Valid 192000Z – 201200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE TONIGHT
ACROSS PARTS OF SOUTHERN IOWA…
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON AND EARLY EVENING ACROSS PARTS OF NEW JERSEY INTO SOUTHERN
PORTIONS OF NEW ENGLAND…
…SUMMARY…
A few damaging gusts will be possible from the Mid-Atlantic into New
England this afternoon, while large hail and damaging gusts are
expected late tonight across Iowa.
…20Z Outlook Update…
Some adjustments have been made to categorical and probabilistic
lines to account for ongoing trends, and the latest observational
data and model output.
A “marginal” risk for severe weather has been extended southward
near the eastern and southern slopes of the southern Appalachians,
mainly to include low probabilities for severe wind gusts. Strong
heating has contributed to rather steep low-level lapse rates along
this corridor, where convective inhibition is also relatively weak
beneath weak mid/upper troughing. This has allowed for the
development of numerous thunderstorms which may continue to pose a
risk for potentially damaging wind gusts, associated with localized
downbursts and perhaps some upscale growing, convectively generated
cold pools through 22-00Z.
Otherwise, the plume of very warm elevated mixed-layer air
overspreading much of the Plains and mid Missouri Valley region may
substantially inhibit convective development into and perhaps
through the 20/03-06Z time frame. Thereafter, it appears that
thunderstorm development may become focused in two general areas.
One of these may be ahead of another cold frontal surge into the
North Dakota/Minnesota international border area by late tonight. A
more prominent thunderstorm cluster may initiate in response to
lower/mid tropospheric warm advection, near/east of the mid Missouri
Valley, near the northeastern periphery of the more strongly
capping elevated mixed layer air.
..Kerr.. 08/19/2019
.PREV DISCUSSION… /ISSUED 1130 AM CDT Mon Aug 19 2019/
…IA late tonight…
Low-level warm advection and moistening will focus across IA
overnight, on the nose of a 30-35 kt southwesterly low-level jet.
The richer moisture observed this morning at TOP and LBF will spread
northeastward beneath a warm elevated mixed layer which will act to
cap the boundary layer through the day. After 06z, ascent and
moistening with the warm advection should be sufficient for elevated
thunderstorm development across IA, and storms will subsequently
spread southeastward and likely evolve into an MCS. Large hail will
be the initial threat with elevated supercells. As storms coalesce
into an MCS, there will be increasing potential for damaging surface
winds to reach the ground, given large buoyancy (MUCAPE at or above
3000 J/kg), steep midlevel lapse rates, and DCAPE at or above 1000
J/kg.
…New England to the Mid-Atlantic this afternoon/evening…
A midlevel shortwave trough will move eastward over PA/NY and New
England this afternoon/evening, with an accompanying surface cold
front. East of a band of thicker clouds near the front, surface
heating and boundary-layer dewpoints ranging from the upper 60s in
southern NH to the mid 70s in NJ will contribute to afternoon MLCAPE
ranging from 1500 J/kg north to 3000 J/kg south. Midlevel
west-southwesterly flow of 25-30 kt and the moderate to strong
buoyancy will favor primarily multicell clusters capable of
producing occasional wind damage with 45-60 mph outflow gusts. More
persistent clouds across Maine will slow surface
heating/destabilization, though deep-layer vertical shear will be a
little stronger with northward extent compared to southern New
England.
Farther south, a weak lee trough will persist through the afternoon
across the Piedmont, with a warm/moist air mass to its east. Strong
surface heating will boost MLCAPE into the 2000-3000 J/kg range east
of the surface trough, where DCAPE near 1000 J/kg will favor
isolated strong/damaging winds with multicell storms in a weak
vertical shear environment.
…Northern Plains through tonight…
Low-level moisture is returning northward from the central Plains to
the Dakotas, east of a diffuse lee trough. Surface heating/mixing
could be sufficient for isolated high-based storms across western ND
this evening, but overall storm coverage is expected to remain
sparse in response to weak forcing for ascent and a lingering cap.
Storm development will be more probable tonight near the
international border along a cold front as an embedded speed max
moves across southern SK/MB. This convection should be elevated,
posing a threat for isolated large hail and a few strong gusts could
reach the ground.
…Central High Plains this afternoon/evening…
Deep mixing with strong surface heating could support isolated,
high-based thunderstorm development this afternoon along the surface
lee trough in the vicinity of the KS/CO border. Inverted-v profiles
will favor gusty outflow winds with any sustained storms or small
clusters for a few hours late this afternoon/evening.
$$