Day 1 Severe Weather Outlook

Valid 311630Z – 011200Z


An extensive squall line of strong to severe thunderstorms is
expected to develop during the late afternoon into this evening,
near and east of the Appalachians from parts of New York to the
Carolinas. At least a few tornadoes and scattered to widespread
damaging winds are possible.

…Northeast to the Carolinas…
Have upgraded peak probabilities for both wind and tornado hazards
with an active period of severe weather anticipated later this
afternoon into the evening.

An amplifying shortwave trough now over IL will accelerate
east-northeast toward the Saint Lawrence Valley through tonight
toward PA/NY, while taking on a more neutral-to-negative tilt with
peak 500 mb wind speeds exceeding 120 kt. Strong surface
cyclogenesis is expected in advance of the amplifying trough, with a
cyclone moving from OH to far southern QC by early Friday. South of
the deepening cyclone, a strong cold front will sweep eastward
across the Appalachians this afternoon, reaching the Atlantic coast
overnight. The cold front will be the primary focus for severe
thunderstorms through this evening.

Northward advection of low-level moisture will continue with mid 60s
boundary-layer dew points up the OH Valley west of the Appalachians
and across southeast PA to coastal southern New England. Poor low to
mid-level lapse rates amid widespread clouds will hinder
destabilization with northern extent in the Northeast. Richer
low-level moisture (upper 60s to low 70s boundary-layer dew points)
and some surface heating in cloud breaks will support a confined
plume of MLCAPE between 750-1500 J/kg across parts of VA into the
Carolinas at peak heating.

Within the warm sector, vertical shear will increase in response to
the approach and deepening of the mid-level trough/surface cyclone.
Long hodographs with effective bulk shear of 50-60 kt are expected,
and there will also be enlarged low-level hodograph curvature for
numerous rotating storms amid effective SRH of 300-500 m2/s2.

Strong linear forcing for ascent along the cold front will be the
primary focus for storm development, mainly east of the Appalachians
later this afternoon/evening. The strong linear ascent will favor
upscale growth into a squall line, though strong deep-layer shear
and enough east component to storm motions should allow embedded
supercell structures to persist. The modest buoyancy and very strong
850-700 mb flow will favor damaging winds as the primary threat.
These could be potentially widespread within longer-tracked bowing
segments. At least a few tornadoes are also anticipated with
embedded supercell and mesovortex structures, particularly from
central NC to central MD.

The severe threat will diminish during the late evening to overnight
given an increasing dearth of surface-based instability with
east/northeast extent.

..Grams/Wendt.. 10/31/2019


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