SPC AC 091233
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0633 AM CST Fri Feb 09 2018
Valid 091300Z - 101200Z
...NO SEVERE THUNDERSTORM AREAS FORECAST...
Thunderstorms are expected to develop from parts of southeast Texas
through the lower Mississippi Valley and the Southeast States,
mainly later today through tonight.
A longstanding, intense, mid/upper-tropospheric cyclone will persist
over the Baffin Island region and adjoining parts of northeastern
Canada. Most progs keep the cyclone over this region for at least
ten more days. However, the configuration of cyclonic flow around
the associated longwave trough will change, starting today, as a
series of shortwaves traverse the back side of the cyclone over
central/western Canada and across the western and northern U.S.
Height falls will occur across a broad swath of the Plains, Rockies
and Intermountain West, as a strong shortwave trough digs southward
and southeastward from coastal BC and the northern Rockies. That
perturbation should reach WY, southern ID and OR by 12Z. By then, a
larger-scale trough will become established from WY to NM, phasing
with a broad, weak, pre-existing, southern-stream height weakness
over northern MX. As that occurs, a series of low-amplitude
shortwave perturbations and vorticity maxima will pivot cyclonically
across northern MX, with a few ejecting northeastward over
south/east TX toward the lower Mississippi Valley.
At the surface, a cold front from northern IL across KS is forecast
to shift southward/southeastward through the period. By 12Z this
front should extend from a weak wave low near EVV southwestward
across AR, north-central/west-central TX and east-central NM. A
separate baroclinic zone, demarcating well-modified Gulf marine
layer from a more stable, continental/polar air mass, will remain
quasistationary through much of the period over the northern and
western Gulf before drifting northward over LA shelf/coastal waters
...Southeast TX to Southeast States...
Isolated to widely scattered thunderstorms are possible episodically
through the period across a long swath of the southeastern U.S. A
lack of both more robust theta-e and stronger deep-layer instability
(including boundary-layer lapse rates) keeps severe potential too
small and conditional to warrant categorical probabilities at this
Gradual moistening and destabilization of the elevated return-flow
plume is expected as a broad sheet of warm advection becomes
established across the Gulf Coast region. As this occurs, potential
will increase for favorably moist parcels to reach an LFC. This
will occur initially in three belts that should overlap spatially in
the outlook area, even if somewhat offset temporally:
1. Warm advection, moisture transport and at least weak low-level
convergence atop a relatively stable boundary layer today, mainly
over portions of GA and SC. The early stages of this process appear
to be underway from southeastern AL into southwestern GA, with
expansion forecast as the ongoing precip/UVV plume shifts
east-northeastward into deeper buoyancy.
2. A roughly east-west corridor of ascent today and tonight from
the Mississippi Valley to the Carolinas, orthogonal to the southerly
component of the sheet of southwesterly flow.
3. A southwest-northeast swath with greater moisture and CAPE from
east TX and LA to the Arklatex, growing across the Mid-South and
shifting eastward late tonight, within a generally 35-50 kt LLJ.
Although midlevel lapse rates will remain modest, these processes
over time should yield sufficiently tall buoyant layers to support
mixed-phase lightning-production zones in deepening convective
turrets. Thunderstorm coverage should increase accordingly,
especially tonight in combined regimes 2 and 3. By the end of the
period, a well-established low-level convergence zone should exist
over the lower Mississippi Valley and south-central/southeastern LA,
with thunderstorm coverage increasing to at least scattered.
Forecast soundings suggest effective-inflow parcels may become close
to surface-based the last few hours of the period around
southeastern LA, but still with a shallow stable layer near the
surface related to relatively cool shelf waters. This regime should
continue to destabilize gradually into early day-2 with marginal
severe probabilities, as discussed in that outlook.
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