High Wind in Oregon, April 6 to 7, 2018

Area Forecast Discussion
National Weather Service Medford OR
746 PM PDT Fri Apr 6 2018

.DISCUSSION...Satellite imagery depicts a deepening low pressure
system offshore, and this will bring high-impact weather to the
forecast area over the next 24 hours. First, moderate to heavy
rain will impact much of the forecast area tonight, heaviest at
the coast and in Siskiyou County. We have issued a Flash Flood
Watch for this evening through very early Saturday morning for the
Chetco Bar wildfire burn scar area in Curry County and extreme
Western Josephine County. This does NOT include the cities of
Brookings, Harbor, Pistol River, or any cities in the Illinois
Valley. Enough high-resolution guidance supported 0.5 to 1 inch
per hour rainfall rates over the burn scar to warrant the Watch.
The main concern is for debris flows on and immediately downstream
from the burn scar.

Heavy precipitation will progress inland through the night. Areas
east of the Cascades will again see anomalously heavy rainfall
overnight for the month of April. The Alturas ASOS observation
already recorded the highest daily rainfall for the month of April
with 0.85 inches so far today, and more will be added to that before
midnight. Lakeview recorded an impressive 0.95 inches in the past 24
hours. Rivers east of the Cascades are rising, and the Pit River at
Canby is forecast to reach levels just below bankfull on Sunday. All
other area major rivers will rise, but are not expected to reach
bankfull. Small creeks and streams could rise and potentially flood,
and ponding of water on roadways is likely overnight.

As the previous forecast discussion below outlined, wind impacts are
the other main concern with this storm. We have expanded the Wind
Advisory area for Saturday to include Joesphine and Douglas County
(elevations generally above 1000 feet in Douglas County) based on
high-resolution guidance and expected strong 5000 foot winds
(forecast to be top 1 percent in terms of windspeed for this time of
year) that could mix down to the surface Saturday afternoon. Given
that trees are starting to leaf out and soil will be saturated, tree
damage could be enhanced in some of the windiest areas, leading to
the potential for power outages. We have added a slight chance of
thunderstorms west of the Cascades Saturday afternoon, and any
thunderstorms could bring down very gusty winds to the surface.

We will be looking closely at snow levels and amounts during this
shift for tomorrow night over the High Cascades. At this time it
looks like the best chance for snow impacts is from Highway 140
northward and especially from Crater Lake northward above 5000
feet.

Please read the forecast discussion below for more details on this
high-impact storm system.

&&

.AVIATION...07/00Z TAF CYCLE...A strong front is approaching the
area and will move inland tonight and early Saturday. This system
will bring wind and rain impacts to the area. Areas of MVFR
cigs/vis with local IFR cigs are expected through late this
evening through around 15z Saturday, then expect VFR with local
MVFR cigs. Widespread terrain obscurations are expected through
Saturday morning followed by areas of mountain obscurations
Saturday afternoon. Winds will increase this evening and tonight
and expect periods of LLWS at area terminals, as a cold front
approaches from the southwest. Saturday morning into the
afternoon, very strong and gusty winds will develop across the
area. The strongest winds are expected along the coast as well as
for areas east of the Cascades where gusts of 50 to 60 mph are
possible. -CC

&&

.MARINE...Updated 800 PM Friday 6 April 2018...A powerful frontal
system will impact the area this evening through Saturday. Very
steep hazardous seas and southerly gales will develop across the
waters this evening and tonight then wind will peak on Saturday.
Widespread strong gales are expected across the waters by early
Saturday morning and continuing through the afternoon. Winds may
near storm force north of Cape Blanco beyond 10 nm from shore. Wind-
driven and very steep seas will build tonight into Saturday, peaking
at 20 to 28 feet afternoon and evening. Winds will gradually
diminish Saturday night into Sunday as high pressure briefly
builds in, but expect hazardous seas from fresh swell through
Sunday ...possibly continuing into Monday. Then the active pattern
continues early next week with additional fronts Tuesday and
Wednesday.

&&

.PREV DISCUSSION... /Issued 245 PM PDT Fri Apr 6 2018/

DISCUSSION...The heaviest rain is mainly south of our forecast area
and it will remain there into this evening. Meanwhile light rain
will continue in northern California and southern half of Klamath
and Lake Counties. Elsewhere, most hours will be dry until early
this evening, then precipitation will arrive in southeast Oregon
later this evening.

The latest IR image clearly shows a developing low near 40N and
140W, this low will continue to deepen as it moves northeast later
this evening through tonight. The low will move inside of 130W west
of Astoria Oregon by Saturday morning. It will be about this time
when the low will bottom out in terms of central pressure, then
gradually rise as the low begins to fill in as it turns more to the
north during the day Saturday. Moderate to strong winds and heavy
rain are the two main impacts from this storm each one will be
discussed below.

