Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0755 AM CDT Tue Oct 30 2018
Valid 301300Z - 311200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
WEST-CENTRAL TEXAS LATE OVERNIGHT...
...SUMMARY...
Thunderstorms late overnight into the predawn hours Wednesday may
produce severe hail over parts of west Texas.
...Synopsis...
In mid/upper levels, a progressive pattern will prevail over the
forecast area. The most important synoptic feature is a broad area
of cyclonic flow and related troughing now over the western U.S.,
extending southwestward from a cyclone over the MB/SK border to the
lower Colorado River Valley. Several initially lower-amplitude,
embedded shortwave perturbations now across the Pacific Northwest
and Great Basin will phase into a more coherent, strong, basal
shortwave trough by 12Z from the Four Corners region across southern
AZ and northern Sonora. Meanwhile the Canadian cyclone will
progress to far northwestern ON near the MB border, its shortwave
trough extending southward then southwestward over the Upper
Mississippi Valley region. A broad fetch of low-/middle-level
southwesterlies and related low-level warm-advection regime will
spread across much of the mid-Mississippi and Ohio Valleys and
vicinity.
At the surface, 11Z analysis showed a low over southern MN, with
cold front southwestward across west-central IA, northeastern to
south-central KS, central TX Panhandle, and northeastern NM. By
00Z, the low should weaken somewhat as it reaches northern Lake
Michigan, with cold front southwestward over northern IL,
southwestern MO, south-central OK, west-central TX, and southeastern
NM. By 12Z, the front should reach northwestern OH, southern IL,
the AR Ozarks, north-central TX, and the Edwards Plateau.
...West-central TX late overnight...
Widely scattered to scattered thunderstorms are forecast to develop
behind the surface cold front, during the last few hours of the
period, over parts of the Permian Basin region into west-central TX.
As this activity moves eastward to northeastward toward the Big
Country and Concho Valley regions, it will encounter increasingly
high-theta-e low-level trajectories atop the frontal surface.
Related low-level warm advection will contribute to steepening
midlevel lapse rates ahead of the amplifying Four Corners
perturbation, resulting in increasing elevated buoyancy, amidst
strengthening deep shear. By the 09-12Z time frame, forecast
soundings yield MUCAPE in the 1500-2000 J/kg range, collocated with
45-60 kt effective-shear vectors. To the extent storm modes can
stay relatively clean and discrete long enough, large hail will be a
concern. At this time, that modal conditionality precludes more
than a marginal hail outlook, but at least isolated severe hail now
appears probable over the region during the predawn hours.
...Eastern IA/northern IL/southern WI region this afternoon...
Low-level warm advection and modest, early-stage moisture return
will support isolated to scattered convective development near the
front this afternoon into evening, from the IA/WI/IL area to eastern
OK, potentially backbuilding into parts of north TX tonight. The
largely front-parallel nature of flow aloft, anafrontal character of
the progressive boundary with respect to convective development, and
lack of more robust moisture should limit the potential for
organized severe overall in this regime day-1.
Strong thunderstorms with isolated, mainly subsevere hail may occur
this afternoon from eastern IA across the WI/IL border region, as
the low-level frontal zone and some weak large-scale ascent from a
northern-stream perturbation encounter the northwestern rim of the
elevated warm-advection/moisture-transport belt. Forecast soundings
suggest the potential for up to about 1200 J/kg of elevated MUCAPE
to develop in the most aggressive moistening scenarios above the
stable boundary layer. Convection-allowing guidance reasonably
indicates a rather messy, streaky, clustered mode in this regime,
which in tandem with limited inflow-layer moisture content, would
tend to temper duration and size of hail. At this time the risk
appears too uncertain for an unconditional marginal-severe area.
..Edwards/Peters.. 10/30/2018
https://www.spc.noaa.gov/products/outlook/day1otlk.html
posted by: Ciela Marie Acala
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