SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0752 AM CDT Mon Jun 21 2021

Valid 211300Z - 221200Z


Isolated to scattered severe thunderstorms are expected from the
upper Ohio Valley and Mid-Atlantic into northern New England,
offering damaging gusts, sporadic severe hail, and perhaps a couple
of tornadoes.

A high-amplitude mid/upper-level pattern is in place and will
continue through the period -- featuring:
* A longwave trough from north of Hudson Bay to the northern Gulf of
* Ridging from northern MX northwestward to WA/ID, and
* A quasistationary to northward-drifting cyclone off the CA

Numerous shortwave troughs and related vorticity maxima (some
convectively generated/enhanced) will pivot through the cyclonic-
flow field from the northern/central Rockies to the mid/upper
Mississippi Valley and northeastern CONUS.  The most prominent is a
perturbation now apparent in moisture-channel imagery from Hudson
Bay across northwestern ON to northern MN.  This feature will pivot
across the upper Great Lakes through this evening, before turning
northeast and weakening late tonight.

At the surface, 11Z analysis showed a strong/992-mb surface low near
PLN, with cold front southward over northeastern IN then
southwestward across southern IL, southern MO, southwestern OK, the
TX South Plains, and central NM.  As the low ejects northeastward
over western QC by 00Z, the front should reach western portions of
NY/PA/WV, southwestward to mid TN, northern MS, and central/
southwest TX.  By 12Z, the front should reach ME, NJ, the Blue Ridge
region of VA/NC, to northern AL, south-central TX, and Chihuahua.

Remnants of small MCSs are noted this morning over northwestern PA
and northern NY, each trailing outflow boundaries extending over
western NY and north-central OH.  The MCV from the trailing area may
aid convective potential over western/northern NY later this

Otherwise, scattered thunderstorms are expected to develop from late
morning through early afternoon across western parts of the outlook
area, along and ahead of the surface cold front.  Initial severe
potential should arise from the Lake Erie vicinity across portions
of OH, western PA and western WV, as the boundary layer destabilizes
diabatically (and in some areas near the Great Lakes, recovers
behind morning convection).  Activity should move eastward to
northeastward through the rest of the afternoon.  Most of the
convection should evolve into broken to solid lines, though
early-stage supercells are possible given the favorable wind
profiles.  Damaging gusts (including some potentially exceeding
50-kt severe limits) will be the main concern, though isolated
severe hail may be observed in any supercells, along with a tornado
or two.  An isolated, localized threat for QLCS tornadoes also may
develop where any line of convection encounters higher SRH -- mainly
near boundaries left behind by the morning activity.

Rich moisture and weak MLCINH will support early development and
afternoon-long maintenance of convection across the outlook area.
Lower-elevation surface dew points should be commonly in the
mid/upper 60s F, with 70s east of the mountains in the lowlands of
the Potomac/Chesapeake Bay/Delaware Valley areas.  Diurnal heating
will be more sporadic/fragmented in northern parts of the outlook
area due to passing areas of cloud cover from earlier/upshear
convection.  Still, a broad prefrontal corridor of 1000-1500 J/kg
MLCAPE should develop, locally near 2000 J/kg.  Tightening height
gradients aloft -- both related to general/regional  synoptic height
falls and the upper Great Lakes shortwave trough -- will strengthen
mid/upper winds over the region.  However, the strongest flow aloft
should remain behind the front, and mid/upper winds will carry a
substantial front-parallel component.  Forecast soundings suggest
preconvective effective-shear magnitudes generally in the 35-45-kt
range over NY/PA/northeastern OH, and 25-35 kt southward from there.

...KY/TN to central/south TX...
Widely scattered to numerous thunderstorms will form on both sides
of the front today.  Locally damaging/isolated severe gusts are
possible from frontal and warm-sector convection, primarily this
afternoon into evening.  Severe concern will be widely dispersed and
primarily along/ahead of the front, though a rogue hail-producing
cell cannot be ruled out behind the front across the southern
Plains.  A general increase in diurnal heating and low/midlevel
lapse rates, but decrease in vertical shear and frontal slope, will
be noted with southwestward extent along/ahead of the front, from
the northern/central Cumberland Plateau southwestward to south TX. 
Rich low-level moisture will support MLCAPE in the 1500-2500 J/kg
range from the Mid-South northeastward, and 2000-3000 J/kg farther
southwest where midlevel lapse rates are greater in closer proximity
to the EML.  Water-loaded downdrafts and mesobeta-scale cold-pool
surges (with low predictability at this range) will pose the
greatest concern for damaging wind.

...Interior OR, extreme northern CA...
Late this afternoon into evening, isolated to widely scattered
high-based thunderstorms are expected to develop across areas near
the OR/CA line and move northward to northwestward, with the threat
for isolated severe gusts.  This potential will grow in step with
increasing low-level moisture through the afternoon, as the upper
ridge moves east of the area and strongly difluent flow sets up
northeast of the Pacific cyclone.  Moist advection should offset the
effects of antecedent vertical mixing processes more with time,
increasing PW to between 0.75 and 1 inch tonight.  Weak deep shear
will limit overall organization, but gust potential will be aided by
a deep, well-mixed subcloud layer supporting downdraft acceleration.
See the SPC day-1 fire-weather outlook for concerns regarding dry
thunderstorms over this region.

..Edwards/Broyles.. 06/21/2021

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