SPC 2000Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0158 PM CST Sun Dec 13 2020

Valid 132000Z - 141200Z


A few damaging wind gusts and a couple tornadoes will remain
possible across parts of southeast Texas and southwest Louisiana
through the early evening.

...20Z Update...
No changes have been made to the Slight Risk across parts of coastal
southeast TX and southwest LA. A band of pre-frontal showers and
thunderstorms should move northeastward across this area through the
remainder of the afternoon. Any isolated damaging wind and tornado
threat should remain focused along/near the coast, as widespread
precipitation aided by low-level warm advection has hampered the
northward advance of a warm front inland across southeast TX and
southern LA. For more information on the near-term severe threat
across this region, see Mesoscale Discussion 1845. It remains
unclear whether sufficient destabilization will occur farther north
into central LA and southwestern MS to support surface-based storms.
Any strong/gusty threat across this area will likely remain marginal
and quite isolated.

Farther east, mid to upper 60s surface dewpoints should advance
northward across southern AL and the western FL Panhandle late
tonight into early Monday morning ahead of a surface low and related
cold front moving generally eastward across the Southeast. Even
though low-level moisture will be increasing across these areas, it
appears that instability will be tempered by a low-level inversion
and poor low/mid-level lapse rates. With stronger forcing for ascent
attendant to a shortwave trough likely remaining to the north of
this region, robust convective development late tonight may not
occur. Have opted not to include low severe probabilities across
southern AL and the FL Panhandle given these potentially limiting

..Gleason.. 12/13/2020

.PREV DISCUSSION... /ISSUED 1023 AM CST Sun Dec 13 2020/

...Southeast Texas into southwest Louisiana...

Late this morning a warm front extends from a weak surface low in
southeast TX, eastward to near Palacios TX and into the northwestern
Gulf. The front will move north through the day, likely reaching the
Houston metro early this afternoon and continuing into the southwest
LA coastal area by late afternoon. Rich low-level moisture with
upper 60s to 70 F dewpoints reside along and south of the warm
front, and will contribute to MLCAPE from 800-1200 J/kg. Numerous
showers and scattered thunderstorms are developing across southeast
TX within corridor of isentropic ascent north of this boundary. This
activity will remain elevated with minimal severe risk. A somewhat
greater threat for severe storms should evolve this afternoon. Some
CAMs suggest surface-based storms will develop along confluence
boundaries just offshore. A couple of these storms might move inland
and interact with the warm front where low-level hodographs with
200-400 m2/s2 storm relative helicity and strong effective bulk
shear will support potential for supercells capable of producing a
tornado or two. Otherwise, additional surface based storms will be
possible as the cold front advances southeast and interacts with the
moist warm sector. Storms might eventually evolve into linear
structures along the cold front this evening as they spread east
into LA with a threat for mainly a few strong to damaging wind
gusts. The marginal thermodynamic environment with weak lapse rates
and widespread low clouds, along with tendency for deeper forcing
for ascent to remain north of warm sector, could potentially serve
as limiting factors for a more robust severe threat.

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