Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 302051
SWODY1
SPC AC 302050

Day 1 Convective Outlook AMEND 1
NWS Storm Prediction Center Norman OK
0350 PM CDT Thu Mar 30 2017

Valid 302043Z - 311200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE OHIO VALLEY AND MIDWEST...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE MIDWEST
SOUTHWARD INTO THE FLORIDA PANHANDLE...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A LARGE
PART OF THE MIDWEST INTO THE SOUTHEASTERN STATES INCLUDING COASTAL
SC/NC...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE EASTERN GREAT BASIN...

AMENDED FOR NC/SC AREA LATE TONIGHT

...SUMMARY...
Scattered severe storms are most likely across the Midwest and Ohio
Valley this afternoon into evening, with other severe storms along
the central Gulf Coast. Damaging winds, isolated weak tornadoes, and
hail will be possible in these areas.

...Midwest/Ohio Valley...
Isolated cells currently exist across central Indiana near a sharp
e-w oriented warm front. Locally backed surface winds along this
front may enhance tornado potential briefly for any storms
interacting with it, before they cross over into the cooler more
stable air. Otherwise, scattered storms continue to evolve along the
main cold front to the west, from southern IL into the lower MS
valley with wind and marginal hail threat. Tornado threat looks to
be reduced across much of KY and TN due to strong boundary-layer
mixing which both reduces moisture and truncates the lower parts of
the hodograph. For more information see MCD 379.

...Alabama into the Florida Panhandle...
A renewed cluster of cells has developed across southern AL in a
region of strong heating and where lapse rates are quite steep along
with long hodographs favorable for supercells and large hail. These
cells were ahead of an approaching MCS, which continues to surge
eastward across southern AL and into the western FL panhandle. While
hodographs are long, low-level shear is not particularly strong,
suggesting low end tornado threat. However, the orientation of the
deep-layer shear vectors are largely parallel to the approaching
line of storms, such that it remains possible that a brief/weak
tornado could still occur. Otherwise, hail would appear to be the
main threat. For more information see MCD 380.

...Coastal Eastern SC and southern NC Late Tonight...
Visible satellite imagery shows a boundary extending eastward from
southern SC with mid 60s dewpoints to the south of it.  As the upper
trough approaches later tonight, this moisture will move farther
inland, an orient with an increasing low level jet with 30-40 kt at
850 mb. Several CAMs show the potential for isolated supercells to
form, perhaps over the water and move northward through early Friday
morning. Instability will not be strong but perhaps sufficient to
support a localized severe risk. Forecast soundings show favorable
hodographs for supercells, and perhaps a localized tornado threat.
Thus, have upgraded the tornado probabilities in this area.

..Jewell.. 03/30/2017

.PREV DISCUSSION... /ISSUED 1150 AM CDT Thu Mar 30 2017/

...Midwest and Ohio/Tennessee Valleys...
A closed mid/upper-level low will continue a slow general eastward
progression from the lower MO Valley and Ozarks toward the lower OH
River Valley through tonight. A related surface low will continue to
develop east-northeastward from east-central MO into central IL,
while a warm front advances northward across eastern IL and much of
IN/OH. Overall buoyancy will remain modest today and cloud cover has
remained semi-prevalent into midday across the warm sector. However,
some cloud breaks are noted near/just east of the MS River with
mid/upper 50s and some lower 60s F surface dewpoints otherwise
expected to expand northward in tandem with the warm front.

As gradual air mass destabilization occurs, storms may first
intensify across far eastern MO into southern IL just ahead of the
surface low and in concert with the aforementioned cloud breaks at
late morning. Additional warm sector or near-warm-frontal-related
development should also occur this afternoon into IN/OH this
afternoon, while other storms should form near the cold front across
KY/TN. While buoyancy will not be robust in most areas, long/largely
unidirectional hodographs, albeit with some near-1km AGL hodograph
curvature, will support a mixed mode including supercells and fast
northeastward-moving bows. Severe hail and damaging winds will be
possible, with a least some tornado risk as well, particularly near
the warm front where low-level SRH will be maximized.

...Gulf Coast States/lower MS River Valley...
12Z Upper-air data and contemporary surface observations/visible
satellite imagery show that considerable convective overturning has
occurred across parts of the region via an overnight/early morning
MCS. Ahead of a slow-moving convective cluster across southeast MS
and coastal eastern LA at late morning, the main severe risk should
be confined to far southeast MS into southern AL/FL panhandle and
eventually southwest GA. For additional short-term details, see
Mesoscale Discussion 377.

Farther west, in the wake of the early-morning MCS, some additional
development may occur near the north/south-oriented cold front this
afternoon. However, persistent cloud cover, modest
moisture/buoyancy, and limited near-frontal convergence all imply
that any near-frontal severe risk later this afternoon/evening
should remain limited across the lower MS Valley vicinity.

...Great Basin...
A band of showers and isolated storms is expected to form along a
strong cold front attendant to an amplifying shortwave trough.
Although buoyancy will be scant, well-mixed thermodynamic profiles
ahead of the front should support downward momentum transport of
robust flow aloft and may yield isolated severe wind gusts.

$$



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