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ACUS01 KWNS 301255
SWODY1
SPC AC 301254

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0754 AM CDT Tue Jul 30 2019

Valid 301300Z - 311200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY OVER PARTS
OF THE NORTHEAST...NORTHERN GREAT PLAINS...AND CENTRAL/SOUTHERN
ARIZONA...

...SUMMARY...
Marginally and/or isolated severe thunderstorms are possible today
over parts of the Northeast, northern Great Plains, and
central/southern Arizona.

...Synopsis...
In mid/upper levels, the synoptic pattern will continue to be
dominated by a quasistationary single wave over the CONUS, whose
troughing will extend over the Great Lakes, Ohio Valley and
southward to the central Gulf.  As a cyclone moves northeastward
across the James Bay region and deepens, the synoptic trough to its
south/southwest will drift eastward toward the Appalachians through
the period, with a chain of embedded vorticity maxima near or just
ahead of the height axis.

Upstream, the ridging portion of the wave will extend from an upper
high anchored over the southern Rockies northward across the
northern High Plains.  Weak, convectively aided/induced vorticity
maxima are evident over the northern Rockies of ID/MT, and are
expected to move generally eastward across MT and the Dakotas today
amidst amplifying larger-scale ridging aloft.  Around the
southwestern rim of the anticyclone, similarly low-amplitude/
convectively modulated perturbations are apparent in satellite and
composited radar imagery over portions of AZ (contributing to
ongoing clouds/precip there) and southward across Sonora to the
eastern Gulf of California.

At the surface, 11Z analysis showed a cold front extending from the
area of its related James Bay mid/upper cyclone southward over
northeastern ON to Lake Huron, then southwestward across
northwestern IN, southern IL, and southeastern MO, becoming
quasistationary from the western MO/AR Ozarks across northern OK.
The western part of this boundary should become diffuse today, while
the remainder moves slowly southeastward across the lower Great
Lakes, OH, KY, western TN and northern/central AR.  A prefrontal
surface trough was drawn across central/northern NY and east-central
PA into northern VA, and should remain near that corridor through
the day.  A wavy lee trough, with several weak surface lows
attached, was drawn from central/southeastern MT southward across
eastern portion of WY/CO.  The MT part of the trough should realign
more meridionally today over east-central MT.

...Northeast...
Widely scattered to scattered thunderstorms are expected to develop
from late morning through afternoon, both near a surface trough
related to the mid/upper perturbation, and in associated with
prefrontal lake-breeze boundaries near the southern shores of the
lower Great Lakes.  The most intense cells may produce damaging
wind, and isolated severe gusts cannot be ruled out as well.
Favorable moisture is forecast region-wide, with dew points
generally in the 60s F, and corridors of sustained heating to weaken
MLCINH favorably, between areas of lingering cloud cover.  This
should offset modest midlevel lapse rates enough to yield peak
preconvective MLCAPE in the 1000-1500 J/kg range.

More densely concentrated convection my occur sooner near the lake
breezes, in a regime of slightly stronger winds aloft and deep-layer
forcing for ascent than farther southeast near the trough.
Adjusting to this regime is the main change this outlook cycle.
Upper-level/ventilating winds will be stronger over this area, with
some organized multicellular structures possible, and brief/messy
and conditional supercell potential cannot be ruled out where
vorticity enhancement from the lake boundaries can get involved.
However, vertical shear still appears modest, especially in low
levels.  Meanwhile, low-level lapse rates and DCAPE will be greater
farther southeast near the trough over central portions of NY/PA,
supporting gust potential, but with even weaker large-scale lift and
deep-layer flow.

...Northern Great Plains...
Isolated to scattered thunderstorms should develop this afternoon
near the surface trough, in a narrow corridor of favorably
juxtaposed boundary-layer moisture and strong surface heating from
eastern MT across portions of eastern WY to the Black Hills region
and northwestern NE.  Isolated severe hail and gusts are possible.
If confidence increases in a relative concentration of convection
within the broader marginal-risk area, perhaps over parts of eastern
MT once 12Z RAOB-assimilating guidance is processed, a subset of the
area may be upgraded to a 15%/slight outlook.  However, at this
time, mixed prognostic indicators of convective coverage/longevity,
and the well-advertised presence of an amplifying upper ridge,
preclude such an upgrade.

In the aforementioned near-trough corridor, surface dew points in
the 50s to low 60s F are expected, amidst a somewhat offsetting
combination of moist advection and strong vertical mixing.  Forecast
soundings accordingly assume steep low/middle-level lapse rates
(e.g., nearly dry adiabatic in the boundary layer and 7.5-8 deg C/km
from 850-500 mb).  This yields MLCAPE 1500-2000 J/kg atop a well-
mixed subcloud layer, and generally high cloud bases with LCLs in
the 1800-2500 m AGL range.  Long, nearly straight hodographs with
small 0-1-km positive SRH are anticipated, amidst 50-55-kt
effective-shear magnitude, suggesting a supportive kinematic
environment for both splitting supercells and organized multicells.
In the absence of more-robust forcing aloft, the severe potential
should abate over most of the area within the first few hours after
sundown, as the boundary layer progressively cools/stabilizes.

...Central/southern AZ...
Scattered thunderstorms are expected to develop this afternoon
across much of the higher terrain of central/southern AZ, with more
potential for activity to move out across well-heated/mixed desert
boundary layers over southern AZ.  Isolated severe gusts will be the
main concern.  A limiting factor for much of the day, and source of
uncertainty on overall timing/coverage, may be the ongoing
clouds/precip over portions of the region. However, satellite and
radar imagery show that activity to be decreasing in coverage,
indicating a delay but not prevention of favorable destabilization.
As such, sufficient heating time should occur, amidst increased
boundary-layer moisture, to permit favorable buoyancy and negligible
MLCINH over both mountains and deserts this afternoon.  This,
combined with a relatively deep layer of substantial easterly flow
component in midlevels southwest of the inveterate upper high, will
support discrete/outflow-driven westward propagation across parts of
the desert.  Well-mixed boundary layers, beneath 500-1500 J/kg
preconvective MLCAPE, will foster the gust potential late this
afternoon into early evening..

..Edwards/Dial.. 07/30/2019

$$


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