Small Craft Advisory 259, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
945 AM PDT Tue Oct 25 2016

.SYNOPSIS...A series of low pressure systems will lift north along
the Pacific Coast over the next several days. This will keep Western
Washington in a generally unsettled and wet pattern with temperatures
near or slightly above normal.

&&

.SHORT TERM...The coastal radar and lightning detection continues
to indicate bands of heavy showers and thunderstorms over the
coastal waters this morning. This activity appears to be associated
with a vorticity max at 700 mb and a dark slot can be see on water
vapor imagery indicating an embedded short wave lifting north-
northeastward. The UIL sounding and forecast sounds for the coast
show a strong veering wind profile with the short wave providing
dynamic lift. This is favorable for rotation which is being observed
by radar in some cells offshore. This activity looks to mainly stay
over the coastal waters but some of the strong cells could
potentially reach the central/north coast over the next few hours.
Cannot rule out the possibility of waterspouts, but so far the cells
have weakens upon reaching the coast. Will be monitoring these
closely, but this activity is expected to begin decreasing by
midday.

Winds have picked up across the area, especially over the north
interior. The 980-985 mb surface low near 133w/50n is lifting
northward allowing southeasterly gradient to increase over the
north. A few land wind gusts to 45 mph were observed this past hour
and a wind advisory is now in effect until 1 pm for Western Whatcom
and the San Juans. Isolated power outages from downed tree limbs can
occur with gusts of this strength. Winds will be breezy to locally
windy in other spots, such as the coast and Admiralty inlet which
could have gusts 35 to 40 mph. Puget Sound will stay breezy today
and southerly gusts should remain below 30 mph.

The forecast was updated to increase pops across the interior today.
Showers over the coastal waters will weaken and move into parts of
the interior today. It will be a hit and miss type shower pattern
but heavy rain/thunderstorms are not expected over Western
Washington. There will be a lull in showers over the area tonight
with loss of diurnal instability and the low moving further north
away from the area.

Models show a 990-995 mb surface low just inside 130 w lifting north-
northeast off the Washington coast on Wednesday. This system will
bring strong warm advection rainfall to the entire area beginning
across the far south interior/south coast prior to day break and
overspreading the rest of the area through the morning. 50+ kt
southerly winds at 850 mb coupled with high precipitable water
values will favor heavier rain amounts along south facing mountain
slopes, especially the Olympics. See the hydro section below for
details. Lowland rain amount Wednesday will range from about .25 to
.75 inches with heaviest amounts up to 2 inches on south facing
slopes of the Olympics. Heaviest rainfall is expected during the
early morning hours so expect ponding of water on roadways to cause a
slow morning commute.

Some showers will linger into Thursday but activity will decrease
significantly. Expect some partial sunshine between passing showers.
Highs will remain close to 60. Mercer

.LONG TERM...No update from previous discussion...The general idea
of northward progressing surface features continues through the
weekend and into early next week. The GFS and Euro are in good
agreement on this and the general 500 mb pattern. This agreement
holds through Monday. Both models show a 500 mb trough sliding
through the area, with the GFS being deeper and slower with the
feature. For now will just continue the ongoing inclusion of pops
through all periods in the forecast. The progression of weak surface
lows and occasional increase in shower chances as they draw in moist
air from the south will continue. However none of these low pressures
look remarkable in terms of strength or impacts. Pressure gradients
remain weak as the lows aren't strong, so winds not expected to be
significant except over the coastal waters. Rain amounts will
steadily add up but over a several day period. Amounts of 1-3 inches
from now through next Monday will be spread across several days to
minimize impacts. The general theme is temperatures near to above
normal with chances of rain every day for the next week. Look for
many areas to close out with a top 3 wettest October on record across
Western Washington.

Johnson

&&

.HYDROLOGY...Will need to monitor the Skokomish river with the wet
Wednesday storm system. Rain amounts around 2 inches are expected by
Wednesday evening along south facing slopes. This should be not be
sufficient for flooding on the Skokomish river. However, if rain
amounts are higher than expected, some minor flooding could occur
Wednesday night and Thursday. No watch is warranted at this time but
will look at this again once all the 12z model data is in. Otherwise,
flooding is not expected on any rivers over the next 7 days.

&&

.AVIATION...A longwave upper trough will persist between 130W and
140W through Wednesday. Moderate southwest flow aloft today,
becoming strong on Wednesday. A deep surface low will move
northward on Wednesday about 250 miles off the Washington coast.
Lower pressure offshore will support a drying easterly component
to the low-level winds, which will either inhibit or prevent low
cloud development. A warm front extending east from Wednesday's
low will cause rain to spread northward through western Washington
on Wed morning. The air mass today is unstable and somewhat
moist...becoming stable and more moist on Wednesday.

KSEA...More breaks in the mid-level cloud deck expected this
afternoon and evening. Southerly winds continuing through about 6
pm, or 01z, then backing around to an easterly direction by late
evening. Easterly wind component for the next 30 hours will
prevent or inhibit low clouds. A warm front will affect the
terminal on Wed morning, brining reduced vsby in moderate rain at
times...but the drying easterly surface wind component will help
hold cloud cigs above 030 on Wed, even in the midst of rain. Haner

&&

.MARINE...A 979 mb low centered near 50N 133W at 8 AM will
continue to move N-NW and weaken for the rest of today, with
easing pressure gradients over the waters. Thunderstorm cells over
the coastal waters this morning have shown well-defined rotation
at times, indicating a waterspout threat. The threat of
thunderstorm-induced waterspouts should end around noon as
instability and wind energy subside.

A second low - near 994 mb - will track northward inside 130W
late Wednesday afternoon into Wednesday night. There is the
potential for another round of gales with this low Wednesday and
Wednesday night along the coast.

Low level flow will turn offshore on Thursday and remain
light offshore on Friday. 	Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory until 1 PM PDT this afternoon for San Juan County-
     Western Whatcom County.

PZ...Small Craft Advisory until midnight PDT tonight 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.

     Gale Warning until noon PDT today 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-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 5 AM PDT Wednesday for
     Grays Harbor Bar.

     Gale Warning until 3 PM PDT this afternoon for East Entrance
     U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory until 9 PM PDT this evening for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PDT this evening for Admiralty
     Inlet.

     Small Craft Advisory until 3 PM PDT this afternoon for Puget
     Sound and Hood Canal.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html