Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 201731
SWODY2
SPC AC 201730

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1230 PM CDT Thu Jul 20 2017

Valid 211200Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE NORTHERN
PLAINS THROUGH THE UPPER MIDWEST AND GREAT LAKES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
PLAINS THROUGH THE OHIO VALLEY AND MID ATLANTIC AREA...

...SUMMARY...
Strong/severe storms will be possible from the northern Plains and
central Plains east across parts of the Great Lakes and Midwest. A
few severe storms will be possible over the Central Plains are.

...Synopsis...

Synoptic pattern will change little Friday with upper ridge
remaining firmly in place over the central and southern plains and
belt of moderate mid-upper flow across the northern tier of states.
A series of low-amplitude impulses will move through the belt of
stronger flow from the Northern Plains into the Great Lakes and
Upper Midwest. A more significant shortwave trough will move through
the southern Canadian Prairies near the international border and
affect ND Friday night.

At the surface a quasi-stationary or warm front should extend from
weak surface low over SD southeastward through the Upper Midwest and
into the Ohio Valley. Western portion of this boundary could make
some northward progress into the upper MS Valley during the period.
A cold front will accompany the shortwave trough into the Dakotas
Friday night. A lee trough will extend from the surface low in SD
southwestward into NE and CO.

...Northern Plains through the Upper Mississippi Valley, Great Lakes
and Ohio Valley...

A few areas of showers and thunderstorms will likely be ongoing
within zone of warm advection and isentropic ascent north of the
warm front from parts of the Dakotas into the upper MS and OH Valley
regions. This activity will be elevated above a stable surface
layer, but could pose at least a marginal risk for hail as it shifts
eastward. The moist warm sector in vicinity of the
warm/quasistationary front will destabilize during the day with
moderate to strong instability possible during the afternoon. A
capping inversion associated with eastern extension of the elevated
mixed layer may limit more robust initiation over western portion of
region until mid-late afternoon when additional storms may develop
over eastern SD near triple point supported by a destabilizing
boundary layer and a progressive shortwave trough.  Activity should
expand eastward into southern MN along nose of southwesterly low
level jet, possibly consolidating into a forward propagating MCS
that will continue southeastward along instability gradient through
the upper Midwest region. Should this scenario transpire, a corridor
of higher concentration of damaging wind would be likely. Given
lingering uncertainty in how the mesoscale will evolve, will
maintain 15% category at this time, but a enhanced might be
introduced in later outlooks.

Otherwise, additional storms may develop farther east along boundary
over the OH valley region. Later in the period, any MCS that
develops upstream could eventually affect the Ohio Valley and Friday
Night.

...Northern Minnesota and North Dakota...

Additional storms may develop over ND most overnight in association
with approaching cold front and shortwave trough. This activity will
spread east through a portion of ND and possibly northern MN posing
a risk for damaging wind.

...Central Plains...

High-based multicell storms may develop over the higher terrain of
CO and within the deeply mixed regime over the central high plains
in vicinity of lee trough posing  a risk for downburst winds during
the afternoon.

...Southern Appalachians through Middle Atlantic region...

Storms will likely form over the higher terrain of the southern
Appalachians as well as in association with a lee trough across
central VA within a moderately unstable atmosphere. This region will
remain south of the stronger winds aloft with multicells the likely
storm mode. The stronger storms may produce a few downburst winds
from late afternoon through early evening.

..Dial.. 07/20/2017

$$



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