Severe Weather Outlook, Day 2

284 
ACUS02 KWNS 301736
SWODY2
SPC AC 301735
Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1235 PM CDT Sun Apr 30 2017
Valid 011200Z - 021200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR PARTS OF
WESTERN NEW YORK AND CENTRAL PENNSYLVANIA...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
THE LOWER GREAT LAKES SOUTHWARD INTO EASTERN WEST VIRGINIA AND
VIRGINIA...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AND INCLUDING PARTS OF THE CAROLINAS INTO SOUTHERN
GEORGIA...
...SUMMARY...
Scattered to numerous severe thunderstorms will be possible from
portions of western New York southward into parts of West Virginia
and Virginia.  Isolated strong to severe thunderstorms may occur in
the Carolinas and Georgia.
...Synopsis...
A closed 500-mb low is forecast to move from the middle MS Valley
into the northern Great Lakes during the period.  Midday Sunday
water-vapor imagery showed a well-defined mid-level vorticity lobe
over OK/TX and this feature is forecast to shear northeastward and
reach the upper OH Valley by midday Monday.  An occluded low over
northeast IA will develop northeast through northern Michigan.  A
cold front will extend from the OH Valley through the southern
Appalachians and northeast Gulf Coast Monday morning and sweep
eastward and into the western Atlantic from the Mid-Atlantic states
south into northern FL.
...Much of western and central NY/PA southward into VA/eastern WV...
The latest model guidance continues to indicate most of the region
is expected to remain free of precipitation through the early
afternoon.  A band of convection may be ongoing along the front
Monday morning but modest mid-level lapse rates coupled with a
destabilizing boundary layer (i.e., surface heating and dew points
in the lower 60s) is forecast to result in weak buoyancy ahead of a
potent mid- to upper-level trough/cold front by early afternoon. 
Despite this somewhat marginal thermodynamic environment, intense
low/mid-level wind fields veering with height will result in
enlarged low-level hodographs yielding 300-500 m2/s2 0-1 km SRH. 
The overall severe risk will be somewhat dependent on the magnitude
of buoyancy (MLCAPE most probable ranging from 200-800
J/kg)---yielding highly nonlinear effects on updraft strength and
convective mode.  A cellular mode will be favored by the wind
profile but the strength of mid-upper forcing for ascent (DCVA and
associated 60-120 m 12 hr height falls) and initial storm
development probably on the front casts considerable uncertainty
whether cellular storms over the Enhanced Risk will form.  The most
likely scenario is a mix mode of a few cells/linear segments with a
transition to primarily linear bands of storms evolving towards
evening.  If the boundary layer can destabilize more than currently
projected, in turn promoting the development of a few pre-line
supercells, a greater tornado risk may develop.  Nonetheless,
convection will likely organize into small bands/bowing structures
and it appears a corridor of higher potential for 50-70 mph gusts
and associated wind damage will occur during the mature phase of
broken linear segments across PA/NY as 50-60 kt 700-mb flow
overspreads the warm sector.  Farther south over MD/VA/WV, slightly
richer low-level moisture with dewpoints in the middle 60s is
forecast.  A strong wind profile will support storm organization
with the stronger updrafts and scattered damaging winds, marginally
severe hail, and a tornado are possible with this activity before
weakening during the evening hours.
...Carolinas and southern GA...
While mid-level flow and forcing for ascent will be weaker farther
south, richer low-level moisture (surface dew points in the
mid/upper 60s) will yield more substantive MLCAPE (upwards of 1000
J/kg) by afternoon/early evening. Convection will likely intensify
within pockets of heating amidst remnant cloud cover from overnight
convection to the west. As it does so, effective shear of 30-40 kt
will likely yield at least a few stronger storms, capable of
isolated severe hail and strong/damaging gusts during the afternoon
and evening hours.
..Smith.. 04/30/2017
$$