SPC AC 010625
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1225 AM CST Sun Jan 01 2017
Valid 021200Z - 031200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS MONDAY INTO
MONDAY EVENING ACROSS PARTS OF EASTERN LOUISIANA...CENTRAL AND
SOUTHERN MISSISSIPPI...AND MUCH OF WESTERN ALABAMA...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING
AREAS OF THE WESTERN AND CENTRAL GULF STATES...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS ADJACENT
PORTIONS OF THE MID SOUTH AND LOWER OHIO/TENNESSEE VALLEYS...
Severe thunderstorms are expected Monday across much of the western
through central Gulf states into portions of the Mid South,
accompanied by the risk for potentially damaging wind gusts and a
Ridging to the east of a blocking mid/upper high, centered near the
Gulf of Alaska, appears likely to expand eastward and southeastward
through portions of the western Canadian provinces and portions of
the Pacific Northwest during this period. As this occurs,
downstream troughing over the northwestern U.S. may undergo
considerable deformation, with at least one significant short wave
impulse forecast to emerge from it, and accelerate east
northeastward toward the upper Great Lakes region.
In lower latitudes, subtropical ridging is forecast to elongate east
to west, with the center of highest heights shifting from the
Caribbean into the vicinity of the Mexican Gulf Coast. On the
northern periphery of this feature, into larger-scale ridging over
the southeastern U.S., it still appears that a significant remnant
short wave impulse, emerging from the Southwest, will weaken as it
accelerates east northeast of the southern Plains. However, the
extent to which this occurs prior to reaching the southern
Appalachians/south Atlantic coast region, remains unclear.
Associated with/in response to these developments, models indicate a
significant low-level cold intrusion will nose southward to the lee
of the northern Rockies on Monday. Primary surface cyclogenesis may
commence across parts of the Upper Midwest/Great Lakes region, well
removed from the Gulf Coast region, where a substantive return flow
of moisture is expected to be ongoing. However, guidance remains
suggestive that a fairly significant secondary surface low may
accompany the southern mid-level impulse, eastward/northeastward out
...lower Plains to Appalachians...
Substantive low/mid-level moisture return, and at least weak
destabilization, appears possible as far north as the middle
Mississippi/Ohio Valleys and southern portions of the Great Lakes
region. However, much of this may occur above a residual
cool/stable near surface layer with little appreciable severe
weather potential currently evident. Richer low-level moistening,
and significant boundary layer destabilization, probably will remain
confined to portions of the western and central Gulf States, with
return to areas farther north impeded, at least in part, by
convective development associated with the short wave emerging from
the southern Plains, and downstream warm advection.
Guidance continues to indicate that thermodynamic profiles across
much of the western into central Gulf Coast states, perhaps areas as
far north as the Mid South, will become characterized by modestly
steep lower/mid-level lapse rates and CAPE of 500-1000+ J/kg.
Higher CAPE is expected to be focused across central and southern
portions of Louisiana and Mississippi into Alabama, where surface
dew points likely will reach or exceed the mid 60s F.
Due to lingering variability among the model output, among other
issues, the potential convective evolution Monday into Monday night
remains unclear. However, given the magnitude of the potential
instability, the environment should be conducive to severe storm
development. Models are suggestive that southern short wave impulse
will be accompanied by considerable strengthening of lower/mid
tropospheric wind fields, including southerly 40-50 kt flow around
850 mb, veering to west/southwesterly at 50-70 kt around 700 mb.
This should provide more than sufficient low-level and deep layer
shear for organized severe convection, including supercells.
Furthermore, guidance now seems suggestive that forcing for ascent
may remain sufficient to maintain vigorous storm development through
the day Monday, into at least Monday evening, spreading across much
of Louisiana and Mississippi into Alabama. This will partially be
driven by low/mid-level warm advection, which may support or
maintain a progressive organized mesoscale convective system with
potential for damaging surface gusts. Sizable clockwise curved
low-level hodographs appear likely to support a risk for tornadoes
as well, particularly in discrete cells preceding any evolving
convective system. A strong tornado or two may not be out of the
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