Day 1 Severe Weather Outlook

Valid 101200Z – 111200Z


Thunderstorms with severe wind gusts, large hail and a tornado or
two are possible across parts of Nebraska, South Dakota and
east-central Wyoming from late afternoon through evening. Some of
the hail could be very large, with a few significant damaging wind

An upper trough will amplify over the western states with
significant cooling aloft and increasing shear developing over the
Great Basin into the central Rockies. A cyclonically curved upper
jet with midlevel speeds of 50-60 kt will nose into NV and UT late
in the day, while shortwave ridging occurs over the Plains. At the
surface, low pressure will deepen over northeast CO, where strong
heating will occur. A warm front will develop northward toward the
NE/SD border through evening, with eastward extension of this
boundary becoming nearly stationary along the MN/IA border. South of
this front, warm southerly winds will bring upper 60s F dewpoints
northward, resulting in substantial instability from the northern
High Plains eastward to southern WI.

…Northern Plains…
Southerly low-level winds and heating within the large-scale warm
advection regime will aid the northward transport of mid to upper
60s F dewpoints across NE and into southern SD. Weakening easterlies
north of the warm front will also maintain 50s F dewpoints into
southwest MT and eastern WY, where heating will lead to an uncapped
air mass. Increasing convergence and upslope flow will lead to storm
initiation over eastern WY by around 21Z, with long hodographs
supporting large hail into western SD and NE. Additional storms are
expected to develop late in the day and into the evening near the
warm front, where substantial low-level shear and SRH will develop.
A few supercells are possible over northern NE and southern SD, with
very large hail and a couple tornadoes possible before storms grow
upscale into an MCS with the primary risk damaging wind gusts. The
damaging-wind threat may persist eastward into MN and IA where warm
advection with the low-level jet will aid lift.

…Northeast NV…northern UT…southeast ID…western WY into
southern MT…
Strong heating near the surface trough will result in scattered
thunderstorms during the late afternoon from northern NV into
southern ID and western WY. Forecast soundings show steep lapse
rates and lengthening hodographs which will support cells or small
bows capable of marginally severe hail or wind. Storms are also
expected to increase in coverage from eastern NV into UT during the
evening as large-scale lift with the upper trough is maximized.
Despite the time of day, marginal hail and a few severe wind gusts
will remain possible as instability remains sufficient, given cold
temperatures aloft.

..Jewell.. 09/10/2019


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