SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0757 AM CDT Fri Aug 23 2019
Valid 231300Z - 241200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
NORTH CAROLINA AND VIRGINIA...AS WELL AS THE CENTRAL HIGH PLAINS...
...SUMMARY...
Organized severe thunderstorms appear most probable today over
portions of North Carolina and Virginia, as well as the central High
Plains.
...Synopsis...
The upper-air pattern over the CONUS this period will be dominated
by a fairly high-amplitude northern stream across roughly the
northern half, along with several slow-moving shortwaves and related
MCVs from the central/southern Plains to the Tidewater region of the
Atlantic Coast. A synoptic cyclone over eastern QC will devolve
into an open-wave trough, extending southwestward over the lower
Great Lakes and interior Mid-Atlantic region. This will contribute
to the eastward ejection of a subtle, convectively enhanced
vorticity lobe, now apparent in moisture-channel imagery over the
Appalachians between southern WV and northeastern GA. This
perturbation should move east-northeastward across VA and northern
NC today, then offshore from the Delmarva Peninsula by 00Z.
Upstream, high-amplitude ridging will shift eastward from the upper
Mississippi Valley across the upper Great Lakes. The trailing
region of large-scale height falls will be populated by several
slowly eastward- to northeastward-moving MCVs and small shortwave
troughs over the central/southern Plains. A substantial shortwave
trough is located from a closed 500-mb low over southwestern MT
southward across UT to northern AZ. This trough will move eastward
to central/eastern portions of WY and CO through the period.
At the surface, 11Z analysis depicted a cold front from offshore New
England across DE, northern VA, central WV, southern IL and central
MO, becoming quasistationary northeastward across east-central/
north-central NE and southwestern ND, to a low over northeastern MT
north of GDV. As a surface anticyclone over Lake Superior shifts
eastward, the front will move southeastward/southward over VA/NC/KY,
reaching SC, TN, and northern AR by 12Z. The western segment of the
front will move little over NE while shifting northeastward slowly
across the Dakotas as a warm front.
...NC/VA...
Scattered thunderstorms are expected to develop along/ahead of the
front from midday through afternoon, mainly over western and
northern VA. This activity should expand and intensify while
encountering a favorably moist and destabilizing air mass across
central/southern VA, the southern Delmarva and northeastern NC. A
few severe gusts will be possible, along with more-widespread
damaging wind approaching severe limits. Isolated large hail also
may occur, especially over northern portions of the outlook in
association with colder air aloft and less-messy convective modes.
This area will reside under the southern rim of the stronger
midlevel winds aloft, related to the eastern Canadian synoptic
trough and the ejecting southern Appalachians shortwave trough, the
latter also providing support through subtle DCVA/ascent in
midlevels. The associated mass responses should encourage enhanced
convergence in the frontal/prefrontal air mass, amidst weak ambient
MLCINH. Favorable buoyancy will occur despite modest mid/upper-
level lapse rates, thanks to
1. Rich boundary-layer moisture (70s F surface dew points and PW of
1.75 to 2.25 inches being common), and
2. Areas of intermittent (but cumulatively favorable) diabatic
surface heating in the warm sector.
Peak preconvective MLCAPE should vary from around 500-1000 J/kg in
the northern and western parts of the outlook to around 1500-2000
J/kg over northeastern NC. 25-35-kt flow is forecast in much of the
700-400-mb layer, with marginally supercell-favorable effective
shear in northern areas. However, deep shear will diminish markedly
with southward extent. Upper-level winds will be weaker over most
of the area, however, contributing to potential for messy convective
modes after early semi-discrete phases. One or more bowing segments
may evolve within a band of mostly multicellular thunderstorms
across VA and into northeastern SC late this afternoon into early
evening before activity weakens and/or moves offshore.
...Central High Plains...
Scattered thunderstorms should develop this afternoon over
well-heated higher terrain, near that part of the I-25 corridor
between CPR-RTN. This activity is forecast to move eastward to
northeastward across the adjoining High Plains into a favorable
moist, diabatically destabilized air mass, offering damaging gusts
and sporadic hail. A tornado cannot be ruled out as well, this
threat depending strongly on mesobeta- to storm-scale processes that
remain rather uncertain/unfocused in predictability terms as this
time.
Mid/upper-level support is forecast to be atypically strong for this
time of year, with large-scale ascent from the shortwave
perturbations and strongly difluent upper flow. Steep midlevel
lapse rates and moist/warm advection in low levels will contribute
to destabilization as well, west of a well-defined moist axis that
parallels the frontal zone from central/eastern KS to the western
Dakotas. Surface dew points 50s to low 60s F should become common
across the central High Plains, and at least near and east of the
axis of the outlook area. These factors will contribute to peak
MLCAPE values ranging from around 1500 J/kg over parts of eastern WY
to the 2000-3000 J/kg range over much of the NE Panhandle, and parts
of eastern CO and western KS.
Given the strong veering with height expected in the low/middle-
level wind profiles, low-level bulk shear and hodographs will be
favorable for supercells and hail, though mid/upper winds will be
modest amidst the aforementioned difluence. Effective-shear
magnitudes 25-40 kt should be common. Modal uncertainties include
timing of the transition from discrete to clustered with respect to
storms moving from higher-elevation genesis areas into greater
moisture. Still, at least a few cold-pool-driven arcs or bands of
convection should develop to contribute to the wind potential, while
moving into well-mixed subcloud/inflow layers.
..Edwards/Gleason.. 08/23/2019
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