Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
910 PM PDT Friday Jul 3 2015
Synopsis...sunny and hot days will continue through the
Independence day Holiday weekend...and likely into the middle of
next week...as an upper level ridge combined with a thermally
induced trough of low pressure at the surface sits over western
Short term...it appears that hot weather will continue over the
interior of western Washington through about the middle of next
week. The hottest day of the Independence day Holiday weekend will
likely be on Sunday. Another hot day appears likely on Wednesday of
next week before the pattern finally shifts into one more normal for
this time of year.
A 590 Dam Ridge is centered around 45n 140w. This ridge is giving
warm and dry northwesterly flow aloft. An upper level low over
northern Alberta is dropping southeast toward the northern plains. On
Saturday...high pressure at the surface is expected to develop in
the wake of this upper trough over north central British
Columbia...then push into southeastern British Columbia and western
Montana on Sunday. This high will increase low level offshore flow
and allow the development of thermal troughing west of the Cascades.
Upper level heights may fall a bit on Saturday...and 850 mb
temperatures may fall a degree or two...but any cooling aloft will
be offset by weak thermal troughing from the main heat low over southeast
Washington into Vancouver Island on Saturday. A more well defined
thermal troughing will be near or over the coastal waters on Sunday
as offshore flow increases. The 18z GFS is more aggressive with the
850 mb cooling on Sat than most of its ensemble members.
The 12z European model (ecmwf) is warmer on Sat than the GFS and is much warmer for
sun. MOS has had a cool bias over the past few days in the interior.
Forecasts high temperatures Saturday and Sunday were pushed 2-4
degrees hotter from about Everett or so southward in the interior
and the heat advisory for the central Puget Sound sound region was
extended until midnight Sunday night. Albrecht
Long term...it is starting to appear that Monday will be nearly as
hot as Sunday. Thermal troughing over the coastal waters appears
likely to shift back into a position that extends from Pasco Washington into
Vancouver Island. Little change is expected on Tuesday...then
Wednesday may see an increase in temperatures as a short wave upper
ridge moves across the region. Good cooling with a shift in the
pattern and strong onshore flow will likely not occur until
Wednesday night of next week. Albrecht
Aviation...an upper ridge centered offshore will keep the flow
aloft from the west-northwest. Low level weak onshore flow will become flat
overnight. There will probably be some localized LIFR ceilings/visibilities on
the coast after 0700 UTC.
Ksea...VFR. Northerly winds of 5-9 knots will become light and variable after
Marine... weak onshore flow will become flat overnight. High pressure
over British Columbia and lower pressure over Oregon will result in
northerly flow on Saturday. Offshore flow will develop Saturday
night and continue into Sunday due to low pressure setting up along the
Washington...heat advisory for Seattle...Bellevue...Tacoma...and east Puget Sound
sound lowlands zones.
Pz...small craft advisories are in effect for the coastal waters
from Cape Flattery to Cape Shoalwater from 10 nm out to 60 nm
and the central/eastern Strait of Juan Delaware fuca.