Probability of a tornado within 25 miles of a point. Hatched Area: 10% or greater probability of EF2 - EF5 tornadoes within 25 miles of a point.
Day 1 Tornado Risk
Area (sq. mi.)
Some Larger Population Centers in Risk Area
St. Louis, MO...Montgomery, AL...Springfield, IL...Peoria, IL...Tuscaloosa, AL...
New Orleans, LA...Birmingham, AL...Baton Rouge, LA...Mobile, AL...Columbus, GA...
Probabilistic Damaging Wind Graphic
Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Day 1 Wind Risk
Area (sq. mi.)
Some Larger Population Centers in Risk Area
New Orleans, LA...St. Louis, MO...Birmingham, AL...Baton Rouge, LA...Montgomery, AL...
SPC AC 011257
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0657 AM CST Sat Dec 01 2018
Valid 011300Z - 021200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE MIDDLE MISSISSIPPI VALLEY AND CENTRAL GULF COAST STATES...
Severe storms are possible today across parts of the middle
Mississippi Valley and central Gulf Coast States.
In mid/upper levels, the synoptic- to large-scale pattern for the
CONUS is transitioning toward a blocky phase with strong ridging
over the northeastern Pacific, and broadly cyclonic flow from West
to East Coast by the end of the period. This will occur as a
pronounced 500-mb cyclone -- initially centered over KS -- deepens
somewhat and moves northeastward to southeastern IA. Meanwhile, an
upstream shortwave trough -- now apparent in moisture-channel
imagery offshore OR and northwestern CA -- will move southeastward
across the Sierra to the southern NV/northwestern AZ region by 12Z.
A subtle, slow-moving southern-stream perturbation over the
north-central/northeastern Gulf will move northeastward across the
central/northeastern Gulf Coast region today.
At the surface, the 11Z analysis showed a deep/988-mb surface low
over east-central KS, with Pacific cold front arching diffusely
southeastward/southward over AR and northwestern LA behind earlier
convection, and then across southeast and deep south TX. Two warm
fronts were drawn:
1. Synoptic warm front from the low generally eastward over central
MO, southern IL, and eastern KY, and
2. Gulf/marine boundary from near MEM southeastward over eastern MS
to near MOB then southeastward offshore from the west-central FL
The surface low is forecast to shift northeastward through the
period largely in step with the mid/upper-level cyclone, with which
it nearly is vertically stacked already. The cold front should move
eastward to northern GA, southeastern AL, the western FL Panhandle,
and the west-central/southwestern Gulf by 12Z, decelerating as it
becomes largely parallel to winds aloft. The poleward/synoptic warm
front will proceed northeastward to the southern upper Great Lakes
and lower Great Lakes. The southern/marine warm front is expected
to move slowly northeastward/inland today, reaching south-central AL
and southwestern GA this afternoon into early evening.
...Middle Mississippi Valley...
A curving, northwest/southeast-aligned arc of strong-severe
thunderstorms (including a few supercells) is expected to form
midday through afternoon and move northward to northeastward across
the outlook area, offering a sporadic, full-spectrum severe threat.
Activity may affect portions of extreme southeastern IA,
northeastern MO, and western/central IL before weakening in the
evening with loss of boundary-layer instability.
East of the cold front, behind the morning clouds/precip, and south
of the northern/synoptic warm front, a relatively narrow corridor of
favorable destabilization is expected. Low/middle-level lapse rates
will steepen considerably through the day from a combination of
cooling/DCVA aloft related to the mid/upper cyclone, low-level
warm/moist advection and areas of direct insolation/diabatic
heating. The result should be a corridor of 500-1500 J/kg MLCAPE,
with forecast soundings showing surface-based effective-inflow
parcels even for temperatures in the low 60s F. Strong deep shear
also is expected across the outlook area beneath 60-90-kt 500-mb
winds, while backed flow near the warm front yields favorably
enlarged low-level hodographs.
...Gulf Coast States...
Thunderstorms will pose a risk of damaging gusts, hail and a few
tornadoes in two main regimes that may have some spatial overlap
over the course of the entire period, even as they are somewhat
separated at any given time. The aforementioned southern-stream
perturbation has been associated with an area of convection now
south of the AL and western FL Panhandle coasts, that has persisted
for about 29 hours since originally forming over the west-central
Gulf. This activity has included occasional supercells, based on
longer-range radar characteristics, in turn indicating a favorable
kinematic and thermodynamic environment south of the warm-frontal
zone as well. Backed flow near the marine front, as well as
favorably large boundary-layer theta-e , will support a buoyancy/
shear parameter space suitable for supercells with any of that
convection that can move ashore along or south of the warm front.
Farther west, isolated to widely scattered severe storms may develop
along/ahead of the surface cold front, with damaging gusts being the
main concern and a tornado or two also possible. Buoyancy and shear
conditions in the foregoing warm sector will be favorable for
supercells, with surface dew points mid 60s to low 70s F and diurnal
heating supporting MLCAPE 1500-2500 J/kg, amidst 50-60-kt effective-
shear magnitudes. One concern will be some veering of prefrontal
surface winds which will act to reduce both low-level shear/SRH and
low-level convergence, potentially mitigating convective coverage/
longevity. Amount and persistence of low-level forcing for ascent
is uncertain, but may best support sustained updrafts in prefrontal
confluence/convergence zones across southern MS/AL into central AL.
As such, relatively maximized tornado potential is maintained across
that area and extended to parts of the Gulf Coast discussed above.
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