Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0400 AM CDT Wed Apr 21 2021
Valid 241200Z - 291200Z
...DISCUSSION...
There is good agreement in medium-range guidance that an upper
trough will continue eastward across the lower MS Valley and
Southeast on Day 4/Saturday. A warm front should lift northward
through the day across parts of central AL/GA and into SC as a
surface low develops eastward across the lower OH Valley to the
central Appalachians. There is some concern that a strong low-level
jet will shift quickly eastwards across these areas, possibly
outrunning the better low-level moisture return. Boundary-layer
winds veered to nearly westerly should occur as the low-level jet
core develops eastward. Still, the increasing low-level moisture,
strong deep-layer shear, and some eastward advection of steepened
mid-level lapse rates should overlap along/south of the warm front.
Any thunderstorms that can form in this regime and remain surface
based will pose a threat for all severe hazards. Additional severe
convection may develop Saturday afternoon along an eastward-moving
cold front. Convection will likely be ongoing Saturday morning given
the strong ascent/isentropic lift associated with the low-level jet,
and these morning thunderstorms may slow the northward progress of
the warm front. Regardless, given latest trends in guidance
suggesting a farther north position of the warm front by Saturday
afternoon, have expanded the 15% severe area to include more of
central AL/GA and southern SC.
The upper trough should continue eastward into the western Atlantic
on Day 5/Sunday, with rich low-level moisture confined to parts of
the FL Peninsula. Given minimal large-scale ascent forecast over
this region, overall severe potential ahead of a cold front appears
too low to include any severe probabilities. Medium-range guidance
remains in fairly good agreement that another large-scale upper
trough will advance eastward from the western CONUS to the Plains in
the Day 6/Monday to Day 7/Tuesday time frame. The past several runs
of the deterministic ECMWF show a very favorable setup for organized
severe thunderstorms capable of producing all severe hazards across
parts of central TX into OK and southern KS, with moderate to strong
instability forecast to overlap rich low-level moisture in the
presence of strong deep-layer shear. There has been a consistent
enough signal in guidance suggesting a substantial severe threat may
develop across these areas next Tuesday to introduce 15% severe
probabilities. Greater probabilities may be needed if current model
trends continue. Finally, some severe threat may continue on Day
8/Wednesday across parts of the lower/mid MS Valley as the upper
trough moves eastward. But, there is too much uncertainty regarding
the evolution of convection on Tuesday and the quality of low-level
moisture return with eastward extent to include a 15% severe area
for next Wednesday.
..Gleason.. 04/21/2021