Day 1 Severe Weather Outlook

Valid 261300Z – 271200Z


Damaging to marginally severe thunderstorm gusts are possible this
afternoon, over portions of southern New England and Long Island.

The mid/upper-level pattern will remain active through the period,
especially in the northern branch of westerlies. A slow-moving,
synoptic-scale cyclone will meander around western Hudson Bay, while
a strong 250-300-mb jet core extends from the Gulf of AK across BC,
southwestern Canada, the northern Plains, and the Great Lakes. An
embedded/strong shortwave trough — evident in moisture-channel
imagery across portions of Lake Huron, Lower MI and Lake Michigan —
will move eastward today and maintain amplitude, while its positive
tilt decreases somewhat. By 00Z the trough should reach eastern NY
and NJ, perhaps western New England. The trough should exit eastern
ME shortly after 06Z.

At 11Z, surface analysis showed a cold front from Lake Ontario
southwestward over southwestern OH, extreme southern IL, to extreme
northern AR and north-central OK, where its position is ambiguated
by outflow, then across the TX Panhandle and northeastern NM. By
00Z, the front should move to a position near the NH/ME line,
eastern MA or Cape Cod, southeastern VA, northern AL, and southern
AR. The front should become still more poorly defined over OK,
amidst return flow occurring in advance of the Western height falls.
The return flow will be part of a broad warm-advection regime, with
elevated moisture transport and a strong low-level jet supporting
thunderstorms the last few hours of the period, over parts of the
mid/upper Mississippi Valley region. Small hail is possible there.

…Southern New England, Long Island…
A strongly forced band of low-topped convection, with isolated to
widely scattered embedded lightning, should develop very near the
cold front this afternoon. Activity should move rapidly eastward
across the outlook area, offering locally damaging wind, with
isolated gusts near severe (50 kt) criteria possible.

A lack of cloud cover evident in satellite imagery is expected to
persist through much of the day, indicates that surface temperatures
should rise into the mid/upper 70s F. Meanwhile moist advection
should offset boundary-layer mixing enough to support an increase of
dew points into the 60s over a greater area than currently analyzed.
This will offset weak lapse rates above the boundary layer enough to
yield 100-500 J/kg prefrontal MLCAPE — locally/briefly higher. A
narrow corridor of preconvective overlap is possible between
surface-based buoyancy and strengthening of both deep-layer forcing
and midlevel winds, in support of organized, marginally severe
downdrafts in the convective band. This convection should sweep
eastward through the outlook area by around 00Z.

A cut-off mid/upper low — now located near YUM, is expected to
meander around the lower Colorado River Valley for most of the day,
while continuing to fill gradually. Tonight, it should devolve into
an open wave. As a series of shortwaves cross the northwestern
CONUS and heights fall from there across much of the Great Basin,
the low/trough over AZ will move eastward, gradually rejoining the
westerlies. A few strong thunderstorms may accompany the cyclone
through this afternoon, but given weakening of associated mass
fields with time, and only modest buoyancy, severe potential is too
isolated and conditional for a categorical outlook.

..Edwards/Leitman.. 09/26/2019


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