Area forecast discussion
National Weather Service Seattle Washington
330 PM PDT Wednesday may 22 2013
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...persistent rain over the Seattle area has ended but
there are still plenty of showers over western sections. The rain
and cloudiness along with the ever present upper level low has left
the Puget Sound sound region on the cool side. Ksea has been holding
steady at 50 degrees for a few hours and may break the record low
maximum of 54 degrees for this date.
Showers will taper tonight but may get energized again Thursday
afternoon and evening. There is some instability in the models so
have kept the idea of isolated thunderstorms. There should at least
be some sun breaks so temperatures will rise...at least into the low
60s over the interior.
The upper level low finally starts to fill Thursday night and
Friday. The GFS moves it away from the area on Saturday though the
Euro does not. Have kept basically chance probability of precipitation in the forecast with
a slow warming trend. Burke
Long term...Sunday could be kind of a nice day if you believe the
GFS. The Euro still has the upper low over the area. Put a mention
of sun in the Sunday forecast but kept the chance of showers. The
GFS brings a weak system into the area Monday through Tuesday while
the Euro still has the same or possibly a different upper low.
Confidence is low in all details but went with the GFS solution and
boosted probability of precipitation into the likely category Tuesday and Tuesday night. By
Wednesday probability of precipitation drop back to chance. Burke
&&
Aviation...an upper low will remain over the Pacific northwest...
centered mainly over north central Oregon or south central
Washington tonight and shifting into the coastal waters Thursday.
The air mass over western Washington will be moist and weakly
unstable. Showery precipitation will continue...and isolated
thunderstorms are possible mainly Thursday afternoon and evening.
The Cascades will remain mostly obscured.
There is a still a fairly wide range of conditions across the
forecast area this afternoon. MVFR ceilings are common over much of
the Puget Sound sound region where conditions have been slowly
improving...with VFR conditions over most of the rest of the
forecast area. At 2 PM showers were mainly to the south and west of
the Puget Sound sound region...with locally MVFR visibilities and
ceilings. Generally speaking these conditions should continue this
evening...with some deterioration tonight and then improvement again
Thursday afternoon.
Ksea...southwest wind 6-12 knots...becoming south to southeast 4-10 knots
tonight and southwest again Thursday. The threat of thunderstorms at
ksea for this afternoon and evening looks minimal at this time...but
there is a 15 percent chance of thunderstorms Thursday afternoon and
evening. Mcdonnal
&&
Marine...a 1010 mb low near along the northwest side of Vancouver
Island will weaken as it moves slowly south across the Washington
offshore waters tonight and Thursday. It will likely be over the
north Oregon offshore waters Thursday night.
The models have been showing a wide range of solutions with regard
to the movement of the low and especially the winds it will produce.
That is still the case. The uw WRF-GFS still indicates spotty
marginal Small Craft Advisory southeast winds Thursday over the
coastal waters...and in the Strait of Juan Delaware fuca and adjacent
inland waters. However the 12z run has backed off somewhat from
previous runs. Our forecast will use a model blend...resulting in
winds 20 knots or less over the coastal and inland waters. That looks
like the best forecast at this time...but certainly Small Craft
Advisory winds similar to the uw WRF-GFS are possible.
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 through tonight.
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. A front will approach the
forecast area from the west Monday. Mcdonnal
&&
Sew watches/warnings/advisories...
Washington...none.
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
