of the
Yucatan Peninsula today, its path curving toward a day-2 landfall
across the southeastern LA area.  Associated wind fields favorable
for potentially tornadic supercells are expected to remain offshore
through this period, but will affect portions of the Gulf Coast east
of its track on Wednesday; see the SPC day-2 convective outlook for
more details.

In the meantime, the boundary-layer response to the approaching
hurricane -- and indirectly, the ejecting Southwestern cold-core
cyclone -- will foster early stages of an inland spread of
increasingly well-modified return-flow air, both at and above the
surface.  This will support at least isolated general thunder
potential over a broad swath of the Gulf Coast States and Southeast,
in addition to antecedent potential across the FL Peninsula. 
Weaknesses of lapse rates, buoyancy and shear will preclude
organized severe.  See NHC advisories for latest track/intensity
guidance and tropical watches/warnings related to Zeta.

..Edwards/Mosier.. 10/27/2020

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SPC Oct 27, 2020 Day 4-8 Severe Weather Outlook

Day 4-8 Outlook

Day 4-8 Outlook Image

Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0332 AM CDT Tue Oct 27 2020

Valid 301200Z - 041200Z

A quiescent period for severe thunderstorms will begin on Friday
(day 4) and continue through next Tuesday (day 8) as the large-scale
upper-air pattern features a mean trough over the eastern U.S. for
much of the extended period.  Dry offshore flow will occur over the
Gulf of Mexico through the period and lead to hostile conditions for
thunderstorm development.

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SPC Oct 27, 2020 0730 UTC Day 3 Severe Thunderstorm Outlook

SPC 0730Z Day 3 Outlook

Day 3 Outlook Image

Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0208 AM CDT Tue Oct 27 2020

Valid 291200Z - 301200Z


A conditional risk for a tornado may exist Thursday morning for
adjacent parts of Alabama, Georgia, and the Florida Panhandle in
association with tropical cyclone Zeta.  An isolated threat for
damaging thunderstorm gusts may develop over the Carolinas and
southern Virginia Thursday night.

A fairly complex scenario with relative higher uncertainty is
forecast for the day 3 period over the Southeast into the Carolinas
and VA.  Tropical cyclone Zeta will move from the AL/GA vicinity
west of Atlanta and accelerate rapidly northeastward reaching
Chesapeake Bay by early Thursday evening according to the latest
National Hurricane Center forecast.  The risk for a tornado may
linger during the morning from parts of the FL Panhandle
northeastward into central GA before low-level flow veers as Zeta
becomes increasingly displaced from the region.  Only marginal
instability is expected to penetrate the areas north of the
immediate coastal counties with less-available instability farther
north into southwest GA.  Nonetheless, a conditional risk for a
supercell or two capable of a tornado could continue from the late
Day 2 period into Day 3 across this region.  

By early evening, a mid-level low will open and feature a
strengthening jet (100 kt at 500 mb) moving from northern AL
northeastward across the Carolinas into southeast VA by early Friday
morning.  Significant mid-level height falls (120-180 m at 500 mb)
will overspread central NC Thursday night.  Concurrently, a cold
front is forecast to sweep eastward across the southern Appalachians
with intensifying frontal forcing in lee of the higher terrain. 
Model guidance currently indicates a moist boundary layer will
become conditionally unstable with a cluster or band of storms
potentially developing after dark.  The transport of higher momentum
aloft to the surface could materialize with a risk for damaging

..Smith.. 10/27/2020

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SPC Oct 27, 2020 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1240 AM CDT Tue Oct 27 2020

Valid 271200Z - 281200Z


Severe thunderstorms are not expected across the contiguous United
States on Tuesday.


Early-morning water-vapor imagery depicts a well-defined upper low
moving south toward the AZ/Mexico border. This feature is forecast
to begin advancing east along the international border later this
morning, then into extreme southwest NM by late afternoon. Very cold
continental surface high has settled into the central Plains with
the leading edge of this air mass now to the coastal plain of TX.
Latest model guidance continues to suggest low-level warm advection
will focus along a corridor from the Edwards Plateau-northwest
TX-central OK. This zone will be the primary area of concern for
elevated convection through the period.

