Day 1 Severe Weather Outlook

Valid 191630Z – 201200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE TONIGHT
ACROSS PARTS OF IA…

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
FROM NJ INTO SOUTHEAST 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.

…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.

..Thompson/Coniglio.. 08/19/2019

$$

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