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.
Day 1 Wind Risk
Area (sq. mi.)
Some Larger Population Centers in Risk Area
Garden City, KS...Liberal, KS...Guymon, OK...
Amarillo, TX...Fort Collins, CO...Greeley, CO...Longmont, CO...Rapid City, SD...
Minneapolis, MN...St. Paul, MN...Lubbock, TX...Sioux Falls, SD...Rochester, MN...
SPC AC 281259
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0759 AM CDT Mon May 28 2018
Valid 281300Z - 291200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS PRIMARILY OVER
THE CENTRAL HIGH PLAINS...
Severe thunderstorms with isolated very large hail, wind damage and
a tornado threat are expected across parts of the Great Plains. A
couple of brief tornadoes will also be possible in the Southeast in
conjunction with Alberto.
The main upper-air feature for this forecast will be a broad cyclone
-- its center evident in moisture-channel imagery over western UT.
As the associated synoptic-scale trough moves slowly eastward
through the period, a series of shortwave troughs and vorticity
maxima will pivot through the southern and eastern quadrants of the
cyclone -- most notably a perturbation now located over AZ. That
feature is expected to eject northeastward over CO toward the CO/WY
state line by 00Z. This should become the primary vorticity center
and 500-mb low overnight across eastern WY.
Meanwhile, Subtropical Storm Alberto is forecast by NHC to move
inland over the western FL Panhandle today and AL thereafter, moving
generally north-northwestward into a longstanding mid/upper height
weakness over the east-central U.S. Please refer to NHC advisories
for specific track/intensity guidance on Alberto, as well as related
At the surface, 11Z analysis showed a wavy frontal zone was analyzed
from the Delmarva Peninsula across central PA to a low over southern
ON, then westward over southern/central Lower MI, being overtaken by
a convective outflow pool across WI. The frontal zone was evident
again from a low over southern MN southwestward to eastern CO.
except where effectively shifted by mesoscale and smaller convective
processes, the central U.S. frontal segment should move little
through this evening. A dryline -- initially drawn from
southeastern CO roughly southward across the TX/NM border region to
between FST-MRF, is expected to move just slightly eastward today
across southeastern CO, the western OK/TX Panhandles and the Llano
...High Plains region...
Convective coverage on the High Plains is expected to be even
greater than yesterday as the cyclone aloft and associated shortwave
perturbation approach, and low-level mass response occurs amidst
generally greater boundary layer moisture content from eastern
CO/southwestern NE southward. Scattered to locally numerous
thunderstorms are forecast to develop this afternoon near the
dryline, as well as the Palmer Divide area and Front Range to its
north. Initial convection may assume supercellular characteristics,
offering the potential for a few tornadoes, along with damaging/
significant hail greater than 2 inches in diameter. Activity should
aggregate with time, forming outflow-dominant/cold-pool-driven
clusters across portions of CO/southeastern WY and NE, and a
generally meridional belt of severe convection from southwestern NE
across western KS into the TX Panhandle. Coverage generally should
diminish with southward extent across the Panhandle toward the South
Plains and adjacent Caprock.
Upslope/post-frontal flow across the CO/WY portion of the outlook
will provide a combination of enhanced storm-relative flow, enlarged
low-level hodographs and lift in support of the severe potential
there, with MLCAPE generally 1000-2000 J/kg, beneath strengthening
large-scale lift and southwesterly deep-shear vectors. Meanwhile,
somewhat weaker shear but substantially greater buoyancy is expected
ahead of the dryline. In that regime, warm-sector dew points 60s F
and strong diabatic heating, beneath steep deep-tropospheric lapse
rates, will drive preconvective MLCAPE to around 3000 J/kg, locally
higher. For what relatively discrete storms remain, a window of
relatively maximized tornado potential may develop within a couple
hours after sunset as the LLJ enlarges low-level hodographs.
Thereafter, overall severe potential should diminish through the
evening, as the airmass stabilizes from a combination of diabatic
cooling and expansive outflow coverage. However, a few clusters of
strong-severe storms may linger into the overnight hours across
portions of the central/northern Plains.
Isolated, transient supercells are possible east through north of
the center of Alberto as it penetrates inland today, and a tornado
or two may occur. Any more-concentrated tornado potential remains
too conditional for a categorical upgrade at this time, but
mesoscale boundary and convective trends will be monitored for the
emergence of a more tightly focused zone of potential within this
rather broad marginal risk.
IR and early VIS imagery show a broad area of relative breaks in
cloud cover over much of the existing marginal-risk area, indicating
favorable duration/intensity of diabatic destabilization. Despite
modest lapse rates aloft -- characteristic of landfalling tropical
and subtropical cyclones -- boundary-layer heating with surface dew
points commonly 70s F may boost MLCAPE into the 1000-2000 J/kg
range. Kinematically, areas forecast to experience the strongest,
most sustained heating away from the central/core convective region
of the cyclone also will have weaker near-surface winds, with
generally smaller hodographs and low-level shear. The possible
exception would be differential-heating/precip-reinforced
boundaries, and any inner bands that have access to inflow air from
the more strongly heated sector. Otherwise, an inner-band supercell
may form every few hours or so in relatively weak buoyancy but
stronger low-level shear, offering at least conditional/brief
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