Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 220558
SWODY1
SPC AC 220556

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1256 AM CDT Tue Aug 22 2017

Valid 221200Z - 231200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM NORTHERN
PA INTO CENTRAL AND EASTERN NY...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK FROM WESTERN NEW ENGLAND TO THE OH VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK FROM THE NORTHEAST STATES TO THE OZARK PLATEAU TO THE
SOUTHERN PLAINS...

...SUMMARY...
Severe thunderstorms are possible this afternoon and evening across
the lower Great Lakes region and Ohio Valley into parts of the
Northeast, accompanied mainly by the risk for damaging wind gusts.
A tornado threat also exists today from Pennsylvania into eastern
New York and western Vermont.  Scattered strong thunderstorms may
also impact parts of the Ozark Plateau into the south central
Plains, with at least some risk for severe weather.

...Synopsis...
An upper trough centered over northern Ontario/Upper Great Lakes at
12Z today is expected to deepen and evolve into a closed low as it
shifts east into southern Quebec.  Strengthening cyclonic flow
regime is expected from the upper MS Valley through the OH Valley to
the Northeast States this forecast period, with mid-level impulses
embedded in this flow spreading across the OH Valley to the
Northeast.  Farther to the southwest, impulses of monsoonal origin
will spread from NM into the southern Plains, while additional
impulses migrate from the southern Great Basin to the central
Rockies region.  At the surface, substantial cyclogenesis is
expected across Quebec in response to the approaching deepening
closed low aloft.  A trailing cold front will advance to the
southeast and south into the OH Valley, Ozarks and southern Plains
this afternoon, and into the Northeast States tonight to early
Wednesday morning.  Ahead of this front, a warm, humid airmass will
exist and support development of numerous showers and thunderstorms
in mainly multiple bands from midday onward across the OH Valley and
Northeast and west/southwestward into the southern Plains.

...Northeast States to the OH Valley...
Rich moisture (precipitable water ranging from 1.5-2.0 inches) is
expected to spread to the east/northeast across the rest of the OH
Valley into the Northeast States this forecast period contributing
to strong destabilization, especially from the OH Valley into
southern PA.  Farther north into northern PA, NY to western New
England, there remains some uncertainty with the degree of overall
destabilization, partly due to weak mid-level lapse rates.  However,
stronger forcing for ascent expected across NY into New England may
prove sufficient to compensate for weaker instability and support
organized line segments spreading to the east as deep-layer westerly
shear vectors strengthen (effective bulk shear at 35-45 kt).  A
linear storm mode should be predominate with damaging winds expected
as deep-layer winds strengthen, including 40-50 kt at 850-700 mb,
with the greatest threat occurring from northern PA into central and
eastern NY.  Given the deepening surface low into Quebec this
afternoon, low-level winds from eastern PA to eastern NY should
remained backed, enhancing low-level shear for a tornado threat
(especially with any discrete storms).

...Southern Plains eastward to Missouri/Arkansas...
This region will remain displaced from stronger mid/upper flow
associated with the Canadian/Northeast U.S. trough, resulting in
mainly weak deep-layer shear.  Despite this factor, strong
instability is expected in response to strong surface heating within
a moist environment (precipitable water ranging from 1.75 to 2+
inches).  Forcing for ascent with the cold front and attendant to
several weak impulses shifting to the east/southeast across these
areas suggest scattered thunderstorms should develop with at least
an isolated risk for hail and damaging downburst winds.  These
storms could evolve into one or two loosely organized clusters
before weakening during the evening.

..Peters/Gleason.. 08/22/2017

$$


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