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Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1233 AM CDT Tue Sep 03 2024

Valid 031200Z - 041200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
INTO THIS EVENING ACROSS PARTS OF CENTRAL IDAHO...MUCH OF SOUTHERN
MONTANA...WYOMING...MUCH OF NORTHERN UTAH AND ADJACENT NORTHWESTERN
COLORADO...INTO PARTS OF THE WESTERN DAKOTAS...

...SUMMARY...
Thunderstorms developing across the northern Rockies and adjacent
portions of the Great Basin by this afternoon may produce scattered
strong to locally severe wind gusts, before spreading eastward
toward the northern Great Plains into this evening.

...Synopsis...
Downstream of split mid/upper flow across the eastern Pacific,
Alaska and Canadian Yukon vicinity, the stronger westerlies remain
confined to the higher latitudes, with weaker zonal flow prevailing
across the northern tier of the U.S., on the northern periphery of
weak/suppressed subtropical ridging.  Seasonably moist
boundary-layer air has generally beco*e confined to the southern
tier of the U.S., to the south of notable surface ridging.  Little
change appears likely through this period, though the center of the
ridge may shift from the lower Great Lakes/upper Ohio Valley into
the northern Mid Atlantic.

It does appear that the westerlies may undergo some amplification
across the northeastern Pacific into Canada later today through
tonight, including building mid-level ridging offshore of the
Pacific Northwest and British Columbia coast and digging mid-level
troughing to the lee of the Canadian Rockies.  As this occurs, a
remnant mid-level trough over the interior northwestern U.S. is
forecast to slowly shift into and across the northern Rockies.

...Northern Intermountain Region and Rockies...
Models indicate that the mid-level trough will weaken while shifting
eastward today, but it still appears that mid-level forcing for
ascent and cooling will remain sufficient to support continuing
convective development, including considerable thunderstorm activity
this afternoon and evening across the higher terrain.  Near the
mid-level cold core, destabilization, including mixed-layer CAPE on
the order of 500-1000 J/kg, might support a few thunderstorms
capable of producing marginally severe hail across the mountains of
southwestern Montana.  Otherwise, more modest CAPE, based above a
warm and deeply mixed boundary layer, may still beco*e supportive of
locally strong to severe downburst winds across the higher terrain
of western Wyoming and northern Utah, into the Great Salt Lake
Desert.  

Eastward into the northern high plains, severe weather potential,
and the extent to which destabilization is able to maintain
thunderstorm development, remains more uncertain.  However, some
model output, perhaps most notably the past few runs of the NAM, has
been suggestive that convective development near/north of the
Absaroka Range and Big Horn Basin vicinity may generate sufficient
consolidating outflow to support a substantive east-southeastward
propagating surface cold pool to the lee of the higher terrain.
Although 03/00Z NAM forecast soundings, for example, are not
particularly indicative that vigorous thunderstorm development will
be maintained, they suggest that 35-50 kt winds may acco*pany a
surging gust front, in this run across southeastern Montana and
adjacent northeastern Wyoming by late evening and into western South
Dakota overnight.

..Kerr/Moore.. 09/03/2024