Day 1 Severe Weather Outlook

Valid 111300Z – 121200Z


Severe thunderstorms are not expected today and tonight.

Flow in mid/upper levels is forecast to become more zonal through
the period, as heights rise across the Great Lakes and Ohio Valley
following the departure of a strongly cyclonic subsynoptic wind
swath now present there. A strongly positively tilted, southern-
stream perturbation — now extending from the ArkLaMiss region
across southeast/south-central TX to the state of Sinaloa in MX —
will shift eastward and weaken through the period. By 12Z, the
remnants of the associated 500-mb vorticity axis should extend from
the coastal Carolinas across the FL Panhandle to shelf waters off
the TX coast. A well-defined shortwave trough — now evident in
moisture-channel imagery over northwestern NV, the central/southern
Sierra and southward past SBA — will move east-southeastward to the
central/southern High Plains by 12Z.

At the surface, a cold front was analyzed from a weak low offshore
ILM southwestward across north-central FL and the central/
southwestern Gulf. This front is forecast to move southeastward
down the peninsula through the period, decelerating and perhaps
stalling by 12Z over extreme southern FL.

Isolated to widely scattered thunderstorms are possible this
afternoon along/ahead of the front. Weak low/middle-level winds and
lack of vertical shear suggest activity will be unorganized. Rich
low-level moisture (manifest as upper 60s and 70s F surface dew
points), beneath modest midlevel lapse rates, will combine with some
diabatic surface heating in the warm sector to support 1000-1500
J/kg MLCAPE this afternoon, similar to the 12Z MFL RAOB. Although
frontal forcing is expected to weaken with time, it may suffice to
support isolated convective development, as would any sea-breeze/
outflow boundaries preceding the front.

…Pacific Northwest…
An upper-level speed max — detaching from the southern rim of a
large Gulf of Alaska cyclone — is expected to shift over the OR
coast during the latter half of the period, with 250-mb westerlies
nearing 150 kt. North of this jet streak, a series of small
midlevel vorticity lobes and related cooling aloft will spread over
the WA coast and Olympic Peninsula this evening and tonight. As
this occurs, a layer of marginal buoyancy (MUCAPE mostly around 250
J/kg or less), rooted in the boundary layer over the Pacific, will
extend gradually upward into progressively colder layers than -20
deg C. This will support sporadic, isolated lightning with cells
moving onshore, mainly during the latter half of the period.

..Edwards/Broyles.. 12/11/2019


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