U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

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acus03 kwns 130731 
swody3 
Storm Prediction Center ac 130730 


Day 3 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0230 am CDT Thu Jul 13 2017 


Valid 151200z - 161200z 


..there is a slight risk of severe thunderstorms across portions of 
the upper Mississippi Valley and western Great Lakes region... 


..there is a marginal risk of severe thunderstorms surrounding the 
slight risk area across portions of the upper Mississippi Valley and 
western Great Lakes region... 


..there is a marginal risk of severe thunderstorms across portions 
of the Carolinas...Georgia...and vicinity... 


..there is a marginal risk of severe thunderstorms across portions 
of central and southern Arizona... 


... 
On Saturday, isolated to widely scattered severe hail and wind will 
be possible across portions of the upper Mississippi Valley and 
western Great Lakes region. 


... 
Embedded within the southwestern rim of broadly cyclonic mid-level 
flow over the northeastern states and eastern Canada, a speed 
maximum is forecast to advance southeastward from the Canadian 
prairies to the Great Lakes region. The low-level mass response 
attendant to this speed maximum will facilitate an 
east-northeastward flux of greater low-level Theta-E air into the 
upper Mississippi Valley and western Great Lakes region, ahead of a 
southward-moving cold front. Elsewhere, an elongated frontal zone 
will extend from portions of the mid-Atlantic to the Central High 
plains, with the richest moisture south of this boundary 
concentrated across the southeast states. Meanwhile, monsoon 
moisture will persist across portions of the southwest states, as a 
mid-level anticyclone builds across the vicinity of the central 
Great Basin. 


..portions of the upper Mississippi Valley and western Great Lakes 
region... 
in association with the influx of greater low-level Theta-E air into 
the region, beneath an eastward-spreading elevated-mixed-layer 
plume, moderate to strong destabilization is anticipated -- to the 
south of the northern front. The combination of modest frontal 
ascent, ascent along a weak pre-frontal trough, and strengthening 
deep ascent in association with the mid-level speed maximum, is 
forecast to support isolated to widely scattered deep convective 
development during the afternoon and evening -- from parts of 
southeast Minnesota into WI, and Upper Michigan. While not particularly strong, 
antecedent capping accompanying the elevated mixed layer may delay 
convective development to some extent. However, strengthening deep 
ascent from the northwest should allow thunderstorm development to 
occur by early evening. With some component of the mid-level flow 
oriented orthogonally to zones of convective initiation, and given 
35-45 kt of effective shear, robust and organized convective 
structures, including possible supercells, are expected to occur. 


The greatest severe risk will exist from central WI to southeast Minnesota 
and vicinity, where the strongest destabilization is anticipated. 
Severe hail and wind are expected to be the primary hazards with 
this activity. However, sufficient low-level hodograph length and 
curvature could support some -- albeit limited -- tornado risk. 


Convection will spread southeastward and southward through the 
evening and overnight hours -- aided by modest low-level isentropic 
ascent. With time, localized instances of merging cold pools could 
Foster areas of convective clustering. The overall severe risk will 
lessen by late in the night, as convective inflow becomes 
characterized by increasingly substantial static stability. 


..portions of the Carolinas, Georgia, and vicinity... 
Moderate to strong boundary-layer destabilization of the moist air 
mass, near and south of the southern front, is anticipated. Frontal 
ascent, orographic circulations, sea-breeze boundaries, 
differential-heating zones, and residual outflow boundaries will 
serve as foci for convective development. Buoyancy will be 
sufficient for some robust convective cores capable of producing 
isolated damaging wind gusts. However, weak deep shear is expected 
to minimize convective organization, thus limiting the overall 
severe risk. 


..portions of central and southern Arizona... 
Diurnally enhanced orographic circulations will favor diurnal 
convective development along the Mogollon Rim and over other higher 
terrain, amid the persistent monsoon moisture. Steepening low-level 
lapse rates, during the diurnal heating cycle, may support this 
activity spreading across the lower deserts. Sufficient deep shear 
is expected to exist to support convective organization, with 
multicell clusters potentially yielding isolated severe potential. 
The magnitude of the severe risk will be dependent upon the degree 
of air-mass recovery, following the potential for prior days of 
convective episodes. If greater destabilization were to occur, then 
the severe risk could increase given favorably enhanced vertical 
shear profiles. 


.Cohen.. 07/13/2017 


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