FXUS66 KSTO 291058
AFDSTO
Area Forecast Discussion
National Weather Service Sacramento CA
358 AM PDT Fri May 29 2020
Synopsis...
A cooling trend begins today and continues into the weekend as a
low pressure system moves through. This will bring widespread
showers and thunderstorms tonight through Saturday, with the
potential for heavy rain over northwestern Shasta County. Chances
for some mountain showers and thunderstorms continues into early
next week.
.DISCUSSION
...
The heat wave is ending as upper level ridging shifts eastward
and onshore flow develops with a low pressure system off the
California coast. Mainly cooler temperatures are evident across
much of the area this morning. The biggest change has been for the
Sacramento metro area, where temperatures are 10 degrees cooler
than at 3 am yesterday. This is due to a decent marine layer and a
moderate Delta Breeze, which at Travis AFB was gusting out of the
southwest at 32 mph. For the Sacramento area, the 4 day
consecutive stretch of triple digit temperatures ended Thursday,
with highs forecast only into the mid 80s for this afternoon. The
4 day stretch of 100`s was the first on record for downtown
Sacramento in the month of May. Triple digit temperatures were
evident across the Valley. The northern Sacramento Valley hasn`t
seen as much cooling yet this morning, but should be a little less
hot today, with highs in mid to upper 90s.
The main weather issue shifts to showers and thunderstorms this
weekend, beginning this evening over the mountains and becoming
widespread tonight as the low moves inland. This is a wet system,
with Precipitable Water more than double normal levels, around 1.2
inches. Abundant atmospheric moisture plus dynamic lift and
steep lapse rates will combine to bring heavy rain showers over
portions of NorCal, especially northwestern Shasta County. CAMs
models show a band of heavy showers and thunderstorms moving
northward through that area around 5 am to 11 am. This could
bring localized flooding, as well as possible burn scar issues.
Scattered afternoon thunderstorms develop across the area in the
wake of this band as the core of the low moves over Northern
California. Surface CAPE levels around 200-500 j/kg are expected.
Low level shear doesn`t look strong, with heavy rain the main
threat. A convergence band sets up over Shasta County, bringing
more heavy rain. A general storm total of around 1 to 2 inches is
forecast, locally higher possible.
The shower and thunderstorm threat will diminish by late
Saturday afternoon and evening, with a residual chance of showers
and storms into Sunday over the Northern mountains and northern Sierra
with the approach of a Pacific trough. This will continue onshore
flow and cloud cover over the area on Sunday, with some lingering
showers and afternoon thunderstorms limited to Shasta County, dry
elsewhere. The trough will bring more widespread mountain showers
and afternoon thunderstorms on Monday.
Temperatures on Saturday will be relatively cool, 7 to 15 degrees
below normal levels. Valley and Delta highs will be in the 70s,
into the mid 60s for the foothills. Sunday and Monday will see
Valley highs a little warmer, into the upper 70s and lower 80s. EK
.EXTENDED DISCUSSION
(Tuesday THROUGH Friday)...
A rather pronounced subtropical to mid-latitude blocking pattern
unfolds next week which will stagnate features on the map. Most
notably, an upper low pinches off in the vicinity of far northern
Baja California. This system is forecast to stall through at least
mid-week before being nudged eastward from broad upstream
troughing. Depending on how far north this circulation center
stays, embedded shortwaves may provide an impetus for scattered
convection over the foothills and mountains each day. These should
generally follow the diurnal heating cycle which favors these
showers and isolated thunderstorms during the afternoon to early
evening hours. By late next week, broad troughing fills out the
eastern Pacific with a gradual lowering of heights along the West
Coast by next Friday/Saturday as shown by the well clustered
ensemble means. Regarding temperatures, a gradual warm up is
anticipated as a Four Corners ridge attempts to build heights
north of the Baja California low. The 00Z ECMWF is decidedly
warmer than other solutions which would support readings well into
the 90s. For now, will continue to play the pattern
conservatively with highs in the upper 80s to low 90s before
cooling down by next Friday given the approach of a trough. ~BRO
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