U.S. Severe Weather Forecast (Convective Outlook)

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acus01 kwns 201257 
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Storm Prediction Center ac 201255 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0655 am CST Wed Feb 20 2019 


Valid 201300z - 211200z 


..there is a slight risk of severe thunderstorms over portions of 
southeastern Louisiana to southwestern Alabama... 


... 
Strong to severe thunderstorms are possible through early afternoon 
from southeastern Louisiana across southern Mississippi and 
southwestern Alabama. 


... 
In mid/upper levels, a longwave trough will persist over the western 
Continental U.S. Through the period, but with geometric adjustments on smaller 
(synoptic to subsynoptic) scales. A weak cyclone now centered over 
Wyoming will pivot/re-develop eastward across the northern plains. 
Meanwhile, a basal shortwave trough -- initially located over the 
Central Plains and southern High Plains -- will eject northeastward, 
reaching portions of Minnesota, WI and Lake Michigan by 00z. Thereafter, 
that perturbation and the cyclone will merge, with the combined 
trough weakening and ejecting across the upper Great Lakes. An 
upstream shortwave trough -- now apparent in moisture-channel 
imagery over the interior Pacific northwest -- will dig south- 
southeastward and amplify, reaching coastal central/Southern California by 
12z. 


At the surface, 11z analysis showed a weak low over west-central MS, 
with cold front southwestward across portions of south-central/ 
southwestern la and the northwestern Gulf. A marine/warm front was 
drawn from the low southeastward across southeastern MS to Gulf 
waters about 60 nm south of mob, then east-southeastward to central 
Florida. The low should migrate erratically eastward across central Alabama 
through the period, likely weakening further as the already-distant 
upper support moves away. The cold front will proceed eastward to 
western al, southeastern MS, and southeastern la by around 00z, 
reaching northern Georgia and south-central Alabama by 12z. The front should 
become quasistationary farther southwest tonight, across 
southeastern la and the northwestern Gulf. The warm front will move 
slowly northeastward over Alabama and the western Florida Panhandle through 
afternoon, gradually losing definition thereafter as its vertical 
slope gets shallower and baroclinicity weakens. 


..southeastern la to southwestern Alabama... 
Widely scattered to scattered thunderstorms will pose a risk of 
damaging gusts and perhaps a tornado or two into early afternoon, as 
loosely organized convective bands shift eastward across the area, 
with embedded elements moving northeastward. For details on the 
near-term threat, refer to Storm Prediction Center mesoscale discussion 114. 


Ahead of the western longwave trough, a broad fetch of southwesterly 
flow will persist over the area, with neutral height tendencies 
(isallohypses near zero). This will maintain favorable deep/speed 
shear and moisture transport across the region, but amidst little or 
no mid/upper-level forcing for ascent. As such, supportive lift 
must arise from near-frontal boundary-layer processes, both 
kinematic (convergence) and thermodynamic (boundary-layer warm 
advection and weak diabatic heating). Very gradual deepening and 
increasing coverage of convection has been noted overnight across 
the eastern la/southwestern MS region. This is consistent with the 
slow rising and weakening of a previously intense midlevel inversion 
noted in perusing soundings from 00z, 06z (vortex-southeast support set), 
an 09z Ulm launch at Breaux Bridge near lft, and available 12z 
raobs. 


The shallow near-surface stable layer and the 650-700-mb inversion 
evident in the 12z lix sounding, for example, and each should weaken 
the remainder of this morning amidst the aforementioned frontal/ 
thermodynamic lift. Both the stable layers were less-prominent 
farther inland near Jan, amidst 400-500 effective srh and 65-kt 
effective-shear vectors. Given these trends, a relative peak in 
convective intensity is forecast the next 3-6 hours, after which the 
threat should diminish as overall deep-layer lift weakens and 
low-level shear diminishes. 


.Edwards/Jewell.. 02/20/2019 


$$ 

Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 201229 
alz000-msz000-laz000-201500- 


Mesoscale discussion 0114 
National Weather Service Storm Prediction Center Norman OK 
0629 am CST Wed Feb 20 2019 


Areas affected...southeast Louisiana...much of southern Mississippi 


Concerning...severe potential...watch possible 


Valid 201229z - 201500z 


Probability of watch issuance...40 percent 


Summary...a damaging wind or tornado risk may develop later this 
morning, and convective trends are being monitored. 


Discussion...showers and thunderstorms continue to gradually 
increase in coverage early this morning as a warm front lifts 
northward across MS. Recently, the Jan observation has warmed 
significantly, and this trend will continue across the remainder of 
central and eastern MS as well as western Alabama later today. 


Morning soundings from the region indicate the inversion above 700 
mb has weakened, with a deepening of the moist boundary layer. 
Modified 12z soundings indicate that low to mid 70s f temperatures 
will result in surface-based parcels, and shear profiles are 
favorable for supercells including a tornado risk with effective srh 
of 300-400 m2/s2 in place. While diurnal heating will be limited due 
to clouds, continued warm advection and deeper ascent near the 
approaching cold front may support an increase in storm intensity 
primarily after 15z, with a few supercells possible. 


.Jewell/Edwards.. 02/20/2019 


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


Attention...WFO...mob...Jan...lix... 


Latitude...Lon 32159060 32899012 33238981 33268905 32848853 32248843 
31768844 31378843 30898845 30428851 30138877 29898929 
29648973 29669019 29739056 29899101 30229138 30509156 
30789157 31819103 32159060