Day 1 Severe Weather Outlook

Valid 121630Z – 131200Z


A few tornadoes, a couple possibly strong, and significant damaging
winds will be likely across southeast Iowa through central Illinois,
mainly this evening.

An upgrade to Moderate Risk for tornado and/or wind may become
necessary in later outlooks today.

A leading MCV is tracking east into eastern IA with a lingering
surface cyclone centered over west-central IA. This latter feature
should move east into northern IL through this evening. Diffuse
convective outflow from remnant leading MCS over southern IL has
been shunted across southeast to northwest MO. Nevertheless, air
mass recovery is underway north of the boundary into extreme
southern IA and west-central IL. Further erosion of the prior cold
pool amid cloud breaks should yield at least weak to moderate
surface-based buoyancy by late afternoon in IA to northern IL. More
robust destabilization is expected farther south where greater
unimpeded insolation occurs while a plume of 77-81 F surface dew
points is advected north from the Mid-South. Despite relatively warm
mid-level temperatures, the very rich boundary-layer moisture and
surface heating should yield a plume of large buoyancy with MLCAPE
likely reaching 3000-4000 J/kg across much of MO into
central/southern IL.

Renewed thunderstorm development is probable towards late afternoon
near the surface cyclone in southeast IA and during the early
evening to the southeast in central IL within a corridor of
strengthening low-level warm advection. The initial storms will
likely be supercells capable of producing all hazards, given the
enhanced low-level shear/hodograph curvature near the cyclone/warm
front. Hodographs aloft should tend to favor relatively quick
upscale growth, but even semi-discrete supercells will pose a risk
for at least a few tornadoes, a couple of which be may strong. A
forward-propagating MCS should eventually evolve along the MLCAPE
gradient from central IL into central IN where severe wind gusts
will remain a threat before weakening overnight.

…TX Panhandle to southern KS/northern OK…
A remnant midlevel trough will drift slowly east-northeastward over
the TX Panhandle, as a weak cold front moves slowly
southeastward from KS/CO. Surface heating and deep mixing will
result in inverted-v profiles along and south of the front, and will
support the potential for isolated strong downburst winds.
Convection will tend to develop east-northeastward tonight along the
front, with strong downdrafts still possible in an environment of
lingering large CAPE/DCAPE.

A midlevel trough over James Bay will pivot east-southeastward to
Maine overnight. In advance of this trough, a weak pre-frontal
trough at the surface and band of weak ascent will provide a focus
for thunderstorm development this afternoon. Though moisture and
lapse rates will be modest, increasing deep-layer westerly shear and
surface heating on the south edge of the thicker clouds could
support storms capable of producing isolated damaging gusts and
marginally severe hail.

Widespread clouds and rain are spreading slowly eastward across the
eastern Dakotas in association with a couple of diffuse MCVs from
overnight convection. A little clearing of clouds and surface
heating will be possible from west to east this afternoon, in
advance of a larger-scale trough moving east-southeastward from MT
to ND. Isolated thunderstorm development and a low-end severe
threat will be possible this afternoon if sufficient near-surface
destabilization occurs, given enough vertical shear for organized

..Grams/Dean.. 08/12/2019


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