Severe Storm Outlook Narrative (AC)
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078
ACUS01 KWNS 251944
SWODY1
SPC AC 251942

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0242 PM CDT Tue Jul 25 2017

Valid 252000Z - 261200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN SD
TO CENTRAL MN...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
WESTERN NE TO NORTHWEST WI...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EASTERN CO
TO LAKE SUPERIOR...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE GREAT BASIN...

...SUMMARY...
Scattered severe storms are expected across parts of the Upper
Midwest during the late afternoon and evening. Large hail, damaging
winds, and a couple tornadic storms will be possible.

...Upper Midwest...

No appreciable changes have been made to the 1630z outlook.

Earlier thoughts regarding the evolution of convection along a
boundary draped from northern NE into MN remain. The last six or so
runs of the HRRR model are very consistent in thunderstorms
initiating over south-central SD where surface temperatures have
soared above 100F. This breach of convective temperature should
result in thunderstorm development by 21-22z. Subsequent development
northeast along the boundary into MN will occur by early evening.

For more information refer to MCD #1420.

..Darrow.. 07/25/2017

.PREV DISCUSSION... /ISSUED 1124 AM CDT Tue Jul 25 2017/

...Upper Midwest...
Pronounced capping as sampled by the 12Z Aberdeen sounding will
inhibit surface-based storm development until late afternoon. Hot
temperatures from 100-104 degree F will be necessary to minimize
MLCIN, and this should occur across western NE into south-central SD
where a thermally-induced cyclone should form along a
quasi-stationary front. Storm initiation should occur in this
vicinity amid a deeply mixed thermodynamic profile characterized by
steep tropospheric lapse rates. Frontal convergence and warm-air
advection across the boundary should yield scattered storm
development to the east-northeast across eastern SD into
central/southern MN during the evening. Here, large buoyancy with
MLCAPE of 2000-3500 J/kg should be present amid upper 60s to lower
70s surface dew points.

Deep-layer shear will be sufficient for organized updrafts on the
southern periphery of moderate mid/upper-level westerlies associated
with a Manitoba shortwave trough. However, weaknesses in the
hodograph and orientation of the front should foster a predominately
messy storm mode, with cells likely evolving rapidly into clusters.
This setup should result in a mix of large hail and damaging winds.
A couple tornadic storms will be possible with embedded but probably
transient supercell structures, most likely in parts of eastern SD
to central/southern MN. The severe threat should diminish late
evening into the overnight as convection becomes further
disorganized with the shortwave trough shifting east in northern ON.

...Western Great Basin...
Scattered storms will again develop this afternoon and evening
downstream of a nearly stationary mid-level low along the northern
CA coast. Winds below the 500-mb level will be light, with a belt of
50-60 kt high-level southwesterlies confined to the upper portion of
the convective cloud layer. Effective shear will likely remain weak,
below 20 kt. This will minimize the risk for severe wind gusts from
organized storms. However, localized severe wind gusts will be
possible where robust diabatic heating occurs, to the north/west of
pervasive cloud cover over the southern/eastern Great Basin.

$$



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