Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 271642
SWODY1
SPC AC 271641

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1141 AM CDT Tue Jun 27 2017

Valid 271630Z - 281200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF NEBRASKA AND SOUTHERN SOUTH DAKOTA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
NORTH-CENTRAL PLAINS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
NORTHERN ROCKIES/HIGH PLAINS TO THE UPPER MIDWEST...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
NORTHEAST STATES/NEW ENGLAND...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE FL
PENINSULA...

...SUMMARY...
Severe thunderstorms capable of large hail, damaging winds and a
tornado or two are expected over much of Nebraska and South Dakota
late this afternoon and evening, with the possibility that a few
gusts may exceed hurricane force. Other strong to severe
thunderstorms will be possible across parts of the Northeast States
as well as the Florida Peninsula.

...Central/northern High Plains to middle MO Valley...
A shortwave trough and related mid/high-level speed max are readily
evident across the eastern ID/southwest MT/western WY vicinity and
adjacent Great Basin per water vapor imagery and 12Z upper-air
analysis. This eastward-progressive shortwave trough will reach the
Dakotas and middle MO valley by this evening amid a steadily
strengthening belt of mid-level westerlies.

An initial increase in widely scattered strong thunderstorm
development should occur this afternoon across interior WY (and far
southern MT), where modest moisture/buoyancy will support some
potential for severe hail and severe-caliber wind gusts. As forcing
for ascent spreads eastward, additional development should occur by
late afternoon in vicinity of the north/south-oriented lee trough,
particularly across eastern WY/NE Panhandle and Black Hills
vicinity. Large hail and damaging gusts will be possible for the
first few hours, although very steep lapse rates and a relatively
well-mixed environment should result in an upscale-growing and
eastward-accelerating complex primarily across NE this evening. As
mentioned in the prior outlook discussion, localized hurricane-force
thunderstorm gusts are possible from the most intense surges of
convection this evening.

...Northeast States/New England...
Aided by a northeast-moving shortwave trough, scattered low-topped
thunderstorms will continue to increase and spread
east/northeastward across the region this afternoon. Relatively
steep lapse rates, accentuated by cool mid-level temperatures (near
-20C at 500 mb) will support stronger low-topped storms capable of
hail, with a few low-topped supercells possible across southern New
England where deep-layer shear will be somewhat stronger. For
additional short-term information, see Mesoscale Discussion 1160.
Convection should weaken as it moves eastward over either land-based
evening diabatic cooling and/or the Atlantic marine layer.

...Florida...
Ample insolation and a moist environment will lead to moderate
buoyancy this afternoon across the FL Peninsula near and south of a
stalled front. A few thunderstorms could produce strong/locally
severe-caliber downdrafts this afternoon within a weak deep-layer
shear environment.

..Guyer/Dean.. 06/27/2017

$$



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