RSS Mesoscale Discussions from Storm Prediction Center

SPC Feb 18, 2019 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook
Day 1 Outlook Image
Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0656 AM CST Mon Feb 18 2019

Valid 181300Z - 191200Z


Thunderstorms are possible over the Four Corners, western Gulf
Coastal Plain and Mississippi Delta regions.

A mean trough will continue in mid/upper levels across the western
CONUS, with a series of shortwaves traversing the associated/
expansive cyclonic-flow field.  One of those embedded perturbations
-- now apparent in moisture-channel imagery over the southeastern
NV/northwestern AZ region -- is expected to pivot slowly across the
Grand Canyon/Lake Powell areas to southeastern UT/northeastern AZ. 
That will occur as another perturbation and associated vorticity
banner aloft dig south-southeastward down the CA coast and adjacent
waters toward and across northern Baja, reaching northwestern MX and
southwestern NM by 12Z.  As these features contribute to synoptic-
scale amplification of western troughing, gradual downstream height
rises are likely from the central/western Gulf Coast to the lower
Missouri and Ohio Valleys. 

At the surface, 11Z analysis showed an occluded low near BGM, with
occluded front extending to a triple-point low near Cape May.  A
cold front arched southward over the Atlantic before crossing
southeastern GA, the central FL Panhandle, and the northwestern
Gulf.  That portion of this front over the north-central/
northwestern Gulf should stall this afternoon, before moving
northward as a warm front overnight.  However, the surface boundary
should remain offshore from the northern/western Gulf Coast.  The
remainder of the front should proceed offshore from the Atlantic
Coast except for the part moving southward to central FL. 

...Four Corners vicinity...
The principal contributor to convective potential over this region
will be the leading substantial shortwave now over the LAS area, as
the latter pivots over the AZ/UT border.  Satellite imagery shows a
pronounced baroclinic leaf and associated zone of difluent
mid/upper-level flow ahead of the trough already, across much of
northern AZ and southern UT.  Associated large-scale lift, related
both to DCVA and a precursory belt of low-level warm advection,
combined with frontogenetic processes above the surface, should
provide the lift and lapse rates to support scattered to widely
scattered convection.  This will include isolated to widely
scattered, episodic thunderstorms -- some of which may be surface-
based on the high elevations of the Colorado Plateau, despite cold
surface temperatures.  Forecast soundings suggest MUCAPE of 100-300
J/kg is possible, much of which may become MLCAPE this afternoon.

Elsewhere over the Great Basin and central/northern Intermountain
regions, very isolated lightning cannot be ruled out, given the very
cold air aloft (500-mb temperatures commonly colder than -30 deg C)
and pockets of marginal midlevel moisture.  However, coverage/
duration of any such convection should be too meager to warrant a
10%/general areal outline.

...Western Gulf Coastal Plain and Mississippi Delta...
Isolated to widely scattered thunderstorms may develop during the
last few hours of the period across the outlook area, within a broad
zone of elevated, low-level warm advection and moisture transport. 
Planar progs and time series of forecast soundings reasonably
suggest that increasing thetae of parcels in the 700-800-mb layer
will occur after about 06Z, gradually reducing MLCINH such that
isentropic lift to LFC is enabled.  The bulk of thunder potential is
after 09Z, when nearly uninhibited MUCAPE of 200-500 J/kg may
develop.  Favorable boundary-layer thetae and associated surface-
based effective-inflow parcels should remain well offshore until the
following period, when marginal severe potential develops, per the
most recent day-2 outlook.

..Edwards.. 02/18/2019

Read more

SPC Feb 18, 2019 0830 UTC Day 3 Severe Thunderstorm Outlook

SPC 0830Z Day 3 Outlook
Day 3 Outlook Image
Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0217 AM CST Mon Feb 18 2019

Valid 201200Z - 211200Z


Strong thunderstorms may continue to impact parts of the lower
Mississippi Valley into the day Wednesday, accompanied by at least
some risk for severe weather.

Models indicate little change to the general large-scale pattern
from Tuesday into Wednesday.  A belt of mid-latitude westerlies
likely will remain amplified across the eastern Pacific into western
North America, while flow remains broadly confluent downstream,
across eastern North America, to the north of prominent subtropical
ridging centered to the east of the Bahamas.

While another strong short wave impulse continues to dig along the
U.S. Pacific coast, toward the base of the larger-scale western U.S.
troughing, a downstream impulse emerging from the troughing is
forecast to progress around the crest of the larger-scale eastern
U.S. ridging.  At the same time, a gradual erosion of cold surface
ridging appears likely to proceed to the east of the Mississippi
Valley, and it appears that a modest cyclone may evolve within
deepening surface troughing accompanying the lead short wave
impulse, across the Upper Midwest and Great Lakes region Wednesday
through Wednesday night.

Considerable convective development, including embedded
thunderstorms, may be ongoing early Wednesday across the lower
Mississippi and Ohio Valleys, largely driven by warm advection and
moisture return above the residual cold/stable surface-based air. 
Forcing for this elevated destabilization and convection is expected
to spread east-northeastward across the Appalachians and northern
Mid Atlantic region through the period, while additional convective
development persists along a trailing quasi-stationary baroclinic
zone across the Tennessee and lower Mississippi Valleys into the
Texas Gulf coast vicinity.

Much of this convection may remain rooted above a cold/stable layer.
However, a corridor of weak to modest boundary-layer instability
developing across parts of the lower Mississippi Valley by 12Z
Wednesday may linger into at least midday, while slowly spreading
eastward.  It appears that vertical shear will remain strong enough
across this region to maintain an environment at least marginally
conducive to organized severe thunderstorm development, perhaps
including supercells with the risk for a tornado or two.

..Kerr.. 02/18/2019

Read more

SPC Feb 18, 2019 Day 4-8 Severe Weather Outlook

Day 4-8 Outlook
Day 4-8 Outlook Image
Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0356 AM CST Mon Feb 18 2019

Valid 211200Z - 261200Z

Models continue to indicate that blocking will remain prominent
within the mid-latitude westerlies across the northeastern Pacific
through this period.  Downstream, it appears that flow may
transition to a broadly confluent regime across the Pacific coast,
Rockies, and Plains, and a less amplified regime in general across
the southern tier of the U.S., by late next weekend into early next
week.  As this takes place, one more vigorous short wave impulse is
forecast accelerate east-northeastward out of the Southwest.  

There appears increasing consensus, within and among the various
model output, that this feature will provide support for strong
surface cyclogenesis to the lee of the southern Rockies by this
weekend.  Guidance generally indicates that the cyclone center will
rapidly migrate from the Texas/Oklahoma Panhandle region into the
Great Lakes on Saturday.  This probably will include intensifying
lower/mid tropospheric flow (to 50-70 kt in the 850-500 mb layer)
across an evolving warm sector boundary layer that will become at
least weakly unstable, across lower portions of the southern Plains
through the mid to lower Mississippi Valley region.  This
environment may become conducive to the evolution of an organized
mesoscale convective system, and perhaps discrete supercell
development ahead of it, accompanied by the risk for damaging wind
gusts and tornadoes.  

It remains unclear whether destabilization eastward and
southeastward, toward the Mid Atlantic and Southeast, on Sunday will
remain sufficient to support an appreciable continuing severe
weather threat.

Read more