- Day Three
acus01 kwns 221456
Storm Prediction Center ac 221454
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0854 am CST Wed Feb 22 2017
Valid 221300z - 231200z
..no severe thunderstorm areas forecast...
the risk for severe thunderstorms appears negligible across the U.S.
Today through tonight.
A progressive upper-air pattern will continue over the contiguous
U.S. Through the period, dominated by two primary perturbations:
1. Closed southern-stream cyclone now centered near mob and
covering most of the area from the Tennessee Valley and southern
Appalachians across the central/eastern Gulf and Florida. The associated
500-mb low is forecast to dig southeastward across the northeastern
Gulf then pivot eastward across southern Florida during the last few
hours of the period.
2. A trough, accompanied by a broad area of cyclonic flow, now
apparent in moisture-channel imagery approaching the West Coast from
northern California northward. This trough, through which several
embedded/smaller-scale vorticity maxima should move, should reach
the northern rockies, Utah and Arizona by the end of the period.
At the surface, a diffuse, wavy frontal zone was evident at 11z from
New England across the lower and upper Great Lakes, to a low over
Central Lake Superior, then across southern Minnesota to central/eastern
Montana. A weak low over the Gulf south of aaf should migrate erratically
southeastward across FL, likely remaining ill-defined given
convective/precip modification of the air mass to its near east and
southeast. Lee troughing should intensify through tomorrow morning
over the Central High plains just east of the Front Range. By 12z,
an elongated/closed area of low pressure likely will be in place
over portions of Colorado and WY, along a warm to stationary frontal zone
extending southeastward over southwestern Kansas and northern OK.
Moisture still should be insufficient for a thunder threat this soon
with this system, which will be the nascent form of the cyclone
forecast to eject across the Central Plains and upper Midwest days
Within a broad precip plume now entering the western peninsula and
Lower Keys, isolated to widely scattered thunderstorms are possible.
Isolated to widely scattered thunder also is possible with
convection accompanying the cold-core vortex this evening and
overnight, away from the deepest dry-slot penetrations into the
inner region of the cyclone.
The most favorable cape and shear parameter space should develop
today and tonight well east of the area, over the northern Bahamas
and adjoining waters. Over FL, lack of both vertical shear and more
robust low-level moisture, along with (in the leading precip plume)
weak midlevel lapse rates, will preclude an organized severe threat.
However, strong gusts from water-loaded downdrafts may accompany the
most vigorous cores, primarily over open-water exposures. The 12z
tbw sounding was launched in precip, but the mfl sounding appeared
representative of the preconvective environment with a choppy cape
profile, thanks to stable layers aloft. The eyw sounding, though
partly contaminated by cloud/precip cover, still contained
surface-based buoyancy as well.
Generally diminishing thunder potential is expected northward into
GA, with distance from the strongest midlevel destabilization
associated with the mid/upper cyclone. A plume of warm advection
and moisture transport will continue ahead of the dry slot, which is
evident in mid/upper levels off the back edge of the precip plume --
roughly 60 west of pie and 85 west of eyw. MLCAPE mostly should
remain less than 700 j/kg amidst generally small low-level
..upper Midwest/Great Lakes...
A compact, northern-stream shortwave trough and embedded vorticity
Max -- currently evident near the Montana/SD/ND border confluence -- will
progress eastward across northern South Dakota by 00z then over Northern Lake
Michigan by dawn tomorrow. Though moisture is very limited in the
warm-advection plume to its east, large-scale lift and low-level
frontal ascent together should support the potential for isolated,
episodic thunderstorms through tonight. This includes the initial
area of convection now apparent roughly between bis-mbg, within
which sporadic lightning has been detected. The thunder potential
will shift eastward across the outlook corridor with time and likely
will be unevenly distributed in space and time -- but isolated
..West Coast states...
A large area of cold air aloft and embedded shots of DCVA-enhanced
midlevel destabilization will help to maintain steep
low/middle-level lapse rates across the outlook area. Isolated
thunder is possible where low-level moisture and that instability
aloft are vertically juxtaposed, amidst weak cinh.
acus11 kwns 202038
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 202037
Mesoscale discussion 0198
National Weather Service Storm Prediction Center Norman OK
0237 PM CST Mon Feb 20 2017
Areas affected...parts of Louisiana and into far southwest
Concerning...severe potential...watch unlikely
Valid 202037z - 202230z
Probability of watch issuance...20 percent
Summary...a slow increase in storm intensity is ongoing, along with
low-probability risk for an isolated damaging wind gust or two, or a
brief tornado. At this time, it appears that risk will remain
sufficiently limited to preclude the need for ww issuance.
Discussion...latest surface analysis shows persistent southeasterly
flow off the north-central Gulf of Mexico, contributing to both
onshore advection of upper 60s to low 70s dewpoints, as well as some
enhancement to the low-level shear profile. As a semi-organized
band of storms continues crossing western Louisiana, a weakly
heating/destabilizing airmass (characterized by around 750 j/kg
mixed-layer cape) is contributing to some increase in updraft
strength -- both within the line, and with isolated showers
developing ahead of the main band of storms.
While potential for additional destabilization is limited, low-level
and deep-layer shear are both sufficient to support a few stronger
cells, and at least weak/mid-level rotation. Indeed, a few cells
immediately ahead of the main convective band have exhibited weak
rotation per area WSR-88D data, and this will likely continue over
the next several hours. Overall risk, however, appears likely at
this time to remain marginal, and thus likely not requiring watch
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 29749036 29169024 29079132 29409283 30819321 32059330
32479261 32109152 30889136 29749036