SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 AM CDT Fri Aug 20 2021
Valid 201300Z - 211200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
UPPER MIDWEST TO LOWER MISSOURI VALLEY...
...SUMMARY...
Damaging winds, large hail and a few tornadoes are possible with
episodes of thunderstorms today and tonight, from parts of the Upper
Midwest to lower Missouri Valley.
...Synopsis...
The CONUS portion of the large-scale mid/upper-level pattern will
feature a mean trough in the West, with two strong shortwaves (one
exiting, one entering):
1. The initial shortwave trough -- evident in moisture-channel
imagery from central MT to western CO -- is forecast to eject
northeastward across the central Rockies, central High Plains and
northern Plains through 12Z tomorrow. Though deamplifying, this
will remain a strong perturbation for the rest of this period. By
00Z the trough should extend from southeastern SK across the
western/central Dakotas to the NE Panhandle. By 12Z, its axis
should lie near a line from Winnipeg southward along the Red River
of the North to near FSD.
2. An upstream trough -- initially positioned over coastal BC
between the AK Panhandle and Vancouver Island -- will dig
southeastward, forming a closed 500-mb low over the interior Pacific
Northwest by the end of the period.
To the east, a broad area of modest cyclonic flow is apparent around
a trough located over extreme southern QC, northern NY, and
southwestward through the central/southern Appalachians. A closed,
cut-off low is progged to develop tonight over WV, while ridging
builds to its north from Hudson Bay across eastern ON. The
associated cold-core cyclone and trough will influence motion and
evolution of Tropical Storm Henri over the Atlantic, per latest NHC
discussions, but with no associated threat for organized severe
storms through day-1.
At the surface, 11Z analysis showed an area of low pressure
elongated on a frontal zone from southeastern MB to central/
northeastern SD, with a cold front southwestward across southwestern
NE. The low is expected to consolidate over central ND, then move
northeastward along the frontal zone, across the northwestern corner
of MN and over adjoining southeastern MB this evening. By 00Z the
cold front should extend from that low across western MN,
northwestern IA, southeastern NE, and south-central KS, to a low
near the Raton Mesa and CO/NM border. By 12Z, the front should
reach the MN Arrowhead, western WI, northeastern MO, southeastern
KS, to the trailing low over the northern TX Panhandle.
...Upper Midwest to lower Missouri Valley...
Scattered thunderstorms should develop today and tonight, at first
in a blend of discrete and banded modes, then evolving into one or
two dominant quasi-linear swaths of convection with southward growth
through the evening. As this activity moves eastward to
northeastward across the outlook area, all severe modes (damaging
gusts, large hail, and a few tornadoes) are possible.
Animated satellite and radar imagery have shown good continuity with
a field of large-scale lift and associated precip just ahead of the
southern portion of the shortwave trough. This UVV plume is
apparent from northeastern CO across the NE Panhandle to
southeastern SD, and should shift northeastward in step with the
motion of the parent perturbation. A relatively maximized zone of
severe-wind threat may develop across parts of eastern SD,
southeastern ND and western MN, as the zone of relatively maximized
large-scale ascent approaches the front, and in combination,
impinges on a warm sector that could destabilize favorably from both
theta-e advection and diurnal heating. Prefrontal surface dewpoints
in the upper 60s to lower 70s F remain common, except over central
NE, where a now-decayed MCS has left a mesoscale, low-level theta-e
deficit that should advect/deform northward and become narrower in
zonal extent.
Given that antecedent boundary-layer disruption, enough uncertainty
lingers to preclude an unconditional "enhanced"-level/30% upgrade at
this time, related to:
1. How much of both heating and warm/moist advection will occur,
and how fast, behind the morning clouds/precip, and
2. Support for robust cold-pool organization on the mesoscale,
given a very deep layer of southerlies from just above the surface
through upper levels. The southerlies will contain favorable bulk
shear, but also, will be aligned strongly parallel to the quasi-
linear corridor of convective forcing.
Mesoscale trends and shorter-fused progs will be assessed for more
confidence in such a wind area, if needed, during the day.
Regardless, the axis of greatest wind potential appears displaced
eastward from that of hail. The latter should be maximized earlier
in the convective cycle, when the potential is greater for discrete
to semi-discrete supercells. As such, within the broader
categorical outlook, these probabilities have been refined
accordingly. Low-level shear will be favorable for maintaining a
swath of 150-250 J/kg effective, inflow-layer SRH during both
phases, increasing some after dark as the LLJ increases (but QLCS
mode also becomes more dominant). This will support potential for a
few tornadoes throughout the afternoon and evening, including as
trailing convection develops near the front and builds southward
into the western IA/eastern NE/northeastern KS/northwestern MO area.
A more-pristine boundary layer will exist over those areas, with
rich low-level moisture and still-favorable frontal lift, but also,
displacement from stronger mid/upper forcing and the gradual
nocturnal surface cooling as counterbalancing influences.
..Edwards/Gleason.. 08/20/2021
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