SPC AC 251716
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1216 PM CDT Tue Sep 25 2018
Valid 261200Z - 271200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
APPALACHIANS NORTHWARD INTO NEW ENGLAND...
Scattered thunderstorms associated with a wind damage threat are
expected to develop on Wednesday from the central Appalachian
mountains to the Northeast. A tornado threat will also be possible
mainly from eastern Pennsylvania northeastward into New England.
A longwave trough centered over Hudson Bay will result in fast,
broadly cyclonic flow aloft across much of the Continental U.S.
throughout the forecast period. One significant shortwave will
migrate through the trough across Ontario/Quebec during the
afternoon, while another series of troughs will migrate
southeastward across Oklahoma and New Mexico. At the surface, a low
centered over southwestern Quebec will continue to strengthen while
migrating northeastward. A trailing cold front will sweep eastward
throughout the forecast period and extend from central New York
state southwestward to the Mid-South and central Texas by 00Z.
Ahead of this front, a warm, humid airmass (characterized by upper
60s and 70s F dewpoints) will persist, supporting scattered
thunderstorms across much of the southern and eastern U.S.
...Portions of the Appalachians northward to the Northeast...
Scattered storms may be ongoing along and ahead of the advancing
front across portions of Ohio and Kentucky at around 12Z. As
surface heating commences ahead of the approaching front, models
suggest that one or two bands of storms will organize and migrate
northeastward through the discussion area within an environment
characterized by weak to moderate surface-based instability
(1000-1500 J/kg MUCAPE) and very strong shear (owing to 50-70 kt
mid-level flow - strongest over portions of New York and northern
New England). Shear/instability parameters suggest a damaging wind
threat with bands of storms migrating quickly northeast.
Supercellular structures may also develop and result in a few
tornadoes - especially if isolated cells can materialize ahead of
any lines (as hinted at in a few of the convection-allowing models).
Instability will be a limiting factor for a higher severe-weather
threat, although portions of region (particularly eastern New York
and Vermont) may need to be upgraded in later outlooks.
The threat should shift eastward into New England and the
mid-Atlantic Coast through the evening, with any lingering severe
threat diminishing with time due to nocturnal boundary-layer
...Tennessee and Alabama...
Guidance suggests that a few forward-propagating bands of storms may
develop during peak heating, although profiles suggest a more
limited severe threat due to areas farther north given weaker shear.
Isolated wind gusts should be the primary threat with diurnally
driven convection in this area.
Forcing aloft associated with a mid-level shortwave trough
approaching the area will foster development of a few storms during
the afternoon and early evening, with small hail and perhaps
isolated damaging wind gusts beneath any storms that can develop.
This threat should be too sparse for even low probabilities,
however, given weak instability profiles.
...MAXIMUM RISK BY HAZARD...
Tornado: 5% - Slight
Wind: 15% - Slight
Hail: 5% - Marginal
NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 0600Z