Severe Storm Outlook Narrative (AC)
Issued by NWS

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000
ACUS01 KWNS 212002
SWODY1
SPC AC 212001

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0301 PM CDT Thu Sep 21 2017

Valid 212000Z - 221200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A PORTION
OF THE SOUTHERN PLAINS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A PORTION
OF THE UPPER MIDWEST...

...SUMMARY...
A marginal risk for a few large hail events exists over a part of
the Upper Midwest tonight into Friday morning. Isolated strong to
severe wind gusts are possible from west Texas into southcentral
Kansas between 4 and 9 pm CDT.

No changes needed with either of the Marginal risk areas.  Previous
forecast reasoning remains valid for this outlook issuance.

...West Texas through southwest/south-central KS...
Observational data indicated gradual weakening of surface-based
inhibition, given ongoing strong diabatic heating in vicinity of the
dryline.  Trends in visible satellite imagery showed cumulus clouds
forming within this environment across western TX into southwest OK,
and this development will persist as additional surface heating
supports further weakening of the cap.  Meanwhile, water-vapor
imagery showed a shortwave trough tracking from southwest to
west-central TX this afternoon, as also indicated by backing 850-mb
winds across the southern Plains, and veering 500-mb winds across
southwest TX with the trough passage.  Attendant forcing for ascent
should prove favorable in aiding eventual stronger updrafts across
this Marginal risk area later this afternoon into the early evening.
 Weak deep-layer shear and DCAPE values exceeding 1500 J/kg suggest
locally strong downburst winds should be the primary severe threat.

...FL Panhandle into northern FL/extreme southern GA...
Weak deep-layer shear will limit organized storm development, though
moderately strong buoyancy and sea breeze/convective outflow
boundary mergers suggest additional storms will be possible.  High
precipitable water values and high DCAPE suggest locally strong wind
gusts will be the greatest threat.  The lack of storm organization
precludes the introduction of severe probabilities.

...Eastern NC...
Forcing for ascent and cold 500-mb temperatures (-11 to -12C)
attendant to a nearly stationary mid-level circulation centered over
eastern NC today combined with steep low-level lapse rates and
precipitable water exceeding 1.5 inches suggest additional storms
will be possible into the early evening.  Weak deep-layer shear will
limit storm organization, though DCAPE exceeding 1000 J/kg across
southeast NC and high precipitable water could support locally
strong winds.

..Peters.. 09/21/2017

.PREV DISCUSSION... /ISSUED 1129 AM CDT Thu Sep 21 2017/

...West Texas through southwest Kansas...

Insolation will result in a corridor of strong instability
(2000-3000 J/kg MLCAPE) from the eastern portion of west TX through
central KS where steeper lapse rates associated with the elevated
mixed layer overlap western fringe of richer low-level moisture.
Other than what appears to be a weak vorticity maximum moving
through northern Mexico and southwest TX, forcing aloft will remain
weak. However, deeper mixing and convergence in vicinity of the
dryline should result in at least isolated multicell storms, a few
of which could produce downburst winds through early evening.

...Upper Midwest...

Strong low-level theta-e advection will be underway today resulting
in northward destabilization into the upper Midwest. However, the
combination of rising heights and northeast advection of warm air
within base of the elevated mixed layer should effectively cap the
atmosphere. Therefore, thunderstorm initiation is not expected until
later tonight, most likely across northern MN just north of a warm
front, where isentropic ascent will increase in association with a
strong southerly low-level jet. These storms will be elevated, but
instability should be sufficient for at least a marginal threat for
large hail.

$$


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