Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
300 am PDT Friday Jul 11 2014
Synopsis...high pressure aloft will remain over the region through
early next week. This high will give well above normal high
temperatures to the interior with possible record warmth this
weekend. A weak upper level low will develop off the Oregon coast
Sunday and move inland Sunday night giving a chance of showers for
the southern portion of the area.
Short term...a long wave upper level ridge is centered over western
North America from about eastern British Columbia into Arizona.
Global models generally retrograde this long wave ridge westward
across the region over the next couple of days. By the middle of
next week the ridge axis is expected to shift out as far as 145w.
The pattern is rather amplified for this time of year with abnormal
warmth building over the Pacific northwest and cool conditions
developing over the Midwest.
For the next couple of days...the worst of the heat will be centered
over the Columbia Basin in eastern Washington. This will keep low
level pressure gradients weak or slightly onshore through Sunday.
The weak onshore flow will result in the warmth in the interior of
western Washington being accompanied by rather high dewpoint
temperatures in the middle 50s to around 60. Nighttime low temperatures
over the urban corridor from about Everett down to Olympia will have
a hard time dropping below the lower 60s...and will only fall to
those relatively comfortable levels for a brief time in the late
night or early morning hours. Highs across much of the interior will
be around 90 this weekend. Night and morning low clouds and sea
breezes will keep the immediate coastal zones cooler. The coastal
beaches will mostly top out around 70 degrees.
A heat advisory will remain in effect for the urban corridor of
interior Washington through early Monday morning.
A very weak disturbance in the middle levels over northern California
is lifting northward today. While moisture is generally
lacking...there is a slight chance of a thunderstorm developing this
afternoon and evening over the Cascades. Water vapor imagery shows a
weak...but slightly more significant...upper low sitting centered
between 35n and 40n latitude and 135w and 140w longitude. Models all
lift this upper low northeastward across western Washington late
Sunday afternoon and Sunday night then off to the east into the
chimney of Idaho on Monday. An increase in middle and upper level
moisture with this feature along with modest lift and an unstable
atmosphere will result in a threat of thunderstorms over mainly the
southern half of western Washington Sunday afternoon and Sunday
night as this feature moves across the area.
The incoming 06z nam12 is showing a moderate but brief marine push
Sunday night as the upper low moves across the area. But rapid
ridging over the coastal waters Monday afternoon will likely result
in conditions warming right back up into the upper 80s to around 90
late Monday afternoon. Monday forecast high temperatures may be a
couple degrees too warm...but a 50 percent nudge toward a 00z model
blend did not result in significant cooling. Albrecht
Long term...the latest 00z GFS shows a rather flat 500 mb ridge
along 42n with continued light onshore flow that gradually increases
middle week. The GFS extended MOS as a result lowers temperatures into
the lower to middle 80s on Wednesday and mainly into the 70s for the
end of next week. The 00z operational European model (ecmwf) shows the same overall
pattern but has a bit more amplitude in the ridge and keeps
temperatures a couple of degrees higher than the GFS in the
Tuesday-Wednesday time period. By about Thursday...the long term
solutions generally agree that the long wave ridge will west of the
area far enough to allow some increase in low level onshore flow and
lower heights in Washington. While temperatures will cool during the
week...they will likely remain above normal through the period.
No changes were made to the extended forecast. Albrecht
Climate...the record for consecutive days with highs 90 degrees
plus in Seattle is 5. It has happened twice once at sea-tac 8/7-11,
1981 and once at the federal building downtown 7/14-18, 1941. The
record for consecutive days 85 degrees plus is 9, 8/5-13, 1977 and
8/12-20, 1967. The record for consecutive days 80 degrees plus is
15, 7/30-8/13, 1977.
Aviation...light southerly flow aloft will veer to southwesterly this evening. The
air mass is stable. Coastal low clouds and fog are slowly
encroaching down the Strait towards kclm and will linger nearby
through the morning. Low clouds are also making their way inland
into the southwest interior...and there is a chance kolm may see
some reduced visibilities and LIFR ceilings for a few hours this
morning...however the chance is low so I am leaving it out of the
taf for now.
Ksea...cirrus is moving in from south. SeaTac should stay VFR this
morning...however low clouds and fog pushing inland may make it to
within 10 nm of the southern approahces to the terminal. Expect
a northerly breeze to pick up Sat afternoon and evening. Jsmith
Marine...higher pressure is offshore with lower pressure
inland...but pressure gradients are weak. The typical pattern of
afternoon and evening onshore flow will continue through early next
week. A Small Craft Advisory has been issued for this afternoon and
evening based on strong diurnal heating and forecast gradients near
1.5 or 2 mb. Stratus and fog on the coast is continuing to push
inland this morning down the western Strait of Juan Delaware fuca. It may
also reach the south Puget Sound sound waters. Small Craft Advisory level
northwest winds are possible starting Sunday afternoon on the coast
as offshore pressure gradients increase. Jsmith
Washington...heat advisory in effect for much of the Puget Sound sound region
and southwest interior from 11 am Saturday until 6 am Monday.
Pz...Small Craft Advisory central and eastern Strait of Juan Delaware fuca
3 PM PDT this afternoon through late tonight.
For the graphical afd...visit