Area Forecast Discussion
Issued by NWS Sacramento, CA

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FXUS66 KSTO 181844 CCA

938 AM PDT Thu Sep 18 2014

Temperatures cooling to below normal today with a chance of
showers and thunderstorms. Warmer and drier weather returns for
the weekend.


An upper trough will shift further onshore today and spread cooler
temperatures and light showers/thunderstorms throughout much of
interior northern California. The base of the 571dm upper trough
is directly along the northern coast of California extending from
the North Bay to Eureka. The upper low is forecast to dig southeast
towards Southern California and transition into a closed low before
ejecting into the Great Basin via the westerlies late in the

Overnight precipitation brought light accumulations to portions
of the coastal range, extreme northern Sierra, and Shasta county.
Correlation coefficient radar imagery indicates light showers
moving in ahead of the base of the trough. Additional
accumulations are expected this afternoon as the showers push
further inland. Short-term model data suggests considerable
instability, shear, and lift for this time of the year, so
isolated thunderstorms throughout the area are possible beginning
mid-day and lasting into the late afternoon. The NAM is the most
aggressive, with widespread convective potential throughout the
region. The WRF is more conservative, and the more likely scenario,
indicating enhanced potential over the higher terrain of Shasta
county and the Sierra Nevada.

The King Fire experienced aggressive growth on the northern
perimeter yesterday and expanded by over 50000 acres due to low
humidities and moderate winds. Conditions today will be slightly
better with higher humidities and lower winds, however only
minimal amounts of precipitation, if any, are expected to fall
over the fire. The heat from the fire should cause lighter
precipitation to evaporate before it even reaches to ground, but
this process does increase the local relative humidity. The main
threat today is additional starts from any isolated lightning. Smoke
from this fire will continue to impact nearby communities and
could sink into the valley after Friday when the winds shift
following the trough. DRP

Previous Discussion... Upper level trough will be moving
inland today. Radar imagery overnight has shown shower activity in
Shasta County near I-5 and westward to the NorCal coastline. At 4
am, most of the showers were west and north of our CWA due to a
dry slot moving into our region. On water vapor imagery, moisture
offshore is digging slightly southward and should impact our
region later today. Once the trough moves inland, models are
indicating increased instability over NorCal which will result in
a slight chance to chance of showers and thunderstorms across much
of our CWA. In the morning and early afternoon hours, instability
looks best over the Coastal and Sierra mountains north of I-80
including the adjacent foothills into the valley. The WRF model
echoes this by showing shower activity across Lake County
northward along the coastal range...across Tehama & Shasta
counties...and from Chico & Oroville eastward into the Sierra. By
late afternoon into early evening, instability continues along the
Sierra and spreads southward of I-80, however, the WRF shows
showers diminishing in areal coverage. So have stuck with the
forecast of having isolated showers/t-storms south of I-80 and
scattered showers/t-storms north of I-80 today with showers
limited to higher terrain for tonight.

Daytime highs today will be cooler than normal. Our CWA will
generally range 5 to 15 degrees below average. This will translate to
valley highs in the upper 70s to low 80s with higher terrain
warming into the 60s and 70s. However, on Friday into the weekend,
temperatures will warm again to normal and above normal.

The King Fire is still likely to cause smoky conditions as it
doesn`t look like there is a high probability for heavy rain to
dampen this wildfire. With southwest to westerly winds this
morning, smoke will spread towards the Tahoe region again. On
Friday, winds should be northerly in the morning which will spread
smoke southward. (See Previous Discussion below)

.Extended Discussion (Monday through Thursday)

For Monday into Tuesday, weak ridging develops over our CWA which
will maintain dry weather with daytime highs near to above normal.
The extended models (ECMWF, GFS, and GEM) are in good agreement
that another trough will dig southward and swing into NorCal
Wednesday and Thursday. Have increased pops and lowered temperatures to
account for this cooler, wetter pattern.



Mainly VFR conditions next 24 hours with showers possible over
Shasta County and Coastal Range and spreading SE over the
northern/Central Sac valley, northeast foothills, and Sierra by
this morning. Local SWerly sfc gusts up to 30 to 45 kts over
higher mountain peaks today, decreasing by this evening.


.STO Watches/Warnings/Advisories...

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