Area Forecast Discussion
Issued by NWS Eureka, CA

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FXUS66 KEKA 192208

National Weather Service Eureka CA
208 PM PST Sun Nov 19 2017

.SYNOPSIS...Rain will spread inland tonight, and then increase in
intensity during Monday. Rain will become light by Tuesday.
However, the potential for wet and unsettled weather will likely
continue through late week.


.DISCUSSION...Latest satellite imagery and model guidance shows a
longwave trough extending southwest from NWRN Canada toward
Hawaii. An atmospheric river, characterized by PWAT values from
1.5 to 2.5 inches, and very high freezing levels (at or above
10000 feet), was observed spreading northeast from the tropics
toward the Pacific Northwest region. Short term model trends
indicate the anomalously moist airmass will become collocated with
a zone of increasing WAA/isentropic upglide over NWRN CA tonight,
resulting in the development of rain. By midday Monday, rainfall
rates are forecast to rapidly increase as intense westerly
moisture flux encounters the coast mountains. The zone of strong
moisture flux is forecast to shift northward with time, eventually
exiting across Oregon by late Monday night. Once that happens,
rain will become light over NW CA.

After Monday night, the atmospheric river (AR) will take aim on
Oregon and Washington, leaving NRN CA mainly dry Tuesday.
Thereafter, remnants of the AR may impact the EKA CWA during
Wednesday and Thursday, resulting in another period of rain. By
late week (Saturday/Sunday), model guidance shows a surface
cyclone ejecting northeast toward the west coast, which would
increase the potential for rainfall once again.


.AVIATION...High clouds in advance of the next system have moved
over all terminals and these ceilings will continue to lower
through the afternoon and evening. Strong winds aloft will also
lead to low level wind shear concerns later this afternoon and
persist through tomorrow for ACV and CEC. Light rain will begin
for all terminals tonight and gradually pick up in intense through
the early morning hours. Periods of heavy rain tomorrow will lead
to reduced visibilities at all three terminals. However, these
conditions will be more prevalent for ACV and CEC. With the clouds
and rain ground fog will not be able to form tonight as has been
the case the past few days. /WCI


.MARINE...Southerly winds gusting to near gale force are expected
over the northern outer waters tonight into Monday, with solid
small craft southerlies expected over the remainder of the coastal
waters. The increase in wind speeds will generate steep seas, and
when combined with a west-northwesterly swell, will yield a
confused sea state. Southerly winds and steep seas will continue
through mid week, followed by weaker winds and lower seas during
late week.


.HYDROLOGY...Heavy rainfall appears probable across much of
Humboldt, Del Norte, and possibly Trinity county during Monday.
Light rain is forecast to spread across northwest California
Sunday night, and then dramatically increase in intensity by late
Monday morning. Average rainfall totals are forecast to range from
1-2 inches over coastal plains and interior valleys...with higher
totals upwards of 2-5 inches probable over the windward side of
mountains and ridges. However, latest high resolution model
guidance shows localized extreme rainfall totals of 6-10 inches
Sunday night through Monday night over west facing terrain. Given
moderate rainfall totals are very probable, and extreme rainfall
totals showing up in the tail of potentiality, a flood watch has
been issued for all of Humboldt, Del Norte, and Trinity counties
(excluding coastal plains). Small stream/localized urban flooding
and rock/mud slides will be the primary impacts associated with
areas of heavy rain.


CA...Flood Watch from 10 PM PST this evening through Monday evening
     for CAZ102-104>108.

     PM this afternoon to 4 PM PST Wednesday for PZZ470.

     Small Craft Advisory from 4 AM to 4 PM PST Monday for PZZ450-455.

     Small Craft Advisory from 11 PM this evening to 4 AM PST
     Wednesday for PZZ475.



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