SPC - No watches are valid as of Mon Sep 28 22:01:01 UTC 2020

No watches are valid as of Mon Sep 28 22:01:01 UTC 2020.


SPC - No MDs are in effect as of Mon Sep 28 22:01:01 UTC 2020

No Mesoscale Discussions are in effect as of Mon Sep 28 22:01:01 UTC 2020.


SPC Sep 28, 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
0254 PM CDT Mon Sep 28 2020

Valid 282000Z - 291200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM A PORTION
OF THE GULF COAST STATES INTO THE OHIO VALLEY...

...SUMMARY...
Isolated strong wind gusts remain possible this afternoon into the
early evening from a portion of the Gulf coastal states into Ohio.

...Gulf coastal states through eastern Ohio...

Recent radar trends show an increase in thunderstorm intensity along
a portion of the front primarily from the Gulf coastal states into
middle Tennessee where instability is a bit stronger than farther
north across the OH Valley. Destabilization of the surface layer has
occurred in pre-frontal warm sector, and vertical wind profiles in
vicinity of the cold front are marginally sufficient for storm
organization over the Gulf Coast states. Farther north into the OH
Valley, wind profiles are more favorable for organized storms, but
the thermodynamic environment is very marginal and will remain a
limiting factor. Will maintain MRGL risk category for this update
with primary threat being isolated strong wind gusts. See SWOMCD
1722 for more information.

..Dial.. 09/28/2020

.PREV DISCUSSION... /ISSUED 1127 AM CDT Mon Sep 28 2020/

...OH to the central Gulf coast...
Within a large-scale trough over the Great Plains/MS Valley, an
initial mid-upper speed max will rotate north-northeast from IL/IN
into southern Ontario. Pronounced cyclogenesis is expected through
tonight from the Lake Huron vicinity north towards James Bay. The
trailing cold front will continue to move east across the OH Valley
to central Gulf Coast regions. Poor mid-level lapse rates are
expected within the majority of the warm sector, and the richer
low-level moisture (boundary-layer dewpoints in the low-mid 70s)
will be confined to the Gulf Coast. Thus, buoyancy will be rather
shallow and limited in magnitude to the north in the OH Valley
(MLCAPE < 250 J/kg), with moderate buoyancy (MLCAPE of 1500-2500
J/kg) expected near the Gulf coast.

Ongoing scattered to broken showers from the TN Valley north within
the zone of ascent along the cold front may deepen into low-topped
thunderstorms by peak heating, most likely centered on the OH
Valley. Here, strong deep-layer shear may offset the limited
buoyancy/poor lapse rates aloft to yield a brief cluster capable of
producing locally strong gusts. Wind profiles will be weaker from AL
to southern LA, though thermodynamic profiles will be much more
favorable for deeper convection, with an attendant threat of
isolated damaging wind gusts during the late afternoon to early
evening.

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SPC Sep 28, 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
1228 PM CDT Mon Sep 28 2020

Valid 291200Z - 301200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
CAROLINAS INTO A PORTION OF THE COASTAL MIDDLE ATLANTIC...

...SUMMARY...
A few severe thunderstorms with isolated damaging wind and a couple
of tornadoes are possible across the eastern Carolinas into the
coastal Middle Atlantic Tuesday.

...Eastern Carolinas into the coastal Middle Atlantic...

Synoptic upper trough will undergo significant amplification over
the eastern states today through early Tuesday with deep layer flow
becoming increasingly parallel to the accompanying cold front. This
front should extend from a surface low in southeastern Canada
through the western Carolinas and into the central Gulf by 12Z
tomorrow. The front will continue slowly east and off the Atlantic
Seaboard toward the end of day 2. A significant vorticity maximum is
forecast to rotate through the base of the synoptic trough then
northeast into the Carolinas and Middle Atlantic Tuesday night.
Forcing accompanying this feature will remain mostly in the post
frontal zone, but will be sufficient to induce a weak cyclone that
will move from the Carolinas through the Middle Atlantic overnight,
before reaching the Northeast States late Tuesday night.

Rich low-level moisture with low 70s F dewpoints will reside in the
pre-frontal sector, but very weak lapse rates, widespread clouds and
ongoing areas of showers and storms will limit potential for
significant destabilization with MLCAPE from 400-800 J/kg possible.
While a few strong storms with gusty winds might be possible with
activity developing along and ahead of the front during the day
given sufficient (40-45 kt) unidirectional shear, the best severe
threat may be during the evening and overnight when frontogenetic
forcing and low-level shear increase in association with an
intensifying southerly low-level jet from the eastern Carolinas into
the Middle Atlantic. This process may result in the development of a
more robust line of storms along the cold front with potential for a
few embedded organized structures capable of damaging wind and
perhaps a couple of tornadoes. Given an expected marginal
thermodynamic environment, will maintain MRGL risk category this
outlook and continue to monitor for a possible SLGT risk upgrade in
later updates.

...Southern New England...

The stronger frontogenetic forcing might result in development of a
low-topped line of convection with little to no lightning across
southern New England late in the period. Though a few strong wind
gusts cannot be ruled out, will preclude introduction of severe
probabilities this update given expected very limited instability.

..Dial.. 09/28/2020

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SPC Day 2 Fire Weather Outlook

SPC Day 2 Fire Weather Outlook
          
Day 2 Fire Weather Outlook Image
Day 2 Fire Weather Outlook  
NWS Storm Prediction Center Norman OK
0218 PM CDT Mon Sep 28 2020

Valid 291200Z - 301200Z

...Central and northern High plains...
Models have come into better agreement on the character of the dry
post frontal airmass overspreading the region Day2/Tuesday. Despite
cooler temperatures, soundings show a relatively dry airmass with
deep mixing heights intersecting stronger winds aloft. This should
result in humidity of 17-25% and sustained winds of 15-25 mph. The
combination of gusty winds, low humidity, and receptive fuels will
support widespread elevated and locally critical fire weather
conditions over much of the area.

...Southern California...
Lingering offshore flow from the ongoing Santa Ana event will
support elevated to locally critical fire weather conditions over
the Transverse, Peninsular, and the southern Sierra ranges through
midday Tuesday. An elevated area will be maintained over portions of
southern California as gradients begin to wane as the upper-level
ridge shifts over the West Coast.

..Lyons.. 09/28/2020

.PREV DISCUSSION... /ISSUED 0159 AM CDT Mon Sep 28 2020/

...Synopsis...
Some offshore gradient is expected to persist Tuesday morning with
elevated winds and relative humidity continuing in the southern
California mountains. These winds should weaken by mid-day as the
Great Basin warms and the pressure gradient relaxes. Elsewhere,
winds will remain light as a strong upper-level ridge builds over
the western CONUS. Elevated/near-elevated winds may return Tuesday
night/Wednesday morning as high pressure strengthens slightly in the
Great Basin.

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

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