Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
940 PM PDT Monday Jul 6 2015
Synopsis...a strong upper level ridge over the region will
gradually weaken this week. Low level onshore flow will give
slightly cooler weather on Tuesday. It will then warm a few degrees
Wednesday and Thursday as onshore flow weakens. A large upper level
trough will develop offshore Thursday night through next weekend
giving onshore flow...an increase in night and morning clouds...and
temperatures closer to normal.
Short term...a weak upper trough extending from central Alberta
into western Washington is helping to increase onshore flow this
evening. Another weak upper trough is dropping southeast from northwest
British Columbia this evening and will clip the north Cascades on
Tuesday. The 18z gfs40 shows sufficient instability with tuesdays
trough to produce isolated ts over the north Cascades Tuesday afternoon.
On the larger scale...the strong ridge along 138w that has helped to
produce very warm conditions across the region for over the past
week is finally showing signs of weakening. This upper high...now
around 591 dam at 500 mb...is forecast to shift east into western
Washington on Thursday as it collapses to the lower 580 dam range. By
later Thursday into Friday...the pattern over the eastern Pacific and
western US will shift to an upper trough offshore with ridging
centered over the eastern half of the US...and low level flow will
become rather strongly onshore over the region.
At the surface...a weak ridge of high pressure over the coastal waters
combined with a heat low in the Columbia Basin of eastern Washington
is producing low level onshore flow this evening. The onshore flow
today resulted in highs 5-8 degrees cooler today than yesterday. The
onshore flow is stronger this evening than 24 hours ago with near
gale force westerly flow in the Strait of Juan Delaware fuca and west
winds over 20 miles per hour at Shelton. This will allow coastal stratus to
push into the Puget Sound sound region late tonight into Tuesday morning
for several hours. Temperatures on Tuesday should top out 2-4
degrees cooler than today in the interior. An update for highs on
Tuesday will be made to the zones and the gridded forecast.
Onshore flow Tuesday night will be weaker as the offshore upper
ridge approaches. The lack of stratus in the interior Wednesday
morning and subsidence with the incoming upper ridge will allow
temperatures to get into the 80s in the interior...and possibly into
the lower 90s south of Seattle.
The incoming nam12 hints at onshore flow ramping up Thursday
afternoon and highs topping out in the middle 80s. Strong onshore flow
Thursday night will start a period of temperatures close to normal
with late night and morning clouds marine typical of a more normal
Long term...from the afternoon long term discussion...SW onshore
flow will increase Thursday night as the large 135w upper level
trough approaches the region. Fairly strong SW onshore flow will
develop Thursday evening ahead of the trough... which should cover
most of the interior lowlands in stratus Friday morning. Combined
cooling from the approaching trough and onshore flow should drop maximum
temperatures back to the 70s. Over the weekend...onshore flow will be
weaker compared to Thursday night...but will be strong enough to
maintain the normal Summer pattern the pattern of night and morning
low clouds and afternoon sunshine through the weekend. Kam
Aviation...little change is expected in the overall pattern through
Tuesday. Continued northwest flow aloft and low level onshore flow. Smoke layers
from numerous wildfires over British Columbia will continue to
stream across the area.
Meanwhile...the onshore pressure gradient was tighter than 24 hours
ago...thus anticipate stratus to be more widespread Tuesday morning. The
clouds should reach ktcm and kpae by 1300 UTC. Ceilings/visibilities should
mainly be in the IFR category... although there will be areas of
Ksea...some stratus /cigs near 1k feet/ will likely reach the terminal
between 1300 and 1500 UTC.
Marine...higher pressure off the Washington coast with lower pressure east of
the Cascades will result in onshore flow of varying strength this
Fire weather...from the afternoon fire weather discussion...
forecast concerns in both the short term and the long term. Short
term...enough instability will be over portions of the north
Cascades for a thunderstorm threat Tuesday afternoon and evening.
However...models not only keep this instability brief but also
pretty limited area-wise. Thus...will keep current isolated wording
from inherited forecast and will not opt for any fire weather
watches or red flag warnings at this time...although will put a
headline on the forecast for this isolated lightning threat...given
the extremely dry nature of fuels out there. The long term issue
comes toward the end of the week...starting Thursday...as a trough off
the West Coast begins to impact the area. At this time...will put in
a chance for thunderstorms...but will reevaluate the threat
depending on how well the short term issue Tuesday pans out. Smr
Pz...Gale Warning central and east Strait of Juan Delaware fuca.
Small Craft Advisory in effect for the Admiralty Inlet and
northern inland waters until 2 am Tuesday.
You can see an illustrated version of this discussion...
temporarily not available...at