Currently, the leading edge of large-scale forcing for ascent
appears to be spreading across the southern Rockies into far west
TX, per moistening observed at mid levels in water vapor imagery.
Scattered showers are now developing across the southern High Plains
in response, and these showers should eventually deepen sufficiently
for lightning. Much of this activity will develop atop sub-freezing
boundary-layer temperatures and significant icing may ultimately be
noted along the aforementioned corridor.

Farther southeast across the Gulf Coast region, a strong mid-level
Bermuda high will forcing deep southerly trajectories across the
Gulf Basin ahead of Hurricane Zeta. While Zeta will not reach the
central Gulf Coast during the day1 period, moisture will begin to
spread inland such that isolated thunderstorms will be possible,
despite the poor low-level lapse rates and overall weak buoyancy.

..Darrow.. 10/27/2020

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SPC Oct 27, 2020 0600 UTC Day 2 Convective Outlook

SPC 0600Z Day 2 Outlook

Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1226 AM CDT Tue Oct 27 2020

Valid 281200Z - 291200Z


A couple tornadoes are possible beginning Wednesday afternoon
through Wednesday night over parts of the central Gulf Coast states
in association with Zeta.

...Central Gulf Coast...
Tropical cyclone Zeta will approach the coast and subsequently move
inland during the period.  The influx of a tropical airmass
(characterized by mid 70s surface dewpoints) will gradually spread
inland over southeast LA and the immediate coastal areas of MS/AL/FL
Panhandle during the day.  Forecast soundings show hodographs
enlarging initially over southeast LA and later along the MS/AL/FL
Panhandle coasts by late afternoon.  Sufficient buoyancy and
enlarged hodographs (0-1 km SRH 300-600 m2/s2) will probably yield
the development of several mini supercells in the outer bands of
Zeta.  It appears the greatest tornado risk will be confined to far
southern AL and the FL Panhandle where larger CAPE (500-1000 J/kg)
and SRH will spatiotemporally overlap.  Considerable uncertainty
remains how far inland a low/conditional supercell-tornado risk will
develop late overnight (i.e., east-central AL and adjacent parts of

..Smith.. 10/27/2020

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SPC Oct 27, 2020 0100 UTC Day 1 Convective Outlook

SPC 0100Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0740 PM CDT Mon Oct 26 2020

Valid 270100Z - 271200Z


Organized severe thunderstorms are not expected across the
contiguous U.S. through tonight.

...01z Update...

Minor changes have been made to the 20z outlook this evening, namely
to lower thunder probabilities across most of the FL Peninsula.
Earlier convection that developed across the southwestern portions
of the Peninsula have propagated northwest with most lightning now
offshore. 01z product will reflect this evolution.

Farther west across the south-central US, isentropic ascent atop
cold boundary layer will be the primary forcing mechanism for
elevated convection later tonight. Earlier thunderstorm activity
that developed across the southern Plains has moved into the Ozark
Plateau region and weakened with minimal lightning observed.
However, buoyancy remains across the southern Plains and the next
short-wave trough will approach this region after midnight. At that
time, renewed thunderstorm activity is expected.

..Darrow.. 10/27/2020

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SPC Oct 26, 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
0244 PM CDT Mon Oct 26 2020

Valid 262000Z - 271200Z


Organized severe thunderstorms are not expected across the
contiguous U.S. through tonight.

...20Z Update...
No major changes have been made to the ongoing forecast. Isolated to
scattered elevated thunderstorms will remain possible across parts
of the southern Plains into the Ozarks through tonight.

..Gleason.. 10/26/2020

.PREV DISCUSSION... /ISSUED 1108 AM CDT Mon Oct 26 2020/

...Southern Plains through Tuesday morning...
A shallow, cold air mass continues to move southward across central
TX.  Bands of elevated thunderstorms (some producing freezing rain
and sleet) are ongoing in association with warm advection and
frontogenesis in the 850-700 mb layer atop the frontal surface.  The
larger buoyancy aloft and stronger forcing for ascent are expected
to shift gradually southeast of the I-44 corridor in OK through this
afternoon into this evening.  Elevated convection should again
increase over northwest TX and spread northeastward into OK in
response to speed maxima ejecting northeastward in advance of the
closed mid-upper low forming over AZ.  Much of this convection after
06z will produce substantial sleet and freezing rain accumulations
where boundary-layer temperatures will be below 32F (along and
northwest of I-44).

