Debris Flow in California, February 18, 2021

Area Forecast Discussion
National Weather Service Medford OR
400 AM PST Thu Feb 18 2021

.Updated AVIATION Discussion...

.DISCUSSION...Satellite imagery is showing the next fast-moving
frontal system approaching the coast this morning. Some light
returns are evident on radar, but this precipitation is virga at
this time since it is falling into a significant dry layer
between 3000 and 6000 feet (as evidenced by 5% RH at 5000 feet on
KMFR sounding launched a little while ago). Also, surface
observations at Blue Ridge RAWS in western Siskiyou County at 3am
indicated RH of 8%.

The atmosphere will moisten quickly this morning as a warm front
pushes in from the Pacific. Expect precipitation to overcome this
dry layer and break out 6-9 am along the coast, then 8-11 am for
most other areas along and west of the Cascades. The leading edge
of precipitation should push east of the Cascades by early
afternoon, reaching far east side areas by mid-late afternoon.

There was a tiny concern for light freezing rain at the start of
precipitation west of the Cascades this morning, but this appears
nil now since cloud cover has held temperatures above freezing. We
still have some winter impacts to deal with today, however. Snow
levels will be around 2500-3500 feet this morning, then will rise
quickly and surge well above 5000 feet (near 7000 feet) this
afternoon west of the Cascades. From the Mt. Shasta region
northeastward, however, snow levels will peak in the 4000-5000
foot range. We expect a period of heavier precipitation to result
in snowfall rates of up to 1 inch per hour over the Cascades and
perhaps some of those other areas in NorCal and the east side late
this afternoon and evening to coincide with the evening commute.
So, we have winter weather headlines out at WSWMFR. If traveling
during this time frame, please check road conditions and "know
before you go".

In terms of rainfall, overall, we're expecting 0.50-2.00 inches
along the coast, with perhaps a little more than that in the
adjacent Coast Ranges, where orographics are maximized. Inland
west side valleys probably have 0.25-0.50 of an inch.

Winds pick up along the coast this morning with gusts of 45-55
mph possible through this afternoon, highest near Cape Arago and
also south of Cape Blanco. We also expect some wind in the Shasta
Valley and the usual suspect areas east of the Cascades, like
along Highway 31 near Summer Lake. These locations have wind
advisories up at NPWMFR for wind gusts to 50 mph. The cold front
will push onshore this evening, then east of the Cascades
overnight.

Expect precipitation to become more showery behind the front with
a corresponding drop in snow levels to around 3500-4500 feet on
Friday, then to 2500 feet Friday night into Saturday morning.
-Spilde

Extended forecast from yesterday afternoon's Area Forecast
Discussion (Saturday through Wednesday)...A progressive pattern
continues with generally weak fronts and upper level disturbances
affecting the area this weekend into next week. On Saturday, a
surface low along the northern California coast will move
southeastward and inland while a shortwave upper trough moves over
the region. There is good agreement between the deterministic and
ensemble solutions showing a showery pattern across the area on
Saturday. Snow levels are forecast to be in the 2500 to 3500 ft
range during this time. Breezy west winds are expected from the
Cascades eastward Saturday afternoon and evening as well. On
Sunday, a high pressure ridge centered offshore nudges into the
area while a warm front moves inland to the north. The southern
portion of this front will bring some chances for light
precipitation, mainly to northern areas of the CWA in Coos,
Douglas and northwest Klamath Counties.

The upper ridge flattens and a zonal pattern aloft briefly sets up
over the area on Monday. Late Monday into Tuesday, a weakening west-
east oriented front will shift southward and into the area. This
will bring precipitation to the area with the best chances from the
Southern Oregon Cascades west. Overall, precipitation totals are
relatively light through with this weakening front so significant
impacts are expected. Also snow levels will be initially high,
around 6000 feet, lowering to 3500 to 4500 feet Tuesday morning
with the frontal passage. Ensembles then indicate an upper upper
trough deepening over the PacNW Tuesday night and Wednesday. There
is variability though on the strength of the upper trough and if it
deepens into a closed low over the region. However, ensembles are in
general agreement with a cool and showery pattern over the area
Tuesday night into Wednesday with best chances in Oregon zones and
less chances into northern California. Also, breezy to gusty west
winds are possible from the Cascades east with this pattern. -CC

&&

.AVIATION...For the 18/12Z TAF Cycle...VFR conditions will prevail
through much of the morning. A strong, quick moving cold front will
move through the area later this morning into this evening.
Conditions will lower to MVFR and widespread terrain obscurations
will develop. As the front moves through, expect moderate to heavy
rain to bring a mix of MVFR/IFR conditions. East of the Cascades,
precipitation will be mostly snow, and this will result in IFR
conditions due to low ceilings and poor visibilities. /BR-y

&&

.MARINE...Updated 200 AM PST Thursday 18 Feb 2021...A strong, quick
moving front will affect the area today. Relatively calm conditions
early this morning will worsen as the day progresses. South winds
increase to gales, peaking this afternoon and seas become wind
driven and very steep and hazardous. The strongest winds and
steepest seas are expected north of Pistol River and beyond 5 nm
from shore. Winds ease this evening behind the front, but seas will
remain steep and transition swell driven as multiple northwest swell
trains move into the waters through the weekend. Active weather
continues into the weekend, but systems will be weaker than today's
front. /BR-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 10 AM this morning to 10 PM PST
     this evening above 4500 feet in the for ORZ029>031.
     Wind Advisory from 1 PM this afternoon to 1 AM PST Friday for
     ORZ031.
     Winter Weather Advisory from 10 AM this morning to 10 AM PST
     Friday above 4500 feet in the for ORZ027-028.

CA...Winter Weather Advisory from 10 AM this morning to 10 PM PST
     this evening above 4500 feet in the for CAZ082-083-085.
     Wind Advisory from 1 PM this afternoon to 10 PM PST this evening
     for CAZ081.

Pacific Coastal Waters...Small Craft Advisory until 4 PM PST Saturday for
     PZZ350-356-370-376.
     Gale Warning from 10 AM this morning to 7 PM PST this evening
     for PZZ350-356-370-376.

MAS/MNF/CC