U.S. Severe Weather Forecast (Przegląd konwekcyjny)

Day Three

acus01 kwns 221456 
Storm Prediction Center ac 221454 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0854 am CST Wed Feb 22 2017 

Valid 221300z - 231200z 

..no severe thunderstorm areas forecast... 

the risk for severe thunderstorms appears negligible across the U.S. 
Today through tonight. 

A progressive upper-air pattern will continue over the contiguous 
U.S. Through the period, dominated by two primary perturbations: 
1. Closed southern-stream cyclone now centered near mob and 
covering most of the area from the Tennessee Valley and southern 
Appalachians across the central/eastern Gulf and Florida. The associated 
500-mb low is forecast to dig southeastward across the northeastern 
Gulf then pivot eastward across southern Florida during the last few 
hours of the period. 
2. A trough, accompanied by a broad area of cyclonic flow, now 
apparent in moisture-channel imagery approaching the West Coast from 
northern California northward. This trough, through which several 
embedded/smaller-scale vorticity maxima should move, should reach 
the northern rockies, Utah and Arizona by the end of the period. 

At the surface, a diffuse, wavy frontal zone was evident at 11z from 
New England across the lower and upper Great Lakes, to a low over 
Central Lake Superior, then across southern Minnesota to central/eastern 
Montana. A weak low over the Gulf south of aaf should migrate erratically 
southeastward across FL, likely remaining ill-defined given 
convective/precip modification of the air mass to its near east and 
southeast. Lee troughing should intensify through tomorrow morning 
over the Central High plains just east of the Front Range. By 12z, 
an elongated/closed area of low pressure likely will be in place 
over portions of Colorado and WY, along a warm to stationary frontal zone 
extending southeastward over southwestern Kansas and northern OK. 
Moisture still should be insufficient for a thunder threat this soon 
with this system, which will be the nascent form of the cyclone 
forecast to eject across the Central Plains and upper Midwest days 

Within a broad precip plume now entering the western peninsula and 
Lower Keys, isolated to widely scattered thunderstorms are possible. 
Isolated to widely scattered thunder also is possible with 
convection accompanying the cold-core vortex this evening and 
overnight, away from the deepest dry-slot penetrations into the 
inner region of the cyclone. 

The most favorable cape and shear parameter space should develop 
today and tonight well east of the area, over the northern Bahamas 
and adjoining waters. Over FL, lack of both vertical shear and more 
robust low-level moisture, along with (in the leading precip plume) 
weak midlevel lapse rates, will preclude an organized severe threat. 
However, strong gusts from water-loaded downdrafts may accompany the 
most vigorous cores, primarily over open-water exposures. The 12z 
tbw sounding was launched in precip, but the mfl sounding appeared 
representative of the preconvective environment with a choppy cape 
profile, thanks to stable layers aloft. The eyw sounding, though 
partly contaminated by cloud/precip cover, still contained 
surface-based buoyancy as well. 

Generally diminishing thunder potential is expected northward into 
GA, with distance from the strongest midlevel destabilization 
associated with the mid/upper cyclone. A plume of warm advection 
and moisture transport will continue ahead of the dry slot, which is 
evident in mid/upper levels off the back edge of the precip plume -- 
roughly 60 west of pie and 85 west of eyw. MLCAPE mostly should 
remain less than 700 j/kg amidst generally small low-level 

..upper Midwest/Great Lakes... 
A compact, northern-stream shortwave trough and embedded vorticity 
Max -- currently evident near the Montana/SD/ND border confluence -- will 
progress eastward across northern South Dakota by 00z then over Northern Lake 
Michigan by dawn tomorrow. Though moisture is very limited in the 
warm-advection plume to its east, large-scale lift and low-level 
frontal ascent together should support the potential for isolated, 
episodic thunderstorms through tonight. This includes the initial 
area of convection now apparent roughly between bis-mbg, within 
which sporadic lightning has been detected. The thunder potential 
will shift eastward across the outlook corridor with time and likely 
will be unevenly distributed in space and time -- but isolated 

..West Coast states... 
A large area of cold air aloft and embedded shots of DCVA-enhanced 
midlevel destabilization will help to maintain steep 
low/middle-level lapse rates across the outlook area. Isolated 
thunder is possible where low-level moisture and that instability 
aloft are vertically juxtaposed, amidst weak cinh. 

.Edwards.. 02/22/2017 


Mesoscale Discussion

acus11 kwns 202038 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 202037 

Mesoscale discussion 0198 
National Weather Service Storm Prediction Center Norman OK 
0237 PM CST Mon Feb 20 2017 

Areas affected...parts of Louisiana and into far southwest 

Concerning...severe potential...watch unlikely 

Valid 202037z - 202230z 

Probability of watch issuance...20 percent 

Summary...a slow increase in storm intensity is ongoing, along with 
low-probability risk for an isolated damaging wind gust or two, or a 
brief tornado. At this time, it appears that risk will remain 
sufficiently limited to preclude the need for ww issuance. 

Discussion...latest surface analysis shows persistent southeasterly 
flow off the north-central Gulf of Mexico, contributing to both 
onshore advection of upper 60s to low 70s dewpoints, as well as some 
enhancement to the low-level shear profile. As a semi-organized 
band of storms continues crossing western Louisiana, a weakly 
heating/destabilizing airmass (characterized by around 750 j/kg 
mixed-layer cape) is contributing to some increase in updraft 
strength -- both within the line, and with isolated showers 
developing ahead of the main band of storms. 

While potential for additional destabilization is limited, low-level 
and deep-layer shear are both sufficient to support a few stronger 
cells, and at least weak/mid-level rotation. Indeed, a few cells 
immediately ahead of the main convective band have exhibited weak 
rotation per area WSR-88D data, and this will likely continue over 
the next several hours. Overall risk, however, appears likely at 
this time to remain marginal, and thus likely not requiring watch 

.Goss/guyer.. 02/20/2017 

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


Latitude...Lon 29749036 29169024 29079132 29409283 30819321 32059330 
32479261 32109152 30889136 29749036