Area forecast discussion
National Weather Service Seattle Washington
940 PM PDT Sat may 18 2013
Synopsis...an upper trough over the Cascades this evening will
continue to move east. An upper ridge will start to build in on
Sunday...then cross the area on Monday...leading to warmer and drier
weather. A large upper low will drop into the area on
Tuesday...causing a strong cold front to push inland during the day.
The upper low will sit in place through the end of the
week...maintaining showery weather and below normal temperatures.
&&
Short term...satellite imagery shows upper level trough axis over
the Cascades this evening. A few precipitation echoes left on the
Doppler radars with the showers moving northwest to southeast
indicating that northwesterly flow aloft has developed over the
state. With the cloud cover at 9 PM pretty narrow temperature range
across the area with most places in the middle 50s.
Flat upper level ridge already evident in the satellite imagery out
around 130w will build into western Washington on Sunday. In the
meantime with plenty of low level moisture and a slightly unstable
air mass will keep the chance of showers going into the morning
hours on Sunday. Have updated the zones and taken the chance of
showers out of The Lowlands for the afternoon hours on Sunday. If
the ridge builds in a little quicker look for the chance of showers
in the morning to be dropped in the morning forecast package. With
the light onshore flow becoming more northerly in the afternoon it
will not be a very warm day with high temperatures hovering near
normal for this time of the year...mostly in the 60s.
Upper level ridge remaining over western Washington Sunday night
into Monday. Temperatures aloft do not warm much with the 850 temperatures
only a couple of degrees c warmer than Sunday but model cross
sections show the winds in the 1000-850 layer becoming light
northeasterly Monday morning. This puff of offshore flow combined
the warmer temperatures aloft should result in highs about 5 degrees
warmer on Monday versus Sunday with many places from Seattle south
getting into the lower 70s.
Conditions deteriorate rapidly on Tuesday as a cold upper level low
drops down from the north bringing with it wet and unseasonably cool
weather for the middle of the week. The record low maximum for Seattle
on Tuesday is 52 degrees which does not look to be threatened at
this point but the record low maximum on Wednesday is 54 degrees and
that has the potential to be broken. By Tuesday night 850 mb
temperatures will drop below 0c and snow levels could fall as low as
3000 feet for snow at all of the passes. Felton
Long term...previous discussion follows. On Wednesday...the upper trough
will morph into a cutoff low...due to the development of an upper
ridge over the Yukon and northern b.C.. this will effectively bring a
blocking pattern and cut the upper low off from any steering flow
which could push it out. So...we will be stuck with the upper low
from Wednesday through the rest of the new week. Safe to say that a
showery weather regime will be in place with below normal
temperatures. This pattern could also bring a few inches of snow to
the higher passes...such as Rainy Pass and Chinook Pass...which have
already re-opened for the season. Models show the upper low becoming
more rag-tag next Friday and Sat as it slowly fills and the low-level
air mass ever so slowly modifies. Temperatures will slowly rise back to
within a few degrees of normal by day 7...and shower coverage may
decrease. One caveat for Thursday through Sat is that if the upper low
center meanders further south into northwest Oregon...it will open up the door
to more widespread deformation-style rainfall spreading in from the
east. Have seen this in past Mays...and it is one way of bringing a
light all-day rain to western Washington this time of year. However...details of
individual waves or rounds of showers within the upper low are
impossible to pin down 5-7 days in the future. Haner
&&
Aviation...the upper level trough pushed east of the Cascades this
evening while high pressure moves in from the west. The air mass across
western Washington is stabilizing but remains moist with low level onshore
flow. Isolated light showers across the region will diminish after
midnight tonight...mainly affecting the Cascades. Ceilings will hover
around 3000-5000 feet tonight. High pressure will remain over the region
on sun for dry/stable weather. Low clouds will scatter by the
afternoon for VFR conditions. 33
Ksea...light showers in the vicinity will diminish after midnight
tonight. Surface flow is light southerly. Ceilings 3000-5000 feet
tonight. 33
&&
Marine...high pressure over the NE Pacific will maintain northwesterly
flow across western Washington tonight through sun. The central and east
Strait of Juan Delaware fuca may see Small Craft Advisory winds late Sun
afternoon and evening.
The next frontal system will form offshore on Monday...then move inland
on Tuesday. But models disagree on the exact tracking of the surface low
as it moves ashore. The GFS brings a 1009 mb low into the Washington coast
Tuesday morning...while the European model (ecmwf) and NAM solutions are both weaker and
farther south /1017 mb low over northern CA/. The Canadian is closer
to the European model (ecmwf) and NAM...so will likely nudge the forecast toward
these solutions. 33
&&
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
