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Area forecast discussion 
National Weather Service Seattle Washington
255 am PDT sun Apr 19 2015

Synopsis...high pressure aloft will give dry and warm weather to
the region through Monday. The high will move east Monday night
allowing marine moisture to move inland. Tuesday will see much
cooler conditions and an increase in cloudiness. Weak cold fronts
and upper level troughs will move across the area Wednesday through
the end of the upcoming week. These systems will give mostly cloudy
conditions...a chance of showers at times...and slightly below
normal temperatures for late April.

&&

Short term...a ridge of high pressure at the surface and aloft sits
along 130w early this morning. This ridge will move slowly eastward
through Monday...finally departing to the east of the Cascades
Monday night. Other than some cirrus clouds moving over the top of
the ridge...expect mostly sunny skies. Some late night and morning
fog may be seen on the coastal beaches...like what is seen on
satellite imagery around Grays Harbor County early this
morning.

Model time sections show temperatures between 2000 and 5000 feet
increasing a degree or two c today over what they were on
Saturday...and another 2 degrees c or so on Monday. This will result
in high temperatures 2 degrees f or so warmer today and another 4
degrees f or so warmer on Monday as the upper level ridge passes
overhead. Forecast temperatures fall between the GFS and NAM
guidance values. Low temperatures early Monday will be 2-3 degrees
higher than this morning.

The main forecast challenge continues to be the timing of the
expected marine push Monday night or Tuesday. With the upper ridge
axis passing off to the east Monday night...500 mb heights falling
early Tuesday morning...warm temperatures Monday afternoon
increasing west to east thermal gradients...and west to southwest
pressure gradients ramping up Tuesday morning...it seems that the models are
too slow in shifting marine air inland Monday night and Tuesday
morning. The 06z NAM shows rather strong westerly gradients after 18z Tuesday
and the European model (ecmwf) is also fast in increasing onshore flow Monday night.
Prefer the European model (ecmwf) MOS guidance high temperatures for Tuesday that
show 60 degrees at SeaTac Airport and middle 50s on the coast. For now
will nudge forecasts about 75 percent in the direction of the European model (ecmwf).

The European model (ecmwf) shows a split trough moving across western Washington Tuesday
night and Wednesday and now shows more energy and cold air in the northern
portion of the trough over Washington. Southerly flow in advance of the
approaching trough supports the idea of a chance of showers over the
coastal waters Tuesday and in the mountains Tuesday afternoon and evening.
Height falls...cold air advection aloft..and somewhat diffluent southerly
flow aloft supports the idea of a chance of afternoon and evening
thunderstorms over the Cascades on Tuesday. Albrecht

Long term...upper troughing passes to the east of the area late
Wednesday. The European model (ecmwf) is stronger with another short wave trough and
associated frontal system on Thursday than the GFS...but both models show
a feature moving through in the Thursday time frame. A rather deep trough
sets up just offshore for next weekend with a frontal system over or
just to the west of the area. Will maintain the current forecast of
mostly cloudy and a chance of showers with temperatures near or
slightly below normal. Albrecht

&&

Aviation...an upper level ridge over the Pacific northwest will lead to dry
and stable weather across western Washington today. The flow aloft is
northwesterly. VFR conditions are expected...although may see patchy
shallow fog along the coast this morning. Otherwise skies will
remain mostly clear with just few-scattered cirrus aloft. 33

Ksea...VFR conditions today. Expect north winds to 10 knots. 33

&&

Marine...low pressure along the West Coast will maintain northerly
flow across western Washington through Monday. Low pressure will shift inland Monday
night with weak onshore flow developing. Onshore flow will increase
on Tuesday as a strong cold front moves inland...models are showing
gales possible through the Strait of Juan Delaware fuca. Onshore flow will
continue through the end of the week. 33

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...none.

&&

$$

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