Valid 302000Z – 311200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
NORTHEAST…AND ACROSS THE BLACK HILLS REGION OF WYOMING AND SOUTH
A few severe thunderstorms are possible today over parts of the
Northeast, and from northeast Wyoming into western South Dakota.
More isolated strong storms may affect parts of the Gulf Coast, and
…WY into SD and NE…
Surface observations as well as satellite trends support the notion
of a cluster of severe storms forming this afternoon over northern
WY, and persisting into western SD and northwest NE by evening.
Plentiful deep layer shear, steep lapse rates and easterly surface
winds suggest supercells capable of large hail are likely. As such,
the area has been upgraded to Slight Risk. Localized damaging winds
are also possible.
For this update, extended the Marginal Risk farther south into WV
and VA as stronger storms move into a warm air mass with steep
low-level lapse rates which may support wind gusts. Small hail is
also possible as temperatures aloft are cool. For more information
see mesoscale discussion 1613.
.PREV DISCUSSION… /ISSUED 1120 AM CDT Tue Jul 30 2019/
A broad upper trough is present today over eastern Canada and the
Great Lakes region, with 30 knot southwesterly midlevel winds
across parts of the Northeast. Visible satellite imagery shows
mostly clear skies and strong heating occurring across much of PA/NY
and New England, which combined with dewpoints in the mid/upper 60s
will yield afternoon MLCAPE values of 1000 J/kg. A weak MCV is
noted over western PA, which should help to organize convection over
northern PA and central NY this afternoon. More isolated convection
is expected farther east over eastern NY into VT. Forecast
soundings throughout this area show only modest midlevel lapse
rates, but sufficient low-level lapse rates and steering flow to
pose some risk of damaging wind gusts. Have therefore upgraded the
corridor of greatest confidence to SLGT risk.
…Northern High Plains…
A large upper ridge remains in place over the southern Rockies, with
westerly midlevel flow of 30-40 knots across much of MT/WY into the
Dakotas. A weak vort max is noted within the flow over western WY
may help to organize storms as it emerges into the High Plains later
today. Isolated supercell storms are expected along a corridor from
eastern MT into eastern WY – spreading into western ND/SD/NE this
evening. Damaging wind gusts and hail will be possible in the
strongest cells. Given the large scale ridging over the area,
confidence in a more focused severe event remains low so will not
upgrade to SLGT risk.
A cluster of thunderstorms over LA is forecast by a consensus of 12z
CAMs to build westward into the hot/humid air mass over western LA
and southeast TX. Deep layer shear is weak, but strong CAPE and
steep low-level lapse rates will pose a risk of locally
gusty/damaging winds in the stronger cells today.
Skies are clearing across central AZ, likely leading to a day of
strong heating and moderate CAPE values across southern and central
AZ. Easterly/southeasterly flow aloft is also increasing, which
will help storms that form over the higher terrain of eastern AZ to
drift westward this afternoon and evening. This scenario should
result in a few storms capable of damaging wind gusts across the
region. Visible satellite imagery suggests a weak MCV over far
southeast AZ that may also focus development. Mesoscale convective
trends will be monitored in this area for need to upgrade to SLGT in