Day 1 Severe Weather Outlook

Valid 071300Z – 081200Z


A marginal tornado threat exists this afternoon across portions of
northern California.

In mid/upper levels, a more-zonal height/flow field will take shape
over most of the CONUS, downstream from a strong eastern Pacific
trough that will be the most relevant upper-air factor in this
forecast. Based on moisture-channel imagery, that trough is
estimated at 12Z near 130W from the BC coast southward to 46Z, then
south-southwestward through a vorticity max (and formerly closed
low) near 41N131W, becoming more positively tilted southwestward to
38N137W. The trough is expected to weaken slightly with time, but
still, should be a high-amplitude feature when it moves ashore
northwestern CA and the Pacific Northwest between 00-06Z. Trailing
portions of the 500-mb trough will cross the northern CA coastline
obliquely into the early hours of day 2, as the northern segment
crosses the interior Pacific Northwest.

…Northern CA…
Isolated to widely scattered thunderstorms are possible through
early evening across this region, supported by at least marginally
moist low-level profiles beneath cooling midlevel temperatures
related to the approaching trough (and related steepening of
low/middle-level lapse rates). A field of strong midlevel flow is
apparent already in the 12Z OAK RAOB and is expected to expand
gradually further inland over the area, contributing to favorable
deep shear, with actual effective shear varying substantially (30-55
kt vectors in forecast soundings) according to depth of CAPE.
However, favorable low-level shear should develop, especially across
parts of the Sacramento Valley, as orographically forced backing of
near-surface winds enlarges boundary-layer hodographs. SRH through
the 0-1-km layer may reach the 150-300 J/kg range in support of
rotation in any sustained, relatively discrete cells.

As often is the case with similar patterns and synoptic timing, the
main uncertainty involves thermodynamics — specifically the amount
of low-level destabilization that can occur around antecedent and
ongoing areas of clouds/precip. Inland, the size and duration of
patches of relative clearing will be the greatest influence, as only
a degree or two C shift in the lowest 1-3 km thermal profile can
boost MLCAPE above 500 J/kg or reduce it to near zero. Surface
temperatures in the upper 50s to low 60s F would realize the former,
most-unstable scenario. With the outlook still strongly conditional
on realizing favorable low-level destabilization at mesobeta and
smaller scales, no changes are warranted on this outlook cycle.

..Edwards/Broyles.. 12/07/2019


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