SPC 2000Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0255 PM CDT Mon Jul 26 2021

Valid 262000Z - 271200Z



Supercells and bowing structures with large hail and damaging winds
are expected this afternoon/evening from eastern North Dakota into
northern Minnesota.  Other locally severe storms capable of damaging
winds will affect portions of Virginia and North Carolina.

Only change this outlook update is to include 5-percent wind
probabilities in central OK into far south-central KS.  The 18z
Lamont, OK raob showed a very unstable airmass with an 18 g/kg
lowest 100 mb mean mixing ratio and 3300 J/kg MLCAPE.  Despite very
weak flow through the troposphere and indicative of a pulse
thunderstorm mode, the steepened 0-2 km lapse rates and high PW will
favor water-loaded wet microbursts (localized gusts up to 65 mph)
with the most intense cores.  This activity will diminish by
early/mid evening.

..Smith.. 07/26/2021

.PREV DISCUSSION... /ISSUED 1148 AM CDT Mon Jul 26 2021/

..Eastern ND/MN/northern WI/Upper MI this afternoon into tonight...
A broad upper ridge is prominent across much of the CONUS today,
with stronger westerly flow stretching from MT to the Great Lakes
region.  At the surface, a moist air mass with dewpoints in the mid
60s extends as far north as ND/MN, where strong daytime heating will
yield MLCAPE values of 1500-2500 J/kg.  Deep-layer shear profiles
will also be quite strong across this region, with effective shear
values around 40 knots.  These parameters will result in a
conditionally favorable environment for supercells and bowing storm
complexes capable of very large hail and damaging winds.  The
primary forecast uncertainty involves the timing and placement of
storms later today.  12z CAM guidance shows considerable diversity,
and given the subtle/weak forcing mechanisms, confidence in the 30%
hail area is not particularly high.  Nevertheless, a few HREF
members continue to indicate an active convective day over the
SLGT/ENH risk area, so will leave risk areas mostly unchanged.

A hot and very humid low-level air mass is present today over
southern VA and northern NC, with dewpoints in the mid 70s.  Strong
heating will result in an axis of MLCAPE over 2500 J/kg from western
NC into south-central VA, where scattered afternoon thunderstorms
are expected.  Vertical shear is not particularly strong, but may be
sufficient for a few convective clusters to become organized and
pose a risk of damaging winds and hail.  Activity will persist into
the early evening and the onset of diurnal cooling.

...Northern New England...
Moderately strong west-northwesterly mid-level flow stretches across
the northeast states today, where mostly clear skies are present. 
This will allow for the development of relatively steep low-level
lapse rates.  Isolated thunderstorms are expected to form over the
region, and across eastern ON/QB and move across the MRGL risk area.
 The strongest cells could produce locally gusty winds.

The corridor of enhanced mid-level winds has shifted northward into
NV/UT today.  Yesterday's convection over this region may limit high
temperatures somewhat and lessen the damaging wind potential.  But
given the mostly clear skies and consistent forecast of an axis of
thunderstorm development later today, will add a MRGL risk for
locally damaging winds.

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