SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0755 AM CDT Tue Jul 27 2021

Valid 271300Z - 281200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM WESTERN NEW
YORK ACROSS PARTS OF NEW ENGLAND...AND OVER PORTIONS OF THE UPPER
MISSISSIPPI VALLEY REGION...

...SUMMARY...
Severe thunderstorms are possible today from western New York across
parts of New England, and mainly tonight over portions of the upper
Mississippi Valley region.

...Synopsis...
In mid/upper levels, slight amplification is expected in the
western-ridge/eastern-trough pattern that will continue through the
period. The closed 500-mb anticyclone over the central/southern
Rockies and adjoining Plains will expand in size and intensify
gradually, with the high strengthening to around 597 dm overnight
over the central High Plains.  Meanwhile, height falls should occur
over much of the Northeast, both with gradual amplification of the
slow-moving synoptic trough, and assorted vorticity maxima and
shortwaves pivoting through the associated cyclonic-flow field.  The
most pertinent such perturbation for this forecast appears to be an
MCV -- now apparent in composited radar imagery crossing from
northern/central Lower MI to Lake Huron.  This feature, preceded by
a field of large-scale lift and somewhat enhanced deep shear, should
move east-southeastward across southern ON through 18Z, and NY/New
England through this evening.

At the surface, 11Z analysis showed a cold front from northern ME
across southern ON to a weak frontal-wave low over northern Lower MI
-- which also may be a surface manifestation of an MCV nearby.  The
cold front should move slowly southeastward through the period into
northern NY and more of northern New England.  An outflow boundary
was drawn across central Lower MI, southern portions of Lake
Michigan and WI, to central MN.  This boundary should move southward
more over southern WI and Lower MI before stalling later today.

...NY/New England...
Surface-based thunderstorms are expected to develop by midday over
parts of western NY and/or adjoining ON, along and south of the
front.  Activity should increase in coverage and move mostly
eastward across central/eastern portions of NY and south-central New
England through the afternoon, its movement aligned with an
instability gradient and boundary-layer moist axis.  Damaging to
locally severe wind gusts will be the main concern.  Confidence has
increased that enough large-scale lift (related to the MCV) and
diurnal low-level destabilization will occur to support maintenance
of the threat eastward into New England.  The 15% wind area has been
extended accordingly.

Surface dewpoints generally in the low/mid 60s F will be common in
the preconvective environment across the region, with minimal
MLCINH.  Accordingly modified RAOBs and forecast soundings suggest
that -- despite weak midlevel lapse rates -- MLCAPE around 500-1000
J/kg will develop (locally/briefly higher), atop a reasonably
well-mixed boundary layer with steep low-level lapse rates.  A
roughly  unidirectional deep-layer wind profile is expected, with
effective-shear magnitudes in the 30-40-kt range.  Given the weak
capping and progressive regime of ascent preceding the MCV, some
cold-pool aggregation may occur in upscale growth of convection,
enabling forward propagation to impinge on the destabilized boundary
layer, potentially as far east as the MA and perhaps NH coastlines
given the westerly flow that can advect favorable boundary-layer
theta-e that far.

...Upper Mississippi Valley region...
Isolated severe gusts/hail may be noted from a cluster of
thunderstorms that developed over the cold pool, and has been moving
southeastward over west-central WI.  See SPC mesoscale discussion
1363 for near-term details.

Severe hail and gusts will be expected from any thunderstorms that
can develop from late this afternoon through tonight over this
corridor.  The MCS that crossed WI last night produced a pronounced
outflow boundary whose backing cold pool since has been reinforced
by rounds of elevated convection to its north, including the ongoing
activity.  The associated baroclinic zone -- both at the surface and
elevated over the cold pool to the boundary's north -- will be the
main focus for additional convective potential through the period.

Greater certainty exists in evening convective development in and
near the somewhat-expanded eastern parts of the outlook area, where
the boundary will be stronger and CINH weaker.  A preponderance of
numerical guidance -- from synoptic models to assorted CAMs, support
this general concept, even if differing in timing/location details
on the mesoscale as should be expected.  Farther west, the outlook
becomes more conditional -- with respect to both coverage and timing
of storms -- in a volatile near-boundary environment that will favor
severe with any sustained convection that can develop.  The southern
MN part of the outlook in particular is more of a "boom or bust"
scenario with convective-development/maintenance uncertainties in a
thermodynamically very favorable parameter space (e.g., 70s F
surface dew points beneath steep midlevel lapse rates, 3500-5000
J/kg MLCAPE, and 30-40-kt effective-shear magnitudes supporting
organized potential).  Additional development is possible late
tonight into early tomorrow morning over parts of west-central/
southern NM as well, with coverage/intensity uncertainties.

..Edwards/Gleason.. 07/27/2021

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