Area forecast discussion
National Weather Service Seattle Washington
312 am PDT Tuesday may 21 2013
Synopsis...a cold upper level low off the western tip of Vancouver
Island early this morning will drop southeast into northwestern
Oregon this afternoon then will lift into western Washington tonight
through Thursday. The upper low will give cool and showery weather
to the area through Thursday. The upper low will gradually weaken
Thursday night through the Memorial Day weekend giving slowly
moderating temperatures and a gradual decrease in shower activity.
&&
Short term...a cold upper level low sits just west of the
northwestern tip of Vancouver Island early this morning. As upper
level ridging amplifies along about 150w...this low is forecast to
drop southeast into far northwestern Oregon this afternoon then lift
back north into western Washington tonight through Thursday. The
frontal system associated with this upper low is now sitting over
western Washington and will slowly push into the Cascades midday.
The coldest air aloft...with 850 mb temperatures near -5c...is
headed down into north central Oregon. Some thunderstorm activity
can be expected where the air is most unstable this afternoon and
evening over northwest Oregon and extreme southwest Washington.
Conditions farther to the north over our forecast area will be more
marginal for thunderstorm activity as cape values are generally less than
200 j/kg at most. Will include isolated thunderstorms this afternoon and
evening over the far south interior and Chehalis gap area to match
up well with the Portland office. Temperatures through tonight will
cool as the upper low moves in. Highs today will stay in the middle and
upper 50s and will fall into the middle and upper 40s tonight. Snow
levels in the mountains will fall to 4500 feet today...and to 3500
feet later tonight. Snowfall amounts in the mountains will be below
advisory criteria...but since we are moving into the latter part of
may...we will refresh the Special Weather Statement for the Cool
Mountain weather and relatively low snow levels.
With the upper level low over western Washington on Wednesday...the
air mass will be cool...unstable...and showery. Nam12 bufr soundings
for 21z Wednesday show enough instability Wednesday for an isolated thunderstorm just
about anywhere. But conditions remain rather marginal and the models
have been wavering...so will not include the thunder chances at this
time.
Cool and showery weather will continue into Thursday as the upper
low remains over the area. Albrecht
Long term...the upper low will continue to wobble around the area
Thursday night through the Memorial Day weekend. As the upper low
slowly fills and the air mass gradually moderates...expect shower
activity to gradually diminish in coverage and intensity and high
temperatures to rise into the upper 50s to middle 60s. Some sunshine
can be expected in The Lowlands during the afternoon hours as we
move into the upcoming weekend. Albrecht
&&
Aviation...a deep upper level trough will settle over the Pacific northwest
today through at least Wednesday. Light to moderate SW flow aloft
will become light westerly later today and tonight. Moist at all levels
and somewhat unstable. There is a risk for an isolated thunderstorm
mainly this afternoon but the odds are too low to include at any
given taf site. A mix of MVFR and IFR ceilings this morning with light
to moderate rain under the initial leading frontal bad. Ceilings should
improve to MVFR or VFR this afternoon.
Ksea...a north/S oriented frontal band stalled over the terminal will
produce MVFR or occasionally IFR ceilings the next few hours through 7
am. Rain will become showery and ceilings will gradually improve to 2-3k
feet later this morning and eventually above 3k feet this afternoon.
Sunbreaks are possible but showers will continue through the evening
hours. A 10 percent chance of a thunderstorm from midday through
early evening. Dtm
&&
Marine...onshore flow today...gradually weakening tonight. Marginal
small craft winds will occur in the central/east Strait today. Northwesterly
flow will also increase over the coastal waters this morning with
small craft winds developing...except the north inner coastal waters.
Elsewhere across the interior...winds generally less than 15 knots. The
north winds in the northern parts of the sound will switch southwesterly this morning
to 15 knots.
The next surface low will track southward through the coastal waters Wednesday
and Thursday. While winds could reach low-end small craft for a short
time as the low moves S...the main effect will be a switch from a
southwesterly direction to northeasterly. Dtm
&&
Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory central and east Strait of Juan Delaware fuca
through Tuesday afternoon...
Small Craft Advisory for Admiralty Inlet and the northern
inland waters through 5 am PDT Tuesday.
Small Craft Advisory coast...except the northern inner
portion...from 5 am PDT Tuesday through the afternoon.
&&
$$
You can see an illustrated version of the forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html
