Valid 011200Z – 021200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE OH VALLEY…NORTHERN/CENTRAL PLAINS…AND SOUTHERN/CENTRAL AZ…
…SUMMARY…
Isolated severe storms may occur mainly this afternoon across parts
of the Ohio Valley, this evening/tonight across portions of the
northern/central Plains, and this afternoon/evening over parts of
southern/central Arizona.
…Ohio Valley…
Within broadly cyclonic flow over eastern Canada and the Great
Lakes, a weak shortwave trough will move across parts of the OH
Valley today. Modest ascent associated with this feature should
encourage at least scattered storm development by the afternoon from
eastern IN into northern KY and OH. This convection should then
spread eastward across parts of WV into western PA through the early
evening. Even though mid-level lapse rates will remain modest,
diurnal heating will contribute to around 1000-2000 J/kg of MLCAPE
across most of this region by this afternoon. A veering wind profile
in low levels combined with around 20-30 kt of mid-level
west-southwesterly flow may support occasional storm organization.
Small multicells/clusters capable of producing isolated strong to
damaging winds appear to be the main threat. This risk should
quickly diminish with eastward extent into PA by late evening as
instability wanes.
…Northern/Central Plains…
Upper ridging will remain prominent across much of the
northern/central Plains through the period. An upper trough/low over
BC will move eastward into AB and gradually erode the upper ridge
over the northern Rockies and vicinity late tonight. At the surface,
a low will develop eastward across central/eastern MT from this
evening through early Monday morning. An EML with steep mid-level
lapse rates will gradually overspread much of the northern/central
Plains though tonight. Greater low-level moisture will likely remain
confined along/south of a surface boundary that will be located near
the SD/NE border through this evening.
Considerable uncertainty remains regarding possible convective
initiation during the day, as subsidence associated with the upper
ridge will probably tend to limit overall storm coverage across the
northern/central Plains. A southerly low-level jet should strengthen
to around 30-40 kt across western KS into central NE in the 00-06Z
time frame. Warm air advection and related lift may support at least
isolated storm development this evening across parts of
western/central NE and vicinity. Although any storms that develop
will probably be slightly elevated, steep mid-level lapse rates and
rich low-level moisture will result in strong MUCAPE across this
area, potentially in excess of 3000 J/kg. Around 35-45 kt of
effective bulk shear will likely promote rotating updrafts with
these storms, and isolated large hail may occur with any elevated
supercells that form initially.
Farther north, a strengthening south-southwesterly low-level jet
late in the period (06-12Z) may encourage isolated convection to
develop over northeastern MT/southern SK and subsequently spread
southeastward along an instability gradient into parts of
western/central ND early Monday morning. Overall coverage still
remains uncertain, but there appears to be enough of a convective
precipitation signal in short-term guidance to support inclusion of
5% probabilities for isolated instances of large hail occurring with
elevated strong to severe storms given the strong shear that will be
present.
…Southern/Central Arizona…
An upper high will remain centered over the Four Corners region
today. Orographic lift should be the main driver of isolated to
widely scattered convection that should develop by early afternoon
across the higher terrain of western NM and southern/central AZ.
However, unlike the past couple of days, around 20 kt of easterly
flow at 500 mb may prove sufficient to move these storms off the
higher terrain and into lower elevations of southern/central AZ
through the evening. A deeply mixed boundary layer is expected to be
in place across this area, which will encourage strong/gusty
downdraft winds potentially producing isolated damage. Marginally
severe hail may also occur with more discrete initial development.
These storms should move westward through the evening posing mainly
a wind threat before eventually weakening as convective inhibition
slowly increases.
…Florida…
Strong low-level shear and outer rain bands associated with
Hurricane Dorian are forecast to remain generally offshore and to
the east of the southern/central FL Peninsula through the period.
See the latest advisories from the National Hurricane Center for
more information on other hazards associated with Hurricane Dorian.
..Gleason/Squitieri.. 09/01/2019
$$