Area forecast discussion
National Weather Service Seattle Washington
915 am PDT Thursday may 23 2013
Synopsis...a slow moving cold upper-level low will continue to
produce showers with isolated afternoon and evening thunderstorms
mainly over the mountains today. The upper low will wobble around
the Pacific northwest into the Memorial Day weekend as it slowly
fills and weakens. The cool air mass will gradually moderate into
the weekend and showers will gradually diminish starting tonight.
&&
Short term...upper level low is spawning widespread showers over
the coast and southern areas with scattered showers elsewhere. There
are notably more sun breaks today and high temperatures should be
higher than yesterday...mostly 55-60. As the low pivots east then
south western Washington will be under an area of divergence aloft.
Models show negative lifted indices this afternoon and evening
mainly over the Cascades but also to some extent over the interior
lowlands. Isolated thunderstorms with small hail are possible.
Both the NAM and the GFS lift the upper low northeast and fill it on
Friday. This would keep scattered showers in the forecast especially
over the mountains. Then these models dampen the upper low further
into a broad upper trough on Saturday and just zonal flow Sunday.
The Euro is not in complete agreement with these ideas and keeps
some semblance of the upper low over the area through Sunday and
beyond. However the latest run of the Euro has gone some way toward
the other models. So the forecast of slow warming and lowering of
probability of precipitation as time progresses has some support. Highs over the interior
will be in the low 60s Friday and the middle 60s Saturday and Sunday.
Some sort of weak system is likely Monday. Burke
Long term...previous discussion...another upper level low will
bring more rain to western Washington on Memorial Day. But there are some
timing differences in the models. The GFS is the wetter model. The
European model (ecmwf) is slower with precipitation holding off until the afternoon...and
mainly affecting areas south. So exact details are still unclear at
the moment. Models keep a longwave trough over the region through
midweek for additional showers with temperatures near or slightly below
normal. 33
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Aviation...a deep upper low centered over western Washington this
morning will remain over the region through tonight. Flow at the
surface and aloft will be generally light and variable. The air mass
will be moist and weakly unstable. Showery precipitation is mainly
along the coast and in the southwest interior this morning. It will
continue through tonight especially over the southern two-thirds of
the forecast area. There is roughly a 15 percent chance of
thunderstorms this afternoon and evening.
There was a wide mix of conditions across the forecast area at 8 am
-- IFR due to fog at kbli where skies were clear part of the
night...low-end VFR and MVFR over most of the Puget Sound sound region...
and mainly lower conditions in showers along the coast and in the
southwest interior. This kind of variability will be the rule
through the forecast period...making confidence in forecast details
rather low.
Ksea...south to southwest wind 4-8 knots. Roughly a 10 percent chance
of a thunderstorm this afternoon and evening. Mcdonnal
&&
Marine...a 1014 mb surface low will move slowly southward across
the Washington offshore waters today...then weaken further as it
moves southeast into Oregon tonight. This feature is producing south
to southwest winds up to 20 knots over the coastal waters this
morning...and the plan to keep winds 20 knots or less -- ie below Small
Craft Advisory threshold -- looks correct.
Onshore flow will develop Friday and continue Saturday and Sunday.
Small Craft Advisory west winds are possible in the central Strait
of Juan Delaware fuca and East Entrance Strait each day during the late
afternoon and nighttime hours. A front will probably approach the
Pacific northwest from the offshore waters Monday. Mcdonnal
&&
Sew watches/warnings/advisories...
Washington...none.
Pz...none.
&&
$$
You can see an illustrated version of the forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html
