SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0651 AM CST Wed Feb 17 2021

Valid 171300Z - 181200Z


Damaging thunderstorm gusts, a few tornadoes, and perhaps some hail
are possible this evening and overnight near the central Gulf Coast.

In mid/upper levels, a longwave trough will be maintained over the
CONUS as a series of subsynoptic perturbations pass across of near
its axis over the Plains States.  In the southern part, vorticity
maxima now over OK and northwest TX will eject northeastward across
the mid Mississippi and Ohio Valley regions and weaken.  Meanwhile,
somewhat separate perturbations over NM will move more slowly
eastward through the period, reaching from the Ozarks to north-
central TX by 12Z tomorrow.  A shortwave trough -- apparent in
moisture-channel imagery over the inland Pacific Northwest -- will
dig southeastward to the large-scale trough position over extreme
northern MX, south of west TX, by 12Z.  

At the surface, a decelerating cold front was analyzed across parts
of south FL, arching westward as a quasistationary front over the
east-central/central Gulf, then a warm front west-northwestward to a
frontal-wave low.  The low was drawn offshore from the lower TX
Coast between BRO and rig station KHHV.  The low is expected to move
northeastward today to just south of the south-central LA coastline,
then across the Mississippi River Mouth area between 00-03Z tonight.
By 12Z, the low should reach the west-central FL Panhandle or
adjoining extreme southeastern AL.  The warm front will arc a short
distance eastward, then southeastward from the low across the
northeastern Gulf, potentially reaching the east-central FL
Panhandle/AAF area around the end of the period.  A cold front will
trail southwestward from the low, moving eastward across the
northwestern/north-central Gulf.

...Central Gulf Coast...
Thunderstorms should organize by this afternoon over the Gulf near
the surface low, trailing cold front, and along an elevated low-
level convergence zone extending northward from the low.  Activity
near the low and cold front will post a threat for damaging gusts, a
few tornadoes, and isolated marginally severe hail, as it crosses
parts of extreme southeastern LA and coastal areas from MS eastward

Little direct upper support is expected in terms of substantial
shots of DCVA or jet-level large-scale forcing, south of the main
250-300-mb southwesterly jet axis, and with the shortwave train
remaining well west and north of the area.  As such, midlevel lapse
rates will remain modest.  However, subtle height falls and
tightening of height gradients aloft will contribute to favorable
deep/speed shear, while persistent low-level warm advection and
return flow destabilizes the air mass north of the warm front (in
support of elevated convection).  

From the low eastward then southeastward along the warm front,
low-level winds will be most backed for surface-based effective-
inflow parcels, relatively maximizing low-level shear/SRH/vorticity.
Effective SRH in the 150-250 J/kg range is possible along and just
north of the warm front, where forecast soundings also depict 50-60
kt effective-shear magnitudes.  Any supercells or well-organized
main-line segments interacting with the warm front will pose the
greatest tornado potential.  The main concern remains instability,
where 60s F surface dew points south of the front may contribute to
500-1000 J/kg MLCAPE in the warm sector.  However, extent of
warm-sector/warm-frontal penetration inland tonight is uncertain,
with enough variability around a consensus near the present northern
bound of the categorical "slight" probabilities to maintain the
general shape of the outlook lines.  Two minor shifts are:
1.  Eastward on the western end in southeastern LA, where airmass
recovery is more in doubt, especially with such intense static
stability and offshore flow in place and potentially more
southeastward low track, and 
2.  The eastern portions shifted slightly east, to allow more room
for destabilization and associated severe potential in the last few
hours of the period (09-12Z).

..Edwards/Smith.. 02/17/2021

Read more
There’s more click here.

Comments are closed.