U.S. Severe Weather Forecast (Convective Outlook)

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Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0234 PM CDT sun Sep 21 2014 


Valid 212000z - 221200z 


..no severe thunderstorm areas forecast... 


... 
scattered showers and a few thunderstorms with gusts approaching 
severe levels are expected from the eastern Great Lakes across the 
central Appalachians to the middle south. Scattered thunderstorms with 
isolated strong thunderstorm wind gusts are also expected across 
parts of the southwest and Great Basin this afternoon and evening. 


..nrn Alabama...TN...KY...WV... 
a cold front continues to spread southeastward across lower MS/OH/TN river 
valleys and into the lower Great Lakes region. Heating ahead of this 
front and strengthening wind fields may still allow for isolated 
severe wind gusts this afternoon before sunset. For more information 
see mesoscale discussion 1754. 


..NV into Utah... 
Heating is leading to an increase in storm coverage across Nevada into 
southwestern Utah under the upper trough with cool temperatures aloft. 
Relatively cool surface temperatures exist across northern areas thus 
have shunted the 5% wind probs farther southwestward. Veering and increasing 
winds with height should favor a few strong cells capable of small 
hail and perhaps strong wind gusts. For more information see 
mesoscale discussion 1755. 


.Jewell.. 09/21/2014 


Previous discussion... /issued 1125 am CDT sun Sep 21 2014/ 


... 
A strong shortwave trough over the Great Lakes will pivot 
east-northeast with the greatest 500mb height falls expected across 
the St. Lawrence Valley and adjacent areas of New York/Vermont through tonight. 
A belt of 60-75kt cyclonic middle-level flow will translate through the 
eastern flank of the trough...from Ohio...across the central 
Appalachians...and then over northern New England...through early 
Monday. A weaker and warmer disturbance east of the Outer Banks is 
forecast to accelerate northward and skirt southeast New England 
coastal waters before becoming absorbed by stronger westerlies over 
the Canadian maritime provinces late tonight. 


Latest indications provide no compelling evidence to increase severe 
weather probabilities ahead of the Great Lakes/Ohio Valley upper 
trough and its associated strong surface cold front. A leading band 
of showers moving east from northeast New York to eastern WV Panhandle 
appears to be decaying as it becomes removed from deep-layer forcing 
for ascent to the west. Despite generally clear skies and surface 
heating immediately ahead of this leading convective band...latest 
guidance and radar trends across western PA/New York support the idea of 
new development occurring along the surface cold front. This 
activity will spread east into a relatively stable airmass in the 
wake of the leading band of showers/clouds. Despite strengthening 
dynamics and the presence of 40kt vertical shear...new convection 
will likely struggle given lack of greater buoyancy. A few line 
segments/persistent storms may move east at up to 40kt and produce 
minor wind damage such as downed limbs/trees but greater organized 
severe weather potential appears limited. 


..Delmarva/central Appalachians southwest to Tennessee Valley... 
The trailing portion of the middle/upper trough will spread south from 
the Ohio River valley with a smaller secondary height fall center 
forecast across the southern Appalachians/Tennessee Valley by Monday 
morning. Widely scattered thunderstorms should develop along/ahead 
of the cold front as diurnal heating contributes to modest 
destabilization. Stronger instability /SBCAPE around 1500 j per kg/ 
is forecast with southwestward extent along the front but these 
areas also become increasingly removed from stronger large-scale 
forcing and vertical shear needed for more persistent/organized 
convection. While stronger storms with some potential to produce 
wind damage may be realized just about anywhere along the extensive 
frontal zone...from the central Appalachians and middle-Atlantic 
southwest across the Tennessee Valley...the sporadic/limited nature of this 
threat does not appear to warrant higher severe thunderstorm wind 
probabilities with this outlook. 


... 
Thunderstorms are expected to continue across much of Utah today 
amidst relatively deep moisture and broad zone of persistent lift on 
the eastern flank of slow-moving closed low over Nevada. Additional 
diurnal storm development may occur within/near the cyclone center 
over Nevada where latest visible satellite imagery depicts strong 
surface heating now underway beneath relatively colder temperatures 
aloft. Pockets of stronger unidirectional deep-layer shear may lend 
support for a couple of more persistent fast-moving cells/lines over 
Utah... while a deeper mixed boundary layer over Nevada results in areas 
of higher dcape. Both regimes could support isolated damaging gusts 
through early evening. 

Mesoscale Discussion


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Mesoscale discussion 1755 
National Weather Service Storm Prediction Center Norman OK 
0215 PM CDT sun Sep 21 2014 


Areas affected...portions of central/eastern Nevada into far southwest 
Utah and northwest Arizona 


Concerning...severe potential...watch unlikely 


Valid 211915z - 212045z 


Probability of watch issuance...20 percent 


Summary...a few strong storms may produce gusty winds this afternoon 
across parts of central and eastern Nevada into southwest Utah and far 
northwest Arizona. The severe threat is expected to remain limited and a 
watch is not expected. 


Discussion...thunderstorm coverage is increasing across southern and 
central Nevada early this afternoon as daytime heating results in weak 
destabilization. While high temperatures will remain on the cool 
side this afternoon /mainly 70s/...steepening low level lapse 
rate...increasing dcape and modest deep-layer shear may allow for a 
few strong thunderstorm wind gusts as cells quickly migrate toward 
the N/NE. Midlevel lapse rates will remain modest at best except 
near the upper low center where colder temperatures aloft reside. 
This...coupled with weak instability /less than 1000 j per kg 
MLCAPE/ should generally limit longevity/intensity of individual 
cells and any severe threat should remain brief and isolated. A 
Severe Thunderstorm Watch is not expected. 


.Leitman/carbin.. 09/21/2014 


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


Attention...WFO...slc...vef...lkn...rev... 


Latitude...Lon 36481403 36511342 36701302 37041297 37661271 38281289 
39401336 39901398 40141461 40391540 40511622 40451685 
40031737 39551762 39181779 38891782 38241752 37591696 
37171656 36851571 36661482 36481403