SPC Jul 26, 2017 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0755 AM CDT Wed Jul 26 2017

Valid 261300Z - 271200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
NORTHEASTERN KS TO SOUTHWESTERN WISCONSIN AND NORTHERN ILLINOIS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
CENTRAL KANSAS TO WESTERN INDIANA AND SOUTHERN/EASTERN WISCONSIN...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE WESTERN GREAT BASIN...

...SUMMARY...
Scattered thunderstorms, some with locally strong to severe wind
gusts, are expected over portions of the Midwest and mid/upper
Mississippi Valley regions this afternoon and evening.  Storms may
also produce a few locally strong to damaging wind gusts over parts
of the western Great Basin.

...Synopsis...
The mid/upper level pattern will be categorized generally by:
*  A zonally elongated anticyclone shifting westward from the
southern plans across the southern Rockies; 
*  Height falls over much of the eastern U.S., as a series of
shortwaves traverse a broad zone of troughing and associated
cyclonic flow;
*  A series of mostly low-amplitude shortwaves moving through a mean
northern-stream ridge over the northern Rockies/northern High Plains
region, and through downstream northwest flow aloft. 

The well-defined low aloft -- currently evident in moisture-channel
imagery over northwestern CA, has begun to weaken and move
east-northeastward in response to height falls over the northeastern
Pacific.  This perturbation should devolve to an open wave through
the remainder of the morning, continue gradual weakening, and slowly
eject northeastward across northeastern CA and northwestern NV
through the remainder of the period.  Meanwhile, a shortwave trough
with convectively induced/enhanced vorticity maxima -- now located
over the central Plains -- will move to near a DBQ-CNU axis by 00Z,
then to an axis from southern IN to southeastern/south-central MO by
12Z. 

At the surface, a cold front was analyzed from a cyclone over far
northern ON southwestward across the MN Arrowhead, southwestern MN,
to central NE, through a weak frontal-wave low over the HLC-MCK
region, to southeastern CO.  Outflow boundaries preceded this front
from IA to central KS.  The front should shift eastward to eastern
Lake Superior, southeastern IA, through a low over the TOP/MKC
region, to the OK Panhandle by 00z, again preceded by outflows.  By
12Z, the front should reach Lake Huron, northern/western IL, central
MO, southern KS, and the TX Panhandle. 

...Midwest...
Ongoing, initially non-severe convection is evident in an extensive
complex from western IA south-southwestward across extreme
southeastern NE to portions of central/northeastern KS.  A separate
area of elevated thunderstorms is evident within the rim of a
warm-advection plume over parts of WI.  These morning convective
clusters are expected to move eastward with general weakening
through midday.  However, some redevelopment or re-intensification
of embedded elements is possible to the east of their current
positions from midday into afternoon as the associated plume of UVV
impinges on a boundary layer slowly destabilizing from a combination
of WAA and cloud-muted diabatic heating.  Additional convection may
develop over western parts of the outlook area near the front, in
the wake of the morning convection/precip, yielding a conditional
wind/hail threat.  A combination of the two potential regimes, with
considerable spatial overlap, yields the outlook area as a whole.

Uncertainties exist regarding the duration and magnitude of
supportive destabilization (especially with northward extent) behind
the morning activity, whose pace of weakening and of eastward
movement will be important.  Where sustained surface heating can
occur, forecast soundings suggest 1500-2000 J/kg MLCAPE is possible,
locally near 3000 J/kg where deep-layer flow aloft will be somewhat
weaker in the southwestern parts of the outlook area.  Mesoscale
enhancement to deep shear and large-scale ascent will be possible,
aiding the potential for organized multicell structures, small bows,
and perhaps messy/heavy-precip supercell types of generally short
longevity.  Effective-shear magnitudes 25-40 kt are common in
forecast soundings.  Outflow and differential-heating zones yet to
be determined will play a major role in focusing the prefrontal and
frontal severe threats more precisely across this region. 

...Western Great Basin...
Widely scattered thunderstorms are expected to develop again this
afternoon over the western/central parts of the outlook area. 
Activity should move generally northeastward over patches of
boundary-layer air heated enough to yield favorably well-mixed
subcloud thermodynamic profiles, beneath about 500-800 J/kg MLCAPE. 
The main concern will be isolated severe/damaging gusts.  The area
will reside under difluent flow aloft and subtle large-scale
ascent/destabilization in midlevels ahead of the ejecting northern
CA perturbation, in support of convective potential.

..Edwards/Smith.. 07/26/2017

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