Severe Storm Outlook Narrative (AC)
Issued by NWS

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

000
ACUS02 KWNS 240451
SWODY2
SPC AC 240450

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1150 PM CDT Wed Aug 23 2017

Valid 251200Z - 261200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FRIDAY NIGHT
ACROSS PARTS OF LOWER/MIDDLE TEXAS COASTAL AREAS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FRIDAY EVENING
ACROSS PARTS OF CENTRAL AND SOUTHERN SOUTH DAKOTA INTO NORTHERN
NEBRASKA...

...SUMMARY...
The circulation center of Harvey may reach the lower or middle Texas
coast by late Friday night, accompanied by some risk for tornadoes.
Otherwise, a cluster of thunderstorms may develop across parts of
the Black Hills and Badlands of South Dakota, into portions of the
middle Missouri Valley, accompanied by at least some risk for severe
weather Friday evening.

...Synopsis...
Fairly sharp cyclonic mid-level flow is expected to linger either
side of an axis extending across the Canadian Maritimes, southward
through the southern Appalachians/southern Mid Atlantic Coast
region, through this period.  However, models indicate that the
significant upstream short wave trough, reaching the Canadian
prairies by early Friday, will pivot northeastward into the
Northwest Territories by late Friday night.  As it does, the
stronger mid-latitude westerlies appear likely to retreat north of
the western and central Canadian/U.S. border area, while subtropical
ridging begins to build northward through the Southwest, and
southern portions of the Great Basin, Rockies and Plains.  Within
generally weak westerly/northwesterly mid and upper flow on the
periphery of this ridging, at least one minor perturbation may
progress east southeastward across parts of the Dakotas and mid
Missouri Valley region.  Within/beneath the upper ridging, extending
along an east/southeast axis into the Caribbean and Bahamas, it
still appears that a significant tropical cyclone will migrate into
the vicinity of the lower/middle Texas coast by late Friday night.

...Texas coastal areas...
Harvey may reach hurricane intensity as it approaches lower/middle
Texas coastal areas late Friday/Friday night.  As it does, sizable
clockwise curved low-level hodographs may develop across coastal
areas well in advance of its arrival, but until the near surface
flow veers from northeasterly to more of an easterly or
southeasterly component, to the right of its forward motion,
relatively stable boundary layer air may minimize any tornado
potential.  Appreciable boundary layer destabilization along coastal
areas may not occur until late Friday evening, or later.  Once
instability begins to increase, strong low-level shear will probably
become supportive of at least some risk for tornadoes with
convection associated with Harvey.  The extent of this risk remains
unclear, and will strongly hinge on mesoscale developments which are
still characterized by low predictability this far in advance.

...Parts of the northern Plains...
Isolated to widely scattered strong thunderstorms may develop near
the surface trough across the region during the late afternoon
hours, perhaps aided by forcing associated with the minor short wave
perturbation.  While westerly deep layer mean flow will be weak,
sufficient veering of winds from lower through mid-levels may
contribute to vertical shear marginally sufficient for organized
convective development.  Any severe weather threat seems likely to
remain rather isolated across most areas, but guidance appears
increasingly suggestive that forcing associated with modest
nocturnal low-level jet strengthening (to around 30+ kt at 850 mb)
may enhance convective development spreading east of the Black
Hills/Badlands area of South Dakota by Friday evening.  This is
where a remnant zone of stronger differential surface heating,
beneath at least modestly steep mid-level lapse rates supportive of
CAPE on the order of 1000-2000 J/kg, may contribute to an
environment conducive to an evolving cluster of storms accompanied
by a risk for potentially damaging wind gusts.

..Kerr.. 08/24/2017

$$



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.