Valid 181300Z – 191200Z
…NO SEVERE THUNDERSTORM AREAS FORECAST…
…SUMMARY…
Severe thunderstorms are unlikely through tonight.
…Synopsis…
The mid/upper-level pattern through the period will be rather
progressive, except for a persistent, cut-off low that will remain
west of the Baja spur through the period. Shortwave troughs moving
over the upper Mississippi Valley, upper Great Lakes and off the
southern Atlantic Coast will cross over a low-level air mass too dry
and stable for thunderstorms, behind a large cyclone located over
the western Atlantic. A strong shortwave trough — initially
evident in moisture-channel imagery well west of WA/OR near 137W —
is expected to move onshore of the coastal Pacific Northwest around
06Z.
…Pacific Northwest…
A few thunderstorms are possible across portions of the coastal
Northwest and Cascades area — mainly after 00Z. As the Pacific
shortwave trough approaches, related large-scale lift/DCVA and
cooling aloft will steepen low/middle-level lapse rates — initially
over the marine layer near the coast, then inland over the Cascades,
atop generally weaker theta-e with eastward extent. Time series of
forecast soundings suggest substantial destabilization immediately
preceding the trough in the 400-600-mb layer, which also corresponds
to thermal profiles colder than -20 C and suitable for lightning
generation once buoyancy can extend into that profile. Even with
max lapse rates, MUCAPE should remain under about 250 J/kg, and
lightning flashes should be isolated.
..Edwards/Mosier.. 11/18/2019
$$