Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 201238

National Weather Service Eureka CA
538 AM PDT Tue Mar 20 2018

.SYNOPSIS...An offshore storm will bring periods of light to
moderate rain to Northwest California through much of the week.
Cooler air and lower snow levels are expected toward the end of
the week into the early portion of the weekend.


.DISCUSSION...High and mid layer clouds have been streaming over
the region through the night in advance of a positively tilted
upper trough centered well offshore to our southwest. There have
been radar returns aloft over the coastal waters through the night
and brief light rain or sprinkles will be possible this morning.

Guidance continues to indicate moist southwesterly flow ramping
up this afternoon and evening as a shortwave trough moves over
the area. GFS, ECMWF, GEM and higher resolution models continue
to indicate the best potential for modest rainfall south of the
King Range with this first wave of rain tonight.

The offshore upper trough will start to transition eastward toward
the coast on Wednesday and light to moderate rain should steadily
increase through the day into the evening hours. The cold front
will move across the region Wednesday night into the early morning
hours of Thursday. Expect widespread rain and perhaps a period of
heavy rain as the boundary intersect the coastal mountains. Snow
levels will remain fairly high during this time period and do not
anticipate any winter weather concerns.

Snow levels will start to fall to pass levels on Thursday behind
the cold front, however the precipitation will most likely
diminish in frequency and intensity. The break will be short
lived. Guidance continues to indicate a cold upper low parking
offshore the Pacific Northwest Thu through Sat. Several shortwave
features are expected to pin-wheel southeastward from the low
center toward Northwest California generating bouts of rain, snow
and possible hail. The first wave of colder rain is expected late
Thu night into Friday, followed by a second and perhaps colder
shot on Saturday. There will be a threat for snow over the
mountain passes Friday through Saturday. Exact snow levels are
still somewhat sketchy as this storm does appear to be warmer
compared to the last several that brought low elevation snowfall
to the area. Current estimates are for snow to fall between 2000
and 3000 ft.

Shallow instability and showers will linger on the backside of
the trough on Sunday, so maintained at least a slight chance of
rain and snow showers. Upper ridging/warming aloft and offshore
wind flow should result in drier conditions Sun Night and Monday.
The models indicate a warm front moving over the ridge on Monday
and Tuesday, however the southern extent of the rain is most
likely overdone so will keep the forecast dry.


.AVIATION...High clouds are filtering overhead this morning leaving
us with VFR conditions so far. This will gradually change throughout
the day as a storm system enters the region. Light rain showers will
be possible through the morning hours, but southerly winds should
keep the coast relatively rain free. Rain will become more widespread
by the late afternoon and evening hours which should drop
terminals down to MVFR conditions.


.MARINE...Light winds and low seas will persist through the morning
hours; however, conditions will gradually change this afternoon and
evening as a storm system approaches the region. Winds will be
increasing from the south and becoming fresh to strong in strength
by this evening. The strongest of the winds should occur over the
southern inner waters and may warrant a Small Craft Advisory from
roughly 2PM(21Z) through 2AM(9Z) Wednesday morning. The second and
more potent round of winds and larger seas will arrive on Wednesday
and Thursday and then continue through Friday. Combined wave heights
are expected to grow to the low to mid teens and continue through
the weekend.




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