SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0749 AM CDT Mon Aug 23 2021
Valid 231300Z - 241200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE CENTRAL AND NORTHERN PLAINS TO MID/UPPER MISSISSIPPI VALLEY AND
OVER PARTS OF SOUTHERN NEW ENGLAND...
...SUMMARY...
Isolated severe thunderstorms are possible across parts of the
central and northern Plains to mid/upper Mississippi Valley through
tonight. A marginal tornado threat may develop today over parts of
southern New England.
...Synopsis...
In mid/upper levels, the expansive area of ridging and related
anticyclonic flow will persist from northwestern MS through a 500-mb
high over the southern Plains, to much of the Southeast. To its
northwest, a weak mean trough will linger over the West Coast. On
the north end of that trough, a zonally elongated cyclone is
apparent in moisture-channel imagery over southern BC. This cyclone
is forecast to become a strong, positively tilted shortwave trough
(with shrinking embedded closed circulation) as it moves eastward
across the Canadian Rockies today into this evening. By 12Z
tomorrow, the trough should extend from western SK across the
southeast corner of AB, northwestern MT and northern/central ID. A
precursory perturbation now over the MB/ON border area will continue
weakening and ejecting away from the northern Plains/Upper Midwest
region.
Farther east, in the synoptic scale height fields, the mid/upper-
level remains of T.D. Henri and a previously cut-off cold-core low
have merged over the northeastern Mid-Atlantic/southwestern New
England region, though they still appear distinct thermally on the
mesoscale. These features will continue to blend through the
period, as the primary 500-mb cyclone devolves into an open-wave
shortwave trough and shifts east-northeastward over New England.
Elsewhere at the surface, 11Z analysis showed a wavy, mostly
quasistationary frontal zone from south-central QC across Lake
Ontario and western NY, southern Lower MI and IA, to a low over
central SD. From there, one branch of the front extended west-
northwestward to another low over north-central MT, while another
branch extended across parts of central NE to northwestern KS/
eastern CO. The front should become sharper today farther north
across southern MN to southeastern SD, with outflow boundaries to
its south related to ongoing convection in IA.
...Central/northern Plains...
Isolated to widely scattered thunderstorms with at least marginal
unconditional severe potential are possible in four somewhat-
overlapping regimes within the broader marginal area:
1. The outflow-modulated warm-frontal zone and adjoining warm
sector, across portions of MN/IA to the mid/upper Mississippi
Valley. This may include any of the ongoing convection across
eastern IA that can persist into a favorably heating/destabilizing
low-level inflow regime in the next several hours, and/or
development along related warm-sector boundary(s).
2. Potential initiation along and ahead of the north-south High
Plains frontal segment over NE, which should act effectively as a
dryline this afternoon, with strong heating on both sides and mixing
to its west. Capping will be a limiting factor and strength of lift
is in question. However, in both area 1 and this regime, low-level
moisture will be rich, beneath steep midlevel lapse rates, yielding
MLCAPE commonly in the 2500-3500 J/kg range, albeit with modest deep
shear (25-35 kt effective-shear vectors).
3. Convection forming along the higher terrain of eastern WY and
southwestern SD and moving northeastward, but into limited low-level
theta-e. Some such activity over southeastern WY and western NE may
survive its passage across relatively dry, deeply mixed boundary
layers (in which strong/locally severe gusts are possible briefly)
and reach the moist sector around the Sandhills.
4. Thunderstorms forming overnight in an elevated zone of low-level
warm/moist advection and moisture transport, north of the frontal
zone over the Dakotas, offering episodic and at least isolated hail
near severe limits. Large lapse rates will contribute to MUCAPE
2000-3000 J/kg in parts of this area, with effective-shear
magnitudes potentially reaching 45-55 kt. Greater confidence in a
concentration of hail potential in this regime may warrant an
unconditional upgrade in a subsequent outlook.
Forcing for convection across this entire region still appears
rather nebulous overall, as weak height rises occur amidst a broad
fetch of southwest flow, between the Canadian Rockies and MB/ON
shortwave troughs. Lift along baroclinic boundaries of outflow/
frontal origin may be sufficient to break the cap locally, outside
of any ongoing convection that can last into the more-favorable
period of diurnal boundary-layer destabilization. The extent/
strength of such frontal/outflow lift with respect to initiating and
maintaining convection remains quite uncertain at this outlook
cycle, given its innate small-scale variability and the
aforementioned dearth of upper-air support. As such, a large
unconditional "marginal risk" area is maintained this cycle, and
mesoscale trends will be monitored for any denser, better-focused,
more consistently progged convective threats within.
...Southern New England...
A marginal and conditional tornado threat is apparent in the
middle-outer eastern semicircle of the remnants of T.D. Henri,
mainly this afternoon into early evening.
Prior/00Z 500-mb analysis showed the surface cyclone located between
a remnant warm pocket (-3 to -4 C) to its northeast over northern
New England, and the old midlatitude low's cold core (-8 to -10 C)
to its southwest over the Chesapeake Bay/Delmarva region -- but
slightly closer to the warm core. Such would remain the case for
any convection that can develop in the eastern side of the remnant
low-level circulation today, given model analyses and 12Z RAOBs from
ALB/OKX/GYX -- but not prohibitively warm aloft as was true for much
of this sector the day before. Cloud breaks already apparent in
visible satellite imagery, and that may develop further through the
day, should provide enough boundary-layer heating to boost MLCAPE
into the 300-800 J/kg range. Forecast hodographs indicate a brief
window (a couple hours or so) at any given locale where 0-1-km SRH
may reach the 150-200 J/kg range, which is on the lower side for TC
tornadoes, but still within the distribution.
Confidence is not strong in getting sustained favorable
juxtaposition of the areas of greatest surface heating, steepening
midlevel lapse rates and favorable boundary-layer hodographs.
Still, the marginal unconditional outlook area represents where such
overlap may occur, rendering a favorable supercell environment for
any mature convection that can move through.
..Edwards/Gleason.. 08/23/2021
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