National Weather Service Raw Text Product

Displaying AFOS PIL: AFDSTO Received: 2017-02-19 13:24 UTC


417 
FXUS66 KSTO 191324
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
524 AM PST Sun Feb 19 2017


.Synopsis...
Wet weather with renewed flooding concerns into the middle of 
next week with wettest storm of the bunch expected for Monday
through Tuesday. Gusty winds likely again Monday. Mountain travel
will also be impacted at times due to heavy snow. Valley
thunderstorms possible Tuesday and Wednesday. 

&&

. Discussion...
Large upper level trough in the northeast Pacific will continue to
impact norcal weather over the next several days. A shortwave
disturbance pivoting through central California this morning in
southwest flow ahead of the trough is bringing light rain to the
Sacramento and Delta area this morning. A 6 mb surface gradient 
from Redding to Sacramento is bringing breezy winds to the 
northern Sacramento valley this morning with sustained winds 
between 20 and 25 mph north of about Chico. Have issued wind 
advisory through today for this area as model guidance keeps this 
level of winds going through the day. Snow levels this morning 
range from about 3500 feet northern Shasta county to 5000 feet 
over the northern Sierra. Snowfall amounts should be relatively 
light today although chain controls are still in affect for the 
major Sierra passes. More light showers expected this afternoon as
another shortwave disturbance pivots through...this time a little
farther north. 

Things begin to ramp up this evening as a much stronger and wetter
Pacific storm system moves onshore. Widespread moderate to heavy 
rain and snow are expected with gusty winds as this atmospheric 
river weather system moves through. The blended total precipitable
water satellite product shows PW values of and inch and a half 
within the river with locally even higher values. This is a return
interval in the 5-10 year range. Precipitation amounts max out 
during the day on Monday. Between tonight and Monday night...1 1/2
to 2 1/2 inches of rain are forecast for the valley and 
orographically favored areas of the Sierra west slopes could see 
up to 8 inches of rainfall during this time. Snow levels during 
this time will climb to about 7000 feet but are still likely to be
low enough to continue to impact pass level travel over the 
Sierra. Rainfall will likely create even more flood issues during 
and after Monday. A flood warning will remain in place for 
interior Norcal through much of the week. Winter storm warnings 
have been issued for the Sierra Cascade range. As if this is not 
enough...evening stronger winds are expected Monday evening as the
main frontal band shifts through with 925 nb progs from the NAM 
showing over 55 mph winds. This will likely produce another round
of power outages as soggy ground allows even more trees to topple
on powerlines. Focus of heaviest rainfall continues to shift 
southward. Heaviest rainfall still likely over the American river
basin and now southward so needed to increase QPF south of the 80
corridor from previous forecasts. 

Precipitation drops off on Tuesday behind the main front as do 
winds but snow levels drop as colder air filters in so a larger 
area of Sierra will be impacted by snowfall. Have extended the
winter storm warning through Tuesday evening. 

Main upper trough shifts through on Wednesday keeping the showers
going and bringing down snow levels even farther but precip
amounts should be relatively light. Snow levels could be down to
3000 feet by Wednesday evening. Cold air aloft that brings down 
the snow levels will also destabilize the atmosphere so it is
likely that the valley will see isolated afternoon and evening
thunderstorms Tuesday and Wednesday. 
&&

.EXTENDED DISCUSSION (Thursday THROUGH Sunday)

Thursday into Friday is expected to be mainly dry as short wave 
high pressure ridging moves in, with just some lingering mountain 
and foothill showers. Any precipitation is expected to be light. 
Northerly winds should limit fog formation, moist low level 
conditions could allow some fog/mist to develop in sheltered 
areas. 

Extended models are trending towards a wetter scenario for late 
Friday through Saturday night. Models previously were showing a 
less wet scenario, and disagree on the track of a low which is
dropping down from western Canada. If the latest GFS is correct,
this cold be a rather wet system, though quite a bit less than the
early week system. If the ECMWF and GEM are correct, there could
be some precipitation, but not a large amount. Sunday there is a
relative lull in the wet weather, with mainly mountain
precipitation. This may a brief lull, with yet another system
expected on Monday, showing no significant break in the wet
pattern. EK


&&

.AVIATION...

Scattered showers with local MVFR/IFR conditions across the 
Valley and IFR/LIFR conditions across higher terrain today with 
snow. Increasing rain/snow threat again after 12z Sunday. 
Southerly winds increasing today, gusting to around 35 kt over the
northern Sacramento Valley, spreading across the Valley tonight, 
with even stronger winds on Monday/Monday evening.
&&

.STO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 4 PM this afternoon to 10 PM PST 
Tuesday for West Slope Northern Sierra Nevada-Western Plumas 
County/Lassen Park.

Wind Advisory from 10 PM this evening to 7 PM PST Monday for 
Carquinez Strait and Delta-Central Sacramento Valley-Northern 
Sacramento Valley-Northern San Joaquin Valley-Southern 
Sacramento Valley.

High Wind Watch from Monday evening through late Monday night 
for Carquinez Strait and Delta-Central Sacramento Valley-
Northern Sacramento Valley-Northern San Joaquin Valley-Southern 
Sacramento Valley.

&&

$$