SPC AC 241728
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1228 PM CDT Mon Aug 24 2020
Valid 251200Z - 261200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
THE MID-ATLANTIC REGION...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE UPPER
OHIO VALLEY INTO MUCH OF THE NORTHEAST...AND PORTIONS OF CENTRAL
IDAHO INTO SOUTHWEST MONTANA...
...SUMMARY...
Thunderstorms capable of isolated severe gusts (55-65 mph) resulting
in scattered wind damage are probable for portions of the
Mid-Atlantic states.
...The Northeast...
A strong cold front is forecast to shift across the U.S./Canada
border and move quickly eastward across New England, while
progressing more slowly southeastward across the Mid Atlantic region
and southward across the central Appalachians. Isolated general
thunderstorms may be ongoing Tuesday morning and lending some
uncertainty regarding destabilization across lower elevations near
the coast ahead of this activity. However, west-northwesterly flow
in the mid levels is forecast to strengthen during the day across
the region and support relatively fast multicell storms.
It appears the greatest risk for severe gusts and associated
tree/wind damage will occur farther south across parts of eastern
PA/NJ into the Chesapeake Bay vicinity. Strong heating and mean
mixing ratios in the 15-16 g/kg range, will result in a moderately
to very unstable boundary layer by mid afternoon (2000-3000 J/kg
MLCAPE). Effective shear 20-30 kt will favor a multicell convective
mode supportive of clusters and line segments potentially capable of
isolated 55-65 mph gusts and accompanying scattered wind damage.
Farther west, a strong storm cannot be ruled out during the morning
over Lower MI vicinity. However, additional storms either
rejuvenating on earlier-day outflow or associated with a weak
disturbance, will foster a risk for strong to severe thunderstorms
across the upper OH Valley during the afternoon/early evening.
...Northern Intermountain region into the northern Plains...
A series of cyclonic mid-level disturbances will continue to move in
an easterly anticyclonic arc across the northern Intermountain
region and northern Plains, embedded within a belt of 40-50 kt 500mb
flow around the northern periphery of the upper ridge.
Afternoon heating will support the development of steep low-level
lapse rates and isolated to widely scattered storm initiation over
the higher terrain. Given strong mid-level flow promoting
fast-moving storms, and inverted-v profile, evaporative cooling
coupled with stronger momentum in the mid levels will favor
severe-gust potential with the stronger cores.
Farther east into the northern Plains, a dry/capped boundary layer
is expected, that should act to substantially limit convective
development though the afternoon. Still, with fast flow aloft
supporting potential for updraft organization, gusty/damaging winds
would be possible with any sustained storm which could develop.
Overnight, evolution of a southwesterly low-level jet may foster an
increase in convection coverage from the Dakotas into the Great
Lakes region, though storms would likely be primarily elevated above
the capping layer. Some risk for hail would likely exist given
steep mid-level lapse rates supporting ample CAPE above the cap, and
a locally strong wind gust cannot be ruled out.
...Upper Midwest...
The potential for some increase in coverage is expected after dark
as warm advection increases in conjunction with a southwesterly
low-level jet. Given the anticyclonic belt of fast flow overhead, a
couple of stronger storms will be possible, with attendant potential
for locally gusty winds or marginal hail.
...Arizona...
Model forecasts continue to depict small mid-level cyclonic
disturbances moving southwestward across Arizona Tuesday and Tuesday
night, within a belt of enhanced northeasterly mid-level flow.
As the airmass heats/destabilizes 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, potential for locally
damaging wind gusts is evident with a couple of the stronger
storms/storm clusters.
...Central/Western Gulf Coast region...
The latest National Hurricane Center forecasts continue to show
Marco weakening with time as it moves westward along the Louisiana
coast, followed by Laura expected to be moving northwestward across
the eastern and central Gulf of Mexico as a
strengthening/substantial hurricane. With sufficient low-level
shear expected across parts of Louisiana due to the presence of
Marco, risk for a brief/weak tornado or two will persist through the
day. Meanwhile, with Laura expected to reach the north-central Gulf
south of southeastern Louisiana late Tuesday night, additional
strengthening of the low-level wind field is expected. While
questions persist regarding the forecast track of Laura, a continued
risk for a couple of weak/brief tornadoes is expected across parts
of the central/western Gulf Coast region through the evening and
overnight hours.
..Smith.. 08/24/2020
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