U.S. Severe Weather Forecast (Convective Outlook)

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Storm Prediction Center ac 231254 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0754 am CDT sun Apr 23 2017 


Valid 231300z - 241200z 


..there is a slight risk of severe thunderstorms over parts of 
central/western Georgia and extreme southeastern Alabama... 


..there is a slight risk of severe thunderstorms over parts of 
central/eastern South Carolina... 


..there is a marginal risk of severe thunderstorms elsewhere from 
the western Florida Panhandle to northern Florida and extreme 
southern North Carolina... 


... 
Areas of strong-severe thunderstorms are expected over the southeast 
today and overnight. The main threats appear to be damaging gusts 
over parts of western/central Georgia today, followed by potential 
for a few tornadoes and isolated wind damage overnight in parts of 
South Carolina. 


... 
A generally progressive upper-air pattern will feature broadly 
cyclonic flow over the western U.S., And a slow-moving 
cyclone/trough over the southeast. The latter feature -- now 
becoming a closed circulation and centered over western Tennessee -- should 
deepen and move southeastward to western Georgia by 12z. Meanwhile, a 
series of mostly low-amplitude shortwave troughs will be embedded 
within the western U.S. Cyclonic flow, amidst a coupled upper-jet 
structure. Associated large-scale lift and related destabilization/ 
cooling aloft, atop weak low/middle-level moisture, will contribute 
to general thunderstorm potential over a broad swath of the west 
from the northern rockies and northern Great Basin to the northern 
High Plains. 


At the surface, a cold front was analyzed at 11z over the Atlantic, 
east of NC, to parts of eastern SC, becoming quasistationary from 
there westward to northern GA, then arching northwestward into a 
frontal-wave low near cha. A cold front extended from that low 
southwestward across Alabama to near the mouth of the Mississippi River. 
The low should migrate generally southeastward across northern Georgia to 
near ags by 00z, with the cold front over southern Georgia and the 
central Florida Panhandle, and a warm to stationary front across 
central/eastern SC. As the mid/upper-level low approaches, the 
surface low should move slowly out of eastern Georgia into western/ 
central SC by 12z, with very slow northward movement of the warm 
front possible over eastern SC and perhaps into extreme southern NC. 
By the end of the period, the cold front should reach the Georgia coast 
and northern Florida Peninsula. 


... 
Multiple rounds of scattered thunderstorms are possible through the 
period across the outlook area, with the two primary and 
best-organized convective groupings likely being: 


1. From midday through at least late afternoon, along and ahead of 
the cold front, starting in southeastern and perhaps extreme 
east-central Alabama and shifting/spreading east-northeastward across 
portions of western/central Georgia. Though low-level flow will not be 
particularly strong - limiting hodograph size and low-level shear -- 
strengthening upper-level winds will help to ventilate convection 
and contribute to organized multicellular structures. At least 
transient supercell characteristics might occur in a few cells, 
given enough flow aloft to yield pockets of 35-40 kt effective-shear 
magnitudes. Available moisture and pockets of surface heating 
amidst cloud breaks should help to offset modest lapse rates aloft, 
yielding peak MLCAPE in the 500-1000 j/kg range in a narrow/ 
prefrontal corridor. A well-mixed boundary layer will support the 
potential for occasional damaging gusts to reach the surface. 


A pocket of relatively low-Theta-E low-level air -- currently 
manifest by a dry axis in surface dewpoint obs from The Spine of the 
Florida Peninsula into central Georgia -- will only slowly erode and 
shift/deform eastward somewhat through the period as low-level 
trajectories veer ahead of the cold front. Diurnally initiated 
convection over western/central Georgia mostly should weaken as it 
encounters the remains of this Theta-E min, though a marginal/ 
lower-coverage severe threat may persist across The Gap between the 
two relative probability maxima represented by the "slight risk" 
areas. 


2. Overnight as moist and unstable flow increases off the Atlantic 
into parts of central SC and the SC coastal plain, along and south 
of the warm front. This should occur amidst strengthening low-level 
and deep-tropospheric flow related to the approaching mid/upper 
cyclone, leading to the juxtaposition of a favorably moist and 
minimally capped boundary layer with increasing low-level and deep 
shear. Relatively backed surface flow and strengthening 850-mb 
southerlies are expected along the frontal zone, and somewhat 
southward from it into the warm sector, yielding increasing 
hodograph sizes in the 00-06z time frame. 


A roughly north-south band of thunderstorms should evolve by the 
last few hours of the period, preceded by some discrete or semi- 
discrete cells, then perhaps containing some supercells as well, 
early in its development cycle. Forecast soundings suggest surface 
dew points in the mid 60s to near 70 f contributing to MLCAPE in the 
500-1000 j/kg range, despite the time of night, amidst effective 
srh/shear values in the 150-300 j/kg and 35-40 kt ranges, 
respectively. This should favor the potential for a few supercells 
and perhaps qlcs mesovortices. As such, the overnight tornado 
potential is ratcheted up one notch into 5%/categorical-slight 
level. 


.Edwards/Peters.. 04/23/2017 


$$ 

Mesoscale Discussion


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Mesoscale discussion 0549 
National Weather Service Storm Prediction Center Norman OK 
0640 PM CDT Sat Apr 22 2017 


Areas affected...extreme southwest Tennessee /western NC...northwest 
Georgia...portions of central into northeast Alabama 


Concerning...Severe Thunderstorm Watch 158... 


Valid 222340z - 230045z 


The severe weather threat for Severe Thunderstorm Watch 158 
continues. 


Summary...a general weakening trend is expected heading into the 
evening hours. A few strong/marginally severe storms may persist 
another 1-2 hours, producing strong wind gusts and perhaps small 
hail. However, threat will diminish with time and eastward extent 
and a downstream watch is not expected. 


Discussion...scattered strong to severe thunderstorms along a line 
from far western NC/southwest Tennessee through central Alabama into southern MS 
will continue to track east this evening. In general, recent 
weakening trend should continue with loss of daytime heating and a 
stabilizing boundary layer. Modest midlevel lapse rates and adequate 
deep layer shear should sustain storms into the evening, and in the 
short term /next 1-2 hours/ some strong gusts and small hail will be 
possible. Given the marginal nature of the threat downstream of ww 
158 and overall weakening trend, a downstream watch is not expected, 
and ww 158 likely will be able to be canceled before the 02z 
expiration time. 


.Leitman.. 04/22/2017 


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


Attention...WFO...gsp...mrx...ffc...bmx...hun...mob...Jan... 


Latitude...Lon 32238702 32038744 32008793 32128832 32668837 33378757 
34448630 35258490 35408431 35328391 35088375 34848384 
34298447 33978487 33048597 32238702