Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS11 KWNS 211934
SPC MCD 211934
Mesoscale Discussion 0303
NWS Storm Prediction Center Norman OK
0234 PM CDT Tue Mar 21 2017
Areas affected...Portions of the Sacramento and San Joaquin Valleys
Concerning...Severe potential...Watch unlikely
Valid 211934Z - 212130Z
Probability of Watch Issuance...5 percent
SUMMARY...Thunderstorms are increasing in intensity in the
Sacramento and San Joaquin Valleys and will pose a risk for isolated
severe hail and a brief tornado or two...but the threat will remain
isolated enough to preclude a watch.
DISCUSSION...A few bands of thunderstorms have increased in
intensity in the Valley south and east of Sacramento as the broad
upper low and an associated upper-level jet approaches the area.
Boundary layer dewpoints in the upper 50s covering all of the area
are helping to provide MUCAPE of 500-1000 J/kg. Extensive low cloud
cover and showers have limited low-level lapse rates so far...but
convective enhancement has occurred along the back edge of the cloud
shield now moving east of a San Luis Obispo-Sacramento-Redding line.
Veering surface winds at SMF and BAB to the west of the cloud edge,
along with the maintenance of surface south-southeasterlies at SCK
and points south and east, suggests enhanced convergence associated
with differential heating has helped to intensify the convection.
Water vapor imagery suggests subsidence from Sacramento to the coast
at this time, so any convective threat in the next hour or two
should remain with the storms evolving to the east of Sacramento and
San Jose, until another band of ascent pushes onshore toward
Low-to-mid level lapse rates of 7-7.5 C/km and effective bulk shear
of 35-40 kt should support storm organization in the form of line
segments and brief supercell structures. Local VAD wind profiles
suggest the low-level shear is presently sufficient for a brief
tornado or two in the near term, especially where the southeasterly
surface winds are present, although the tornado and hail threat
should remain isolated enough to preclude a watch.
...Please see www.spc.noaa.gov for graphic product...
LAT...LON 39642088 39202052 38762038 37641993 36881996 36322033
36232106 36512137 37222143 38352145 39182132 39642088