Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 221731
SPC AC 221730

Day 2 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
1230 PM CDT Tue May 22 2018

Valid 231200Z - 241200Z



Isolated severe thunderstorms are possible Wednesday afternoon
across the High Plains and eastern portions of Virginia/North

Upper low currently centered over the western Great Basin will
gradually shift northeastward toward the central Rockies today while
becoming even more progressive/open on Wednesday. By 12Z Thursday,
the resulting shortwave trough will likely extend from central
Alberta into northeast MT. In response to this upper system,
cyclogenesis is anticipated across southeast MT with the resulting
low tracking northward across eastern MT Wednesday night/early
Thursday morning.

Farther east, upper ridging initially extending from the Southeast
into the Canadian Prairie provinces will build throughout the day
with some minor eastward progression. At the surface, expansive area
of high pressure will drift from the mid MS/lower OH Valley
northeastward across the Upper Great Lakes.

...High Plains...
Surface lee troughing coupled with persistent southerly low-level
flow will continue to encourage moisture return across the Plains
and High Plains. Additionally, southwesterly mid-level flow will
advect steep mid-level lapse rates into the region with the
resulting airmass characterized by moderate instability (i.e. MLCAPE
around 1500 J/kg). Vertical shear will be fairly weak across
southern portions of the region while slightly stronger mid-level
flow will contribute to modestly higher values across the central
and northern High Plains.

Forcing for thunderstorm development is a bit more nebulous with the
strongest height falls moving through northern portions of the area
early in the period before diurnal destabilization. Primary forcing
for convective initiation across the majority of the High Plains
will likely be surface convergence along the lee trough. There is
some potential for an outflow boundary across SD, resulting from
early period, mostly elevated storms, to provide an additional area
of convergence and lift.

Confidence in the intensity and coverage of thunderstorms is still
too low to increase severe probabilities above 5%. At this point,
the two most probable locations for a potential increase in
probabilities appear to be across eastern MT where a developing warm
front may enhance low-level vertical shear and across far
southeastern WY and adjacent portions of southwest SD and the NE
Panhandle where the strong surface convergence is forecast to align
with relatively higher instability and shear.

Cold front extending southwestward from southern NJ across central
VA to middle TN is expected to shift southeastward throughout the
day. Temperatures ahead of the front will likely reach the mid-80s
amidst dewpoints in the upper 60s, which is supportive of airmass
destabilization and modest instability. Vertical shear supports a
predominately linear multicell mode with isolated damaging wind
gusts possible as a result of water loading within the strongest

Tornado:   2%     - Marginal
Wind:      5%     - Marginal
Hail:      5%     - Marginal

..Mosier.. 05/22/2018

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