Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Seattle Washington
920 PM PM PDT Monday Oct 20 2014

Synopsis...showers will decrease later tonight as an upper trough
moves inland. A strong frontal system will bring rain and wind to
the coast Tuesday afternoon and to the interior of western
Washington by Tuesday night. Wednesday will also be a wet day over
western Washington...especially in the mountains as strong southwest
flow aloft persists. Unsettled weather will continue for the rest of
the week as a series of weather systems move onshore.

&&

Short term...showers and a few thunderstorms over western
Washington this evening will taper off overnight. The main area of
thunderstorms occurred between the south tip of whidbey and the Arlington
area...and the Camano Island radar showed up to 0.75 inches of rain
in some locations of western Snohomish County from the heavy showers.
The next frontal system is fast approaching the area and the leading
frontal band is crossing 130w this evening with a strong westerly
jetstream chasing the system across the North Pacific along 45n.
This next frontal system should produce winds in the category of a
Wind Advisory Tuesday afternoon and evening for the coast and all around
the San Juans. The NAM takes a 996mb low across 130w around 8pm and
that gets down to 991mb by the time it reaches Vancouver Island
around 2am. The 00z gfs40 doesn't show much in the way of secondary
development but the uw wrfgfs hints at a little something. Sustained
winds along the coast and in the northern inland waters will be
gales...and the uw 10m wind gusts forecast shows winds over the
coastal waters perhaps as high as 60kt with 40kt or 45mph gusts over
land. The other thing to watch with this system is the amount of
rain that carries over into Wednesday...especially in the
mountains...in strong SW flow aloft. The rain does appear likely to
be heavy enough to get the skokomish up to flood stage.

Long term...Friday will be in between weather systems with just a
small chance of rain. The GFS and European model (ecmwf) are in pretty good agreement
showing another shot of rain Friday night and Saturday as a frontal
system moves through. A weak upper ridge will move over the area
Sunday and Monday for drier and more tranquil weather. Schneider

&&

Hydrology...a vigorous frontal system is expected to bring heavy
rains to the area Tuesday night. The highest amounts are forecast to
be over the Olympics where up to 5 inches are possible by Wednesday
morning. This will likely push the Skokomish River in Mason County
to flood stage on Wednesday. A hydrologic outlook is currently out
to cover this possibility. For other areas rivers...flooding is
unlikely over the next 7 days.

&&

Aviation...a trough over the Pacific northwest tonight will weaken and shift east
of Tuesday. The next low pressure system will develop offshore
Tuesday afternoon with a warm front lifting across the area. A 990
mb surface low will track into central Vancouver Island Tuesday
night....dragging a front through western Washington. Light to moderate SW flow
aloft...becoming strong S-SW flow aloft by Tuesday afternoon. Moist
atmosphere and unstable...becoming somewhat stable after midnight
tonight. Showers and isolated thunderstorms...then showers
decreasing after midnight. Rain developing Tuesday afternoon with
the next system. Variable ceilings...but generally VFR except MVFR with
heavier precipitation.

Ksea...showers through tonight but decreasing in coverage after
midnight. Isolated thunderstorms possible the next couple hours
becoming unlikely by midnight. Mainly VFR ceilings but MVFR ceilings
2000-2500 feet possible with heavier showers or any thunderstorm. South-southwest
wind 5-10 knots...but variable near thunderstorms. Rain increasing late
Tuesday afternoon and Tuesday night. Dtm

&&

Marine...coastal waters...W-SW swell of 10 to 14 feet will continue
through Tuesday. Combined seas may reach 15-16 feet Tuesday night.
West Entrance...seas 10 to 12 feet. Southerly winds rising to 15 to 25 knots
Tuesday. Small craft advisories for up for the coast and west Strait
through Tuesday....and for Admiralty Inlet northward on Tuesday.

A surface low is expected to approach offshore Tuesday...and move NE
into Vancouver Island Tuesday night as a 990 mb low. This system
will likely produce solid gales over the coastal waters...West
Entrance and inland waters north of Admiralty Inlet. A gale watch
remains effect for late Tuesday and Tuesday night. Dtm

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory coastal waters and West Entrance to the
Strait of Juan Delaware fuca through Tuesday afternoon.
Small Craft Advisory for rough Grays Harbor bar through Tuesday
night.
Gale watch for the coast...west and east entrances to the
Strait of Juan Delaware fuca...and the northern inland waters and the
northern inland waters late Tuesday afternoon and night.

&&

$$

Www.Weather.Gov/Seattle

You can see an illustrated version of this forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html

National Weather Service Glossary of Abbreviations