Scientific Forecaster Discussion
fxus66 ksew 201003
Area forecast discussion
National Weather Service Seattle Washington
303 am PDT Thu Jul 20 2017
Synopsis...a weather system lifting across Vancouver Island today
will bring cooler temperatures, clouds, and the first chance of
rain for many areas in over a month. Another weak weather system
will brush the far northern coastal areas Friday and Saturday.
Then the pattern will return to dry, sunny and warmer than normal,
with morning clouds and afternoon sun.
current radar image shows a narrow band of sprinkles and showers
pushing onshore in coastal areas. Presently a few sites in Grays
Harbor County have measured rainfall of a few hundredths. The band
of showers is associated with an upper level low and weak surface
low pressure which should drift across Vancouver Island today.
The forecast remains for chances of showers across much of the
area today with light rainfall amounts expected. The most likely
spots to see measurable rain will be the coast, the Olympics, and
north Cascades. Showers will be spottier elsewhere and thus
rainfall accumulations light and perhaps not even measurable in a
few spots. With enhanced cloud cover today, highs will be cooler
than yesterday, and be slightly below normal in the 60s to low
Showers will linger through the evening hours but then end
completely across the area by midnight or so. The upper low will
track well east and a quick rise in mid level heights is expected
overnight. All in all, not a terribly impressive system. But since
it's not rained in over a month in most areas, this will be the
forecast focus in the short term. By Saturday and Sunday a
stronger upper low will push into the British Columbia coast, well north of the
area. The models continue to suggest additional rain chances along
the northern Pacific coastal areas of western Washington, but dry
elsewhere. Mid level heights will remain rather high across the
area, exceeding 582 dm nearly everywhere, so not too terribly
excited at this point on likelihood of more rain this weekend.
Temps will moderate Friday and this weekend. Reaching back in the
70s and lower 80s, or slightly above normal for the time of year.
Overnight lows will warm, with more 50s and lower 60s - courtesy
of some increased surface dewpoints.
Key short term messages: a chance of showers today may break the
dry streak, which is now 32 days for SeaTac. The current one is
number 30 all time longest, and is the longest since the Summer
of 2013 when we had 35 consecutive days.
the long term pattern continues to suggest varying degrees of
onshore flow. Early in the coming week, temps will warm well into
the 80s for most spots. Then a cooling trend is anticipated by mid
week as troughing increases in the Pacific just off the coast.
This will support the morning clouds and afternoon sun pattern and
temperatures, while cooler than earlier in the week, still near
normal by later next week. No significant weather or impacts from
weather are expected.
Key long term messages: following the briefly cooler and showery
weather today, there will be a quick return to warmer
temperatures with highs near to above normal for the rest of the
forecast period. No significant systems in the long term forecast
Aviation...southwest flow aloft continues over western
Washington early this morning, ahead of an upper trough along the
Pacific northwest coast. The upper trough will move inland today,
and westerly flow will follow the trough through tonight. At low
levels light onshore flow will become weak to moderate late this
afternoon and evening, following its usual diurnal pattern. The
air mass will be somewhat moist through tonight, especially below
Low clouds have increased over western Washington during the last
few hours, with broken-overcast ceilings 011-018 mainly in the central
Puget Sound region and at the coast. This trend should continue
this morning as the weak trough moves through the area. Low clouds
should lift later today, with VFR ceilings continuing through
Ksea...south wind 7-13 kt, veering to southwest this afternoon,
then eventually northwest 4-8 kt this evening. The ceiling that
developed during the last couple hours should continue through
late morning, followed by improvement to VFR midday. Mcdonnal
Marine...light onshore flow prevails early this morning with a
weak surface trough just off the coast. The trough will dissipate
as it moves slowly inland this morning. High pressure building
weakly over the coastal waters and diurnal forcing will cause
onshore flow to resume this afternoon and evening. Small Craft
Advisory westerlies are expected in the central and eastern Strait
of Juan de Fuca again tonight.
Another weak trough will move into the offshore waters Friday and
dissipate offshore on Saturday. This will be followed by more
onshore flow Sunday and Monday with high pressure offshore and
lower pressure inland. Mcdonnal
Pz...Small Craft Advisory from 6 PM this evening to 3 am PDT Friday
for central U.S. Waters Strait of Juan de Fuca-East
Entrance U.S. Waters Strait of Juan de Fuca.