Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
835 am PM PDT Monday Oct 20 2014
Synopsis...showers will decrease today and tonight in the wake of a
front. A frontal system will bring rain and wind to the coast
Tuesday afternoon and inland Tuesday night. Unsettled weather will
continue for the rest of the week as a series of weather systems
Short term...showers are likely through this evening as an upper
trough moves overhead in the wake of the front from last night.
There is also a slight risk of thunder near the coast where the air
will be most unstable. Showers should become fairly isolated later
tonight and Tuesday morning as the upper trough pushes inland and
weak high pressure aloft brings more stable air.
A vigorous frontal system will bring rain to the coast Tuesday
afternoon and then inland Tuesday night. This system will bring a
good shot of rain...especially to the Olympics which could push the
skokomish to flood Wednesday or Wednesday night. There will also be
some windy conditions with this system...most notably for the coast
and north interior Tuesday night. Wind advisories may eventually be
needed for this.
It looks like there will be a distinct lull in the rains for part of
Wednesday...then another frontal wave will bring more rain later
Wednesday or Wednesday night with showers lingering into Thursday.
Long term...previous discussion...models are now showing more
showers Friday through the weekend as an upper level low crosses
western Washington. Temperatures will likely remain at or below normal throughout.
Both the GFS and European model (ecmwf) try to build a ridge over the Pacific northwest early
next week for a brief period of dry weather...maybe Sun night or
Monday. Then another wet Pacific system will drag through the region Monday
night or Tuesday. 33
Hydrology...forecast models show that heavy rain Tuesday night
through Wednesday is probably enough to bring the Skokomish River to
flood stage. Models are showing up to 5 inches of rain in the
Olympics over this period. The models confine flood-producing
rainfall to the olympic peninsula and probably just to the Skokomish
River basin. Any flooding on the Skokomish River would probably
occur Wednesday...possibly continuing into Thursday. An hydrologic
outlook will be issued to address the flooding potential for the
skokomish later this morning.
For other areas rivers...flooding is unlikely over the next 7 days.
Aviation...a trough will shift across the Pacific northwest through tonight.
Light to moderate SW flow aloft. Moist and weakly unstable. Rain
changing to showers today with isolated thunderstorms possible near
Ksea...rain at times...becoming showers by this afternoon. Southeast wind
4-6 knots...becoming S-SW 5-10 knots after 18z.
Marine...a weak surface trough offshore will produce low-end small
craft winds over the coastal waters today before easing. West-SW swell
of 10 to 14 feet will continue through tonight. The small craft for
hazardous seas continues for the West Entrance to the Strait and
will likely be needed for the coastal waters tonight. Will wait to
issue that with the afternoon forecast package. Winds over the
interior should remain below 20 knots through tonight.
A surface low is expected to approach offshore Tuesday...and move NE
into Vancouver Island Tuesday night. This system will likely produce
gales over the coastal waters and inland waters north of Admiralty
Inlet. A gale watch is in effect late Tuesday and Tuesday night.
Current data suggests may have to expand gale watch to include West
Entrance of the Strait. Again...will make that decision for
afternoon package. Yet another surface low will track near or
through the waters around Friday. The strength and track of this
system remains highly uncertain.
Pz...Small Craft Advisory coastal waters today.
Small Craft Advisory for rough Grays Harbor bar through
Small Craft Advisory for hazardous seas West Entrance to the
Strait of Juan Delaware fuca through tonight.
Gale watch for the coast...East Entrance to the Strait of Juan
Delaware fuca...and the northern inland waters Tuesday night.
You can see an illustrated version of this forecast discussion at