SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0758 AM CDT Tue Sep 14 2021

Valid 141300Z - 151200Z

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

...SUMMARY...
Isolated to widely scattered severe storms are possible today into
tonight across parts of the Midwest and Great Lakes into the
Northeast States. Other severe storms may develop across the
south-central High Plains this afternoon and evening. A brief
tornado risk is also possible in association with Nicholas from
southeast Texas into southern Louisiana.

...Midwest/Great Lakes...
Initially, a band of thunderstorms is ongoing over northern Lower
Michigan this morning. Although an initially cool/stable boundary
layer will limit severe potential, a localized early day severe risk
is a possibility, with moderate instability and ample cloud-layer
shear noted in the 12Z observed sounding from Alpena, MI. 

Meanwhile, a surface low will continue to quickly move from Upper
Michigan into Ontario. A cold front will trail south from the low,
oriented roughly from southeast Lower Michigan into northern
Indiana/central Illinois by 00Z. Ahead of the front, mid to upper
60s F dewpoints will be common, along with strong heating resulting
in 2000+ J/kg MUCAPE by late afternoon.

Deep-layer shear will be most favorable over northern parts of
Michigan. However, instability will be greatest farther south where
vertical shear will be less. The greatest overall severe threat will
be with storms along the front after 21Z from central Lower Michigan
into Illinois. Linear forcing along the front suggests wind may be
the primary concern, but hail will also be possible. 

...Northeast States...
Shortwave ridging will occur today, with a warm front otherwise
shifting north-northeastward across New York and southern New
England. Heating will contribute to instability south of the front,
from western New York into Pennsylvania and New Jersey. A capping
inversion is expected to generally hinder development during the
day, although a few isolated late-afternoon storms could occur over
north-central/northeast New York with some severe risk. Otherwise,
storm coverage will increase tonight as the front approaches after
03Z into western and northern New York, where 500-1000 SBCAPE may
remain. Strong westerly flow with 40-50 kt mid-level winds may
support a few severe wind gusts across western New York, but
eventual nocturnal thermodynamic influences should keep the overall
severe risk somewhat limited.

...Colorado Front Range to Central Plains...
Late-afternoon storms are likely along the stalled front across
Missouri into southern Kansas, with localized severe wind gusts
and/or hail a possibility. Some potential for a few supercells will
exist across southern Kansas where vertical shear should be
stronger.  

Although some questions exist regarding storm coverage and the
downstream thermodynamic environment across the Plains, other
potentially severe storms are expected to develop, at least on an
isolated basis, near the Colorado Front Range this afternoon.
Although the boundary layer will not be overly unstable, ample
effective shear (40+ kt) and steep mid-level lapse rates will
support some supercells capable of large hail. These storms may
gradually merge/organize as they progress southeastward from
east-central/southeast Colorado into southwest Kansas this evening
with an isolated severe hail/wind risk.

...Louisiana/southeast Texas...
Nicholas will continue to weaken and drift east-northeastward along
the upper Texas Coast toward southern Louisiana through tonight.
Lower 70s F surface dewpoints reside in coastal areas coincident
with moderately strong low-level shear, accentuated by low-level
easterly winds near the coast. The scenario will support the
possibility of a brief tornado risk mainly across southern
Louisiana.

..Guyer/Smith.. 09/14/2021

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