Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 280547
SWODY1
SPC AC 280546

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1246 AM CDT Wed Jun 28 2017

Valid 281200Z - 291200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
FAR EASTERN NEBRASKA...FAR NORTHERN MISSOURI...IOWA...FAR
NORTHWESTERN ILLINOIS...SOUTHWESTERN WISCONSIN AND FAR SOUTHEASTERN
MINNESOTA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
ENHANCED RISK FROM PARTS OF THE CENTRAL PLAINS NORTHEASTWARD INTO
THE UPPER MISSISSIPPI VALLEY AND WESTERN GREAT LAKES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OUTSIDE OF THE
SLIGHT RISK AREA FROM PARTS OF THE NORTHERN ROCKIES AND CENTRAL HIGH
PLAINS EAST-NORTHEASTWARD INTO THE GREAT LAKES...

...SUMMARY...
Severe thunderstorms with large hail, wind damage and possibly a
couple of tornadoes will be possible from the Mid Missouri Valley
northeastward into the mid to Upper Mississippi Valley. Isolated
severe storms will be possible in the central Plains with marginally
severe storms possible in the central and northern High Plains.

...Mid to Upper Mississippi Valley/Mid Missouri Valley...
A shortwave trough will move eastward into the mid to upper
Mississippi Valley today. At the surface, a cold front will advance
southeastward across the north-central states and should be located
from central Nebraska northeastward into southeastern Minnesota by
mid afternoon. Ahead of the front, a corridor of maximized low-level
moisture will be located from eastern Kansas north-northeastward
into Iowa where surface dewpoints are forecast to be in the upper
60s and lower 70s F. A linear MCS will likely be ongoing at the
beginning of the period across parts of the mid to upper Mississippi
Valley. This may leave an outflow boundary across parts of the mid
to upper Mississippi Valley on which storms can develop later in the
day. The models show moderate to strong instability from southeast
Nebraska east-northeastward across Iowa by mid to late afternoon
with MLCAPE values reaching the 2000 to 3500 J/kg range. Scattered
thunderstorms should initiate along this corridor by early afternoon
and move eastward away from the instability axis. Additional
thunderstorm activity should initiate further west along the front
during the mid to late afternoon with several clusters of
thunderstorms moving eastward across the enhanced risk area.

Concerning the severe weather potential, the NAM is forecasting a 45
to 55 kt mid-level jet to move eastward across Iowa during the
afternoon. This feature will help to create moderate to strong
deep-layer shear profiles along the instability axis. Forecast
soundings from Des Moines, IA to Davenport, IA show 0-6 km shear in
the 40 to 45 kt range with strong speed shear and some directional
shear in the lower levels. This environment combined with steep
mid-level lapse rates will be favorable for supercells with large
hail. Hailstones of greater than 2 inches in diameter will be
possible with the more dominant supercells. As the low-level jet
strengthens during the late afternoon and early evening, a tornado
threat should also develop. A transition to linear mode is expected
to take place by the mid evening which would increase the
wind-damage threat. An isolated severe threat will be possible in
the late evening and overnight period mainly across southeastern
Iowa, northern Missouri and northwestern Illinois as the convective
system moves southeastward.

...Central Plains and Northern High Plains...
An upper-level trough will move southeastward into the northern
Rockies today as a cold front advances southward across the central
Plains. South of the front, surface dewpoints are forecast to be in
the 60 to 65 F range across central and eastern Kansas with
dewpoints mainly in the 50s F across the central and northern High
Plains. In response, moderate instability should be in place from
northeastern Colorado across much of Kansas during the afternoon.
Thunderstorms are forecast to initiate along the western edge of
moderate instability during the mid to late afternoon in far eastern
Colorado into southwestern Nebraska. This convection should spread
eastward into the central Plains during the late afternoon and early
evening. NAM forecast soundings at North Platte, NE and Goodland, KS
at 00Z/Thursday show MLCAPE in the 1000 to 2000 J/kg range with 35
to 45 kt of 0-6 km shear. In addition, 700-500 mb lapse rates are
forecast to exceed 8.0 C/km suggesting supercells with large hail
will be possible. A wind-damage threat may also develop if cells can
organize into line segments. An isolated severe threat will also be
possible northwestward into the northern High Plains and northern
Rockies but weaker instability should keep any severe threat
marginal there.

..Broyles/Jirak.. 06/28/2017

$$



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