SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0753 AM CDT Mon Jul 25 2022
Valid 251300Z - 261200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN MID-ATLANTIC AND NEW ENGLAND...
...SUMMARY...
Scattered strong-severe thunderstorm gusts are possible across parts
of the northern Mid-Atlantic and New England.
...Synopsis...
In mid/upper levels, northern-stream flow is becoming characterized
by broadly cyclonic character across the northern Rockies to the
Great Lakes and Northeast, with the Great Lakes part being nearly
zonal. This is in response largely to the presence of two dominant
cyclones over Canada:
1. A long-lived gyre over the James Bay region, forecast to move
eastward across northern QC. A shortwave trough over its southern
sector is apparent in moisture-channel imagery from the northern
ON/QC border across northern Lake Huron and portions of Lower MI.
This trough will pivot across southern QC and the St. Lawrence
Valley region today, with its southern fringe glancing the northern
NY/northern New England area. A weaker, trailing vorticity lobe
will move eastward from Lake Erie across NY this afternoon/evening,
then weaken as it crosses New England.
2. A strengthening vortex digging southeastward from the SK/NWT
border region across northern MB. Heights will fall across the
Dakotas late in the period (overnight) as a basal shortwave trough
approaches. In advance of that, a small shortwave trough --
initially apparent over southeastern MT/northeastern WY -- will move
east-southeastward to central SD today, while weakening, then across
southern MN overnight.
At 11Z, the surface analysis showed a cold front from western NY
across central OH, the lower Ohio River Valley, and MO Ozarks,
becoming a warm to stationary front across southern KS to a low
between LBL-GAG. By 00Z, the cold front should reach eastern New
England, southern PA, eastern KY, western TN, then become a wavy/
quasistationary front over southwestern MO and southern KS. By 12Z,
the front should extend across NJ to eastern KY, then
quasistationary near its previous position across MO/KS. A separate
cold front -- related to the trailing northern-stream cyclone over
central Canada -- will move southeastward across the northern High
Plains overnight, reaching from eastern ND to central WY by 12Z
tomorrow.
...New England and northern Mid-Atlantic region...
Scattered thunderstorms are expected to form through this afternoon
along/ahead of the surface cold front, near a prefrontal surface
trough, and farther south over portions of the Virginias, over
strongly heated higher terrain in the warm sector. An existing
plume of clouds/precip and widely scattered embedded thunderstorms,
from northern ME into PA, should continue to break up gradually on
the north end this morning, permitting some destabilization in its
wake, but also setting up localized to mesobeta-scale areas of
differential heating that may aid storm initiation from midday into
the afternoon as well. Scattered damaging gusts, and several
severe/50-kt gusts, are possible with the midday/afternoon activity.
A marginal/conditional tornado threat also may exist over northern
areas, where low-level and deep shear will be the greatest under
relatively maximized flow aloft.
Instability generally will increase with southward/southeastward
extent away from the early cloud cover, as well as between it and
the front, under cooler air aloft. Meanwhile, boundary-layer
moisture will remain favorable across the entire swath (i.e.,
surface dewpoints upper 60s to low 70s F). This will offset modest
midlevel lapse rates to foster a plume of MLCAPE around 1500-2000
J/kg from northern VA and the Chesapeake Bay area, narrowing
northeastward to the southern/eastern coastal areas of New England
away from marine-layer influences. MLCAPE should diminish
northwestward from there, but still may reach 500-1200 J/kg over ME,
except for Downeast areas with onshore flow. Forecast effective-
shear magnitudes range from a supercell-favoring 40-50 kt over
northern New England to around 20-30 kt or less across central VA to
eastern WV, though a more strongly-heated, deeply mixed boundary
layer may develop in southern areas in support of pulse/multicell
gust potential.
...Mid Mississippi Valley region overnight...
Given some capping and lack of large-scale support aloft,
considerable uncertainties exist as to whether (and if so, how much)
convection can develop along or just north of the front this
afternoon. Still, as the southwesterly low-level jet intensifies
into the region north of the Missouri River this evening, warm
advection and isentropic lift should strengthen along the elevated
frontal surface until LFC is reached. With or without antecedent/
diurnal activity moving into western parts of the area,
thunderstorms should develop near the nose of the LLJ where ascent
will be focused strongest. Forecast MUCAPE around 1000-1500 J/kg,
amidst 35-40-kt effective-shear magnitudes and 200-300 J/kg
effective SRH (accounting for elevated inflow) suggest a few
organized cells are possible, particularly early in the convective
cycle before a more front-parallel linear configuration evolves.
Potential upscale growth into an MCS and eastward/forward
propagation across IL is too uncertain to account for in
unconditional probabilities at this stage.
...Central/northern High Plains...
Conditional severe potential (hail/wind) is evident in this region
this afternoon into this evening, with a deeply mixed diurnal
boundary layer, steep low/middle-level lapse rates, and pockets of
favorable/residual moisture all expected to support a plume of
1500-2000 J/kg MLCAPE from parts of western SD into the NE
Panhandle. Any convection that does develop will be in an
environment of modest-magnitude but strongly veering low-level winds
with height, leading to long hodographs and around 40-50 kt
effective-shear magnitudes. However, convective guidance is quite
inconsistent with timing, location and even existence of development
in this region, due in part to the dearth of large-scale support
related to its position behind the aforementioned shortwave trough
moving out of southeastern MT. Surface forcing also appears
nebulous east of a lee trough in eastern parts of WY/MT. Still, if
convective guidance and mesoscale trends concentrate convective
focus better than apparent for the last outlook cycle (or this one),
an area of unconditional severe probabilities may need to be
reintroduced at some point today.
..Edwards/Gleason.. 07/25/2022
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