SPC AC 211259
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0759 AM CDT Sat Jul 21 2018
Valid 211300Z - 221200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF BOTH
THE SOUTHEAST AND THE NORTHERN HIGH PLAINS...
Severe thunderstorms are expected across parts of the southeastern
U.S. and the northern High Plains.
A seasonally high-amplitude pattern characterizes the middle/upper
troposphere over the CONUS. A longstanding, strong anticyclone now
centered over TX Panhandle is forecast to retrograde slowly across
northeastern NM through the period. This will occur as a large,
closed, occluded, essentially stacked cyclone -- now centered over
the Michiana area -- drifts southeastward obliquely across the IN/OH
border. Associated cyclonic flow will cover most of the nation
between the Mississippi River and Atlantic coast. Within that
cyclonic flow, a negatively tilted shortwave trough is noted over
the Carolinas and adjoining Atlantic. This feature should eject
northward over the Tidewater/Delmarva region through 00Z, then wrap
north-northwestward over the inland Mid-Atlantic region overnight.
A series of convectively induced/enhanced, lower-amplitude vorticity
lobes also will orbit upstream areas in the southern semicircle of
the synoptic cyclone.
Elsewhere, a strong northern-stream shortwave trough is apparent in
moisture-channel imagery extending southwestward from a compact
cyclone over western AB, across Vancouver Island to the Pacific west
of WA/OR. The cyclone is progged to devolve to an open wave through
the period. Nonetheless, a strong shortwave trough should reach
southwestern SK and north-central/southwestern MT by 12Z.
At the surface, an occluded low was analyzed at 11Z near SBN, with
occluded front to northern KY, and cold front across western TN into
some outflow from overnight convection over AR. The frontal zone
was quasistationary over OK but should sag southward slowly there
today, while moving southeastward across TN/AL. A cold front
associated with the northern-stream perturbation should cross much
of MT through the period, with a frontal-wave low developing tonight
over eastern MT. Warm frontogenesis already is evident across
southeastern MT into western NE, and this should shift northward
across the northern High Plains through the period.
Two primary/ongoing convection/precip areas related to prior
overnight MCS action will impact the severe potential today. A
decayed western MCS that had produced severe/damaging gusts over
portions of AR/MS has left an area of precip, embedded/subsevere
thunderstorms and outflow that continues to move southeastward over
portions of MS into extreme western AL. Another, still well-defined
arc of thunderstorms is moving southeastward across northern/central
GA and extreme eastern AL, with a trailing outflow boundary still
moving southwestward over parts of northern/central AL (but likely
to decelerate/stall with time). See SPC watch 197 and related
mesoscale discussions for near-term guidance on the eastern MCS.
Should this complex survive an area of relatively strong CINH this
morning over GA, it may re-strengthen over southern GA and/or parts
of SC this afternoon, while impinging on a diurnally heated, very
moist, destabilizing air mass. Damaging gusts and isolated hail
would be the main concerns.
Otherwise, scattered thunderstorms are expected to develop again
this afternoon along and near the boundary left by the eastern
complex, offering a risk of damaging gusts and large hail, and
perhaps a tornado or two. This boundary, whose specific location
several hours from now still is uncertain, but generally
northwest/southeast across parts of GA/AL, will demarcate the
diabatically destabilizing eastern rim of some very richly
moist/high-buoyancy warm-sector air. The rate/intensity of heating
may be hampered somewhat by cloud debris left over from the western
complex, particularly with northward extent and closer to the
synoptic front. Furthermore, the forecast westerly surface
component west of that outflow boundary appears likely to advect
convectively processed boundary-layer air eastward into areas now
covered by a more favorable air mass ahead of the front. As such,
convective development/coverage is more uncertain over parts of
northern AL and MS, and the unmodified warm-sector plume should
narrow from west to east with time.
In that more-unperturbed warm-sector slot nearer to the boundary,
and southward to the Gulf Coast, expect low/mid-70s F surface dew
points, 16-19 g/kg mean mixing ratio and PW 1.75-2 inches (locally
higher). Forecast soundings suggest a corridor of 2500-3500 J/kg
MLCAPE near the boundary -- increasing to around 4000 J/kg near the
FL Panhandle coast, and decreasing gradually eastward/northeastward
around the eastern outflow pool to the Carolinas where lapse rates
aloft will be weaker. Strongest vertical shear also should be near
the boundary where relatively backed flow and west to northwest
winds aloft contribute to 35-45 kt effective-shear magnitudes.
Meanwhile shear will lessen westward into the westerly surface
winds, and northward into a zone of weaker, difluent mid/upper winds
over the central/southern Appalachians, to the left of the jet
...Northern High Plains...
Widely scattered to scattered thunderstorms are expected to develop
this afternoon in a meridional corridor from northeastern WY and
near the Black Hills northward, ahead of the surface cold front,
near a surface trough/confluence line. Activity will pose a risk of
damaging gusts and sporadic large hail before moving into
lower-theta-e air over the Dakotas this evening and weakening. A
narrow but well-defined corridor of rich low-level moisture over the
central High Plains -- relatively unperturbed by assorted MCS
activity the last few days farther southeast -- was evident in
overnight surface analyses and sampled well by 12Z LBF RAOB. This
moist plume, somewhat diluted by lateral and vertical mixing, should
advect northwestward in support of convective potential. Steep
low/middle-level lapse rates and well-mixed subcloud layers are
forecast, with preconvective MLCAPE reaching 1500-2500 J/kg in a
very narrow corridor, juxtaposed with 40-50 kt effective-shear
...Coastal Mid-Atlantic/New England late...
As the mid/upper shortwave trough ejects northward, closer to this
region, the surface low should deepen and move generally up the
coast to NJ by late evening, then inland while weakening overnight.
As this occurs, both low-level moisture/theta-e and low-level
vertical shear will increase over the outlook area, resulting in
weak buoyancy tonight amidst lapse rates only slightly greater than
moist adiabatic in low/middle levels. The latter factor will limit
storm intensity/coverage, despite favorable low-level hodographs. A
brief tornado cannot be ruled out, though the potential appears very
marginal and conditional at this time.
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z