Area forecast discussion
National Weather Service Seattle Washington
300 am PDT Wednesday may 22 2013
Synopsis...a cold upper level low will give showers with isolated
afternoon and evening thunderstorms through Thursday. Snow showers
are expected in the mountains. 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...an upper level low appears to be centered near The
Dalles Oregon early this morning. Radars show showers on the coast
moving east and southeast into the area...showers forming over the
north interior moving southward...and a light to moderate rain
rotating around a vorticity center east of Yakima moving
northwestward. Today promises to be a showery and cool day across
western Washington. Nam12 bufr soundings show some instability this
afternoon and evening as the cool air aloft with the vorticity
center to the southeast moves into the area. While the threat of
thunderstorm activity over the area appears quite small...there were
thunderstorms with the vorticity center to the east earlier...so they cannot
be ruled out.
Showers continue tonight then will diminish in coverage somewhat
Thursday and Thursday night as the upper low wobbles back down into
northern Oregon. The air mass may be a bit more unstable Thursday
afternoon and evening if there are any breaks in the clouds...so
isolated thunderstorms are indicated in the forecast Thursday afternoon and evening.
Snow levels through Thursday will be rather low for this time of year in
the mountains...3500-4500 feet. But snowfall amounts are expected to
remain below advisory criteria.
The upper low will rotate back up into Washington on Friday...but it
will be in the process of weakening and filling. Expect temperatures
to begin to slowly moderate closer toward normal on Friday and the
coverage of showers to decrease somewhat and become increasingly
tied to the terrain during the afternoon and evening hours. Albrecht
Long term...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...upper level low pressure will meander over the Pacific northwest
through Thursday. Lower pressure east of the Cascades will maintain
onshore flow over western Washington. Mainly light west to SW flow aloft. Moist at
all levels and somewhat unstable. Isolated thunderstorms are
possible this afternoon and early evening...roughly a 15 percent
chance at any terminal. Showers today should increase in coverage
later this afternoon with rain at times tonight.
Ceilings vary across the area this morning with wrap around moisture
moving in from the north and east into the greater Puget Sound sound region. An
ill defined convergence band also appears to be affecting the
ksea/kbfi/kpae terminals with either IFR or low MVFR. These
conditions will continue through the early morning hours then ceilings
lifting at most terminals to a low VFR ceiling except MVFR with showers.
Ksea...south-southwest wind 6-10 knots...becoming SW 9-13 knots after 18z.
&&
Marine...a 1006 mb low near the north tip of Vancouver Island will
weaken as it moves slowly S across the Washington offshore waters through
tonight and into the Oregon offshore waters Thursday and Thursday night.
A little stronger onshore flow will develop Friday and continue
Saturday and Sunday. Small craft winds are possible in the central
and east Strait during the afternoon and night-time hours. Dtm
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Sew watches/warnings/advisories...
Washington...none.
Pz...small craft for hazardous seas outer coastal waters.
Small Craft Advisory for rough Grays Harbor bar.
&&
$$
You can see an illustrated version of the forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html
