SPC - No watches are valid as of Tue Dec 12 17:17:02 UTC 2017

No watches are valid as of Tue Dec 12 17:17:02 UTC 2017.


SPC - No MDs are in effect as of Tue Dec 12 17:17:02 UTC 2017

No Mesoscale Discussions are in effect as of Tue Dec 12 17:17:02 UTC 2017.


SPC Dec 12, 2017 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
0652 AM CST Tue Dec 12 2017

Valid 121300Z - 131200Z

...NO THUNDERSTORM AREAS FORECAST...

...SUMMARY...
No thunderstorm areas are forecast forecast across the continental
U.S. today.

...Synopsis...
In mid/upper levels, a highly amplified pattern is forecast to
persist over the contiguous U.S. today, with troughing in the East. 
A blocky Rex pattern will persist in the West, with a broad/weak
cyclone over the Baja region and a closed high over the Great Basin.
Embedded in the eastern cyclonic-flow field, a positively tilted
shortwave trough was noted in moisture-channel imagery from PA/WV
southwestward through its vorticity max over the TN/AL border
region, to the Arklatex.  This trough is expected to elongate and
weaken through the remainder of the period, pivoting across the
corridor from the Mid-Atlantic to Florida by 00Z.  That will occur
as a strong/upstream perturbation -- initially located from
northeastern ON to IN -- digs southeastward across the lower Great
Lakes and south-central Appalachians.  By 00Z, the associated
shortwave trough should extend from a small/closed 500-mb low --
near the southern tip of Lake Huron's Georgian Bay, southward over
the Carolinas.  By 12Z, the low should pivot eastward to the
Adirondacks, with the trough southward over the NYC vicinity and the
Atlantic.

At the surface, a frontal-wave/triple-point low was located over
western NY, with cold front southwestward across the central/
southern Appalachians, southern MS, southern LA, northwestern Gulf
shelf waters, and deep south TX.  The low is forecast to move
northeastward across NY and weaken today.  By 00Z, a new low should
form on an occlusion point near the southwestern Maine or NH coast,
with cold front offshore all but central FL.  The low-level front
should exit southern FL tonight, leaving a stable, continental/polar
air mass in its wake.

...Cape Cod/Nantucket/Martha's Vineyard region...
Isolated lightning cannot be ruled out over this area this afternoon
or early evening, beneath the passage of a marginally buoyant,
elevated warm-advection conveyor and adjacent frontogenetic forcing.
Some forecast soundings depict 100-300 J/kg elevated MUCAPE with a
small fraction located above the -20 C isothermal level.  However,
the bulk of lightning should remain over the Atlantic.  Thunderstorm
potential decreases with northward extent from open ocean waters,
and inland thunder potential -- whine non-zero -- presently appears
too isolated, brief and conditional for an areal outline at this
time.

..Edwards.. 12/12/2017

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SPC Dec 12, 2017 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
1020 AM CST Tue Dec 12 2017

Valid 121630Z - 131200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Isolated lightning strikes will be possible across parts of coastal
Massachusetts and vicinity this afternoon and early evening.

...Synopsis and Discussion...
Large-scale upper troughing over the eastern CONUS will further
amplify today as several embedded shortwave troughs rotate around
its base. At the surface, a cold front will move eastward off the
Atlantic coast by this evening as a low consolidates near coastal
MA/NH. Preceding the front, low-level warm air advection should
increase across coastal MA this afternoon, and recent NAM/RAP
forecast soundings suggest MUCAPE up to 300 J/kg may be attained as
mid-level lapse rates modestly steepen. Large-scale forcing for
ascent will also strengthen across this region in the same time
frame, and isolated lightning strikes may occur with the more robust
elevated convection. These isolated thunderstorm chances will
quickly diminish this evening as the cold front moves offshore.
Dry/stable conditions will dominate across the rest of the CONUS
today, with negligible thunderstorm potential evident.

..Gleason.. 12/12/2017

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SPC Dec 12, 2017 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
1111 AM CST Tue Dec 12 2017

Valid 131200Z - 141200Z

...NO THUNDERSTORM AREAS FORECAST...

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

...Synopsis...
Broadly cyclonic flow aloft will continue across much of the U.S.
Wednesday, as an upper vortex over the Northeast rotates northward
into Canada, while farther west, short-wave troughing digs southward
across the Rockies.  As a result, overall/broad long-wave troughing
will prevail -- east of the strong ridging residing in the vicinity
of coastal western North America.

