SPC 1630Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1114 AM CDT Mon Jun 21 2021

Valid 211630Z - 221200Z


Scattered severe thunderstorms are expected from the upper Ohio
Valley and Mid-Atlantic into northern New England, capable of
damaging gusts, sporadic severe hail, and perhaps a couple of

...Northeast States...
Water vapor imagery shows a broad upper trough digging across the
Great Lakes region, with the associated cold front sweeping across
southeast Ontario and OH.  Mostly clear skies and dewpoints in the
60s to lower 70s ahead of the front, coupled with steep midlevel
lapse rates shown on 12z raobs and model forecast soundings will
yield a moderately unstable air mass by early afternoon (MLCAPE over
2000 J/kg).  It appears thunderstorms will develop by early
afternoon along the front - and in the free warm sector to the east.
 Vertical shear will be sufficient for a mix of multicell and
occasional supercell structures, capable of damaging winds and large
hail.  Low level shear is strongest over Quebec, but may be
sufficient over parts of VT/NY for an isolated tornado or two as
well.  Activity will sweep eastward across the ENH risk area through
the early evening, before weakening as it approaches cooler
marine-influenced air mass over eastern New England and NJ.

Thunderstorms are likely to develop southwestward along the front
from eastern OH into KY/TN this afternoon.  Vertical shear weakens
with southward extent, but given the degree of instability and
midlevel winds of 30-40 knots, will be sufficient for at least
occasionally intense storms capable of damaging winds and hail. 
Therefore, have extended the SLGT farther southwest into these

Several clusters of thunderstorms are expected to develop today
across a relatively large area of TX and the mid/lower MS Valley. 
CAPE will be sufficient for robust updrafts/downdrafts, but weak
vertical shear will limit the convective organization. 
Nevertheless, the strongest cells will pose a risk of locally
gusty/damaging winds and hail.

...Northern CA/OR...
There continues to be a good signal in the 12z model guidance for a
small area of strong thunderstorms this afternoon and evening over
portions of northern CA into central OR.  A deeply mixed boundary
layer and inverted-v profiles suggest that locally damaging wind
gusts are the main threat.

..Hart/Bentley.. 06/21/2021

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