Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 181501

800 AM PDT Sat Apr 18 2015

Very warm weekend with temps in the 80s cooling to around 70 by
mid week next week. Slight chance of mountain showers this
weekend. Cloudy with chance of showers everywhere starting early
next week. Storm is not very strong so precipitation amounts not
expected to be significant.


Water vapor imagery shows the weak mid/upper level system drifting
down through the PacNW this morning. This feature is forecast to
form a weak closed low over the far northern portion of the state
this afternoon, and may provide enough instability for a few late
day showers and thunderstorms over the northern mountains.

Otherwise, the remainder of the area will continue to see warm and
dry conditions as most areas are starting off considerably warmer
than 24 hours ago. Morning soundings show further warming inland,
though the marine layer has deepened to around 1500 ft and offshore
flow has weakened, so we`ll watch to see if we start to see the
delta breeze pick up this afternoon and provide some local cooling.

No updates.


.Previous Discussion...
Little to no update to the going forecast. Another very warm day
today with temperatures in the upper 80s. Models still forecasting a
weak impulse to slide south across the area then stall and linger
around through Sunday. It will create some weak instability and
showers and possibly a stray thunderstorm is possible.

A cool down is still expected starting Monday as a trough slowly
gets carved out over the west coast. This will bring more widespread
instability and clouds with increasing chances of moisture. Still
not looking like a very strong system. Rasch


.Extended Discussion (Wednesday through Saturday)...
Mid range models in fairly good agreement for the middle of next
week and if model solutions verify, An upper low pressure system
should be fairly well established over California on Wednesday. In
addition to the significant cooling expected, resulting instability
from the low will generate scattered showers over most of the north
state. Except for the far north state, farthest from the low center,
most of Norcal will be unstable enough for a chance of afternoon
thunderstorms as well. Thursday will see a bit more cooling as the
low remains over the region and still more cooler air filters in.
Although precipitation with this low is not substantial, some areas
could end up picking up a few tenths of an inch of rainfall under
heavier or more numerous showers. Precipitable water values
associated with this low predicted at around .75 inches so expect
any thunderstorm cells to contain at least some rainfall. Extended
models all lift the low northeastward into the Great Basin next
Thursday night and early Friday but differ on what happens next in
regards to the next upper low dropping out of the Gulf of Alaska.

ECMWF and GEM want to dig this next low into Norcal bringing another
round of convective showers to most of the north state next Friday
and Saturday. GFS shows more of an inside slider situation which
could leave most of Norcal dry although Sierra could see some
showers under this scenario. Have gone with GFS for now as it is
more in line with previous model thinking but trend may be towards a
wetter solution and will have to wait and see how this trend
develops. Either way, still relatively cool daytime temperatures
expected with daily highs near normal for the end of next week.


VFR conditions next 24 hours. Isolated thunderstorms possible over
the mountains after 18z today. Light winds generally below 15 knots
except in vicinity of thunderstorms.


.STO Watches/Warnings/Advisories...None.


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