Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 191731
SWODY2
SPC AC 191730

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1130 AM CST Thu Jan 19 2017

Valid 201200Z - 211200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FRIDAY NIGHT
ACROSS PARTS OF THE LOWER MISSISSIPPI VALLEY AND ADJACENT GULF
COASTAL AREAS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
SURROUNDING PORTIONS OF THE GULF COAST STATES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS NEAR SOUTHERN
CALIFORNIA COASTAL AREAS...

...SUMMARY...
A few severe storms are possible Friday night into early Saturday
across the lower Mississippi Valley and adjacent Gulf coastal areas.
Strong thunderstorms with at least some severe weather potential are
also possible near the southern California coast.

...Synopsis...
Models continue to indicate that a seasonably strong mid/upper jet,
with a number of embedded speed maxima, emanating from the
mid-latitude Pacific will continue to develop inland across the
southern tier of the U.S. during this period.  This appears likely
to include a 110-130 kt 500 mb jet streak, which is generally
forecast to dig southeastward near the southern California coast and
contribute to the development of larger-scale mid/upper troughing
inland of the Pacific Coast through the Mississippi Valley by 12Z
Saturday.  Preceding this feature, an elongating closed low and
associated upper troughing, now over the central and southern
Plains, is expected to become increasingly deformed/sheared while
pivoting north/east of the middle Mississippi/lower Ohio Valley
Friday through Friday night.

In lower levels, temperatures will continue to moderate across most
areas east of the Rockies, with general deepening of low pressure
across much of the Plains and Mississippi Valley into the lower Ohio
Valley.  The primary frontal zone or zones may continue to develop
well inland of the Gulf Coast region.  However, the leading edge of
lower/mid tropospheric warming and drying in the wake of the lead
upper impulse may reach north central Gulf coastal areas and the
northwestern Gulf of Mexico by 12Z Friday, before stalling during
the day Friday.  This may include a convectively generated or
enhanced surface boundary which could advance offshore, before a
significant southerly return flow of moisture develops across the
northwestern Gulf Coast and lower Mississippi Valley Friday night.

...Northwestern/north central Gulf Coast and lower MS Valley...
The exact position of the potential convective outflow boundary and
leading edge of the lower/mid tropospheric subsidence and
warming/drying remains a bit uncertain through much of the day
Friday.  It is possible that it could stall close enough to coastal
areas, from the upper Texas coast through the western Florida
Panhandle, that widely scattered storms could initiate across those
areas in response to daytime heating.

More likely, primary convective development may occur in response to
strengthening low-level warm advection on the leading edge of the
deeper/more favorable low-level moisture return.  This is expected
across parts of the northwestern Gulf of Mexico/upper Texas and
Louisiana coastal areas Friday evening, northeastward through
portions of southern Louisiana and central Mississippi Friday night.
In the presence of strengthening wind fields and vertical shear,
including a southerly 850 mb jet to 40-50+ kt, destabilization is
expected to become sufficient to support a risk for severe storms.
This may be mostly in the form of severe hail, with activity
initially elevated above a residual stable near surface layer.

By late Friday night, forcing for ascent may become strong enough,
southward along the 850 mb jet axis toward southeast
Louisiana/Mississippi/Alabama coastal areas, to overcome inhibition
and support increasing convective development.  In the presence of
increasingly boundary layer based instability, the risk for
supercells with potential for tornadoes, in addition to large hail
and damaging winds, may increase by or shortly after 12Z Saturday.

...Southern California coastal areas...
Some uncertainty exists concerning the exact track of the intense
mid/upper jet streak, and just how cold mid-level temperatures will
be near the coast and coastal waters.  However, at least scattered
convective development appears possible during the day Friday, in
the presence of lower/mid tropospheric warm advection.  Including
west southwesterly 850 mb flow on the order of 40+ kt, vertical
shear in the convective layer may be sufficient for low topped
supercells, with a risk for strong surface gusts and perhaps a
tornado.

..Kerr.. 01/19/2017

$$


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