Scientific Forecaster Discussion
fxus66 ksto 052314
Area forecast discussion
National Weather Service Sacramento California
314 PM PST Thu Dec 5 2019
scattered light showers today before another winter storm will bring
periods of rain and mountain snow as well as hazardous travel
Friday and into the weekend. Dry conditions return next week with
near average temperatures.
cloud cover has been plentiful over much of the region although
early morning fog has long mixed out over the northern San Joaquin
Valley. A general mixed bag of low to mid level clouds have spread
across the region within the moist conveyor belt ahead of the
strong upper low offshore. Kbbx radar has even picked up scattered
showers across the northern Sacramento Valley into the higher
terrain. However, observational data over the past 12 hours show
amounts generally under 0.10 inches has fallen. The true focus in
the weather shifts to the mentioned offshore low set to impact
northern California Friday and into much of the weekend.
Mid-afternoon visible satellite imagery shows a rather impressive
cyclone currently over 750 miles off the coast accompanied by well
defined frontal features. There have even been some spurious
lightning strikes underneath the dome of cold air aloft. All of
this action will shift eastward during the next 24 hours. Before
this occurs, expect another round of early morning patchy fog
from Sacramento southward toward Modesto given all the ground and
soil moisture remaining from prior rainfall. Eventually warm
advection precipitation will propagate from west to east across
the state, roughly hitting the coastal range by around noon Friday
before reaching the Sacramento Valley during the mid/late afternoon.
Conditions over the Sierra should quickly deteriorate as accumulating
snows commence in the early evening. Accordingly, a Winter Storm
Warning has been issued for elevations 6,500 feet and above.
From Friday evening through Sunday morning, multiple facets of
weather will impact the region which includes heavy mountain
snows, moderate to heavy rainfall in the valley, and isolated
thunderstorms. Beginning with the Sierra, heavy snow will make
for another challenging travel period along all the major
thoroughfare. Chain controls are likely along with major travel
delays through Sunday. Gusty winds will significantly reduce
visibilities, potentially reaching whiteout conditions at times. If
travel plans include going across the Sierra, attempt to leave
before Friday morning or wait until Monday. While snow levels
initially sit around 6,500 feet, they will fall down to around
5,500 to 6,000 feet by early Sunday. Expecting anywhere from 1 to
3 feet around pass level, potentially higher in heavier snow bands
and across higher mountain summits. Conditions will turn more
showery by Sunday afternoon as the upper low passes overhead.
Across the valley, a semi-persistent period of moderate to
occasionally heavy rainfall will impact the region Friday
afternoon into early Sunday. While storm total amounts should stay
anywhere from 2 to 3 inches toward Redding and 1 to 2 inches from
Sacramento southward, hourly rainfall rates are likely to stay
under 0.25 inches. This will limit any major hydrologic issues
although cannot rule out localized flooding due to clogged storm
drains nor ponding on roads. As the cold, unstable air aloft moves
through over the weekend, some isolated thunderstorms are
possible. Any breaks in the clouds will maximize instability in
the column which supports a few random lightning strikes.
Confidence is low in placement of such storms but will keep it in
the forecast with later high-resolution models hopefully ironing
out the details. In addition to the rainfall, modest pressure
gradients support an uptick in southerly winds, potentially
gusting 30 to 40 mph, likely higher into the foothills and
elevated terrain. Conditions will gradually improve later on
Sunday as heights slowly begin to build. ~Bro
Extended discussion (monday through thursday)...
not much change to the previous forecast. Ensemble and model
guidance hold firm on ridge building over the Pacific northwest
early in the week. This will lead to at least a short stretch of
dry weather, with the potential for some fog Tuesday morning.
Looking towards to the mid to late portion of the work week, model
and ensemble guidance continue to deviate from each other. Much
like yesterday afternoon's runs, there is uncertainty as to when
and where the next trough sets up. Some ensemble members suggest
we could see light precip over northern California, mainly limited
to the Cascades, coastal range, and northern Sacramento Valley
Wednesday and Thursday. Confidence is low on this occurring for
the time being, thus we'll be keeping a close eye on how the
models and ensembles handle the situation.
Temperatures are forecast to be within +/-5 degrees or so of
their seasonal average for both daytime highs and morning lows.
stratus will be the main issue of the taf period with most sites
around MVFR ceilings, with Sacramento sites southward seeing
periods of VFR later this evening. Some small reductions in
visibility look possible tomorrow morning, but stratus should
limit dense fog.
Winter Storm Warning from 4 PM Friday to 1 PM PST Sunday for
West Slope northern Sierra Nevada-western Plumas County/Lassen