With dry/stable continental air to remain in place east of the
Rockies, thunderstorms are not expected.

..Goss.. 12/12/2017

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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
0946 AM CST Tue Dec 12 2017

Valid 121700Z - 131200Z

...Montana...
The elevated area has been expanded across parts of central/eastern
Montana. Recent observations and high-res guidance suggest
west/northwesterly downslope flow around 15-25 mph will combine with
dry fuels and RH values below 20% to promote elevated to locally
critical conditions in the lee of the Rockies this afternoon.

...Southern California...
Morning observations indicate pockets of elevated/locally critical
conditions ongoing across terrain-favored areas. Winds may weaken
slightly during the day, but very dry conditions will maintain
higher fire-weather concerns. The current elevated area captures the
risk area well, and no changes are needed.

..Picca.. 12/12/2017

.PREV DISCUSSION... /ISSUED 0128 AM CST Tue Dec 12 2017/

...Synopsis...
The overall mid-level pattern across the United States continues to
be a general west-to-east ridge-trough pattern. This will maintain a
very dry airmass across most of the United States. Additionally, the
pattern will maintain surface ridging across the Great Basin and a
generally favorable pattern for offshore winds across Southern
California.

...Southern California...

The long-duration offshore-wind event will continue on Tuesday. Peak
wind gusts should be in the 20-40 mph range as compared to the 60-80
mph range last week. This continued offshore flow will maintain a
very dry airmass across the region, with very poor overnight
recovery. Thus another day of elevated-to-locally critical
fire-weather conditions is expected.

One note, even though the overall large-scale surface-pressure
gradient will be weakening with time, forecast soundings indicate
mid-level-lapse rates will be on the order of 7.5-8.0 C/km. These
lapse rates will support plume-dominated fires. Any fire plume 
would likely modify/enhance low-level winds in the vicinity of the
fire.

...North-central to Southeast Montana...

Strong westerly downslope flow will contribute to afternoon
temperatures warming to around 60F across the area. Surface
dewpoints will be in the teens across the region and then should
fall during the day. The combination of warm temperatures and
falling dewpoints will result in minimum afternoon relative humidity
at or below 20% across much of the area, with local areas even
lower. Additionally, the lack of appreciable precipitation in recent
weeks has allowed fuels to cure and ERC percentiles to rise. Thus,
given the dry, receptive fuels, strong, gusty winds, and the warm,
dry airmass, elevated fire conditions are likely.

...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
1114 AM CST Tue Dec 12 2017

Valid 131200Z - 141200Z

No changes to the ongoing forecast are needed. See below for more
details.

..Picca.. 12/12/2017

.PREV DISCUSSION... /ISSUED 0128 AM CST Tue Dec 12 2017/

...Synopsis...
Ridge-trough pattern from west-to-east across the United States will
persist through Wednesday. A shortwave trough will crest the apex of
the ridge and dive south across the eastern Great Basin and central
Rocky Mountains. The result will be a reinforcement of the cold air
in the Great Basin, which will act to maintain the large-scale
surface-pressure field conducive for offshore winds across Southern
California -- albeit weaker than last week.

...Southern California...

Offshore winds will continue for another day. These winds will
maintain very low relative humidity across the region, and poor
overnight recovery. Elevated to locally critical conditions are
likely.

One note, even though the overall large-scale surface-pressure
gradient will be weakening with time, forecast soundings indicate
mid-level-lapse rates will be on the order of 7.5-8.0 C/km. These
lapse rates will support plume-dominated fires. Any fire plume 
would likely modify/enhance low-level winds in the vicinity of the
fire.


...Central United States...

Repeated surface cold front intrusions have scoured what limited
moisture existed across the central United States. This, along with
limited 60-day precipitation has resulted in very dry fuels across
most of the region. Ahead of yet another surface cold front, the dry
airmass and full sunshine will allow for temperature to quickly warm
into the 60s with afternoon relative humidity ranging from around
30% to (possibly) the low teens. Additionally, north surface winds
will strengthen (as the surface-pressure gradient increases) to 15
mph across southern portions of the highlighted area, to near 25 mph
farther north. The result will be widespread elevated to locally
critical fire-weather conditions.

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

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