Winter Storm Watch 2, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
249 PM PST Sun Jan 15 2017

.SYNOPSIS...An upper ridge will move inland tonight, and a warm
front will spread rain inland Monday and Monday evening. Rain,
heavy at times, will continue Monday night into Wednesday. An
upper trough will bring cooler showery weather Thursday through
Saturday.

&&

.SHORT TERM...A pattern change will bring wetter, warmer and
windier weather back to Western Washington. This change will take
place on Monday as a strong jet lines up over WA. Strong, moist
westerly flow will continue through Tuesday and Wednesday and
models are ramping up the precip totals in the mountains. Heavy
rainfall, with rising snow levels, may cause river flooding. A
Hydrologic Outlook has been issued. However, there is still some
uncertainty on the precip totals in the basins. The models are
showing a wide range and the GFS still has nearly double the
amount of precip in the mountains compared to the ECMWF. It will
be windy in the lowlands over this period as well, especially the
coast and north interior with Wind Advisories possible. The warmer
air mass will cause temps to spike into the lower 50s both Tuesday
and Wednesday...with lows in the 40s. The jet will shift south
Wednesday night and a trough will bring an end to steady heavy
rains, although rivers may still be running high. 33

.LONG TERM...An upper level trough will maintain showers over
Western WA on Thursday. The air mass will be cooler with snow
levels back down to around 3000 ft. Temperatures will be close to
normal with highs in the mid 40s. We may see a brief break in the
showers Thursday night, then more rain on Friday as the next
trough pivots inland. Moist SW flow will continue through the
weekend for at least a chance of showers. 33

&&

.AVIATION...An upper ridge will continue to move eastward today
with the ridge axis making its way into Eastern WA tonight with
upper flow generally being westerly into Monday. Surface flow will
remain light and easterly tonight and Monday morning before turning
more south to southeasterly and increasing Monday afternoon and
evening in advance of an approaching front. The air mass is expected
to remain dry and stable tonight...moistening Monday.

Generally thin high level clouds over portions of the area...with
clouds expected to increase as well as lower tonight and Monday in
advance of frontal system. While mid-level clouds and cigs expected
for Monday...conditions look to remain VFR.

KSEA...High clouds this evening...increasing tonight. Mid-level
clouds developing tonight and increasing by early Monday morning.
Light winds tonight and early Monday morning...increasing to 4-8
kts and becoming southeasterly after 17Z Monday.  SMR

&&

.MARINE...High pressure will be inland with lower pressure
offshore through tonight. A series of fronts will move through the
waters Monday through Thursday...bringing gales to the coastal
waters and small craft advisory winds to most of the remaining
waters...excluding the Central Strait and Puget Sound. Thus...the
Gale Watch has been upgraded to a Gale Warning and small craft
advisories are in place for the afternoon forecast package. The
Sound is likely to be included in the advisories...but current
forecast shows winds not starting there until late Monday so plenty
of time for re-evaluation. Timing on advisories for inland waters
may need extending but will leave that to future shifts to evaluate.
Forecast remains persistent regarding westerly swells of around 20
feet by Wednesday. SMR

&&

.HYDROLOGY...River flooding is not expected through Monday. A warm
front will move through Western Washington Monday through Monday
evening. Rain -- heavy at times -- will continue more or less
Monday night through Tuesday night with the snow level 7000 to
8500 ft. Southwest flow of 40 to 65 kt at 850 mb will prevail
during this period, which is a fairly classic sign of a heavy rain
event especially for the Olympic Mountains.

Precipitation for the 36-hour period from late Monday through
Tuesday night will probably be 4 to 7+ inches over the Olympics,
and around 3 to 5+ inches over the Cascades. There could be
another 1 to 2 inches on Wednesday, as the snow level falls to
around 6000 ft.

A big uncertainty at this point, as mentioned above, is the fact
that the operational GFS run has consistently had the highest QPF
of all the ensemble members; therefore, the GFS and UW WRF-GFS
precipitation amounts could be too high. It means that the NWRFC
guidance for the rivers could also be too high. Another
consideration with regard to the potential for flooding is that
rivers are running low, many of them at their lowest levels since
the end of the summer.

Bottom line is that we're forecasting enough rain to produce some
flooding on at least the more flood-prone rivers in our forecast
area.

Note: Snow is not a significant contributor to flood flows on
Western Washington rivers. Even when there is snow on the ground,
river flooding is almost exclusively caused by runoff from heavy
rain. McDonnal/33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning from 10 AM Monday to 4 AM PST Tuesday for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory from 3 AM to 10 AM PST Monday for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm.

     Small Craft Advisory from noon Monday to 4 PM PST Tuesday for
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 4 PM Monday to 4 PM PST Tuesday for
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 10 AM to 10 PM PST Monday for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 9 AM Monday to 4 PM PST Tuesday for
     Admiralty Inlet.

     Small Craft Advisory from 10 PM Monday to 4 PM PST Tuesday for
     Puget Sound and Hood Canal.

&&

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