Area forecast discussion
National Weather Service Seattle Washington
345 PM PDT sun may 19 2013
Synopsis...an upper level ridge axis will cross western Washington
tonight...bringing a relatively warm and dry day on Monday. An upper
trough will develop offshore on Monday...then move east on Monday
night. This feature will evolve into a cutoff low by Tuesday...
becoming nearly stationary and persisting over the Pacific northwest
through the end of this week. This will push a strong cold front
onto the coast on Monday night...pushing quickly inland early
Tuesday morning. A cool and showery pattern will then persist for
the rest of the week.
&&
Short term...an upper ridge axis will continue to build into western Washington
tonight. The ridge axis should move across western Washington during the
overnight hours. With clear skies tonight...expect cool temperatures.
Should be a few cold pocket locations...such as kolm...down into the
upper 30s late tonight.
On Monday...the ridge axis will be east of the Cascades...and an
upper trough will dig and amplify out around 135w-140w. With
offshore low pressure easing the onshore pressure gradient...and
light southerly flow at 850 mb...Monday should be one of those warm and
dry days in advance of tuesday's strong cold front. Looking for 70+
from ksea on south along I-5 Monday afternoon.
Speaking of Tuesday...the offshore upper trough will advance east between
130w and the coast and then become a cutoff low that sinks southeast off
the Washington coast...reaching northwest Oregon on Tuesday evening. As a result...a strong
cold front will push onshore and inland on Tuesday morning. The front will
be accompanied by a band of briefly heavy rain...followed by
convective showers on Tuesday afternoon and evening given the cold air aloft. In
typical fashion...showers will focus on windward side of mountains and in
the Puget Sound sound convergence zone. An unseasonably cool air mass will
follow the front...with 850 mb temperatures down in the -3c to 0c range by
Tuesday night. This will lower snow levels as low as 3000 feet at sunrise
on Wednesday morning.
On Wednesday and Thursday...the center of the cutoff upper low will be located
to our south. Western Washington will be prone to embedded shortwaves rotating
into western Washington from the southeast and east. Both the GFS and European model (ecmwf) show
wet periods on Wednesday. With cold air in place and some wet weather...the
forecast maximum temperature of 54f at ksea would tie the record low maximum temperature for
the day. Of course...the wet weather with snow levels of 3000-4000
feet will provide snow to the higher mountain passes. Snoqualmie Pass is
unlikely to get more than an inch of accumulation...but the official
forecast shows 30-hour snowfall totals from Tuesday evening through Wednesday
evening of 2.5 inches at Stevens Pass...4 inches at Chinook Pass...5
inches at Rainy Pass...6 inches at Paradise and 9 inches at Harts
Pass along the far north Cascade crest. Those venturing into higher
elevations and the backcountry should be prepared for a quick change
to cold and wet or snowy weather early Tuesday morning. Will issue a
Special Weather Statement to highlight this concern. May eventually
need winter weather highlights for the Cascades above 4000 feet.
Haner
Long term...the cutoff upper low will meander around the inland
northwest on Thursday and Friday...then start to fill and slowly lift out
next Sat and sun. This will lead to a slow warming trend late this
week and next weekend...but temperatures will still struggle to rise back
as high as normal. Shower coverage will also trend downward from Friday
through sun...becoming driven more by daytime heating and diurnally
driven convective elements. However...there will still be enough
moisture and instability to warrant a chance of showers next Sat and
sun. Looking out toward Memorial Day...models hint at either zonal
flow or height falls spreading onshore. At best...this would suggest
deep marine air and clouds. At worst...more organized rain. Haner
&&
Aviation...light northwest flow aloft. Air is stable and drying as high
pressure builds into the region. The layer of stratocu that
persisted for much of western Washington this afternoon should clear up by
evening. Monday should be sunny with just a little patchy River
Valley fog or low clouds around daybreak.
Ksea...skies ought to be scattering out late this afternoon. Light
and variable wind except northerly to 10kt afternoon and evening hours.
&&
Marine...high pressure over the NE Pacific will maintain northwesterly
flow across western Washington today and Monday. We probably will see several
hours of 15-25kt northwest wind coast and Strait this evening...although
the sea level pressure analysis doesn't show a great deal of
gradient this afternoon over western Washington. A weather system will move over
the region Tuesday and Wednesday...in fact an upper level low may persist over
the pacnw through the end of the week. What is likely to happen is
that western Washington will have a regime of onshore flow and a lot of
shower activity...probably a few periods of rain...but the stronger
pressure gradient is likely to stay well offshore or down over
Oregon Tue-Thu.
&&
Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory coastal waters and central and east Strait
of Juan Delaware fuca until midnight.
&&
$$
You can see an illustrated version of the forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html
