Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 171725
SWODY2
SPC AC 171724

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1224 PM CDT Thu Aug 17 2017

Valid 181200Z - 191200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ROUGHLY FROM
WESTERN MN INTO NORTHERN OK...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE MID
ATLANTIC INTO WESTERN NEW ENGLAND...

...SUMMARY...
Scattered strong to severe storms are possible from the mid Missouri
Valley into the central Plains.  A few strong to severe storms are
also possible from the Mid Atlantic into Upstate New York and
western New England.

...Synopsis...
A broad area of cyclonic flow aloft will extend from the northern
Plains into the Northeast, with two primary shortwave troughs and
associated surface boundaries. A leading shortwave will move from
the Upper Great Lakes across the Northeast with surface low over
eastern Ontario by 00Z. A weak front will extend southward from the
low across NY, PA, and into the Mid Atlantic, and will provide a
focus for daytime storms.  Ample moisture will spread northward
ahead of the front with increasing flow aloft supporting some wind
threat.

To the west, a more amplified shortwave trough will move
southeastward out of the northern Plains and toward the mid MS
Valley with a somewhat weaker boundary from western MN southward
across KS and trailing west across the OK/TX Panhandles. Strong
instability over southern areas will support a severe wind and hail
risk mainly late in the day.

...Much of eastern KS, western MO, northern OK...
Scattered early day storms may be ongoing from eastern NE into KS
supported by southwesterly low-level jet, with a marginal wind
threat possible. Any early activity may influence the effective
boundary position for storms later in the day.

Strong heating will occur ahead of the effective front, and lapse
rates aloft will steepen with the upper shortwave trough skirting
the region. One or more clusters of cells are expected to form along
the front/wind shift or perhaps along any preexisting outflow
boundaries, supported by strong instability. Northwest flow aloft
combined with low-level southwesterlies will support a few clusters
of storms capable of both hail and damaging winds. Various model
solutions suggest storms may form anywhere from eastern KS and
western MO into the eastern TX Panhandle. An upgrade to Slight Risk
is possible in later outlooks when predictability is greater
regarding antecedent/early storms and their influence on the
boundary position and air mass quality.

...Eastern VA into upstate NY and western New England...
Early day rain and storms are expected to move across eastern NY and
New England in association with theta-e advection in the low-levels
supported by a broad area of 30 kt 850 mb flow. Instability will be
weak for this initial activity, but gradual destabilization is
expected to the west ahead of the front with isolated strong to
severe storms possible. Instability will increase the most to the
south with heating focused across MD and VA. As such, southern areas
may have the greatest chance of severe storms with wind and isolated
hail. Across the Delmarva area, low-level winds may remain locally
backed, and it will be quite moist. A brief tornado cannot be ruled
out provided a discrete cell moving into that area, but overall wind
profiles suggest a more complex storm mode.

...Western MN area into IA...
An inverted surface trough will exist across eastern SD into western
MN just on the north side of the midlevel jet. Low-level moisture
will remain sufficient for strong storms with dewpoints in the lower
60s F and with 500 mb temperatures around -15 C. Although low-level
winds will be weak, weakly veering flow with height suggest a few
southward-moving cells are likely, perhaps a supercell or two. Large
hail will be the main threat during the afternoon.

..Jewell.. 08/17/2017

$$



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