SPC AC 230732
Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0232 AM CDT Sun Aug 23 2020
Valid 251200Z - 261200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
The greatest risk for severe storms Tuesday will likely reside over
the Northeast. More isolated risk will extend from the northern
Intermountain region to the Upper Great Lakes, and also across
Arizona. Risk for a brief/weak tornado or two may also develop as
Laura approaches southeastern Louisiana.
A strong cold front is forecast to shift across the U.S./Canada
border and into New England early in the period, and then will
advance quickly toward the coast, while sagging more slowly
southward across the central Appalachians/Mid Atlantic region.
Questions exist across New England, with respect to just how much
destabilization will be able to occur ahead of the fast-moving
boundary. Still, with very strong flow aloft promoting rapid storm
motions, risk for damaging winds will exist with any sustained
convection. If more destabilization than currently expected occurs
ahead of the front -- particularly if frontal progression is slower
in future model runs -- greater severe-weather probability would be
likely across this area.
Farther south, greater destabilization is expected ahead of the
sagging front, but flow aloft will progressively weaken with
southward extent. Still, ample CAPE/shear combination is expected
southward across the central Appalachians and into the Mid-Atlantic
region to support risk for damaging winds and hail with stronger
storms/storm clusters through afternoon and evening.
...Upper Great Lakes/Midwest region...
Storms may be ongoing at the start of the period across portions of
the Wisconsin/lower Michigan vicinity, with very limited/lingering
severe potential possible with one or two of the stronger convective
Meanwhile, with daytime heating supporting some destabilization
along the trailing portion of the cold front crossing the Northeast,
a few afternoon storms may redevelop near the boundary. Given the
anticyclonic belt of fast flow overhead, a stronger storm or two --
capable of producing locally gusty/damaging winds or marginal hail
-- will be possible.
After dark, additional isolated/mainly elevated storms may evolve as
a southwesterly low-level jet supports warm-advection-induced ascent
into the upper Mississippi Valley/Upper Great Lakes region
...Northern Intermountain region into the northern Plains...
Multiple, pronounced cyclonic mid-level disturbances are forecast to
move in an easterly anticyclonic arc across the northern
Intermountain region and northern Plains, embedded within a belt of
fast flow around the northern periphery of the upper ridge.
Heating/weak destabilization over the northern Intermountain region
should promote isolated storm initiation over the higher terrain.
Given fast mid-level flow promoting quickly moving storms, and some
evaporative enhancement to downdrafts, locally gusty/damaging winds
will be possible.
Farther east into the northern Plains, a dry/capped boundary layer
should act to temper convective development though the afternoon,
with only isolated storm development expected at best. Still, with
fast flow aloft supporting potential for updraft organization,
gusty/damaging winds would be possible with any storm which becomes
Overnight, development of the southwesterly low-level jet may foster
an increase in convection coverage, though storms would be primarily
elevated above the capping layer. Still, with some risk for hail
and possibly gusty winds, this region will be included in a broad
MRGL risk area across the northern Intermountain region across the
NAM forecasts suggest that small mid-level cyclonic disturbances
will move southwestward across Arizona Tuesday and Tuesday night,
within a weak belt of enhanced northeasterly mid-level flow.
With modest destabilization forecast to occur through the afternoon,
storms should develop over higher terrain/the Mogollon Rim, and then
shift southeastward into the lower deserts. With a very deep mixed
layer promoting evaporative downdraft acceleration, risk for
damaging winds appears sufficient to warrant MRGL/5% risk inclusion
at this time.
...Central Gulf Coast region...
Based on the current track forecasts from the National Hurricane
Center, Laura is forecast to be moving northwestward across the
eastern and central Gulf of Mexico Tuesday, reaching the
north-central Gulf south of southeastern Louisiana by Wednesday
morning. Presuming this track proves accurate, a gradual increase
in the low-level flow field would be expected with time. While the
breadth of convection surrounding the center of Laura remains
uncertain, cells within outer banding, spreading onshore overnight,
could pose low-end risk for brief tornadoes.
CLICK TO GET WUUS03 PTSDY3 PRODUCT
NOTE: THE NEXT DAY 3 OUTLOOK IS SCHEDULED BY 0730Z