Area forecast discussion
National Weather Service Seattle Washington
910 am PDT Wednesday may 22 2013
Corrected to add Winter Weather Advisory.
Synopsis...a cold upper level low will give showers with isolated
afternoon and evening thunderstorms through Thursday. The upper low
will wobble around the Pacific northwest through the Memorial Day
weekend as it slowly fills and weakens. Cool temperatures through
Thursday night will gradually moderate over the weekend and showers
will gradually diminish starting Thursday night.
&&
Short term...upper level low appears to be centered somewhere over
southeast Washington this morning. Showers are rotating around the
low so one batch is moving east to west over Puget Sound sound while
another moves into the South Coast west to east. Ksea has had a
persistent shower this morning with over four tenths of an inch so
far. Today will definitely be showery...with somewhat fewer showers
over the north part. As the low moves north a bit this afternoon the
shower threat will also shift north. There could be isolated
thunderstorms this afternoon and evening...some with small hail.
With substantial persistent cloud cover...temperatures are unlikely
to get out of the 50s today...except possibly in the far north.
Paradise on Mount Rainier has received nearly a foot of new snow
since last night. The snow level has fallen to near 2000 feet around
the mountain...with accumulation occurring at Longmire. Will see how
things are going this afternoon and may keep the Winter Weather
Advisory for the central Cascades going.
The upper level low will move right over western Washington
Thursday. This will keep the shower threat going and again make
isolated thunderstorms a possibility. High temperatures should tick
up a bit as heights rise.
Though the upper low will still be hanging around...models show
considerably less shower activity Thursday night and Friday. There
will probably be some diurnal enhancement Friday afternoon and
evening...especially in the mountains. There will be more sun and
higher temperatures on Friday. Burke
Long term...previous discussion...the upper low will tend to
retrograde to just offshore Saturday then shift back inland Sunday.
This will generally result in diurnal mainly mountain showers with
temperatures a few degrees below normal through much of the Memorial
Day weekend. Memorial Day weather is up in the air as all models
bring a brief weak shortwave ridge through the area sometime late
Sunday night or Memorial Day followed by a stronger frontal system
associated with a deeper upper low offshore by Tuesday. There is
little skill in forecasting these features several days out.
Forecasts Saturday Onward were not altered early this morning.
Albrecht
&&
Aviation...an upper low will remain over the Pacific northwest
through tonight...centered mainly over north central Oregon or south
central Washington. The air mass over western Washington will be
moist and weakly unstable. Showery precipitation will continue today
and tonight. Isolated thunderstorms are possible this afternoon and
evening mainly over the Puget Sound sound region...southwest interior...
and the Cascades. The Cascades will remain mostly obscured.
There is a wide range of conditions across the forecast area this
morning. IFR visibility and ceilings are common over much of the
Puget Sound sound region...with VFR conditions over much of the north
interior and along the coast. These conditions are highly subject to
change according to the development and movement of showers...
however there should be generally an improving trend with low-end
VFR conditions common across the area late this afternoon and
evening. Some deterioration is likely again tonight.
Ksea...south wind 6-12 knots...becoming southwest this afternoon and
evening...then backing to south again tonight. There is a 15 percent
chance of thunderstorms at ksea this afternoon and evening. Mcdonnal
&&
Marine...a 1008 mb low near the northwest tip of Vancouver Island
will weaken as it moves slowly south across the Washington offshore
waters today through Thursday. It will likely be over the north
Oregon offshore waters Thursday night. There is a good chance that
the low will produce Small Craft Advisory south or southeast winds
over the coastal waters Thursday...and marginal Small Craft Advisory
southeast winds are possible in the inland waters zones as well.
Models continue to differ considerably in this regard.
In addition we have marginal Small Craft Advisory conditions over
the coastal waters for hazardous seas through this evening...along
with rough bar conditions at Grays Harbor.
Onshore flow will develop Friday and continue Saturday and Sunday.
Small Craft Advisory winds are possible in the central Strait of
Juan Delaware fuca and East Entrance Strait Friday through Sunday during
the late afternoon and nighttime hours. Mcdonnal
&&
Sew watches/warnings/advisories...
Washington...Winter Weather Advisory central Cascades.
Pz...Small Craft Advisory for hazardous seas coastal waters through
this evening.
Small Craft Advisory for rough bar conditions Grays Harbor
through tonight.
&&
$$
You can see an illustrated version of the forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html
