NOAA Storm Prediction Center

Convective Outlooks

Convective Outlooks
The convective outlooks serve as guidance to the local NWS forecast offices and are used by emergency managers, private sector meteorologists, media, and other weather customers concerned with public safety. Three separate risk areas (slight, moderate, and high) are used to describe the expected coverage and intensity for the categorical severe weather threat on days 1-3 along with severe weather probabilities for the potential threat.

Day 1 Outlook Day 2 Outlook Day 3 Outlook Days 4-8 Outlook

Day 1 Outlook Convective Tornado
Hail Wind
Categorical Day1 1300Z Outlook
  
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 Forecast Discussion


000
ACUS01 KWNS 291254
SWODY1
SPC AC 291252

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0752 AM CDT Thu Jul 29 2021

Valid 291300Z - 301200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
OF CENTRAL/EASTERN PA...NORTHERN MD...DE...AND NJ...

...SUMMARY...
Damaging thunderstorm winds and a tornado or two will be possible
today over parts of the Ohio Valley, central Appalachians and
Mid-Atlantic.

...Synopsis...
The CONUS portion of the large-scale mid/upper pattern will change
little through the period, anchored by a stout anticyclone whose
core region of 594+ dm 500-mb heights will extend from the central
Rockies to the Mid-South.  To its northeast, a belt of northwest
flow aloft will extend from SK/MB across the upper Great Lakes,
becoming cyclonically curved through a trough over the Northeast. 
The most important feature embedded in that flow, for this forecast,
is a shortwave trough initially located over portions of southern ON
to northwestern OH and IN.  Embedded MCVs -- related to the
prior/overnight convective complex in the upper Great Lakes region
-- were apparent east of southern Lake Huron, and over northwestern
OH.  This shortwave trough will pivot southeastward to eastward over
the Mid-Atlantic region and southern New England today, moving
offshore mid/late afternoon.

At the surface, a low influenced by the aforementioned shortwave
trough was analyzed at 11Z near APN, somewhat ahead of a cold front
extending from eastern Upper MI across southwestern WI, northern IA,
western NE and southeastern WY.  The low and front should link
today, with the low crossing Lake Ontario by 00Z and reaching near
PWM by 12Z.  The trailing cold front should reach northern OH,
central portions of IL/IN, northern MO, southern/southwestern NE and
south- central WY by 00Z.  During the day, a warm front initially
drawn across NJ/PA will move northeastward, its western segment near
the low reaching southeastern NY and coastal southern New England
late this afternoon into this evening.

...Mid-Atlantic and vicinity...
Isolated to scattered thunderstorms should develop ahead of the
surface cold front and along or south of the warm front today, with
damaging gusts as the main threat, and a tornado or two possible. 
The greatest potential concentration of both convection and severe
threat should be in a zone of favorably destabilized air ahead of
the MCV/shortwave trough.  The latter will provide large-scale
ascent to boost ambient midlevel lapse rates somewhat, while also
enhancing vertical shear to its southeast over parts of the Mid-
Atlantic.  Low-level shear should be favorable near the warm front,
where surface winds will be relatively backed; however, buoyancy
will be less than farther south amidst greater cloud cover and
related slowing of diurnal heating.

Rich moisture will be present over the area east of the
Appalachians, with warm-sector surface dew points commonly in the
upper 60s to mid 70s F.  The associated high theta-e and diabatic
heating will help to offset the modest lapse rates aloft, enough to
support peak preconvective MLCAPE in the 1000-1500 J/kg range
(briefly approaching 2000 J/kg locally).  Greatest buoyancy should
reside across the Chesapeake Bay/Delmarva and Delaware Valley
regions eastward over NJ, with some extension across small parts of
southeastern NY into southern New England possible prior to
convective passage.  Thunderstorms should concentrate most densely
in and near the 30% wind area, shifted slightly northward to be
closer to the low-level convergence max associated with the low, and
under somewhat stronger deep-layer flow/shear.  Forecast soundings
indicate 35-45-kt effective-shear magnitudes in this area, generally
decreasing southward.

...OH Valley to MO/southeast NE...
Widely scattered to scattered thunderstorms are expected to develop
along the surface cold front and prefrontal outflow boundaries,
mainly over the OH Valley region, but potentially westward across
northern MO/southeastern NE area.  Damaging, isolated severe gusts
and sporadic large hail are possible.

The low levels south of the boundaries will remain very moist, with
strong diurnal heating and a deep troposphere helping to drive
MLCAPE into the 3000-4500 J/kg range over the corridor from NE to
western IL, and 2000-3000 J/kg across portions of IN/OH that can
recover in time from the ongoing clouds/precip.  As buoyancy will
increase westward, mid/upper-level flow and deep shear will
decrease, though the latter still should be sufficient to aid some
organized multicell to transient supercell structures, particularly
over the OH Valley region.  Severe potential, in reality, will not
be uniformly distributed across the lengthy 15%/slight area, with
upscale clustering and related concentration of wind potential in
particular being possible on the mesoscale.

...Central High Plains...
Isolated to widely scattered thunderstorms are expected to form this
afternoon, mainly on higher terrain from the southern Bighorns into
the Laramie Range and vicinity, and more conditionally, east-
southeastward near the front.  Isolated severe gusts/hail are
possible from this activity as it moves northeastward to eastward
late this afternoon and evening, before dissipating in a nocturnally
stabilizing boundary layer.  Overall coverage and longevity
generally should diminish westward from MO into NE, hence the
narrowing outlook area. 

A substantial easterly component of low-level winds will become
established behind the front, enabling upslope lift, westward
moisture/theta-e advection and greater storm-relative boundary-layer
flow.  This also will contribute to substantial veering of flow with
height to augment deep shear, despite the lack of stronger deep-
layer wind speeds near the mid/upper anticyclone.  Insolation of
higher terrain will weaken MLCINH as low-level moisture increases,
contributing to steepening low-level lapse rates and strengthening
buoyancy, with MLCAPE reaching 1000-2000 J/kg in a corridor of
roughly front-parallel instability extending from east-central/
southeastern WY across the NE Panhandle and southwestern NE. 
Meanwhile, 35-40 kt effective-shear magnitudes are possible,
supporting occasionally organized convective structures. Clustering
and cold-pool forcing also may occur on the mesobeta scale for a few
hours, locally augmenting the wind potential, though spatial
uncertainties for such processes are too great to focus larger
unconditional probabilities at this time.

..Edwards/Gleason.. 07/29/2021

$$