SPC AC 211237
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0737 AM CDT Wed Mar 21 2018
Valid 211300Z - 221200Z
...NO SEVERE THUNDERSTORM AREAS FORECAST...
A few thunderstorms are possible this afternoon and evening across
parts of central/northern California and the Great Basin.
Moisture-channel imagery and available upper-air RAOBs revealed
broadly cyclonic mid/upper-level flow over much of the eastern U.S.,
with a low over northeastern KY. As upstream speed/vorticity maxima
move through the cyclone's southern semicircle, the vortex will
expand somewhat and move eastward, its center crossing the Delmarva
area around 00Z. By 12Z, the mid/upper-level cyclone center's path
should curve back toward the ACK/Cape Cod region, becoming more
vertically stacked with the low-level vortex. At the surface, 11Z
analysis showed the main low over Atlantic waters, south of Long
Island and east of Hampton Roads. This low is expected to move
northeastward to just offshore from ACK by 12Z. The cold front was
evident across the northern Bahamas and southern Straits of Florida,
and will continue moving away from the mainland. In its wake, a
low-theta-e, continental/polar air mass will preclude thunder
potential over the central and eastern parts of the U.S.
Ridging aloft will build over the Rockies through day-2, as a
high-amplitude trough strengthens offshore from the Pacific Coast.
A cyclone over the eastern Gulf of Alaska, centered just off the AK
Panhandle, will migrate south-southeastward, while a series of
mostly weak shortwaves in southwest flow cross CA. As this occurs,
midlevel lapse rates should steepen enough to support widely
scattered to scattered showers and at least isolated thunderstorms,
particularly over the Central Valley of CA where pockets of diabatic
heating also will increase low-level lapse rates. Forecast
soundings in the southern Sacramento Valley area show 200-600 J/kg
MLCAPE developing, amidst brief periods of enough low-level shear to
enlarge 0-3-km hodographs and cause some concern for storm-scale
rotation. Small hail cannot be ruled out, though a
consistent/coherent focus for growth/maintenance of any
supercellular structures appears lacking at this time. Given
favorable mesoscale boundaries and destabilization trends,
marginal/brief tornado risk may develop. Currently that threat
appears too small, isolated and conditional for an outlook area.
Very isolated lightning cannot be ruled out farther north across the
interior Pacific Northwest, wherever some weak CAPE can be generated
that extends past about the -20 C isotherm. However, theta-e
appears too weak, coverage too low, and forcing too nebulous, to
draw/extend a general-thunder area over that region at this time.
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z