Area forecast discussion
National Weather Service Seattle Washington
330 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.
Another weather system is expected to arrive on Memorial Day.
&&
Short term...expect more showers across western Washington today as
an upper level low spins overhead. Temperatures will remain below
normal although we will see highs a few degrees warmer compared to
yesterday. The high at SeaTac was only 52 yesterday which set a
record low maximum. The low level flow is turning onshore so looks like
temperatures will range from the middle 50s to lower 60s. Thunderstorms remain
in the forecast this afternoon and evening with the cold pool aloft.
Temperatures at 500 mb are around -30 degree c with 850-500mb lapse rates
around -7 c/km...and slightly negative lifted index. T-storm coverage is
isolated and best shot of seeing any strikes will be over the
Cascades. The snow levels are around 4000 feet so pretty chilly in the
mountains today. The mountains may see an inch or two of snow...but
the heaviest accumulations will be over the South Washington and
Oregon Cascades.
The upper level low will slowly track into eastern Washington on Friday...with
a few more wrap-around showers affecting western Washington. The air mass
will remain cool and unstable but looks like the T-storm threat will
diminish. Models show temperatures moderating a bit with highs in the lower
60s...and a few more sunbreaks.
The low will weaken on Sat and sun as it shifts north into
BC/Alberta...but we still have a trough over western Washington for a
chance of showers. 33
Long term...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
&&
Aviation...a strong upper level low pressure system will remain
stalled over Washington/Oregon through tonight...then start to shift
eastward Friday. Light SW flow aloft becoming westerly on Friday. Moist
at all levels and slightly unstable. The risk for an isolated
thunderstorms remains less than 15 percent. Showers through this
evening will decrease and become more scattered tonight and Friday.
Widespread MVFR clouds and patchy low IFR where more steady rain
develops this morning.
Ksea...SW wind 4-8 knots. Rain at times this morning...then showers
this afternoon and evening. Less shower activity later tonight and
Friday. Roughly a 10 percent chance of a thunderstorm today through
this evening. MVFR ceilings through midday with IFR ceilings below 1k feet at
times...low VFR or MVFR ceilings this afternoon. Dtm
&&
Marine...a 1013 mb surface low over the southern Washington coastal waters will
move S over the northern Oregon waters by midday. The low will stall and
weaken the rest of today...then dissipate 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. Dtm
&&
Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory for rough bar conditions Grays Harbor.
&&
$$
You can see an illustrated version of the forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html
