Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sacramento California
338 am PDT sun Jul 5 2015
gradual cooling next few days along with daily afternoon and
evening thunderstorms mainly over the mountains. Thunderstorm
coverage may increase over the mountains...northern SAC valley and
surrounding foothills on Tuesday and Wednesday as an upper low
over the eastern Pacific moves towards the California coast.
temperatures at press time trending about 3 to 6 degrees cooler than 24 hours
ago in the Delta influenced areas due to a strong Delta breeze and
deepening marine layer over 2 kft per the feet Ord profiler. Onshore
gradients have increased up to 4 mbs with more than 2 mbs up the SAC
valley most likely from the strong thermal contrast resulting from
triple digit temperatures in the valley on Sat. Expect this cooling trend
to continue in the valley today as the Delta breeze finally has some
"freon" to promote significant cooling.
Synoptic scale pattern dominated by the 4-corners high and a large
low pressure cell off the California coast along 130w. Short wave moving northward
over western Nevada is is pushing convection north-northwest...with our Shasta Colorado area
on the southern fringe. Higher resolution quantitative precipitation forecast/radar reference forecasts suggest
some showers/isolated thunderstorm may brush the northestern part of Shasta Colorado this
morning before ending. Otherwise...instability forecasts focus the
convective potential over the Sierra Nevada/southern Cascade zones sun/Monday
afternoons and evenings. Reamplification of the ridge over the
Sierra Nevada in the wake of the short wave will limit the cooling over
the higher Sierra elevations today however...and in some instances
persistence may be followed.
Expect an increase in areal coverage of convection Tuesday/Wednesday as the
aforementioned large low pressure cell slowly approaches the coast.
Upper level difluence...dynamics and moisture expected to become
more focused over norcal resulting in more widespread storms. Began
the upward trend of probability of precipitation during this time in this morning's
forecasts with sufficient moisture available for "wetting rains."
Synoptic cooling Monday-Wednesday this week will lower maximum temperatures below normal
with onshore flow increasing chances of a stratus intrusion into the
valley Tue/Wed...although increasing cyclonic flow will also result
in an increase in cloud cover as well especially over the mountains jhm
Extended discussion (thursday through sunday)
Medium-range models continue to struggle with the timing of the
upper low as it approaches California with the GFS slightly faster
moving it onshore Thursday while the ec is about 24 hours slower
with the timing. Regardless of timing, mountain thunderstorms are
expected daily. Temperatures will moderate to slightly below
average before warming over the weekend as the trough shifts east.
Mainly VFR conds the next 24 hours, except local MVFR/IFR vicinity
mountain thunderstorms 22z-04z. Southerly winds 10-15 knots in the SAC valley
(nwly in the sj vly) after 18z, with SW wind gusts 25-35 knots continuing
near the Delta.