Valid 121300Z – 131200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ALONG THE
COASTAL NORTHWEST…
…SUMMARY…
Small but strong thunderstorms may move ashore along the coastal
Northwest through midday, offering a marginal tornado threat.
…Synopsis…
In mid/upper levels, high-latitude Rex blocking is evident and will
persist over northwestern North America and the adjoining Arctic,
anchored by an anticyclone north of Alaska’s North Slope. To its
south, fast/westerly flow will be maintained across much of the
northeastern Pacific and the Pacific Coast States, gradually bending
southeastward across the Intermountain region and Rockies toward
mean troughing over the central CONUS. A series of weak vorticity
lobes will move through the associated cyclonic flow field,
following a shortwave trough now over the central/southern High
Plains that is expected to eject east-northeastward and weaken.
Over the northeastern Pacific, a weak shortwave trough was evident
in moisture-channel imagery near 132W, west of the OR coastline, and
forecast to move ashore between 18-21Z. A 140-160-kt 250-mb jet max
will spread inland today and this evening south of that perturbation
across southern OR, spreading over northern UT overnight.
At the surface, a wavy/quasistationary front was analyzed from
Atlantic waters just north of the Bahamas, across south FL, westward
to the north-central Gulf, then southwestward across the west-
central Gulf. A frontal-wave low is progged to develop and ripple
slowly northeastward along this boundary this evening and tonight,
passing just south and east of the Mississippi River mouth, but
remaining offshore from the MOB area through 12Z. The boundary to
its east will move northward as a warm front, but also remain over
water through this period. This should keep the surface-based warm
sector south of the FL Panhandle, and associated severe potential
away from land, until day 2.
…Coastal Northwest…
Isolated to widely scattered thunderstorms may move ashore
throughout the period, but most notably through the rest of this
morning, in the zone of enhanced large-scale ascent and low/middle-
level destabilization preceding the low-amplitude shortwave trough.
With low LCL and strong low-level shear likely, a tornado cannot be
ruled out from any relatively sustained/discrete cells. Small hail
and strong gusts also are possible with the moist vigorous cells.
Cooling IR cloud tops have been noted with a broken band of
convection ahead of the trough, between 126W-128W. Sporadic
lightning has been detected in the same area. This or nearby/later-
developing activity may become the best-organized convection to move
ashore in the period, later this morning. Sufficient vertical shear
and weak buoyancy will exist to support potential for a low-topped
supercell or two, within an area of relatively backed near-surface
winds, ahead of the trough. Low-level mass response to the imminent
shortwave will enhance flow near the top of the marine boundary
layer, enlarging both low-level shear vectors and hodograph size.
Forecast soundings suggest a brief window of effective SRH in the
200-300 J/kg range may collocate with weak but surface-based
buoyancy (MLCAPE up to about 250 J/kg) rooted in the marine layer.
After the perturbation passes during midday to early afternoon,
veering low-level flow and decreasing large-scale lift should reduce
already marginal convective organization.
..Edwards/Broyles.. 12/12/2019
$$