U.S. Severe Weather Forecast (Konvektivereignisse)

Day Three

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Storm Prediction Center ac 201243 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0643 am CST Fri Jan 20 2017 

Valid 201300z - 211200z 

..there is a slight risk of severe thunderstorms this evening 
across the Upper Texas coast...and tonight across the lower 
Mississippi Valley... 

..there is a marginal risk of severe thunderstorms surrounding the 
Gulf Coast slight risk area... 

..there is a marginal risk of severe thunderstorms this afternoon 
along the Southern California coast... 

Severe storms will develop this evening and spread across the lower 
Mississippi Valley tonight through Saturday morning, with a few 
tornadoes, large hail, and damaging winds all possible. Strong 
thunderstorms with at least some severe-weather potential are also 
possible near the Southern California coast this afternoon. 

Broad cyclonic flow is now established across much of the contiguous 
U.S., As a series of embedded shortwave troughs move from The 
Rockies to the MS valley in advance of a strong mid-upper jet 
approaching California. A broad/diffuse cyclone will be maintained across 
the plains, which will encourage gradual inland progress of the 
warm/moist sector across the Gulf Coast. A lead shortwave trough 
now over northern Mexico will begin to affect the Texas coast by this 
evening, with its influence spreading east-northeastward across the 
lower MS valley by Saturday morning. 

..TX coast this evening to the lower MS valley overnight... 
Convection on Thursday shunted the Richer low-level moisture 
southward into the northern Gulf, but low-level air mass recovery is 
underway across the northwest Gulf Coast. Today will be quiet in 
terms of convective potential across the lower MS valley. However, 
the risk for severe thunderstorms will increase by this evening as 
the lead midlevel trough from northern Mexico interacts with the 
moisture/buoyancy gradient along the Texas coast. The storm 
environment will support supercells capable of producing large hail 
and damaging winds as convection spreads east-northeastward toward 

The more pronounced mass response to the midlevel trough will occur 
tonight across the lower MS valley, where strengthening low-level 
flow/warm advection will contribute to multiple thunderstorm 
clusters near and after midnight. Boundary layer dewpoints in the 
mid-upper 60s will maintain surface-based buoyancy through the 
overnight hours, while the increasing low-level shear will favor 
supercells and some risk for tornadoes. Otherwise, steepening 
midlevel lapse rates (emanating from northeast Mexico/south tx) will 
promote large hail formation in the stronger/supercell storms, as 
well as a risk for damaging winds. 

..Southern California coast this afternoon... 
The latter system will overspread the central/Southern California coast 
today, with focused ascent in the left-exit region of the mid-upper 
jet. Steepening lapse rates and some increase in low-level moisture 
with onshore flow later today will drive at least weak surface-based 
buoyancy near the Southern California coast. Deep-layer vertical shear and 
buoyancy will be sufficient for low-topped supercells and organized 
line segments capable of producing isolated damaging gusts, and 
perhaps a brief waterspout/tornado along the coast. 

.Thompson/Peters.. 01/20/2017 


Mesoscale Discussion

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sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 201144 

Mesoscale discussion 0078 
National Weather Service Storm Prediction Center Norman OK 
0544 am CST Fri Jan 20 2017 

Areas affected...immediate coastal regions of southern Marin...San 
Francisco...and San Mateo counties 

Concerning...severe potential...watch unlikely 

Valid 201144z - 201245z 

Probability of watch issuance...5 percent 

Summary...locally strong wind gusts (around 40-50 kt) will be 
possible as an eastward-moving line of storms reaches the coastal 
areas of southern Marin, San Francisco, and San Mateo counties by 

Discussion...at 1125z, radar imagery continued to show a fast-moving 
line of storms (speed close to 50 kt) approaching the Pacific 
coastal areas of southern Marin, San Francisco, and San Mateo 
counties, with this activity expected to reach the coast by 12z. 
Area WSR-88D vwp data indicated south-southeasterly low-level winds 
of 40-50 kt. Wind gusts associated with the line of storms could 
reach 40-50 kt at the surface, given momentum Transfer in the 
downdrafts. The threat for the locally strong/damaging wind gusts 
is expected to be confined to the coastal areas, since a more stable 
environment inland with surface temperatures in the upper 40s should 
support a diminishing trend in storm intensity. 

.Peters/Thompson.. 01/20/2017 

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


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