Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
SPC AC 280558
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1258 AM CDT Thu Jun 28 2018
Valid 281200Z - 291200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE NORTHERN PLAINS...
...SUMMARY...
Severe thunderstorms are expected across parts of the northern
Plains Thursday afternoon into Thursday night. The primary threat
will be damaging winds, but large hail and perhaps a couple
tornadoes will be possible as well. Elsewhere, isolated
strong/severe storms will be possible from the mid Mississippi
Valley to the Southeast, as well as over parts of the Northeast.
...Synopsis...
Pertinent features for this convective outlook will generally be
situated around the periphery of an expansive mid-level ridge
anchored over the central Plains. Although 500mb height tendencies
will be neutral/slightly positive across the northern Plains
initially (owing to lower-level warm advection), heights will
gradually fall through Thursday night with the approach of an
upstream trough over the Pacific Northwest. Farther east, a compact
impulse will cross the Hudson Valley and southern/central New
England through the afternoon. Meanwhile, a series of
low-amplitude/convectively augmented impulses will drop southeast
from the mid Mississippi Valley to the Southeast through the period.
...Northern Plains...
In response to continued warm advection over Montana and the
Dakotas, a warm front will lift northward towards the Canadian
border through the day. Meanwhile, a cold front will begin to
accelerate southeast over central Montana during the evening hours.
Between these two surface features, southeasterly flow will
transport mid/upper 60s dew points over the western Dakotas and into
eastern Montana. Closer to the trough / cooler temperatures aloft,
convection should initiate near/ahead of the cold front over the
higher terrain of Montana. Here, steep mid-level lapse rates and
strong effective shear will likely yield a few supercells and/or
bowing segments, capable of large hail and damaging winds during the
afternoon and evening.
Farther east, warmer temperatures aloft and relatively neutral
height tendencies should keep convective coverage more sparse
through the afternoon hours. However, convergence near the warm
front and amplifying large-scale ascent will likely promote a rapid
increase in coverage through the evening and early overnight over
eastern Montana and western North Dakota. Storms should rapidly
become severe, owing to strong buoyancy characterized by MLCAPE
values of 3000-3500+ J/kg. Organized by veering flow with height,
initial cells will likely be supercellular, capable of very large
hail, damaging winds, and a couple tornadoes. Through the
evening/overnight, though, upscale growth into one or more
eastward-accelerating convective systems is anticipated, with a
swath of damaging winds possible. At present, uncertainty regarding
the timing/location of such upscale growth precludes an upgrade to
moderate risk.
...Mid Mississippi Valley to the Southeast...
Severe potential will be highly dependent on initial convective
organization/evolution over the mid Mississippi Valley this morning.
Aided by steep mid-level lapse rates and sufficient effective shear
around 25-35 kt, a few storms this morning will be capable of
isolated large hail and damaging winds near the mid Mississippi
Valley. Some potential exists for a small convective complex to
develop and spread southeast during the morning/afternoon hours,
with an attendant damaging wind potential. Related MCVs may also
focus convective/gusty wind potential across the Tennessee Valley
and Southeast within a warm/moist, but relatively weak flow regime,
during the afternoon. A slight risk could be introduced in later
outlooks, pending greater confidence in the location of any focusing
mechanisms.
...Northeast...
A warm front will lift northeast across New England this morning,
with weak instability/surface-based buoyancy spreading northeastward
in tandem. Warm advection should yield a few showers/thunderstorms
through the morning hours, and a threat may exist for a brief
tornado or localized damaging gusts, owing to enhanced low-level
shear and moist/neutral boundary-layer profiles. A few stronger
storms may also develop through mid-day/afternoon, as the main
shortwave impulse crosses the region and offers some focus for
stronger convection along a surface trough.
..Picca/Nauslar.. 06/28/2018
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