Day 1 Severe Weather Outlook

Valid 191300Z – 201200Z


The severe-thunderstorm threat appears most concentrated tonight
over parts of Iowa, in the form of wind and hail.

The broader mid/upper-level pattern over the CONUS is expected to
amplify somewhat through tomorrow morning, as ridging builds from
the southern High Plains across the central/northern Rockies. A
synoptic cyclone over northern MB is progged to elongate zonally and
redevelop eastward across southern Hudson Bay to northern QC through
the period. As that occurs, general height rises are expected
across most of the Great Lakes region. A related, positively tilted
shortwave trough is located from western NY southwestward across the
Tennessee Valley region. The southern portion of this trough, from
KY southward, will move eastward and weaken considerably. The
northern part will eject northeastward across the remainder of PA,
NY and New England through about 00Z. A weak shortwave trough —
initially apparent in moisture-channel imagery over WY — is
forecast to move eastward across much of southern SD and northern NE
through 00Z, then turn southeastward over IA overnight.

At the surface, the 11Z analysis showed a cold front from
southwestern QC across southern ON to southern/central Lower MI,
becoming wavy and quasistationary southwestward across southeastern
IA, northwestern MO, and central/western KS. The central Plains
segment of this front is forecast to move northward and reorient
from northwest to southeast through the period. By 12Z the
resulting warm front should extend from east-central/northeastern WY
across northern NE and western/southern IA. A dryline should mix
eastward across the High Plains today, reaching the NE Panhandle,
extreme eastern CO/western KS and northeastern NM.

…Northern Plains to IA…
Isolated to widely scattered thunderstorms are expected to form over
the northern Plains part of the outlook area this afternoon into
evening, offering the potential for isolated severe gusts/hail. A
more-robust severe threat is becoming apparent overnight across
portions of IA in association with potential formation of severe
thunderstorms with large hail as the main early-stage threat,
evolving toward damaging wind as activity aggregates upscale. A
forward-propagational MCS is possible into parts of central/southern
IA late overnight.

Return flow on either side of the warm-frontal zone will foster
increasing low-level theta-e throughout the day and into tonight,
with 60s surface dew points becoming common, and values near 70 F
possible on an isolated basis. Large-scale support will be weak,
especially with heights building over and west of the area, but weak
MLCINH and strong surface heating should foster the development of
at least a few thunderstorms in an environment of steep low/middle-
level lapse rates and well-mixed subcloud layers. Increasing
moisture with southward/southeastward extent and backing of low-
level flow near and on the north side of the warm front will augment
deep shear in support of some supercell potential with any sustained
convection. MLCAPE 3000-4500 J/kg is possible. Lack of stronger
large-scale support, and concerns over convective coverage, preclude
more than a marginal risk for most of the region at this time.

However, concerns (and prognostic consensus) for overnight MCS
action are increasing across portions of IA in the zone of strong
low-level warm advection and moisture transport, near the nose of a
40-50-kt southwesterly LLJ. Steep midlevel lapse rates and
increasing low-level moisture will yield MUCAPE increasing to the
2500-3500 J/kg range with decreasing MUCINH. While forecast
soundings depict parcels rooted in the low levels, but above the
surface, the presence of dry air in the subcloud layer suggests the
potential for strong-severe downdrafts to reach the surface —
especially with support from any cold-pool forcing and rear-inflow-
jet processes that can organize on the storm scale. Effective-shear
magnitudes in the 35-50-kt range also indicate the potential for
supercells, especially in the early hours of convective organization
before too much clustering occurs, and an attendant severe-hail

…New England…
Widely scattered thunderstorms are expected to form as early as
midday over portions of central/northern NY and during the afternoon
elsewhere, in a loosely north-northeast/south-southwest-oriented
broken band. Damaging gusts, including isolated gusts to severe
limits, are possible region-wide. A marginal hail threat exists
mainly over ME, and a tornado cannot be ruled out.

Activity should develop behind and between areas of antecedent cloud
cover, near a surface trough, where diabatic surface heating can
destabilize the boundary layer favorably and remove MLCINH. Weak
large-scale ascent also should spread across the area very near the
ejecting mid/upper-level shortwave trough. These factors, along
with moisture represented by surface dew points generally in the
mid/upper 60s F, should support the development of preconvective
afternoon MLCAPE in the 1000-1500 J/kg range. Mid/upper-level winds
will strengthen with northward extent across the outlook area, and
with time through afternoon over northern New England, as the
aforementioned cyclonic processes over Canada tighten the height
gradient aloft. Though low-level hodograph size will be modest,
effective-shear magnitudes of 30-40 kt may favor organized
multicells, small bows, and transient/marginal supercellular
characteristics especially in ME.

…Central High Plains…
Isolated to widely scattered thunderstorms should develop this
afternoon near the dryline and move eastward to northeastward.
Very strong heating/mixing of the boundary layer is forecast, which
will yield nearly dry-adiabatic lapse rates from surface to near 500
mb. Modified model soundings yield around 500-1000 J/kg MLCAPE in
those areas east of the dryline/lee trough where CINH is minimized.
CAPE increases with eastward extent into more robust moisture, but
so will capping. Lack of more robust low/middle-level winds will
limit vertical shear, but the well-mixed subcloud layer will support
isolated severe downburst potential with any sustained convection.

..Edwards/Marsh.. 08/19/2019


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