SPC 1200Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1200 AM CST Thu Dec 31 2020

Valid 311200Z - 011200Z


Severe thunderstorms -- including potential for tornadoes -- are
possible today through tonight from southeast Texas into the lower
Mississippi Valley and central Gulf Coast States.

...Southeast Texas through the lower Mississippi Valley...

Cold front will stall from the TX coastal area through northern LA
and central MS. This boundary might advance slightly northward as a
warm front or remain stationary through a portion of southeast TX
and the lower MS Valley today into the evening. Cyclogenesis  will
commence along the front near the central TX coast later this
morning. This cyclone will subsequently develop northward through
east TX within corridor of deep forcing for ascent associated with a
northward-ejecting shortwave trough.

A moist warm sector will reside south of the front with mid to upper
60s F dewpoints from southeast TX into southern LA and MS. However,
widespread clouds and weak lapse rates will support only a modest
thermodynamic environment with MLCAPE from 500-1000 J/kg. Deeper
forcing for ascent will begin to overtake the western portion of the
warm sector across southeast TX later this morning. This will result
in more robust thunderstorm development in vicinity and east of the
developing surface low near the central TX coast and southward along
the cold front into the western Gulf. Activity will subsequently
spread into southeast TX and southwest and central LA during the day
before reaching southwest LA by early evening. Vertical wind
profiles will strengthen as a coupled jet structure overspreads the
warm sector in association with the ejecting shortwave trough. Mixed
storm modes are likely. Large low-level hodographs with 300-500
m2/s2 0-1 km storm relative helicity and 50+ kt effective bulk shear
will support organized storms, including bowing line segments and
supercells with low-level mesocyclones capable of producing
tornadoes and damaging wind.

...Central Gulf Coast states...

A subtle warm front will move onto the central Gulf coast later
today. The boundary layer will moisten with dewpoints increasing to
the mid to upper 60s F. Showers and thunderstorms are expected to
gradually increase and develop northward through the evolving warm
advection regime later today into the overnight. Primary severe
threat is expected during the evening and tonight as low-mid level
winds increase downstream from the shortwave trough that will remain
well west of this region. Hodograph size will increase substantially
overnight, along with effective bulk shear supporting potential for
a few supercells. The deeper forcing for ascent will remain west of
this region, and most storms might remain slightly elevated given
the most favorable wind profiles will evolve overnight.
Nevertheless, the environment will support a supercell/tornado
threat, assuming updrafts can root in the boundary layer.

..Dial/Wendt.. 12/31/2020

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