Winds: South winds will increase along the coast later this evening.
There's good agreement winds will be strongest for about a 6-8 hour
period late tonight into Saturday morning and mainly from around
Port Orford north. Moderate winds are expected south of Port Orford
to around Gold Beach, but they should remain below high wind
criteria. Inland, it's already windy in the Shasta Valley near Weed
and the Rogue Valley and they are expected to increase later
tonight. Medford could see gusty breezes at times into tonight, but
the stronger winds will remain south. There's good agreement the
front will move inland Saturday morning, then east of the Cascades
Saturday afternoon. The focus for strong winds will then shift east
of the Cascades late Saturday morning through Saturday evening.
However moderate south winds are likely Saturday morning before
shifting southeast late Saturday morning. A new development is the
potential for moderate to strong winds west of the Cascades which
could approach advisory criteria late Saturday morning and
afternoon. This is due to stronger winds aloft mixing down near and
at the surface. Details on all the wind headlines can be seen at
NPWMFR.

Rain: Precipitation will increase from the southeast later this
evening through tonight. Rainfall will be heaviest along the coast
from about Cape Blanco south, coastal mountains and western Siksyou
County. The general consensus is we could get about a 6 hour period
of intense heavy rain along the coast and coastal mountains
overnight tonight which could result in rapid rises of streams and
creeks and possible mudflow debris from the Chetco Bar burn scar.
Inland, rain will be heaviest late tonight into early Saturday
morning. Widespread showers are likely behind the front with the
focus of heaviest precipitation in the coastal mountains, Cascades
and Western Siskiyou County. Snow levels will be high through
tonight, then will lower during the day Saturday. They will start
out around 9,000 feet just before daybreak Saturday, then lowering
to 4500 to 5500 feet Saturday afternoon and night.

Showers will continue through Saturday night with snow levels
lowering further to 4000 to 4500 feet by Sunday morning. The focus
for heavier showers during this time will be along the Cascades and
coastal mountains. Therefore snow cover roads are likely around
Crater Lake and Diamond Lake. Snow covered roads are also possible
near Lake of the Woods by Sunday morning, but amounts will be less.
Snow amounts will be highest in the southern Oregon Cascades north
of Lake of the Woods (including Diamond Lake and Crater Lake), and
on Mount Shasta with around 7 to 14 inches expected in the 24 hours
from Saturday afternoon into Sunday afternoon.

The majority of showers and snow showers will taper off Sunday
afternoon with activity lingering longest from Douglas County
northward. Clouds will diminish on Sunday night with the start of
what will be a brief but very strong warming trend. High pressure on
Monday will produce inland high temperatures in the ballpark of a
dozen degrees above normal. For example were forecasting a high of
77 degrees for Medford. Conditions will remain dry with milder
temperatures Monday as clouds increase ahead of the next front.
-Petrucelli

LONG TERM...Tuesday through Friday night...The extended period will
begin with more active weather. Light to moderate rain is expected
across southern Oregon and northern California as the next front
pushes through on Tuesday. This will move through rather quickly and
showers will linger on the back end. A quick break is expected on
Tuesday night before the next front comes through on Wednesday. This
front will be a bit slower to exit the area, and showers are
expected through Friday before the next quick break is expected
Friday night. None of these systems will provide heavy rainfall like
what we've seen in the near-term forecast, but the accumulation of
each system will could start to weigh in on the snowpack a little
bit.

Snow levels will generally be around 4 to 5000 feet with a few
periods of 2000 foot snow levels on Thursday and Friday, so the
abundance of rain should not affect snow melt on the higher peaks of
the Cascades and Siskiyous. Still, it is worth paying attention to
the small streams and creeks. Additionally, it appears as if our
forecast for the Lost Creek Lake Reservoir should reach full pool
late in the extended period--therefore a few releases could be
possible which could downstream rises.

Overall, confidence is moderate on the pattern and low on the
details. The GFS, as per usual, is a bit more progressive as opposed
to the ECMWF. Both, however, are showing similar strengths in these
systems. As always, the extended forecast will be worth watching as
the pattern evolves. -Schaaf

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 11 AM to 11 PM PDT Saturday for
     ORZ030-031.
     Wind Advisory from 11 AM to 11 PM PDT Saturday for ORZ029>031.
     High Surf Advisory from 5 AM to 11 PM PDT Saturday for
     ORZ021-022.
     High Wind Warning from 11 PM this evening to 5 PM PDT Saturday
     for ORZ021-022.
     Wind Advisory from 9 AM to 8 PM PDT Saturday for ORZ023>026.
     Flash Flood Watch until 4 AM PDT Saturday for ORZ022-024.
     Wind Advisory until 2 AM PDT Saturday for ORZ026.

CA...High Wind Warning from 11 AM to 11 PM PDT Saturday for CAZ085.
     Wind Advisory from 11 AM to 11 PM PDT Saturday for CAZ084-085.
     Wind Advisory until 11 AM PDT Saturday for CAZ081.

Pacific Coastal Waters...High Surf Advisory from 5 AM to 11 PM PDT Saturday for
     PZZ350-356.
     Gale Warning until 5 PM PDT Saturday for PZZ350-356-370-376.
     Hazardous Seas Warning from 5 PM Saturday to 2 PM PDT Sunday for
     PZZ350-356-370-376.

NSK/BMS/CC