FXUS66 KSTO 242158
Area Forecast Discussion
National Weather Service Sacramento CA
258 PM PDT Tue Apr 24 2018
Mainly dry and warm this week except for a chance of a few
afternoon showers or thunderstorms near the Sierra Crest. Cooler
weather Friday into the weekend with increasing chances for
mountain showers and thunderstorms.
Ridge continues to shift eastward as a trough of low pressure
approaches in the eastern Pacific. High level clouds were
beginning to increase as moisture streamed ahead of the
disturbance in the early afternoon. Temperatures continued to be
warm across the region similar to yesterday afternoon.
Latest model runs have backed off significantly to bringing showers
and thunderstorms along the Sierra crest this afternoon. Have
removed the chances in the forecast. However, models continue to
indicate slight chances for showers and isolated thunderstorms
developing along the Sierra crest for Wednesday and Thursday
afternoons. As the low pressure system moves closer to the area,
temperatures each afternoon will cool slightly.
Temperatures Friday will return to near normal with readings
decreasing from between 10 and 20 degrees from today. Chances for
mountain showers and thunderstorms will increase across the Sierra
.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)
Extended models consistent in idea of large upper low over the
western U.S but differ on details. Either GFS or ECMWF would
indicate a threat of showers over most of the forecast area
especially during the afternoon hours. Stability progs indicate
some instability over the northern mountains and Sierra Cascade
crest on Saturday so limited thunderstorm threat to these areas.
Instability spreads westward on Sunday so may see some isolated
thunderstorms dropping down into the northern and central
Sacramento valley. Cloud cover and cooler airmass associated with
the low will bring cooler temperatures over the weekend down to a
little below normal for this time of year. Models diverge early
next week so confidence in forecast lowers but keeping a threat
of showers over the Sierra Cascade range. Upper low should begin
to shift inland by next Tuesday whichever model verifies making
Tuesday the last day of any shower or T-storm threat before upper
ridging builds over the west coast the middle of next week.
Daytime highs start a warming trend with a slight warm up to near
normal on Monday then climb to several degrees above normal on
Tuesday as upper ridging starts to build in. At this point the
second half of next week looks dry.
VFR conditions at the TAF sites the next 24 hours. Winds
generally 10 kts or less.
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