SPC 1730Z Day 2 Outlook

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1120 AM CST Thu Jan 28 2021

Valid 291200Z - 301200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
The risk for severe thunderstorm appears negligible across the U.S.,
Friday through Friday nnight.

...Synopsis...
To the southeast of blocking mid/upper ridging centered across
northern Quebec, large-scale troughing is currently in the process
of evolving near the Atlantic Seaboard into the western Atlantic. 
It appears that this will be maintained through this period, as
renewed amplification occurs within the westerlies across the
mid-latitude central into eastern Pacific.

Models indicate that the latter regime will include deepening
large-scale mid-level troughing across the northeastern Pacific, the
leading edge of which may at least approach British Columbia and
Pacific Northwest coastal areas by 12Z Saturday.  In advance of this
feature, low amplitude ridging may begin to build inland across
California by late Friday evening, as the significant short wave
trough currently approaching the California coast is forced
east-northeastward.

A vigorous smaller-scale perturbation may still be digging within
the lead short wave trough, to the west of the southern California
coast at 12Z Friday.  This feature, and the associated mid-level
cold core (with 500 mb temps around -30C), might not turn completely
inland of the southern California coast until late Friday afternoon
or early evening.  However, a more rapid east-northeastward
acceleration is forecast Friday night, with the larger-scale short
wave trough taking on a negative tilt across and east of the
southern Rockies by Saturday morning.  

In response to the approaching mid-level perturbation, models
indicate considerable deepening of surface troughing to the lee of
the Rockies, with cyclogenesis commencing across southeast Colorado
by late Friday night.  While a southerly return flow off the
southwestern Gulf of Mexico may be underway by Friday morning across
the lower Rio Grande Valley and adjacent southern Great Plains,
substantive boundary-layer moistening (including 60-65F+ surface dew
points) may not reach lower Texas coastal areas until around 12Z
Saturday.  Forecast soundings indicate that the depth of this moist
layer will remain fairly shallow, beneath very warm and strongly
capping elevated mixed-layer air.

...Southwest...
Models indicate that the surface cold front associated with the
inland advancing mid-level short wave trough may be in the process
of advancing into/across the lower Colorado Valley by 12Z Friday,
before continuing eastward across the central/southern Arizona
deserts during the day.  It appears that the front will be preceded
by moderately strong (30-40+ kt) south/southwesterly flow in the
850-700 mb layer, but moistening and mid-level cooling sufficient to
contribute to thermodynamic profiles conducive to thunderstorm
development are forecast to be confined to the post-frontal regime,
after this flow veers and weakens.  With only very weak CAPE
currently anticipated, while some of this activity (which could
impact southern California coastal areas into Friday afternoon, as
well as the Greater Phoenix area by late Friday afternoon) may be
accompanied by small hail/graupel and gusty winds, it is expected to
generally remain well below severe limits.

...Parts of the southern Great Plains...
Increasing deep-layer large-scale forcing for ascent ahead of the
inland accelerating short wave trough is forecast to begin impacting
the higher terrain and high plains of west Texas and eastern New
Mexico by Friday evening.  This will be accompanied by steepening
mid-level lapse rates.  Although forecast soundings suggest that the
most substantive moisture return across this region may be of lower
latitude eastern Pacific origin, and based near or above 700 mb, it
appears that thermodynamic profiles will become conducive to
convection capable of producing lightning.  And it appears that this
activity will tend to develop northeastward into south central
portions of the Great Plains by 12Z Saturday.

..Kerr.. 01/28/2021

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