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  Wednesday December 13, 2017

RSS Mesoscale Discussions from Storm Prediction Center

SPC - No watches are valid as of Wed Dec 13 08:15:01 UTC 2017

No watches are valid as of Wed Dec 13 08:15:01 UTC 2017.


No Mesoscale Discussions are in effect as of Wed Dec 13 08:15:01 UTC 2017.

SPC Dec 13, 2017 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
1036 PM CST Tue Dec 12 2017

Valid 131200Z - 141200Z

...NO THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Thunderstorms are not forecast across the contiguous states on
Wednesday and Wednesday night.

...Discussion...

The persistent high-amplitude ridge-trough pattern across the United
States will undergo slight modification on Wednesday as a shortwave
trough crests the western ridge and digs into the Great Basin. As
this occurs, the western ridge will build northward, helping to
carve out a broader downstream trough across the eastern united
States. At the surface, a fresh intrusion of cool, dry continental
air will push south and east across areas east of the Rockies.

Subsidence beneath the western ridge and a reinforced continental
airmass across the East will maintain a dry atmosphere across the
contiguous United States. As a result, thunderstorms are not
expected Wednesday or Wednesday night.

..Marsh.. 12/13/2017

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SPC Dec 13, 2017 0700 UTC Day 2 Convective Outlook

SPC 0700Z Day 2 Outlook
      
Day 2 Outlook Image
Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1200 AM CST Wed Dec 13 2017

Valid 141200Z - 151200Z

...NO THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Thunderstorms are not expected across the contiguous states on
Thursday and Thursday night.

...Synopsis...
A couple of Pacific shortwave troughs expected to move equatorward
into the Four Corners/southern Great Basin and northern High Plains
today will proceed southward Thursday and Thursday night.  These
troughs are expected to phase this forecast period, resulting in a
positively tiled longwave trough extending from northern Mexico and
the southern Plains to the Great Lakes.  A cold front attendant to a
progressive Day 1 shortwave trough moving through the Midwest, OH
Valley to the mid Atlantic states should extend from the central and
southern Appalachians through the mid South to central TX at 12Z
Thursday.  This boundary will continue to advance south and into the
western and northern Gulf of Mexico Thursday night.

Although moistening is expected across the Gulf Basin ahead of the
cold front, poor lapse rates suggest buoyancy will not become deep
enough for charge separation as the front moves into the western and
northern Gulf Thursday night.  Elsewhere, a dry, stable environment
will preclude convective development along the cold front.

..Peters.. 12/13/2017

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SPC Dec 13, 2017 0830 UTC Day 3 Severe Thunderstorm Outlook

SPC 0830Z Day 3 Outlook
      
Day 3 Outlook Image
Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0212 AM CST Wed Dec 13 2017

Valid 151200Z - 161200Z

...NO THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Thunderstorms are not expected across the contiguous states on
Friday and Friday night.

...Synopsis...
Another day with no thunderstorms is expected across the contiguous
United States for Friday into Friday night.  A split-flow regime is
expected to develop across the central U.S. into Mexico, with a
southern-stream trough/closed cyclone becoming temporarily cut off
as it moves slowly to the east across northern Mexico.  This
upper-air pattern and offshore low-level flow into the Gulf of
Mexico and across the Atlantic coast maintains a dry, stable
environment inland that remains unsupportable for thunderstorms.

..Peters.. 12/13/2017

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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
0104 AM CST Wed Dec 13 2017

Valid 131200Z - 141200Z

...Synopsis...
Persistent ridge-trough pattern across the contiguous United States
will persist on Wednesday. The ridge across the western United
States will maintain cool, dry air across the Great Basin and a
surface-pressure gradient favoring offshore flow across Southern
California. Additionally, the eastern trough will maintain generally
equatorward flow across areas east of the Rockies, which will
maintain the dry, continental airmass in place.

...Southern California and the southern Sierras...

The long-duration Santa Ana event will continue for yet another day.
The persistent downslope, offshore flow has resulted in very low
relative humidity (single digit to low teens) with very little if
any overnight recovery across the area. Therefore, despite a weaker
surface-pressure gradient than last week, elevated to locally
critical conditions will remain on Wednesday.

Farther north, elevated fire-weather concerns will exist for
portions of the Sierras. This will stem from very low relative
humidity (single digits) being advected westward from the Great
Basin. Additionally, winds around 10 mph will be possible.

...Central United States...

A very dry airmass remains in place across much of the eastern
two-thirds of the United States. This dry airmass, coupled with
little precipitation this fall and curing fuels, will set the stage
for elevated to locally critical fire-weather conditions through the
day on Wednesday. 

The dry airmass, full sunshine, and adiabatic warming associated
with a southward moving cold front will combine to allow
temperatures to warm into the 50s and 60s across the region.
Generally, southern and western areas (western Kansas, Oklahoma,
Texas, and southwest Arkansas) within the highlighted area will be
warmer, with the stronger winds across the northern and eastern
areas (Kansas, much of Missouri, northeast Arkansas, western
Tennessee, and western Kentucky). This will result in two different
regimes for elevated fire-weather concerns.

(1) North and East: Here, slightly cooler temperatures will result
in modestly higher relative humidity as compared to areas farther
south. The slightly higher relative humidity (20-35%) will be offset
somewhat by stronger/gustier surface winds (20-30 mph).

(2) South and West: Warmer temperatures will allow for relative
humidity to fall into the teens. However, surface winds are expected
to be only in the 10-20 mph range.

There will be the potential for a few hours of near-critical
fire-weather conditions across western Kansas. Here, relative
humidity will fall into the upper teens to low twenties in the
presence of winds gusting between 20-30 mph. However, concerns
regarding the duration of the stronger winds preclude an upgrade to
critical at this time.

..Marsh.. 12/13/2017

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

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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
0107 AM CST Wed Dec 13 2017

Valid 141200Z - 151200Z

...CRITICAL FIRE WEATHER AREA FOR PORTIONS OF SOUTHERN CALIFORNIA...

...Synopsis...
The persistent ridge-trough pattern across the contiguous United
States will modify during the forecast period (Thursday and Thursday
night). A shortwave trough will crest the western ridge late
Wednesday and dig south into the Great Basin. As this occurs, the
western ridge will build north along the West Coast. The combination
of these two events will reinforce the cool air across the Great
Basin and strengthen the surface-pressure gradient between the Great
Basin and southern California. The increased surface-pressure
gradient will result in an increase in offshore flow on Thursday
across southern California. Given little, if any, recovery from the
single-digit and low-teens relative humidity, the increase in winds
will result in critical fire-weather concerns once again.

Additionally, enhanced north-to-south oriented mid-level flow
associated with the aforementioned digging trough will overspread
southern Nevada, southeast California, and western Arizona. Diurnal
mixing and increased surface-pressure gradient will allow surface
winds to strengthen to around 20 mph. The resulting increase in
low-level flow will combine with relative humidity ranging from the
low teens to mid twenties to result in elevated fire-weather
concerns across the lower Colorado River Valley.

..Marsh.. 12/13/2017

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

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