SPC 1200Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1241 AM CDT Wed Sep 15 2021

Valid 151200Z - 161200Z


Severe thunderstorms capable of damaging winds, hail and an isolated
tornado threat will be possible today from the central Appalachians
into New England. A marginal tornado threat will also exist across
parts of the central Gulf Coast.

...Central Appalachians/New England...
A shortwave trough will move eastward across the lower Great Lakes
and Ohio Valley today. At the surface, a cold front will advance
southeastward into New England and the central Appalachians. Ahead
of the front, surface dewpoints in the 60s F will contribute to
moderate destabilization by afternoon across parts of central and
eastern Pennsylvania, southeast New York and New England. The NAM
and RAP suggest that MLCAPE will peak in the 2000 to 2500 J/kg range
along parts of this corridor. Thunderstorms that develop near and
ahead of the front will move east-northeastward across the western
edge of the stronger instability during the mid to late afternoon.
The magnitude of instability should allow the convection to persist
into the early evening.

Forecast soundings along the axis of strongest instability by 21Z
have supercell wind profiles, with 0-6 km shear in the 35 to 40 kt
range and veering winds with height in the lowest 3 km. Supercells
will be capable of producing wind damage and isolated large hail.
However, most of the convection will likely remain linear suggesting
that wind-damage will be the greater of the two threats. As
low-level winds strengthen during the late afternoon, the most
favorable low-level shear for tornadoes will be located from far
eastern New York into central New England. The tornado threat
appears conditional upon whether any cells can remain discrete late
in the event.

Further southwest into the central Appalachians, a marginal severe
threat is expected to develop this afternoon. However, deep-layer
shear will be considerably less keeping any potential for
wind-damage much more isolated.

...Central Gulf Coast...
The remnants of Nicholas are forecast to continue moving slowly
eastward across the central Gulf Coast today. RAP forecast soundings
around midday show the strongest low-level shear from southeast
Louisiana into far southern Alabama, where 0-1 km shear could be in
the 15 to 20 kt range. This combined with surface heating and
moderate destabilization could be enough for a marginal tornado
threat with cells that remain semi-discrete.

..Broyles/Bentley.. 09/15/2021

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