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SPC Feb 27, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook
      
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0658 AM CST Sat Feb 27 2021

Valid 271300Z - 281200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EASTERN
OKLAHOMA TO THE LOWER OHIO VALLEY...

...SUMMARY...
Isolated severe thunderstorms are possible from eastern Oklahoma to
the lower Ohio Valley this evening into tonight.

...Synopsis...
Mid/upper troughing will amplify and assume more net positive tilt
through the period.  A strong shortwave trough -- initially over the
interior Northwest and northern Great Basin -- will dig south-
southeastward and strengthen, resulting in a sharply defined trough
with nearly closed 500-mb low over UT/AZ by 12Z tomorrow.  As that
occurs, a leading perturbation -- apparent in moisture-channel
imagery over western WY and eastern UT -- will phase with a lower-
amplitude shortwave to its south and pivot across the central
Plains.  To the southeast, a long and broad fetch of southwesterly
flow aloft will be maintained from the southern Rockies and northern
MX across the Mississippi Valley, becoming west-southwesterly to
westerly flow over the Atlantic Coast States.  

At the surface, a wavy warm-frontal zone was drawn at 11Z over
eastern/northern AL, northern/western MS, northern LA, and east TX
near LFK, becoming quasistationary southwestward across the lower/
middle TX Coastal Plain.  This boundary should shift northward
diffusely and erratically through the period, through the Mid-South
and Arklatex, as well as the TN Valley region.  A low should develop
over the central Plains this evening, associated with the ejecting
mid/upper perturbation, and cross IA overnight, with cold front
trailing over eastern MO, eastern/southern OK, and the TX South
Plains by 12Z.

...Eastern OK to Ohio Valley...
A persistent, elongated plume of elevated, low-level warm advection
and moisture transport is apparent in precip fields from AR-WV, and
extends back into east TX.  With the ejection of the WY/UT upper
perturbation displaced well northwest of the area, direct upper
support will be minimal.  Still, as low-level theta-e advection
persists and the Gulf warm-frontal zone moves poleward, the western
part of regime likewise should persist and shift northward across
northeast TX and AR throughout the day, offering mostly weak
convection and showers.

Thunderstorm potential will increase from late afternoon into
evening as the regime continues to shift northward over the
Arklatex, eastern OK, the Ozarks and Mid-South.  This will be
encouraged by strengthening theta-e accompanying a broad, increasing
southwesterly LLJ building to the 45-55-kt range (locally stronger).
Frontal lift and isentropic ascent of increasingly moist/unstable
parcels to LFC will foster a growing, intensifying plume of
convection within which the most intense cells will be capable of
severe hail or damaging to marginally severe gusts.  Despite the
modest lapse rates aloft over much of the area, isolated marginally
severe hail will be possible given a wind profile suitable for
elevated supercells, with 45-55-kt effective-shear magnitude beneath
the southwest-flow field aloft, and LLJ-enlarged hodographs with
250-400 J/kg effective SRH.  A near-surface stable layer will
minimize but not entirely eliminate damaging-gust potential as well.

...Lower Missouri Valley vicinity...
Isolated thunderstorms may develop near the surface low and cold
front tonight across parts of the corridor from northeastern KS
across southeastern NE/northwestern MO into IA.  Though large-scale
and frontal forcing will be strong in this area relative to the
other main convective regime described above, the low-level air mass
will be much more moisture-starved.  At this time, the barely
adequate buoyancy for elevated thunder (e.g., MUCAPE under 200 J/kg)
appears too limited for unconditional severe potential, especially
with a relatively stable near-surface layer remaining.  However, a
better-organized, more strongly forced arc of convection than most
progs indicate could penetrate that layer with strong gusts.

..Edwards/Jewell.. 02/27/2021

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SPC Feb 27, 2021 Day 4-8 Severe Weather Outlook

Day 4-8 Outlook
      
Day 4-8 Outlook Image
Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0320 AM CST Sat Feb 27 2021

Valid 021200Z - 071200Z

...DISCUSSION...
A series of upper shortwave troughs will migrate across the southern
tier of the U.S. during the Day 4-8 period. The first in this
series, located over the southern/central Plains on Day 4/Tue will
shift eastward to the Carolinas/Mid-Atlantic vicinity through Day
5/Wed. At the surface, a weak low will develop near coastal LA on
Tuesday, shifting east/northeast across the southern states and
offshore the Carolinas by Wednesday evening. This will bring a cold
front across the southeastern U.S. and widespread showers and
thunderstorms are expected. Much of this activity is expected to
remain elevated, with instability limited and shear relatively
modest. High pressure will settle across the eastern half of the
U.S. Day 6/Thu, with the surface cold front continuing southward
into the northern Gulf and FL Straits vicinity. 

Late in the period, a second upper shortwave trough/closed low is
expected to develop eastward across the southern Plains and Gulf
coast vicinity around Day 7/Fri-Day 8/Sat. Guidance varies quite a
bit in the evolution of this system, with some medium range guidance
digging the trough fairly far south into the Gulf. This could limit
northward moisture return, keeping any severe potential offshore
from the central Gulf coast. However, this could bring an increase
in severe potential to the FL Peninsula over the weekend. Given
large spread, uncertainty is too great to include severe
probabilities at this time.

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