Day 1 Severe Weather Outlook

Valid 011300Z – 021200Z


Isolated thunderstorms will be possible from the Pacific Northwest
into the northern Great Basin today, with locally strong winds in
the northern Utah/northeastern Nevada area. A few thunderstorms
might also occur from south Texas to western Louisiana tonight.

In mid/upper levels, three initially well-separated features will
phase through the period to form a high-amplitude trough by 12Z,
from MB across the Dakotas and southern Rockies to west-central MX.
A southern stream cyclone — now apparent in moisture-channel
imagery over Baja — is forecast to devolve into an open-wave trough
and pivot eastward across portions of western and northern MX. This
will occur as an initially subtle shortwave perturbation and
accompanying speed max — initially over portions of WA/OR — dig
southeastward and amplify, reaching eastern parts of AZ/UT by 12Z.
Meanwhile, a northern-stream shortwave trough — initially over
portions of central/north-central SK and MB — will move slowly
southeastward across MB and strengthen.

At the surface, 11Z analysis showed a cold front over the northern
Bahamas, becoming quasistationary between the Keys and Cuba, and
losing definition across the central Gulf. Baroclinicity will
become better-defined through the period over the shelf waters of
the western/northwestern Gulf as the open-water marine boundary
layer continues to modify north of the decaying frontal zone. The
inland air mass will remain stable in low levels over most of the
CONUS east of the Rockies.

…Northwestern CONUS to northern Great Basin…
Sporadic, isolated thunder may occur just about anywhere within the
outlook area, as very cold air aloft overlies weak midlevel
moisture, amidst little MUCINH, and areas of locally enhanced
orographic lift. One particular area of interest in the broader
outlook is from near the southern ID border across northeastern NV
and northwestern/north-central UT. The general thunder outlook has
been expanded across this region, and some strong convection may
occur late this afternoon.

Over the latter region, the diurnal thermal peak in the boundary
layer, behind morning clouds/precip, will correspond with
strengthening deep-layer ascent ahead of the amplifying shortwave
trough. This will result in sufficiently steep low/middle-level
lapse rates to yield minimally inhibited MLCAPE in the 100-400 J/kg
range. Despite seemingly cool surface temps of upper 30s to low 40s
F in lower elevations, -23 to -25 deg C 500-mb temperatures are
expected to spread over the area at the same time, to enable such
buoyancy. Depending on elevation, convective squalls and showers of
snow or rain/snow mix are possible, with occasional lightning. The
most vigorous of this convection may enable momentum transfer of
strong/gusty winds to the surface, possibly approaching severe
limits on a brief/localized basis. The potential for severe
convective winds appears too conditional and low for an outlook area
at this time.

…South TX to western LA…
Isolated to widely scattered thunderstorms are possible within an
inland-expanding area of precip overnight. A broad regime of
low-level warm advection and moisture transport will strengthen
through the period ahead of the cold front, from the western/central
Gulf Coast region into the central Appalachians. This will occur in
the mass response to the coalescing/approaching synoptic trough to
the west, gradually strengthening the low-level jet to 50-70 kt
across LA by 12Z. Amidst a still-incomplete/immature return-flow
process, a relatively stable boundary layer will remain across most
of this corridor, with the convective inflow layer elevated above
that. Time series of forecast soundings suggest around 500 J/kg
MUCAPE could develop by 10-12Z over the southeast TX/southwestern LA
near the coast, rooted between 850-700 mb. Effective shear may
become strong at 50-60 kt; however, modest midlevel lapse rates,
likely messy storm modes, and a deep near-surface stable layer
combine to suggest that convection will remain subsevere until the
following day. Refer to the day-2 outlook for a discussion on
post-12Z potential.

..Edwards/Jewell.. 01/01/2020


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