Day 1 Severe Weather Outlook

Valid 311300Z – 011200Z


The greatest concentration of strong-severe thunderstorms today
shoudl be over parts of the Mid-Atlantic and Northeast, from eastern
Virginia to southeastern New York. Damaging to severe winds are the
main concern.

The persistent, intense, mid/upper-level high over NM may make a
temporary eastward diversion toward the TX Panhandle today in
response to a series of convectively generated/enhanced vorticity
maxima moving through the associated anticyclone’s western sector,
across potions of AZ/UT. Meanwhile, eastern CONUS mean troughing
will remain, but become more positively tilted, as the northern part
shifts eastward over the lower Great Lakes and New England, while
the southern part stays across the TN/MS/AL region.

At the surface, 11Z analysis showed a wavy cold front from southern
QC across northwestern PA, central OH, western TN, and portions of
the MO/AR Ozarks, becoming a stationary to slowly moving warm front
over southeastern/north-central NR. The front should move
southeastward across much of New England and the Mid-Atlantic region
through the period, while drifting southward and becoming diffuse
along the segment between the central Appalachians and Mississippi
River. A weak low initially analyzed near MCK is progged to drift
east-southeastward over northwestern/north-central KS through
tonight, possibly linking with the frontal zone over northern KS by
the end of the period. Surface troughing will persist over eastern
MT, with erratic and possibly oscillatory movement.

Scattered thunderstorms are forecast to initiate this afternoon over
higher terrain from central PA to northeastern WV and northern VA,
with more isolated to widely scattered development possible farther
northeast through central/northern NY and portions of New England
near and east of a pre-frontal surface trough. Activity will pose a
risk of damaging wind, particularly in and near the 15%/slight-risk
area where the densest concentration of convection is anticipated.

Rich low-level moisture is expected to persist across the warm
sector in this region today, with surface dew points in the
mid-upper 60s over most of the regional warm sector — highest near
the coast from southern New England southwestward where some 70s
will be measured. Diurnal heating will steepen low-level lapse
rates and strip away MLCINH readily, while the slowly approaching
mid/upper trough contributes subtle DCVA/destabilization aloft.
These factors will contribute to a corridor of around 1000-1500 J/kg
preconvective MLCAPE (locally higher) over the Piedmont and coastal
plain. Weak low/middle-level winds will limit vertical shear,
though ventilating/anvil-level flow will be favorable. The dominant
mode should be multicellular, with some upscale clustering or
quasi-linear aggregation possible across parts of VA/PA/MD.

…Northern Great Plains…
Widely scattered to isolated, mostly high-based thunderstorms are
expected to form this afternoon over parts of extreme eastern MT or
western ND, and southwestward over parts of eastern WY as well.
Isolated severe gusts/hail should occur.

As the mid/upper-level high shifts slightly eastward, broader-scale
ridging should build over the northern Plains, resulting in height
rises today over most of the region. One exception will be across
parts of WY, coming under the influence of the northern parts of the
northeastward-moving vorticity banner now over UT/AZ. Strong
diurnal heating is forecast under that ridging, near the surface
trough, affecting a corridor of relatively maximized low-level
moisture with surface dew points commonly in the 60s F. Vertical
mixing may reduce dew points into the 50s F in some areas, but
favorable buoyancy still is expected amidst steep low/middle-level
lapse rates. MLCAPE of 1500-2500 J/kg is expected (locally near
3000 J/kg over parts of ND), atop very well-mixed subcloud layers
with DCAPE commonly over 1000 J/kg. Meanwhile, despite the ridging
aloft, a corridor of relatively maximized 250-400-mb layer flow
curving across the region will foster favorable deep shear for some
activity to become organized, though weak low/middle-level winds
will limit hodograph size and curvature.

…Missouri Valley, east-central Plains…
The persistent low-level warm-advection and moisture-transport
regime contributing to ongoing, non-severe convection across this
region will persist through the period. Its strength will vary
considerably, weakening into a relative lull much of today, with a
peak expected again this evening into parts of the overnight hours.
Parcels isentropically boosted to LFC this evening will access steep
low/middle-level lapse rates in an elevated buoyant layer, off the
eastern rim of the strongest EML air. Accordingly modified model
soundings depict 1000-1500 J/kg elevated MUCAPE, rooted generally in
the 800-850-mb layer, beneath north-northwesterly flow in mid/upper
levels. The resultant strong directional shear contributes to 50-60
kt effective-shear magnitudes, indicating the potential for some
activity to become organized with isolated severe hail possible.
Strong-severe gusts — penetrating a shallow near-surface stable
layer to ground level — cannot be ruled out either, depending on
the extent of modal aggregation/clustering.

..Edwards/Dial.. 07/31/2019


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