SPC 2000Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0256 PM CDT Tue Sep 14 2021

Valid 142000Z - 151200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
THE GREAT LAKES/MIDWEST AND CENTRAL PLAINS...

...SUMMARY...
Scattered storms with damaging winds are expected into tonight from
parts of the Ohio Valley into Lower Michigan.  Severe storms with
large hail and damaging winds will also be possible late this
afternoon into early tonight from eastern Colorado into southwestern
Kansas.  A tornado or two may occur through tonight across southern
Louisiana with the remnants of Nicholas.

...20z Update...

Earlier thoughts regarding the potential for severe remain. No
appreciable changes are forecast for this update.

One area where a minor change appears warranted is across
south-central KS. Surface front has settled into southern KS early
this afternoon and a substantial cu field has developed southeast of
DDC. Over the next few hours deeper thermals should lead to
thunderstorm development and strong/severe storms should spread
southeast. This activity may spread a bit farther east than earlier
depicted. Large hail should be the primary threat, though gusty
winds are also possible.

Farther north across lower MI, low-level flow has veered
considerably in the lee of northern Lake MI. As a result, the
primary zone of boundary-layer confluence is setting up across
mainly central portions of the State. Have adjusted MRGL Risk a bit
southeast across northern lower MI. Otherwise, no significant
changes are necessary.

..Darrow.. 09/14/2021

.PREV DISCUSSION... /ISSUED 1119 AM CDT Tue Sep 14 2021/

...Lower MI to the lower OH Valley this afternoon/evening...
Elevated thunderstorms moved across northern Lower MI this morning
in association with the primary ejecting midlevel trough over Upper
MI/Lake Superior.  A positive-tilt trough extends southwestward to
eastern IA/northern IL, and this portion of the midlevel wave will
combine with low-level ascent along a surface cold front to support
additional storm development by mid afternoon from Lower MI into
IN/IL.  Surface temperatures warming into the 80s with dewpoints of
66-70 F will support MLCAPE greater than 2000 J/kg, which combined
with straight hodographs and effective bulk shear of 25-35 kt will
support line segments and clusters of storms along the front, with
the potential to produce damaging outflow gusts given the strong
buoyancy, steep low-level lapse rates, and 35-40+ kt midlevel flow
(strongest across Lower MI).  Isolated large hail will also be
possible with the stronger embedded cells.

...Eastern CO/southwestern KS this afternoon into tonight...
A weak upslope flow regime will establish today from western KS into
eastern CO, on the cool side of a slow-moving cold front.  Aloft, a
series of subtle midlevel shortwave troughs will move
east-southeastward atop the low-level upslope regime.  Though
moisture will be rather modest (boundary-layer dewpoints in the mid
40s to low 50s), midlevel lapse rates of 7.5-8 C/km and daytime
heating/mixing will contribute to MLCAPE near 1000 J/kg.  The most
probable area for storm initiation will be from the Palmer Divide
(along a reinforcing frontal surge) southward in the immediate lee
of the Front Range in CO, where convective inhibition will be weaker
compared to the lower plains.  Relatively long, straight hodographs
will favor splitting supercells capable of producing large hail
initially.  Storm interactions could lead to some upscale growth and
an increase in the potential for strong-severe outflow winds later
this evening into early tonight, as storms spread southeastward.

A few storms may also form along the front into southwest KS this
afternoon, in an environment with sufficient vertical shear and
buoyancy for supercells with at least isolated large hail/damaging
winds.

...South central/southeast LA through tonight...
Tropical Storm Nicholas continues to weaken along the upper TX coast
this morning, after making landfall overnight as a minimal
hurricane.  The convective structure of the tropical cyclone has
become quite asymmetric and ragged as a result of westerly shear
aloft, and entrainment of relatively cool/dry air into the cyclone
core.  These factors will contribute to further weakening today into
tonight as the cyclone drifts eastward near the northwest Gulf
coast.  Well east of the cyclone core, there will still be the
potential for more discrete convection in the zone of sufficient
vertical shear for supercells across south central/southeast LA
where buoyancy will be larger compared to father west.

...Northeast OH/northwest PA into western NY this afternoon...
A surface warm front will continue to move northeastward across NY,
with a warm and moderately unstable warm sector established across
western NY into eastern OH.  Though the surface cold front will not
approach this area until overnight, localized lake breeze boundaries
may provide sufficient lift to initiate isolated storms this
afternoon from northeast OH into western NY, with an attendant
threat for isolated damaging winds and some hail, given MLCAPE
approaching 2000 J/kg and effective bulk shear near 30 kt.  An
isolated supercell may also occur where vertical shear will be
stronger along the warm front in NY, though this threat is highly
conditional.

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