Day 1 Severe Weather Outlook

Valid 121300Z – 131200Z

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
THE MID/UPPER MISSISSIPPI VALLEY…SOUTHWESTWARD TO THE
SOUTH-CENTRAL PLAINS…

…SUMMARY…
Scattered strong-severe thunderstorms are expected across portions
of the mid/upper Mississippi Valley, southwestward to the
south-central Plains.

…Synopsis…
The most pertinent mid/upper-level feature for this outlook will be
a well-defined synoptic trough — initially evident in moisture-
channel imagery from a newly redeveloped 500-mb low over
southeastern MT south-southwestward over the Four Corners area. The
cyclone is forecast to deepen and pivot eastward across the Dakotas,
reaching northern MN by the end of the period, when the trough
should extend south-southwestward to central/southern KS.

The associated surface low was analyzed at 11Z near SUX, with cold
front southwestward across eastern NE, west-central KS and
northeastern NM. A warm front — reinforced by antecedent precip to
its north — was analyzed from the low through an area of outflow
over northern IA, to northwestern IL, becoming quasistationary
eastward over Lake Erie, north-central PA, northern NJ, and south of
Long Island. By 00Z, the synoptic low should be occluding over
northwestern MN, with a triple point farther southeast over
southeastern MN. The warm front should extend from there over
northwestern IL, becoming quasistationary from there across
north-central IN, then a slow-moving cold front over parts of PA and
NJ. The western cold front should reach central IA, eastern KS,
western OK, and east-central NM by 00Z. By 12Z, this cold front
should extend from a triple point near or over southern Lake
Michigan across southwestern IL, southwestern MO, southwestern OK,
and the TX South Plains region.

…Southern MN, IA and vicinity…
An ongoing swath of precip and embedded thunderstorms, over the
northern IA/southern MN area, will continue east-northeastward at
through the remainder of the morning with continued gradual
weakening expected. This constitutes the remnants of the MCS from
the Siouxland area discussed in the previous outlook, and appears to
have a gradual rather than sudden increase in stabilizing effects on
the boundary layer with westward extent toward the surface low/
front. As such, some trimming has been performed on the 5% and 15%
severe areas, but additional stabilization may be minimal from
roughly the current position of this convective plume eastward.

Scattered thunderstorms are expected to develop in a generally
north/south corridor over the western parts of the outlook area from
midday through mid afternoon, shifting eastward and increasing in
coverage with time. The relatively undisturbed warm sector and
prospective inflow layer will contain very favorable theta-e as
moisture transport, evapotranspiration and gradual/muted diabatic
heating (under broken cloud cover) all occur, with dew points
commonly upper 60s to low 70s F.

The most favorable low-level and deep shear for supercells will
exist in and just south of the warm-frontal zone, east and southeast
of the surface low, where isallobaric forcing will keep near-surface
winds relatively backed. This will lead to a relative maximum in
storm-relative surface winds and low-level hodograph size, with the
warm front also representing a relative corridor of boundary-layer
vorticity and low LCL along the north rim of most favorable moisture
and buoyancy. Given these factors, any relatively sustained/
discrete convection that can persist in this environment will have
the greatest potential to become supercellular, with tornadoes and
large hail as well as localized severe gusts. Otherwise, the threat
will transition more toward wind with southward extent through IA.
The band of convection should proceed eastward across the
Mississippi River this evening, maintaining some severe threat,
particularly near the warm front where storm-relative winds/
helicity, lift and vorticity will remain relatively maximized.

…MO to southern High Plains…
Widely scattered to scattered thunderstorms should develop near the
front this afternoon and persist into evening, offering the threat
of damaging to severe gusts. A narrow prefrontal corridor of peak
preconvective MLCAPE 1500-2500 J/kg should develop mid/late
afternoon, related to diurnal surface heating/destabilization,
modest but adequate mid/upper-level lapse rates, a deep troposphere,
and rich low-level moisture. This, and minimal MLCINH, will
encourage convection in the frontal convergence zone that should
peak late afternoon into early evening.

Mid/upper-level winds and related deep shear should remain strongest
behind the front, with deep-layer mean flow having a large component
parallel to the boundary. As a result, only short-lived discrete/
semi-discrete convective character is expected with multicellular
mode, ultimately developing a band of convection. This band should
be more dense from about central/west-central OK northeastward
across MO. It also generally should be shorter-lived with
southwestward extent, leading to a tapering outlook geometry. These
factors account for the greater unconditional severe potential from
about central OK northeastward, relative to the more brief/isolated
nature of the threat across the southern High Plains region. With
substantial upper support and the strongest LLJ each remaining
roughly north of I-70, expected nocturnal cooling of the warm-sector
boundary layer, and the front’s being shallower in slope than across
the IA/MN area, the convective/severe threat should wane late this
evening through the overnight hours.

…Mid-Atlantic/Tidewater…
Isolated to widely scattered thunderstorms should form this
afternoon along the front, and a surface trough extending
southwestward across the region near the western rim of the outlook
area. Damaging gusts are expected from the most intense cells, with
isolated severe (50 kt) downdrafts possible. Deep-layer flow/shear
will be weak, limiting overall organization of the severe threat,
However, strong surface heating amidst favorable low-level moisture
(surface dew points commonly upper 60s to low 70s F) will offset
modest midlevel lapse rates enough to support MLCAPE in the 1000-
1500 J/kg range over norther parts of the area, where greater
low-level lift is expected near the boundaries, to 2000-3000 J/kg
across portions of southeastern VA and the Carolinas, amidst weaker
low-level forcing. Water-loaded downdrafts and shallow but well-
mixed boundary layers should contribute to the potential for
strong/isolated severe convective gusts.

..Edwards/Dial.. 09/12/2019

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