SPC 1300Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0630 AM CST Thu Jan 21 2021

Valid 211300Z - 221200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
Some weak thunderstorm activity is possible today across parts of
southern Arizona, and mainly this evening in a corridor across parts
of eastern Texas into the lower Mississippi Valley.

...Synopsis...
Within amplified, branching flow across the eastern Pacific, models
indicate that mid/upper ridging may build further, along a
positively tilted axis from the mid latitudes, near/west of 150 W,
north-northeastward into the Arctic latitudes.  Strongest height
gradients, and wind fields, are forecast to continue to develop in a
sharply anticyclonic regime across the northeastern Pacific, with
one significant short wave perturbation digging on its leading edge
(to the east of the ridge axis).  At mid-levels it appears that this
will include an embedded mid-level low reaching southern Oregon and
northern California coastal areas by late tonight.

Downstream, the westerlies converge into strong west-northwesterly
flow east of the Rockies into the western Atlantic.  Within the
branch emanating from the Arctic latitudes, a mid-level low may
gradually progress southeastward and eastward, to the northeast of
the Great Lakes region.  While an initially broad and deep
associated surface cyclone begins to weaken, as secondary
cyclogenesis commences along a frontal zone off the Mid Atlantic
coast, it appears that a significant reinforcing intrusion of cold
air will overspread much of the upper Midwest/Great Lakes region.

Meanwhile, within the branch emanating from the northeastern
Pacific, an initially closed mid-level low, now turning toward the
Southwestern international border area, is forecast to transition to
an open wave and accelerate northeastward in response to the
approach of the upstream closed low.  Models indicate considerable
shearing/weakening of this impulse by the time it reaches the
southern Great Plains by late tonight, as it begins to round the
broad crest of subtropical ridging centered over the southwestern
Gulf of Mexico into Caribbean vicinity.

Beneath this regime, a narrow plume of low-level moistening is
already well underway within southerly return flow across the
northwestern Gulf coast vicinity.  However, this is mostly elevated
above a residually cool and stable boundary layer (even across the
northwestern Gulf of Mexico).  It is also generally beneath a broad
plume of mid/high-level moisture return emanating from the lower
latitude eastern Pacific.

...East Texas into lower Mississippi Valley... 
Thermodynamic profiles are already in the process of becoming
saturated, and generally weak lapse rates are likely to prevail
through much of the period, beneath relatively warm mid-levels. 
However, strengthening lift focused along a mid-level baroclinic
zone (roughly around 700 mb), perhaps aided by forcing in the right
entrance region of a high level jet around the crest of the
subtropical ridge, is forecast by this evening.  And various model
output (perhaps most notably the SPC High Resolution Ensemble
Forecast) suggest that this might become supportive of at least 
some weak thunderstorm activity.

...Southern Arizona...
To this point, thunderstorm development has remained focused in
relatively close proximity to the northern Gulf of California, aided
by the inflow of more moist and potentially unstable air.  Weak
destabilization does appear underway east-northeastward along the
international border area, as the relatively compact mid-level cold
core of the low crosses the northern Gulf of California.  However,
the extent to which convection capable of producing lightning will
be maintained, as it accelerates east-northeastward through the day,
remains unclear.

...Southern Oregon/Northern Oregon coastal areas...
Convection capable of producing lightning may accompany the
relatively compact mid-level cold core (perhaps to -28 to -30C at
500 mb) of the digging mid-level low.  However, while this could
approach coastal areas late tonight, models are suggestive that any
such activity should remain offshore through at least 12Z Friday.

..Kerr.. 01/21/2021

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