U.S. Severe Weather Forecast (Convective Outlook)

Day Three

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Storm Prediction Center ac 201605 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1005 am CST Sat Dec 20 2014 

Valid 201630z - 211200z 

..no thunderstorm areas forecast... 

thunderstorms not expected over the Continental U.S. Today. 

..synoptic setup/forecast... 
Largely zonal flow will prevail over the Continental U.S. Through early 
sun...downstream from powerful/deep zonal jet topping broad ridge 
over the central and east Pacific. A series of weak disturbances over the 
east central and southeastern states will further deamplify as they continue east 
into the Atlantic. Pattern will therefore remain unfavorable for 
appreciable elevated warm air advection/moisture transport atop shallow cool dome 
over the northestern Gulf of Mexico and the adjacent Gulf CST. Elevated 
convection in that region should therefore remain too shallow/weak 
for thunder. 

In the Pacific northwest...low to middle-tropospheric wind field will 
further strengthen later today...yielding strong...vertically 
veering wind profiles over western Washington as developing surface wave west of 
Vancouver Island continues eastward. Gradual warming at middle-levels and 
limited low-level buoyancy should...however...preclude any attendant 
thunder threat. 

.Corfidi/Gleason.. 12/20/2014 

Mesoscale Discussion

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sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 170924 

Mesoscale discussion 1980 
National Weather Service Storm Prediction Center Norman OK 
0324 am CST Wednesday Dec 17 2014 

Areas affected...portions of coastal Southern California 

Concerning...severe potential...watch unlikely 

Valid 170924z - 171130z 

Probability of watch issuance...5 percent 

Summary...limited risk for a marginally damaging wind gust or two -- 
or possibly a brief/weak tornado -- is evident over coastal portions 
of Ventura/Los Angeles/Orange counties in Southern California early this morning. 
Ww issuance will not be required. 

Discussion...latest radar and lightning data depict a small cluster 
of thunderstorms crossing The Channel islands area and adjacent 
coastal Southern California at this time...coincident with a middle-level cyclonic 
circulation center moving east-southeastward across this area per WV loop. 

Despite surface dewpoints only in the low 50s...relatively steep 
lapse rates through the lower and middle troposphere are 
contributing to marginal cape /a few hundred j/kg per objective 
analyses/...thus fueling updrafts which are extending through the 
mixed-phase /-20 c/ layer. 

While area vwps depict relatively modest /20 to 30 knots/ swlys through 
500 mb...backed/sely low-level winds ahead of the storms seem to be 
aiding in occasional weak/short-lived low-level circulations in 
stronger cells -- as observed by kvtx /Ventura/ WSR-88D. Given the 
sustained/weakly rotating storms...a very isolated/low-end severe 
risk may spread onshore over the next couple of hours. With that 
said...any risk for strong winds and/or a brief/weak tornado should 
remain well below thresholds required to consider ww issuance. 

.Goss/Thompson.. 12/17/2014 

..please see www.Spc.NOAA.Gov for graphic product... 


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