U.S. Severe Weather Forecast (Convective Outlook)

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swody1 
Storm Prediction Center ac 231950 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0250 PM CDT sun Apr 23 2017 


Valid 232000z - 241200z 


..there is a slight risk of severe thunderstorms this evening into 
tonight...mainly across parts of the Piedmont and coastal plain of 
South Carolina... 


..there is a marginal risk of severe thunderstorms across 
surrounding areas of the Carolinas through eastern/southern Georgia 
and the Florida Panhandle.... 


... 
Strong thunderstorms may still impact parts of the Florida 
Panhandle, Georgia and the Carolinas late this afternoon into 
tonight, accompanied by at least some risk for severe weather. This 
includes a possibility for a couple of tornadoes near a frontal zone 
across the Piedmont and coastal plain of South Carolina. 


..20z outlook update... 
Some adjustments have been made to categorical and probabilistic 
lines, mainly to account for ongoing convective trends and latest 
observational data. 


Weak warm sector destabilization remains an appreciable limiting 
factor concerning severe weather potential through the remainder of 
the period. And, south of the stalled portion of the frontal zone 
(extending east of the Atlanta Metro into the South Carolina 
Piedmont and coastal plain), vertical shear is rather modest as 
well. Seasonably high moisture content air (pw of 1.75-2.00 inches) 
is largely confined to a plume spreading across and northeast of 
southern Florida and the Bahamas, in association with an impulse 
within the subtropical westerlies, and potential for significant 
further moistening inland of the Carolina coast through at least 
late tonight seems relatively low. However, models still suggest 
that at least modest strengthening of southerly 850 mb flow (to 30+ 
kt) is possible by this evening, centered roughly across South 
Carolina, as an upper trough axis continues to pivot from a positive 
to more neutral tilt while approaching the region. Given some 
enlargement of low-level hodographs, and some further boundary layer 
heating and moistening within a narrow corridor to the north of the 
surface front, it is possible that the environment could become 
conducive to a risk for tornadoes by this evening, as thunderstorm 
activity develops along/across the frontal zone. 


.Kerr.. 04/23/2017 


Previous discussion... /issued 1130 am CDT sun Apr 23 2017/ 


..Georgia and South Carolina... 
Satellite imagery indicates an upper low has formed over western 
Tennessee, and this feature is forecast to move southeastward 
reaching the Alabama/Georgia border by the end of the period. A 
band of stronger mid/upper level winds curving cyclonically around 
the southern periphery of the circulation is expected to spread 
across parts of Georgia and northern Florida into tonight. 


At the surface, a low over extreme northwest Georgia will move 
mainly eastward along a quasi-stationary front extending across 
east-central Georgia to the central South Carolina coast. The low 
is forecast to reach east-central Georgia this evening and cross 
into South Carolina by 24/12z. A cold front trailing southward from 
the low will progress eastward to the Atlantic coast from Georgia to 
northeast Florida by the end of the period. The frontal boundaries 
are expected to focus the stronger thunderstorm activity through 
tonight. 


Initial storm development is expected along the advancing cold front 
by early afternoon from west central Georgia into the Florida 
Panhandle as it moves into an axis of slightly greater moisture. 
Although mid-level lapse rates are generally weak, visible satellite 
imagery indicates stronger diabatic heating will occur in the warm 
sector where fewer clouds are present. This will enhance low-level 
lapse rates and contribute to modest destabilization with MLCAPE of 
500-1000 j/kg this afternoon. With some cam guidance indicating 
development of qlcs structures along/slightly ahead of the front, 
stronger cells within the line segments will have potential to 
produce damaging downbursts as they move through the focused 
moisture/instability axis. See mesoscale discussion 550 for more 
detailed information. 


Other storms are expected to develop over parts of central/southern 
South Carolina this afternoon near the aforementioned east-west 
front. The lack of clouds south of the boundary is promoting a 
strong differential heating zone where attendant mesoscale 
circulations will support storm potential this afternoon. Isolated 
severe storms will be possible as vertical shear increases over the 
area, especially along and immediately north of the boundary where 
backed surface flow will enhance low-level shear. 


The severe threat over South Carolina is expected to increase 
somewhat tonight as a south-southeasterly low-level jet strengthens 
in response to the approaching upper low. This will result in 
increasing moisture advection from the Atlantic and be associated 
with stronger low-level shear with an attendant risk for supercells 
to develop. Low local levels and moderate-strong srh indicate 
potential for a few tornadoes to occur with any persistent discrete 
supercells that develop. 


..northern/central Utah... 
Isolated weak thunderstorms may develop this afternoon and evening 
in association with an upper short wave trough that will move 
eastward across the Great Basin. Column moisture/total precipitable 
water values are quite minimal /approximately 0.25 inch/ which will 
limit the coverage and intensity of any convection. However, a dry 
sub-cloud layer will enhance evaporative cooling and promote 
Transfer of stronger mid-level horizontal momentum to the surface, 
suggesting potential for convectively-enhanced surface wind gusts. 
Given the aforementioned limiting factors, we will maintain a 
sub-severe outlook at this time over the area. 


$$ 

Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 231824 
scz000-gaz000-232100- 


Mesoscale discussion 0551 
National Weather Service Storm Prediction Center Norman OK 
0124 PM CDT sun Apr 23 2017 


Areas affected...portions of SC and far east-central Georgia 


Concerning...severe potential...watch unlikely 


Valid 231824z - 232100z 


Probability of watch issuance...20 percent 


Summary...an isolated strong/gusty wind and perhaps brief tornado 
threat may exist this afternoon with thunderstorms developing along 
a front. Watch issuance is unlikely in the short term. 


Discussion...a stalled front is draped east-west across SC into far 
east-central Georgia as of 18z per recent visible satellite imagery and 
surface observations. Although large-scale ascent associated with an 
upper low over Tennessee remains displaced to the west, low-level 
convergence along the front has proven sufficient for the initiation 
of isolated showers and thunderstorms in SC over the past hour or 
so. The airmass along and south of the boundary is weakly unstable 
(mlcape up to 750 j/kg), with surface temperatures generally in the 
low to mid 80s and dewpoints in the low to mid 60s. Mid-level lapse 
rates remain poor across this region, around 6 c/km. 


Both low and mid-level flow are generally modest (30 kt or less), 
although strongly backed winds in the 0-1 km layer along and just 
north of the front are contributing to 0-1 km srh around 200-250 
m2/s2 per recent vwps from kcae. A veering wind profile with height 
is supporting 30-35 kt of effective bulk shear, locally higher right 
along the front. The combination of weak instability and shear along 
may allow for some updraft organization with the strongest storms, 
and isolated instances of strong/gusty winds may occur. In addition, 
a brief tornado cannot be ruled out with any storm which crosses the 
boundary and encounters greater effective srh along/just north of 
the front. The overall severe weather threat is expected to remain 
isolated through the afternoon, and watch issuance is unlikely. 


.Gleason/Weiss.. 04/23/2017 


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


Attention...WFO...ilm...chs...cae...gsp...ffc... 


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