Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS48 KWNS 230900
SPC AC 230858
Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CDT Thu Mar 23 2017
Valid 261200Z - 311200Z
An active severe weather pattern should continue across parts of the
CONUS though the forecast period, although predictability decreases
substantially Day 6/Tuesday and beyond.
...Day 4/Sunday - Day 5/Monday...
An upper trough/low is forecast to move eastward across the
southern/central Plains Day 4/Sunday. Latest model guidance remains
in reasonable agreement with the placement of this trough, and its
corresponding surface low position over western OK and the eastern
TX Panhandle by Sunday evening. The degree and quality of low-level
moisture return across the southern Plains remains the primary
uncertainty for Day 4/Sunday, as convective initiation along a
dryline extending southward from the surface low appears probable.
The deterministic GFS is more aggressive compared to the ECMWF and
many GEFS members regarding greater surface dewpoints across
central/eastern OK into north TX. If future model trends show better
agreement on more substantial low-level moisture return ahead of the
dryline, a greater severe weather risk could develop. For now, have
expanded the 15% risk area eastward to encompass more of eastern OK.
The upper trough will continue eastward across the lower MS Valley
and Southeast on Day 5/Monday. Some disagreement regarding the
strength/amplitude of the trough is evident amongst deterministic
guidance, but they remain in general agreement on the
placement/timing. Enough low-level moisture return should occur
across the lower MS Valley to support surface-based convection.
Although some uncertainty with convective evolution from Day
4/Sunday into Day 5/Monday remains, the overall forecast combination
of instability and shear appears to support a severe weather threat
across parts of the Mid-South and lower MS Valley where models
forecast convection to develop by Monday afternoon. A 15% area has
been included across this region for Day 5/Monday.
...Day 6/Tuesday - Day 8/Thursday...
Model spread increases from Day 6/Tuesday through the remainder of
the extended forecast period. Another western CONUS upper trough/low
should develop eastward across the Southwest/southern High Plains
vicinity on Day 6/Tuesday. However, medium-range model guidance
becomes increasingly dispersive regarding this feature and its
eastward/northeastward ejection across the southern and/or central
Plains. The GFS solution would imply any meaningful severe threat
would remain confined to parts of TX and the lower MS Valley in this
time frame, while the ECMWF suggests the southern/central Plains and
lower to mid MS Valley may have some severe risk. For now, this
disagreement is too substantial to justify the inclusion of 15% risk
areas, although they may be needed in later outlooks pending better