SPC Jul 8, 2017 1730 UTC Day 2 Convective Outlook

SPC 1730Z Day 2 Outlook

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1228 PM CDT Sat Jul 08 2017

Valid 091200Z - 101200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF
THE UPPER MISSISSIPPI VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A PORTION
OF THE MIDDLE TO UPPER MS VALLEY AND GREAT LAKES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHERN
GEORGIA THROUGH THE COASTAL CAROLINAS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A PORTION
OF THE NORTHERN ROCKIES...

...SUMMARY...
Severe thunderstorms are possible Sunday primarily across parts of
the upper Mississippi Valley and vicinity. A few strong storms with
downburst winds are expected across the Carolinas and Georgia
coastal plain, and across portions of the northern Rockies and
adjacent High Plains.

...Synopsis...

Blocking synoptic pattern will persist Sunday with an upper ridge
centered over the Great Basin and a downstream trough over the
eastern states. Several smaller-scale vorticity maxima will move
through the periphery of the upper ridge and the eastern U.S. upper
trough, providing foci for more active corridors of thunderstorms.

At the surface a quasi-stationary front will reside from the coastal
Carolinas through the Gulf Coast states then northwestward as a warm
front into the central Plains. Farther north another front is
expected to evolve over the Northern Plains into the upper MS valley
region. A weak lee trough will persist over the High Plains.

...Upper MS Valley region...

Model consensus is that a shortwave trough currently cresting upper
ridge and moving through southern Alberta will reach the upper MS
valley early Sunday, fostering the development of showers and
storms. This activity is not expected to be severe and will
subsequently shift eastward through northern MN and WI during the
day. The boundary layer will gradually moisten Sunday in the
presence of southerly low-level winds with near-surface dewpoints
climbing into the 60s F beneath eastern fringe of steeper mid-level
lapse rates. These processes should result in moderate instability
with diabatic heating boosting MLCAPE to 1500-2000 J/kg during the
afternoon. A capping inversion and weak forcing in wake of early
shortwave trough may delay more robust thunderstorm development
until later in the afternoon when a weaker impulse is forecast to
drop southeast within the northwesterly flow regime during peak
heating.  Thunderstorms might initially develop in vicinity of
boundary across MN and subsequently spread southeastward. Initial
storms will likely be discrete with sufficient vertical shear
supportive of a few supercells. Additional storms may develop
farther south overnight within warm advection regime, possibly
evolving into an MCS as the low-level jet undergoes a modest
increase. Large hail and damaging wind will be the primary threats.

...Northern Rockies and adjacent High Plains...

Deep inverted-V boundary layers and marginal instability will exist
over the northern Rockies. Weak impulses riding the crest of the
upper ridge and orographic forcing should foster development of
thunderstorms during the afternoon. The storms will pose a risk for
isolated downburst winds through early evening.

...Carolinas and Georgia coastal plains...

Thunderstorms are expected to develop in vicinity of frontal
boundary and sea breeze interactions during the day. Diabatic
warming of the moist warm sector will promote moderate instability
and a threat for a few wet downbursts with multicell storms during
the afternoon and early evening.

..Dial.. 07/08/2017

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