Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
000
FXUS66 KSEW 281637
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 AM PDT Tue Mar 28 2017

.SYNOPSIS...A pair of wet frontal systems in quick succession will
bring rain to Western Washington today through Wednesday, along with
locally windy conditions. An upper trough will follow on Thursday
for showers and sun breaks, and an upper ridge will bring dry
weather Friday. Weak fronts will move through the area this weekend
with a chance of rain in the forecast.

&&

.SHORT TERM...A warm front is bringing steady rain to much of
Western Washington this morning except for the rain shadowed areas
northeast of the Olympics. Precipitation is heaviest over the
Olympic Peninsula, especially along the north coast, and south/west
facing slopes of the Olympics which had nearly 2 inches the past 12
hours. This is somewhat heavier and faster that was anticipated.
Rivers running off the Olympics will need to be monitored closely,
mainly the Skokomish but also the Bogachiel which is rising quicker
than forecast. Rain intensity over the Cascades will also pick up
today, and some rivers could approach bankfull tonight. See the
latest hydrology discussion below for details.

Snow levels are near 3000 to 3500 feet this morning with some snow
accumulation likely in the passes, perhaps up to 4 or 5 inches. Snow
levels are expected to rise to nearly 6000 feet this evening which
will bring a switch over from snow to rain at the passes and even at
some higher locations like Paradise. This should keep amounts
generally below advisory criteria. The one spot that could get 6+
inches might be Mount Baker depending on how quickly warm air
arrives giving a mix or change over to rain tonight. However, the
north interior is also partially shadowed off the Olympics which may
also hinder snow amounts. Will not issue an advisory but the north
Cascades of Whatcom and Skagit counties should see the most snow
before switching over. Call 5-1-1 for road conditions prior to
departing through mountain passes as conditions could still get
rather slick with some accumulation expected.

The 06z models trended slightly deeper with the 1006-1008 mb low
tracking into Vancouver Island and also a little closer to
Washington. This could generate a somewhat stronger meso low setting
up over the northeast side of the Olympics on Wednesday as the main
low reaches central Vancouver Island. Breezy conditions today could
become windy in a few spots on Wednesday with gusts up to 40 mph
from around Everett to Admiralty inlet. Will need to evaluate all
the new 12z model data to see if those areas could see advisory
level gusts of 45 mph or higher. Regardless, it will be breezy to
locally wind with saturated soils making it easy for some trees to
topple. Rainfall totals in the lowlands from around Seattle/Everett
southward including the coast, southwest interior, and east lowlands
could reach 1 inch or more through Wednesday, causing further soil
saturation. This will cause the already elevated landslide risk to
be even higher the next 2 days.

Showers will taper late Wednesday and Wednesday night. A few inches
of snow will probably fall in the mountains, including the passes as
snow levels fall back to 3000 feet Wednesday night. An open trough
will cross the region Thursday with some additional showers and the
Puget Sound convergence zone could be active between Snohomish and
King counties. Highs will remain cool in the lower 50s. Mercer

.LONG TERM...From previous discussion...Models agree than an upper
ridge will progress across the Pacific Northwest on Friday, and it
should be a day of dry weather for Western Washington. Westerly flow
will develop behind the ridge Friday night, with a couple relatively
weak systems bringing a chance of rain and some mountain snow to the
forecast area Saturday and Sunday. The snow level should be around
5000 ft Saturday and 4000 ft Sunday. An upper trough will follow the
second system late Sunday or Sunday night for a partly sunny day
with a chance of showers on Monday. McDonnal

&&

.AVIATION...A front will bring rain to Western Washington today with
the mountains obscured and strong westerly flow aloft. There is low
pressure moving into British Columbia and breezy southerly pressure
gradients over the area. There will be periods of rain right through
Wednesday morning and then a cold fropa will occur Wed afternoon.

KSEA...Rainy middling TAF looks good with a decent southerly breeze
through Wednesday morning. Periods of rain will give way to showers
later Wednesday afternoon as a cold fropa occurs.

&&

.MARINE...Pretty decent southerly flow today through Wednesday with
the wind rising to 20-30kt southerlies at times til the cold frontal
passage on Wednesday brings in westerlies. High pressure will build
into the region for Thursday and Friday and then a front may reach
the coast Friday night.

&&

.HYDROLOGY...Up to 2 inches of rain fell the past 12 hours along
south and west facing slopes of the Olympics. Another 1 to 2 inches
could fall there today and another 1 to 2 inches tonight. This could
bring rain totals up to 6 inches which means flooding is likely on
the Skokomish river. Area rivers, especially those running off the
Olympics are rising faster that forecast and will need to make some
adjustments to some rivers, specially the Skokomish river which
could flood by tonight rather than Wednesday. Some rivers in the
central Cascades could also reach bankfull with 2-3 inches possible
in some basins through tonight. Forecasts should be closely the next
couple days for the potential of additional watches and/or warnings.
The Skokomish river in Mason county is the only river forecast to
flood but forecasts for all area at this time. Refer to the latest
flood watch statement on our website at www.weather.gov/seattle

Rain amounts of .5 to 1.5 inches are possible in the lowlands through
Wednesday. When combined with the current saturated soils, the risk
of landslides will be even higher over the next couple days.
Mercer

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch for Mason county.
PZ...Small craft advisories for all waters except the central Strait
     of Juan de Fuca. Small craft advisory Grays Harbor bar for
     rough bar conditions.



&&

$$

www.weather.gov/seattle

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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.