Heavy Rain and Winter Weather in California, May 14 to 15, 2015

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
943 PM PDT Wed May 13 2015

.Synopsis...
Cooler and more unsettled conditions expected across Norcal
Thursday and Friday with accumulating snow impacting the
mountains. Some lingering mountain showers are possible over the
weekend, and again early next week as yet another low pressure
system drops into the area.

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.Discussion...
Showers are tapering off at this time. Some of the models try to
indicate a few showers popping up over the far north end of the
valley over the next few hours but more likely than not we will be
waiting until tomorrow before the activity flares up again.

Another trough centered off the coast will move southward on
Thursday with the core remaining off the coast. Models seem to be
indicating that the rain could start over the coastal areas and
south of I-80 in the mountains first and spread northward during
the day. It seems more than likely that the valley in the morning
will stay dry. The central part of the trough will remain off the
coast and thunderstorms activity may not be that great over the
interior. The best chances look to be over the mountains where
orographics can aid with some additional lift. Precipitation
amounts look to be less than a tenth of an inch over the valley
and up to around half an inch in the mountains. The higher totals
in the mountains will be south of highway 50 with lessor amounts
as you move north. Thunderstorms may produce local higher amounts.

The core of the system will move into Southern California on
Friday but some warp around moisture may occur that could lead to
additional showers and thunderstorms over the mountains. The
valley looks to remain mostly dry with any chances most likely
over the far northern portion of the Sacramento valley.

Snow levels are look to vary between 5000 and 6000 feet for the
most part as the system moves through. Precipitation amounts have
are still not that high around I-80 and US-50 so not expecting any
major impacts for those routes unless a band of thunderstorms
moves through that area during the afternoon to early evening
hours. Some of the southern passes like highways 88 and 4 should
have more snow and some problems with travelers crossing the
crest. Convection will be the main issue that determines issues
with crossing over the Sierra Nevada for the northern routes and
the main time period for convection will be during the afternoon
hours both Thursday and Friday.

On Saturday a weak baggy trough will set up off the northwest
coast. This may help to trigger some showers and isolated
thunderstorms over the mountains but for the most part people in
the interior should expect to remain dry. The delta breeze looks
like it could pick up and become strong by the evening hours.

.Extended Discussion (Sunday through Wednesday)

An upper low off the CA coast Sunday is forecast to dig slowly
then swing into California on Monday. Yet another upper low is then
forecast over the area Tuesday into Wednesday. This will result in
an unsettled weather pattern with the threat of showers and
thunderstorms...mainly afternoons and evenings and over the
mountains. High temperatures expected to be near to a little below
normal through the extended forecast period.

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.Aviation...

Upr low off the NW CA cst with Sly flow alf. Mainly VFR conds ovr
Intr NorCal through 18Z exc isold MVFR poss in shwrs...mnly
omtns. Lcl SWly sfc wnd gsts to 30 kts poss thru Delta and ovr
hyr trrn aft 18Z. Sct shwrs and isold tsra aft 18z with areas of
IFR cigs and vsbys over mtns due to wx.

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.STO Watches/Warnings/Advisories...
winter storm warning from 11 am thursday to 5 am pdt friday
above 5500 feet in the west slope northern sierra nevada.

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