Day 1 Severe Weather Outlook

Valid 021300Z – 031200Z


Isolated severe thunderstorms are possible over parts of central
Kansas to central Oklahoma, and over parts of eastern Idaho to
western Montana.

The large-scale mid/upper-level pattern that has been dominant over
the CONUS for many days will persist through this period: a weak
but lengthy eastern trough, and an intense anticyclone over the
southern Rockies with mean ridging northward toward MT. A strong
shortwave trough — now evident in moisture-channel imagery over
Vancouver Island, BC, will deamplify slightly (but remain intense)
through the period. This trough will move east-northeastward to the
Canadian Rockies and ID Panhandle by 12Z. Meanwhile, an MCV over
northwestern KS and associated weak shortwave trough should move
east-southeastward across most of KS through today, turning
southeastward over portions of eastern OK and AR through tomorrow

At the surface, a weak cold front will move eastward across the
northern Rockies through the period, west of an area of lee
troughing persisting across eastern portions of MT/WY/CO. A weak
yet well-evident low analyzed at 11Z over southwestern KS should
move slowly southeastward over northwestern OK today, with a trough
extending southwestward across the TX Panhandle and northeastward to
an outflow/differential-heating boundary over southern KS and
northern OK.

Ongoing convection, embedded in a large swath of precip from
southeastern NE across east-central KS to north-central/
northeastern OK, generally should weaken through the remainder of
the morning. While a brief/isolated core flareup with marginally
severe hail report cannot be ruled out, the main severe potential
will exist with separate activity developing this afternoon.

Isolated to widely scattered thunderstorms are possible from mid/
late afternoon into evening across portions of central/south-central
KS and perhaps northwestern/central OK. The main foci for this
development in low levels should be the outflow/differential-heating
zone, and convergence near the surface trough. While mid/upper-
level flow is forecast to be modest overall across the region, from
the northwest to north, a mesoscale channel of enhanced midlevel
winds may develop southeast of the MCV/shortwave trough to enhance
deep shear. Meanwhile, strong veering of wind with height will
yield enlarged low-level hodographs, especially near the boundary
and in a regime of backed near-surface winds north of the surface
trough. As such, supercell characteristics are possible, especially
with relatively persistent discrete storms.

Thermodynamically, forecast soundings from the RAP and HRRR appear
to be overly drying/mixing the boundary layer and reducing dew
points too much, particularly with eastward extent toward the
boundary and over parts of KS near the boundary/trough intersection
and boundary-layer moist axis. As such, their progged convective
timing may be late, and coverage too low. Given the moisture in
place across the region, areas of 1500-2500 J/kg MLCAPE appear
likely in a corridor west of the convective/cloud debris and
northeast/east of the legitimately strongly mixed/drying air of
western OK. Once mesoscale boundary trends and perhaps more-useful
input from 12Z RAOB-assimilating guidance become apparent, enough
confidence in a small corridor of greater unconditional severe
threat may develop to warrant an upgrade.

…Northern Rockies…
Scattered to widely scattered thunderstorms are forecast to form
over the western and southern parts of the outlook area during
early-mid afternoon, as strong diabatic surface heating and lift
near the frontal zone combine to weaken CINH preferentially at
higher altitude. This will result in steep low/middle-level lapse
rates and deeply well-mixed boundary layers, with surface dew points
now in the 40s to mid 50s F reducing to generally 30s to low 40s F.
Still, marginally sufficient moisture should exist to support
convection that will encounter an environment characterized by far
more DCAPE (1000-1500 J/kg) than MLCAPE (around 500 J/kg or less in
most areas). As such, damaging to locally severe gusts will be the
main concern as convection moves northeastward, over the ranges and
valleys from the remainder of ID across western MT. Lack of greater
moisture/buoyancy precludes both a stronger wind risk and an
unconditional severe area for hail.

..Edwards/Broyles.. 08/02/2019


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