Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 221731
SWODY2
SPC AC 221730

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1230 PM CDT Thu Jun 22 2017

Valid 231200Z - 241200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FRIDAY INTO FRIDAY
EVENING ACROSS PARTS OF THE TENNESSEE VALLEY INTO THE CUMBERLAND
MOUNTAINS AND SOUTHERN PORTIONS OF THE ALLEGHENY PLATEAU...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
SURROUNDING AREAS OF THE MID SOUTH INTO THE APPALACHIANS...UPPER
OHIO VALLEY INTO WESTERN NEW ENGLAND...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE FRIDAY
AFTERNOON AND EVENING ACROSS PARTS OF THE SOUTHERN PLAINS...

...SUMMARY...
Thunderstorms may impact portions of the Mid South and Tennessee
Valley eastward into the Cumberland Mountains and southern portions
of the Allegheny Plateau Friday into Friday night, accompanied by a
risk for potentially damaging wind gusts and perhaps a few
tornadoes.  Other strong to severe thunderstorm activity is possible
near the southern Plains Red River Valley, and across the Texas
Edwards Plateau/Pecos Valley region.

...Synopsis...
Cyclonic flow within the mid-latitude westerlies appears likely to
amplify southeastward into the middle Mississippi/Ohio Valleys and
much of the Northeast during this period.  Models suggest that this
will be accompanied by significant low/mid-level cooling which may
also nose southward to the lee of the Rockies, within at least a
shallow near surface layer, through much of the southern high Plains
by 12Z Saturday.  As this occurs, the lower/mid tropospheric
remnants of Tropical Storm Cindy are forecast to become increasingly
sheared while merging into the westerlies and accelerating
northeastward ahead of the evolving upper trough axis, generally
across Kentucky/Tennessee and the central Appalachians, perhaps as
far east as the northern Mid Atlantic Coast region by the end of the
period.  Across much of California and the Southwest, eastward
across the Gulf states into the southwestern Atlantic, subtropical
ridging is expected to remain fairly strong.

...Mississippi Valley into Atlantic Seaboard...
Given the presence of the shearing remnants of the tropical system,
the potential for strong/severe storm development along and ahead of
the surface front advance southeastward into the middle Mississippi
and lower Ohio Valley region during the day is becoming more
unclear.  Frontal/pre-frontal low-level convergence may be generally
weak, and early day cloud cover/precipitation may reduce pre-frontal
destabilization.  As a result, severe probabilities have been
lowered across this region.

The primary focus for any appreciable severe weather potential
Friday seems likely to become the belt of stronger southwesterly
lower/mid tropospheric flow along and south of the shearing remnants
of Cindy.  Models suggest that this likely will be on the order of
40+ kt, across parts of the Mid South into the central Appalachians,
and perhaps considerably stronger in closer proximity to the
circulation center tracking across this region.  Although lapse
rates are likely to be weak, insolation beneath the mid-level dry
slot ahead of the circulation may contribute to at least weak
boundary layer instability.  And this may be sufficient to support
convective development accompanied by at least a risk for
potentially damaging wind gusts.  Depending on how fast the
low/mid-level cyclonic circulation weakens, in relatively close
proximity to the east/southeast of it, there may be potential for
low-topped supercells accompanied by a risk for tornadoes, mainly
during the afternoon and early evening hours.

Across Pennsylvania and New York into western New England, any
appreciable severe weather potential on Friday seems likely to
largely hinge on pockets of stronger daytime heating.  Mid-levels
will be relatively warm with weak lower/mid tropospheric lapse
rates, but the environment will be seasonably moist with modest
southwesterly to westerly lower/mid tropospheric wind fields on the
order of 30-40 kt.  With sufficient boundary layer
heating/destabilization, it may not be out of the question that
forcing associated with a weak mid-level perturbation or two could
enhance thunderstorm development across Pennsylvania, to the
southeast of the Allegheny front.  It is also possible that forcing
downstream of a much more significant impulse within the
mid-latitude westerlies could enhance thunderstorm development
across the Adirondacks region of Upstate New York into portions of
western New England Friday evening.

...Southern Plains...
A corridor of strong pre-frontal boundary layer heating may become
the focus vigorous storm development late Friday afternoon and
evening.  Activity may initiate where the stronger heating noses
into higher boundary layer moisture content across portions of the
Red River Valley (and supports large mixed layer CAPE on the order
of 2000-3000+ J/kg), and along the axis of strongest pre-frontal
heating/deep mixing across parts of the Edwards Plateau into the
Pecos Valley.  Both regimes may become conducive to the risk for
severe hail and winds, despite generally weak prevailing deep layer
mean flow/shear.

..Kerr.. 06/22/2017

$$



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