...South FL this afternoon...
Daytime heating within a maritime tropical air mass will support
scattered diurnal thunderstorm development along and south of a weak
front across central FL.  The convection will be a little more
concentrated in the deeper moist profiles across southwest FL.

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SPC Day 1 Fire Weather Outlook

SPC Day 1 Fire Weather Outlook

Day 1 Fire Weather Outlook Image

Day 1 Fire Weather Outlook  
NWS Storm Prediction Center Norman OK
1140 AM CDT Mon Oct 26 2020

Valid 261700Z - 271200Z


Current visible satellite depicts clear skies across California and
the Great Basin, allowing for ample boundary-layer heating and
continued mixing. As such, near-critical to critically low RH is
expected to persist across much of California into the southern
Great Basin through the day. Current METAR observations depict 20-30
mph sustained winds, with higher gusts from the Sacramento Valley in
northern California, down to the southern Transverse Ranges in
southern California and eastward into the Colorado River Basin. In
addition, 60-80 kt mid-level flow continues to overspread California
into Nevada. Upper support, downslope flow, and downward momentum
transport of the stronger mid-level flow will continue to foster
strong northerly/offshore flow across portions of the Sacramento/San
Joaquin Valleys and the southern Transverse Ranges in California
through the day, as also suggested by the latest model guidance
consensus. Critical/Extremely Critical Highlights have been
maintained across California, although widespread Extremely Critical
conditions in the southern Transverse ranges may gradually subside
by late afternoon. Critical conditions however, are still expected
to persist across southern California into tomorrow morning.

At the moment, temperatures are rather cool across parts of southern
Nevada, with temperatures in the upper 30s to mid 40s. However,
temperatures are expected to warm into the 50s by afternoon peak
heating, that combined with continued 25-40 mph sustained northerly
flow and 10-25% RH, will foster Critical wildfire-spread conditions
through the afternoon.

..Squitieri.. 10/26/2020

.PREV DISCUSSION... /ISSUED 0152 AM CDT Mon Oct 26 2020/

A very steep pressure gradient will persist across dry areas of the
West and Southwest today in response to 1) very strong high pressure
over the western Great Basin and 2) surface troughing from the
California coastline southeastward through Arizona and western New
Mexico.  Additionally, very strong flow aloft will persist across
these areas in response to a vigorous mid-level wave expected to be
centered over Utah at 12Z and Arizona in the evening.  Continued dry
fuels and areas of drought will continue to support higher-end
fire-weather conditions particularly in portions of California.

...Southern California...
Models/high resolution guidance continue to indicate very strong
flow across typical, terrain-favored areas of the southern
Transverse Ranges throughout the forecast period.  Northeasterly
surface flow will range from 25-35 mph, with gusts perhaps as high
as 70 mph in spots.  Meanwhile, very low RH will continue given the
dry airmass in place, with 5-15% values becoming common during the
day as surface heating commences.  Fuels remain dry and favorable
for fire spread given continued dry conditions over the past several
months.  Guidance suggests that these conditions will continue even
into the evening hours, owing to poor overnight recoveries and the
continued offshore gradient.

...Northern California and the Bay Area...
Elevated to critical fire-weather conditions will likely be ongoing
at the start of the period as current (05Z-06Z) observations
indicate areas of 25-35 mph northerly low-level flow and 7-15% RH
values, indicating very poor recovery.  The strong surface pressure
gradient and favorable upper support suggest that these conditions
will continue throughout the day, with RH values remaining low amid
surface warming.  A few areas may experience gusts to 70 mph.  Fuels
remain extremely dry/combustible, and a few areas will likely exceed
extremely critical thresholds at times - especially in areas between
Sacramento and San Francisco/Oakland and including the North Bay
Mountains and East Bay Hills.

...Southern Nevada through the Lower Colorado River Valley...
Recent high-resolution guidance indicates that enough surface
warming will occur for temperatures to reach the 60s F during peak
heating hours despite northerly surface flow and modest cold
advection.  The residing airmass across the region is quite dry, and
as temperatures increase, surface RH values will fall into the 5-15%
range during the afternoon.  Additionally, surface winds will
increase into the 25-35 mph range given the surface pressure
gradient across the region.  Critical fire-weather delineations
exist where the aforementioned conditions are most likely to occur
amidst dry fuels/fuel beds.

...Please see www.spc.noaa.gov/fire for graphic product...

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