Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 261752
SWODY2
SPC AC 261751

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1251 PM CDT Mon Jun 26 2017

Valid 271200Z - 281200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS CENTRAL AND
SOUTHERN SOUTH DAKOTA THROUGH NEBRASKA TO FAR NORTHEAST COLORADO...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM YELLOWSTONE
N.P. AREA EASTWARD INTO THE NORTHERN PLAINS AND INTO PARTS OF THE
CENTRAL HIGH PLAINS...

...SUMMARY...
Areas of severe thunderstorms capable of large hail and damaging
winds are expected over much of central and western Nebraska as well
as South Dakota during the late afternoon and evening on Tuesday.
Isolated severe storms are also possible from eastern Colorado into
central Kansas, and from Yellowstone National Park into North
Dakota.

...Synopsis...
The northern branch of the western U.S. upper ridge will remain
progressive from today through Tuesday, as it shifts east through
the northern Great Plains and the Upper Midwest.  However, its
amplitude is expected to undergo further dampening, as phasing day-1
midlevel troughs (southern extent of the British Columbia system and
one moving through the Pacific Northwest) advance east of the
northern Rockies to the northern Plains.

A surface cyclone will develop over the western Dakotas Tuesday by
late Tuesday afternoon, and move east across SD/ND Tuesday night.
Southerly low-level jet will translate east through the northern
Plains this forecast period, and strengthen Tuesday afternoon from
the central Plains into the central/eastern Dakotas.  Further
strengthening of this low-level jet is expected Tuesday night from
the central Plains through the mid MO Valley to MN, as heights fall
with the approach of the northern Rockies/High Plains phased
shortwave troughs.  These southerly winds will result in moisture
return through the central and northern Plains.

...Portions of central and northern Plains...
Surface dew points in the lower 60s are expected to spread poleward
across western and central parts of KS/NE and far eastern CO into
the central Dakotas by late Tuesday afternoon, beneath steepening
midlevel lapse rates.  This combined with diabatic heating will
result in moderate instability.  Storms are expected to form by 21Z
across the central and southern High Plains near the low pressure
area and southward in vicinity of the lee trough/wind shift.  Storms
will move into a slightly capped, but more unstable air mass with
eastward extent, where wind profiles will support supercells, with
very large hail and damaging winds possible. Storms are expected to
merge into one or more MCSs during the evening, with damaging winds
and some hail being the primary threats.  Capping will eventually
become a concern, given the lack of cooling midlevel temperatures
limiting the eastward progression of the stronger storms into the
Upper Midwest early Wednesday morning.

...ND/northeast SD into northwest MN...
Strong low-level warm-air advection developing later Tuesday
afternoon and evening within the nose of the strengthening Plains
low-level jet should support elevated strong to severe storms across
this region as heights fall with the approach of the upstream
shortwave troughs.  Coverage of the severe-weather threat (mainly
hail) is expected to remain as marginal.

...Yellowstone National Park area eastward to the Black Hills...
A shortwave trough will move over western areas during the day, with
cooling aloft and daytime heating resulting in sufficient
instability (despite scant moisture) for a few strong to severe
storms.  Storms are expected to form relatively early, between
18-21Z around the Yellowstone National Park area, and then continue
eastward across southern MT and northern WY during the evening.
Locally severe hail or damaging winds will be possible.

...Central and eastern NY into New England...
A progressive shortwave trough, currently moving through WI per
water-vapor imagery, is expected to track northeast from in lee of
the lower Great Lakes through NY and northern New England.  Although
bulk shear of 30-40 kt will be sufficient for organized storms,
uncertainty in the amount of destabilization should preclude the
inclusion of a marginal severe-weather threat area across this
region.

..Peters.. 06/26/2017

$$



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