Scientific Forecaster Discussion
fxus66 ksew 290447
Area forecast discussion
National Weather Service Seattle Washington
929 PM PDT Tue Mar 28 2017
Synopsis...another warm front will bring more rain and higher
mountain snow to the area later tonight through Wednesday morning.
Low pressure will move into Vancouver Island with the associated
cold front pushing through western Washington by midday Wednesday.
An upper trough will follow on Thursday resulting in showers and
sun breaks. High pressure will build Friday and Friday night
providing mostly dry weather. Weak fronts may affect the region
Sunday and early next week.
Short term...another batch of rain is moving onshore this evening
ahead of a warm front that will move into the area early Wednesday
morning. The warm front will be quickly followed by a cold front
(pic10) associated with a low that is developing well offshore and
moving toward central or northern Vancouver Island midday Wednesday.
Precipitation will be heaviest on the western portion of the olympic
peninsula (where 2 or 3 inches more can be expected) and in the
Cascades while ligher amounts are expected in the central portions
of the Puget Sound area where there will be rain shadowing going on.
The warm air is taking its time getting into the north Cascades this
evening and Mt Baker is still getting snowfall at 5000 feet. The
delay in the warm up is supported by the 00z NAM solutions this
evening. Based on that, will extend the Winter Weather Advisory into
late Wednesday morning.
Rain will become showers on Wednesday as low pressure moves into
Vancouver Island and cooler, somewhat drier air filters into
western Washington. Snow levels will drop back down to most pass
levels and the strong flow give some orographic help in snowfall.
But the lack of moisture should still keep amounts limited to just
a few inches or less, including the passes. A meso low is still
expected to develop on the north Kitsap peninsula. Latest models
are a little stronger that yesterday but the kbli-kpdx gradient
never reaches +10 mb on most models and the WRF-GFS keeps winds
generally sub-advisory from late morning to the afternoon.
Strongest southerly gusts will occur from central/north Puget
Sound to around Admiralty Inlet, reaching 35 to 40 mph at times. A
westerly push down the Strait may also bring some gusts to the
land portions of the Strait and west side of Whidbey Island but
still sub-advisory. With recent rainfall and saturated soil, it
will not be surprising to see some trees topple in gusts of 35 or
The weather begins to quiet down Wednesday night and Thursday. An
upper trough will move across the region bringing showers. A Puget
Sound convergence zone is also possible, with the best chance of
this bringing enhanced showers to Snohomish and King counties.
Showers will taper off Sunday afternoon and most areas will see sun
High pressure aloft builds Thursday night and Friday giving a period
of dry weather. The low level flow will be light and 500 mb heights
over 5700m will support warming temperatures, possibly reaching near
60s from around the central sound to the southwest interior,
especially inland from the water. If sea-tac reaches 60, it will be
the first time this year. Albrecht/Mercer
Long term...from the previous long term discussion: the
GFS/ECMWF are in fair agreement agreement on showing the ridge
flattening Friday night and Saturday as a system rides over the
top of the ridge. Increased cloud cover should be expected, and a
chance of light rain, especially toward the coast and further
North. Western Washington will still be in the warm sector so high
temperatures should again be mild, possibly close to 60. If less
clouds prevail, some areas like the southwest interior could reach
the low 60s. But any stronger onshore flow would certainly keep
the area stuck in the 50s but still nice compared to recent
Models show mainly weak systems to affect the region early next
week. Some solutions keep cut off lows more offshore with less
threat of precipitation while other models are a bit wetter. Neither
the GFS or ECMWF show any significant rainfall so a lull in the
active weather is expected. All signs point toward more typical
Spring like weather with some clouds, some sun, and a chance of some
showers with near average temperatures. Mercer
Aviation...strong wly flow aloft will continue over the region
overnight. The low level flow will remain sly. A weak wmfnt will
move across the area late tonight or early Wed morning (pic10).
Areas of MVFR cigs/vsbys will become more wdsprd overnight.
Ksea...cigs will bounce between VFR and MVFR categories overnight.
Expect ceilings (and occasionally vsbys0 to be in the MVFR category
during the day Wed. Winds will remain sly 10-15 kt.
a warm front will move across the area overnight, followed by a cold
front on Wednesday (pic10). High pressure offshore with lower
pressure inland will result in onshore or westerly flow on Thursday.
Hydrology...the heavy rain that fell on the olympic peninsula has
pushed the Skokomish River over flood stage a few hours earlier
than previously expected. The Flood Warning for the skokomish was
updated to account for the slightly higher rises and the earlier
flood timing. Other rivers will see rises, but are not expected to
Otherwise, rainfall is not expected to be sufficient to cause
flooding on area rivers for the next several days.
The threat of landslides has increased somewhat due to rainfall
over the past 24 hours and expected through Wednesday. A Special
Weather Statement is out to cover this elevated risk. Albrecht
Washington...Flood Warning in effect for the Skokomish River in Mason
Winter Weather Advisory in effect until 10 am Wednesday for the
Cascades of Whatcom and Skagit counties.
Pz...small craft advisories are in effect for all waters except the
central Strait of Juan de Fuca.
Small Craft Advisory for rough bar in effect until 6 am Wednesday
for the Grays Harbor bar.