Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 231716
SWODY2
SPC AC 231715

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1215 PM CDT Wed Aug 23 2017

Valid 241200Z - 251200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS A LARGE
PART OF THE HIGH PLAINS AND VICINITY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
NORTHEASTERN OREGON...NORTHERN IDAHO...AND SOUTHWESTERN MONTANA...

...SUMMARY...
Strong thunderstorms may impact the Front Range of the Rockies, the
Black Hills vicinity and adjacent areas of the central High Plains
late Thursday afternoon and evening, perhaps accompanied by some
risk for severe weather.  A low risk of a tornado or two may exist
late in the forecast period as outer bands of TC Harvey approach the
Texas Coast.

...Synopsis...
A fairly progressive pattern will exist across the northern tier of
the Lower 48 Thursday as a synoptic-scale mid-level trough migrates
eastward across the Northeast and another vigorous trough grazes the
Pacific Northwest and Northern Rockies region.  Farther south, areas
of weak mid-level ridging will exist from the Southwest eastward to
Florida, while another shortwave ridge migrates eastward across the
Dakotas.

At the surface, a broad anticyclone centered over the western Great
Lakes will aid in spreading relatively stable low-level airmass
across a large part of the East.  A surface front will make slow
southward progress across the Carolinas and Deep South throughout
the day.  The circulation associated with TC Harvey will gradually
organize while approaching the Texas Coast (see the latest National
Hurricane Center advisories for more information).  Meanwhile,
southerly/upslope flow will contribute to moist advection in a
corridor along the High Plains along and just ahead of a weak lee
trough.  A relatively ill-defined surface pattern will exist across
the Intermountain West throughout the forecast period.

...Portions of the High Plains and Vicinity...
Aforementioned upslope flow will aid in maintaining an axis of 50s
to 60s F dewpoints along much of the High Plains throughout the day.
This upslope, combined with substantial surface heating, will
support scattered thunderstorm development across the MRGL risk area
by mid-afternoon despite negligible mid/upper support for this
activity.  Low-level and deep layer shear profiles are relatively
weak, but 20-30 knot flow at around 6km AGL may be just enough to
foster some loosely organized, slowly propagating clusters of strong
to severe storms in the afternoon.  These storms will primarily be
diurnally driven, but steep low/mid-level lapse rates will foster a
risk for hail and damaging wind gusts on at least an isolated basis.
Isolated severe thunderstorms may also develop as far east as
northeastern Nebraska along a weak east-to-west oriented surface
boundary in that region.

...Northeastern Oregon into southwestern Montana...
Models are consistent in developing storms during peak heating hours
on the southern periphery of a more vigorous, yet favorably timed
mid-level trough just north of the region.  Steep lapse rates,
around 1 inch precipitable water values, and 30-40 knots of deep
shear will foster a risk of isolated damaging wind gusts and hail
with the strongest cores.  This activity will also be mostly
diurnally driven, with weakening expected after around 03Z or so.

...Portions of the Texas Coast...
Low-level wind fields will begin to gradually increase (especially
after 06Z Friday) associated with the approach of TC Harvey in the
western Gulf of Mexico.  Models suggest that outer bands associated
with this circulation may begin to approach the coast toward 12Z
Friday, although the specific timing of these bands remains unclear
at this time frame.  MRGL/2% tornado probabilities may need to be
added in later outlooks (most likely from CRP to GLS and vicinity)
in later outlooks if it appears that 1) outer bands will reach the
Texas coast sooner than currently forecast and 2) sufficient
instability exists (as currently suggested by the latest 12Z Nam).

..Cook.. 08/23/2017

$$



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