Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 300600
SWODY1
SPC AC 300558

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1258 AM CDT Tue May 30 2017

Valid 301200Z - 311200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
CENTRAL NEW YORK AND PENNSYLVANIA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
NORTHEAST TO THE CAROLINAS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
LOUISIANA COAST...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
PLAINS TO THE OZARKS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
PACIFIC NORTHWEST...

...SUMMARY...
Severe thunderstorms, capable of large hail, damaging winds, and
perhaps a tornado, will be possible from the lower Great Lakes to
North Carolina today. Elsewhere, a few strong/severe storms may
occur across the central Plains/Ozarks, central Gulf Coast, and
Pacific Northwest.

...Synopsis...
Broad cyclonic mid-level flow, with a corridor of 60-kt westerlies
along its southern periphery, will persist across much of the
eastern U.S. today. Within this flow regime, a series of impulses
will translate from the Canadian Prairies southeastward to the Ohio
Valley, and then northeastward across the St. Lawrence Valley.
Farther south, weak impulses embedded within the sub-tropical jet
will translate from northern Mexico to the Gulf Coast and
Mid-Atlantic. Across the western U.S., an amplified ridge will
slowly progress east across the Great Basin and northern Rockies, as
a shearing impulse approaches the Pacific Northwest.

...Northeast to the Mid-Atlantic...
Along the southeastern fringe of the aforementioned cyclonic flow,
relatively robust mid-level southwesterlies (upwards of 60 kt) will
overlie the region. Moreover, an impulse, currently observed in
water-vapor imagery over the mid Missouri Valley, will approach the
lower Great Lakes during the late morning hours. As it does so, a
surface front will reach portions of western New York and
Pennsylvania. Guidance suggests a corridor of modest heating ahead
of the boundary will combine with surface dew points in the
mid/upper 50s to lower 60s to foster MLCAPE values upwards of 1000
J/kg. In turn, storms will gradually organize along/ahead of the
boundary during the late morning and afternoon hours. Relatively
straight hodographs suggest splitting cells and bowing segments will
offer a threat of damaging winds and a few instances of large hail.
Additionally, modestly backed surface flow ahead of the trough may
provide enough storm-relative helicity for a tornado or two, most
likely across parts of Pennsylvania.

Farther south across the Mid-Atlantic and Carolinas, while mid-level
flow will be slightly weaker, effective shear will remain adequate
for occasional updraft rotation in stronger cores. This kinematic
environment, combined with diurnal heating and surface dew points in
the 60s, will support isolated to widely scattered strong/severe
storms capable of large hail and damaging winds during the afternoon
and evening.

...Central Plains to the Ozarks...
Despite marginal boundary-layer moisture (e.g., mean mixing ratios
around 10 g/kg or less), diurnal heating will foster pockets of
MLCAPE around 500-1500 J/kg. The glancing influence of weak impulses
in northwesterly flow (associated with the broad eastern trough), as
well as subtle features emanating from the southern High Plains
overnight, will likely be adequate for isolated/widely scattered
convective initiation by afternoon. Straight hodographs, ample
convective-layer flow, and relatively dry low levels will support
splitting/bowing storms capable of strong/gusty winds.

...Southern Louisiana...
Thunderstorms will likely be ongoing near the coast at the beginning
of the period, aided by warm advection related to a southerly
low-level jet. Convection will likely persist through much of the
day, as the low-level jet slowly shifts east along the coast before
eventually weakening/veering during the evening. While weak
deep-layer flow will keep updrafts disorganized, dry mid-level air
overlying moist low levels may be sufficient for localized strong
wind gusts.

...Pacific Northwest...
As a mid-level ridge departs to the east, cooling temperatures aloft
will allow convection to gradually organize across central
Oregon/Washington during the afternoon. These storms will then
advance north/northeast through the late afternoon and evening, as
southerly mid-level flow increases to around 40-50 kt. This robust
southerly flow will combine with a deep/dry sub-cloud layer to
promote a threat of strong, gusty winds with the strongest cells.

..Picca/Dean.. 05/30/2017

$$



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