Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 191445

745 AM PDT Sun Apr 19 2015

Very warm weather continues across the region today and Monday.
Isolated late day showers and thunderstorms will be possible over
the mountains. Another low pressure system will move into the region
by the middle of the week bringing cooler temperatures and
continuing the threat of showers or thunderstorms into the end of
the week.


Clear skies across the interior of NorCal this morning except for a
few remnant mid and high clouds to the south of Sacramento from
yesterday afternoon`s convection. Temperatures range from the upper
20s and 30s in the colder mountain valleys to the upper 40s and 50s
elsewhere. The warmer thermal belt areas in the foothills and
northern Sacramento Valley are in the 60s.

The surface thermal trough remains over the valley and a moderately
strong northerly pressure gradient across far northern California
has kept enough of a northerly breeze overnight in Redding to keep
the temperatures there in the upper 60s while just to the south,
readings at Chico and Red Bluff managed to lower in the mid and
upper 50s.

Little change today with another warm day expected across the
region. Most areas of the valley are forecast to once again climb
well into the 80s, with even some lower 90s possible. The marine
layer is about the same depth as yesterday morning (around 1500 ft),
though the onshore gradient from SFO to SAC has tightened a bit to
around 2 mbs, so a little more of a Delta trickle may keep Fairfield
down a few degrees from Saturday.

Diurnal mountain convection is expected to be confined to the Sierra
mainly to the south of I-80 today as the weak low backs off the
NorCal coast allowing ridging from the PacNW to cover far northern
California. This is expected to cap off deeper convection over the
northern mountains today. A return to more widespread diurnal
convection over the mountains is forecast Monday and Tuesday.

No updates.


.Previous Discussion...
Weak upper low off the norcal coast which brought isolated mountain
thunderstorms on Saturday drifting to the southwest today. As a
result, instability associated with the low is moving southward as
well. Therefore, only expecting a threat of showers or thunderstorms
over the northern Sierra today south of about interstate 80. Overall
airmass under upper level ridging will be warming slightly today
bringing up high temperatures to near records for the date. The low
then shifts inland towards the Socal coast on Monday in response to
a larger trough dropping out of the Gulf of Alaska and into the
eastern Pacific. With the forecast area well north of the the Socal
low, moister is fairly limited but models show some increased
instability over the mountains Monday so isolated afternoon mountain
thunderstorms are not out of the question. Airmass remains very warm
on Monday but may see highs drop just a bit over Sunday highs. by
Tuesday afternoon, Gulf of Alaska through is forecast to dig in just
off the coast. Should see a few to several degrees of cooling as
upper flow becomes more onshore increasing marine influence.  Mid
range models in fairly good agreement in pivoting some sort of
shortwave out ahead of the trough bringing a threat of showers or
thunderstorms to most areas of the CWA afternoon and evening hours.
General idea of mid range models is to split the trough as it moves
inland into a close low off the socal coast and an open wave over
the Pacific Northwest. Models differ on details but agree on showing
enough instability and moisture over the North state to warrant
shower or thunderstorm threat most areas Wednesday. Daytime
temperatures continue to cool on Wednesday but at this time are
still forecast to remain above normal.


.Extended Discussion (Thursday through Sunday)
The longwave trough will dominate through the latter portion of the
week, keeping temperatures moderated. A closed low that moves
southward along the coast will be over northern Baja by Thursday.
However, the broad low will still provide instability over NorCal
and bring return flow moisture from an easterly direction with
thunderstorms looking like a pretty good bet Thursday. The low will
weaken and move to the east by Friday, but the longwave trough will
likely remain near the west coast with potential showery activity
through the weekend. JClapp


VFR conditions next 24 hours. Afternoon isolated thunderstorms will
develop over the Sierra southward of Lake Tahoe. Light winds will
continue except in vicinity of thunderstorms. JClapp


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


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