Area forecast discussion
National Weather Service Seattle Washington
900 am PDT Monday may 20 2013
Synopsis...an upper level ridge will move across the area today
giving a dry and warm day to the interior. Conditions on the coast
will be cooler with clouds and onshore flow. Big changes will begin
late tonight and persist into next weekend as an upper level low
moves southeast from the Gulf of Alaska then spins around over the
Pacific northwest. Conditions will be moist and cooler than normal.
&&
Short term...there is a fair amount of stratus over the area this
morning...with low ceilings covering the coast and south. Above the
stratus there is just patchy cirrus. Clouds will not increase from
the approaching upper level low until tonight. So most stratus
should evaporate under the may sun. Heights around 570 dm imply high
temperatures in the 60s except on the coast where onshore surface
flow will keep skies mostly cloudy and highs in the 50s.
The much anticipated upper level low is forecast to move onto the
coast Tuesday afternoon and spin around over the Pacific northwest
through at least Thursday. Heights get amazingly low...the NAM has a
540 dm low centered just off the coast Wednesday afternoon...and
there is plenty of moisture. A high pop and unseasonably cool period
is coming up. High temperatures will flirt with low maximum records but
probably not set new records. Low maxes for may 21-23 are 52...54...
and 49. With cold air aloft and late Spring solar heating it does
not seem like convection will be completely out of the picture
either. Burke
Long term...previous discussion...temperatures are likely to slowly
moderate late in the week through next weekend as the upper low
spins around the area. Showers may decrease somewhat toward the end
of the week as well. Forecasting the details will be quite difficult
as small changes in the position and strength of the upper low can
result in big sensible weather changes. Nonetheless it appears that
temperatures will remain at least 5 degree below normal through next
weekend and there will be quite a few showers around the area...
especially over the mountains. Albrecht
&&
Aviation...southwest flow aloft over western Washington this
morning will strengthen and back toward southerly today and
tonight...as a deep upper low moves into the outer Washington
offshore waters late tonight. Low level flow will turn increasingly
onshore during the period. Patchy low level moisture over western
Washington this morning will dissipate midday...then the air mass
will become moist all levels and weakly unstable tonight.
VFR conditions prevail over the north part of the forecast area with
ceilings mainly above 120. Areas of low stratus are giving IFR to
MVFR ceilings this morning from the Seattle area southward with
cloud cover roughly scattered-bkn009 broken-ovc018 and tops around 022. The
low cloud cover should dissipate midday. Rain will reach the coast
this evening and spread inland tonight with conditions deteriorating
across the forecast area to MVFR and IFR.
Ksea...southwest wind 3-8 knots becoming northwest 6-10 knots late
afternoon...then backing to southwest again around midnight. Stratus
this morning should dissipate midday. Rain will likely begin at ksea
around 4 am Tuesday...and conditions will likely deteriorate to MVFR
occasionally IFR. Mcdonnal
&&
Marine...onshore flow will develop across western Washington and
its coastal waters today and increase tonight. Small Craft Advisory
westerlies are likely in the central Strait of Juan Delaware fuca and East
Entrance Strait tonight.
Northwest flow over the coastal waters will increase tonight and
Tuesday as a weak surface low of around 1013 mb tracks
east-southeast across the Oregon offshore waters and into southwest
Oregon. Small Craft Advisory northwest winds are likely over the
coastal waters Tuesday.
A 1005 mb low will move southeast across the Washington offshore
waters Wednesday...becoming nearly stationary over the coastal
waters west of Astoria Wednesday night and Thursday. The exact
timing and tracking of this feature is still unclear. Model output
is rather swirly with the winds. For now will stick with the
predominant west to northwest flow across the region. 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
