Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
250 am PDT Thursday Jul 31 2014
Synopsis...a strong upper level ridge over the Pacific northwest
will bring warm sunny days to western Washington through much of the
next week. Weak onshore flow will result in some night and morning
low clouds from tonight Onward...especially along the coast. Weak
instability aloft Friday through Sunday could result in isolated
afternoon and evening thunderstorms over the mountains.
Short term...a large strong upper level ridge will remain over the
western U.S. For the next week. The ridge axis is currently over The
Rockies which puts West Washington under weak SW flow aloft. The two forecast
problems this morning are the potential for convection over the
mountains and the strength and direction of low level onshore flow
which impacts stratus coverage and daily temperatures.
The upper level ridge will be strongest today with 500 mb heights
around 5880 meters. Stratus cleared from the coastal waters
yesterday and low level onshore flow has been weaker and more
northwesterly than 24 hours ago. In particular the onshore flow up
the lower Chehalis valley has been non-existent overnight. Fog
product imagery at 09z/2 am shows no stratus anywhere. With the
strong ridge overhead and no cooling from the onshore flow maximum temperatures
should be higher today...with the highest readings in the middle to
upper 80s over the central and SW interior.
The NAM boundary layer relative humidity indicates stratus filling in along the
coast tonight then spreading partway inland after midnight...but
probably only reaching the SW part of Puget Sound sound Friday morning.
The next onshore flow cycle Friday night-Saturday should bring
stratus and patchy fog to most of the interior by Saturday morning.
All the models show a weak upper level shortwave trough just west of
the central/southern Oregon coast this morning. Models all agree on
bringing the weak shortwave northward off the Washington coast on Friday and
then over northwest Washington/SW b.C. On Saturday. The shortwave will increase the
instability over West Washington Friday and Saturday and shift the middle level
flow around 700 mb to southerly. The southerly flow will allow
moisture from convection over the Oregon Cascades today and Friday
to advect up over West Washington. The GFS drops lifted indices down to around
-2 both afternoons while the NAM is more unstable at around -5. Both
models also show a pocket of instability over the east Olympics. The
forecast was already in good shape with a chance of thunderstorms so
no changes were made.
Maximum temperatures will cool slightly...back to the middle 70s to middle 80s Friday
and Saturday. This is due to the combination of general air mass
cooling from the upper level shortwave and to the return of stratus
in the shallow marine layer. Kam
Long term...Sunday looks about the same as Saturday over The
Lowlands. Onshore flow will allow morning stratus to form over most
inland areas...followed by a midday burn-off. Maximum temperatures will remain
in the middle 70s to middle 80s inland. The upper level shortwave trough
will shift over East Washington/southeast b.C. With westerly 700 mb flow developing
over West Washington. This should end the potential for convection over the
mountains on Sunday. However I will hold on to a chance of Cascade
thunderstorms for Sunday afternoon and evening a little longer just
A weaker upper level ridge will remain over the Pacific northwest on
Monday...then both the GFS/European model (ecmwf) have the remnant of a weak front
reaching West Washington late Tuesday and early Wednesday. This looks like a
dry feature so the main effect will probably be continuing weak to
moderate onshore flow. 500 mb heights remain around 5860 meters so
maximum temperatures should remain in the 70s and 80s. Kam
Aviation...an upper ridge is over the western U.S. With light southwest
flow aloft over Washington. At the surface there is high pressure just
offshore with thermally induced low pressure east of the Cascades.
Patchy morning fog coast and Strait otherwise clear. The air will be
unstable over the mountains in the afternoon and evening hours each
day and there is a chance of a thunderstorms in the forecast over
the mountains Friday afternoon and evening.
Ksea...clear with an afternoon and evening northerly breeze.
Marine...the pattern hasn't changed with high pressure over the
offshore waters and thermally induced low pressure east of the
Cascades. There is not much marine fog or low clouds but there will
probably be more of that by Friday morning as a shallow marine layer
develops. Race Rocks will flirt with westerly gales each evening...I
think there is a better chance of getting up around 35kt Friday
evening than there is tonight. Along the coast...Destruction Island
gets up around 25kts each evening...and a Small Craft Advisory there
blends pretty well with the Portland marine forecast.
Pz...for this afternoon and tonight...Small Craft Advisory central
and East Entrance to the Strait...and Small Craft Advisory James
Island to Cape Shoalwater and out 10 nm.
You can see an illustrated version of the forecast discussion at