Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Seattle Washington
310 am PDT sun Aug 28 2016

Synopsis...weak higher pressure aloft is expected to build over
western Washington today and Monday resulting in some afternoon
sunshine in many areas. Developing lower pressure aloft well off the
coast is forecast to send a weak system with some showers onshore
Tuesday night and Wednesday along with cooler daytime temperatures.
The low pressure system aloft itself is anticipated to move ashore
late in the week for another threat of showers.

&&

Short term...
a broad trough of low pressure has developed over western Canada
into the NE Pacific this morning with the upstream upper ridge now
further west in the vicinity of 160w. Several embedded shortwaves in
the trough's circulation were evident in satellite imagery. The
first was along about 135w early this morning with the next one near
145w in the Gulf of Alaska.

The progs all offer a similar solution in sagging the upper trough
well offshore through Monday as these shortwaves dig southward
resulting in the flow aloft over Washington becoming more SW today
and southerly by Tuesday. 500 millibar heights rise a bit today into
Monday with low level onshore flow dropping off.

These developments should stall the initial shortwaves arrival and
have adjusted the forecast to offer more afternoon sunshine in many
areas today and limited any precipitation threat to just the north
coast and olympic peninsula. The additional sunshine and nudged
heights aloft should bump high temps up a bit from Saturday. This
trend should continue into Monday.

By Tuesday, the offshore upper trough shows signs of rotating
another shortwave toward the Pacific northwest. Associated clouds should
spread onshore during the day with spotty light showers arriving
Tuesday night into Wednesday. Buehner

Long term...
the mid range guidance begins to offer differing solutions on timing
and intensity of developments beyond Wednesday. The general trend
though is the likelihood of the offshore upper trough moving onshore
by Friday with associated precipitation primarily Thursday night into
Friday. The precipitation threat should ease Saturday as the Labor
Day weekend begins with 500 millibar heights rising. Temperatures
during this period should be near or a bit below early September
seasonal averages. Buehner

&&

Aviation...westerly flow aloft becoming southwesterly tonight as
an upper level trough digs south well offshore. Low level onshore
flow becoming light today and remaining light tonight. Stable and
moist conditions in the lower levels with a drying trend this
afternoon.

Stratus over most of the area early this morning with ceilings in
the 1500-2500 foot range. With the onshore flow weakening during the
day the stratus will lift later this morning then scattering out
early in the afternoon. Just scattered clouds this evening into
early Monday morning.

Ksea...ceilings near 1500 feet at 10z will lift during the morning
hours with the stratus deck scattering out early in the afternoon.
Southerly winds 4 to 8 knots becoming northwesterly late this
afternoon. Felton

&&

Marine...low level onshore flow will become light today with the
light low level flow continuing tonight. Onshore flow will increase
again on Monday as a front stalls over the offshore waters but not
strong enough for any advisories. The front will finally move inland
Tuesday night with Small Craft Advisory winds possible over the
coastal waters with the front. Another front may reach the area late
Thursday. This system could be strong enough for another round of
Small Craft Advisory winds over the coastal waters. Felton

&&

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

Pz...none.

&&

$$

Www.Weather.Gov/Seattle

An illustrated version of this forecast discussion can be seen at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html

National Weather Service Glossary of Abbreviations