SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0744 AM CDT Fri Oct 23 2020

Valid 231300Z - 241200Z


Threat for a few severe storms capable of damaging wind, hail and
perhaps a tornado or two will exist this afternoon into the early
evening over a portion of the Great Lakes and Ohio Valley.

In mid/upper levels, broadly cyclonic flow will persist across
approximately the northern 1/2 of the CONUS, with two substantial/
embedded shortwave troughs this period:
1.  A perturbation now located over portions of SD and NE, forecast
to deamplify somewhat as it ejects northeastward across WI toward
eastern Upper/northern Lower MI through the daylight hours.  The
trough should reach western QC by 12Z tomorrow.
2.  A trough currently evident in moisture-channel imagery over
northern BC and parts of the AK Panhandle, which will move
southeastward down the length of BC and the Canadian Rockies to the
interior Northwest near the international border.

At the surface, 11Z analysis depicted a low near MBL with cold front
southwestward near ORD, SGF, and MLC, to north-central TX, the
Permian Basin, and south-central/central NM.  The low should move
northeastward across Lake Huron, the neck of ON, and southwestern to
northeastern QC through the period, ahead of the aforementioned
eastern shortwave trough.  By 00Z, the trailing cold front should
reach southern ON, western OH, western KY, northwestern MS, ad the
TX coastal plain.  By 12Z the front should extend from northeastern
NY or northern VT across southern PA, eastern TN, and AL, to the
west-central Gulf. 

...Southeastern MI/northwestern OH to Ohio Valley...
Scattered to numerous thunderstorms are forecast to form mainly
along the cold front during midday to early afternoon from Lower MI
at least as far southwestward as the Arklatex region, moving
eastward over the adjoining Great Lakes and Ohio Valley States and
Mid-South to lower Mississippi Valley in the afternoon.  While a
strong gust cannot be ruled out south of the existing outlook areas,
the most favorable parameter space for severe wind/hail and a threat
of a tornado or two still appears to be near the northern part of
this frontal passage, across southeastern MI, eastern IN and
western/northern OH.  Only minor expansive adjustments were made
accordingly to account for observational and model trends seen the
past several hours, and no substantial alternations to the prior
area nor reasoning are required at this stage. 

By the time the front and its relative max in lift reach suitably
destabilizing, minimally capped boundary-layer air in the 17-20Z
time frame, diurnal heating and moist advection will contribute to
around 1500-2000 J/kg prefrontal/preconvective MLCAPE from the
Mid-South to Lower MI.  Deep-layer wind fields and shear generally
will increase with northward extent over the cold front and
adjoining warm sector.  Forecast soundings show effective-shear
magnitudes ranging from only around 15-20 kt near MEM to around 30
kt near IND and 40-45 kt near DTW.  SRH will be limited to some
extent by a westerly surface wind component, but still may reach
150-300 J/kg over the "slight risk" area in association with a 45-
55-kt LLJ.  Rather messy, quasi-linear modes will predominate,
though any sustained, discrete to semi-discrete convection in the
same area may become supercellular, with concordant local
enhancement of hail/tornado threat.  Convection should weaken this
evening and with eastward extent as it encounters progressively
less-favorable inflow-layer theta-e.

..Edwards/Mosier.. 10/23/2020

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