Day 1 Severe Weather Outlook

Valid 111200Z – 121200Z


Severe thunderstorms capable of producing large hail, damaging wind
gusts, and a tornado or two are possible mainly over parts of the
central and northern Plains. More isolated and/or marginal severe
storms are possible extending eastward across the southern Great
Lakes and to the East Coast from Virginia to New Jersey.

A shortwave trough will progress east across the Rockies, with a
50-60 kt midlevel speed max moving from the central High Plains
during the day to the central Plains by Thursday morning. Modest
westerly winds aloft will extend across the Great Lakes, north of a
gradually weakening upper high over the Southeast.

At the surface, low pressure will move from northeast CO across NE,
with a stationary front roughly from northern NE, IA and into
southern WI. Early day storms and outflow may change the position of
this front. During the evening and overnight, a cold front will
move east across the central Plains behind the low.

Dewpoints in the 65-70 F range will be common south of the
stationary front and ahead of the cold front, aiding the development
of ample instability supporting strong to severe storms from eastern
WY into southern WI.

…Eastern WY…far northeast CO…NE…
An outflow boundary now over west-central NE and northeast CO is
expected to mix during the day, and may retreat north as southerly
winds increase ahead of the upper trough. Heating south of the
outflow (and ahead of the developing cold front over eastern WY)
along with ample moisture will lead to strong instability. Low-level
winds will veer with height, but will remain relatively weak below
700 mb resulting in long, mostly straight hodographs as the upper
jet overspreads the High Plains. Storms are expected to form over
eastern WY by early afternoon, and over northeast CO and western NE
near a triple point. Supercells capable of very large hail will be
possible, with perhaps brief tornadoes although effective SRH will
average 100-150 m2/s2 prior to 00Z. Thereafter, storms are expected
to merge into an MCS with damaging wind the main concern.

…Upper MS Valley and Great Lakes…
Storms may be ongoing early in the day as an MCS (now over SD/MN/IA)
continues east along the instability gradient. Southwest winds at
850 mb will also maintain warm/moist advection into the region,
supporting continued development or rejuvenation of the pre-existing
system. Damaging winds appears to be the main concern.

…PA and NJ southwestward into northern GA…
Although beneath an upper ridge, the air mass will remain moist and
the region will experience strong heating. MLCAPE of 2000-3000 J/kg
will develop, and will conditionally favor pulse storms capable of
marginally severe wind or hail over the Appalachians. Storm mode
over northern areas such as PA and NJ may trend toward multicells
with 20-30 kt mid to high-level flow. The severe threat will be tied
to the diurnal cycle, and will dwindle by early evening.

..Jewell/Squitieri.. 09/11/2019


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