Scientific Forecaster Discussion
fxus66 ksew 170356
Area forecast discussion
National Weather Service Seattle Washington
900 PM PDT Tue Jul 16 2019
Synopsis...an organized frontal system approaching the area will
bring widespread precipitation Wednesday and showers Wednesday
night. Scattered showers will linger across the area Thursday and
Friday before dry and warmer weather return over the weekend through
the middle of next week.
Short term /tonight through Friday/...a few isolated showers
lingering over the area this evening with increasing mid and high
level clouds ahead of an approaching frontal system. This system
will dig southeast into the central and eastern Gulf of Alaska overnight
with warm frontal precipitation starting to work onto the North
Washington coast early Wednesday morning. Relatively widespread
showers associated with the warm front will spread across western
and northern portions of the County Warning Area through Wednesday morning. The
trailing cold front will follow quickly pushing another round of
organized precipitation from northwest to southeast through the day Wednesday.
Models continue to advertise shadowing downstream of the Olympics -
mainly areas around Puget Sound, including the Seattle Metro area.
The frontal forcing will shift south of the area late Wednesday
afternoon leaving residual showers over much of the area. There
again remains some chance for the formation of a convergence zone
behind the front Wednesday evening, and with the onshore flow may
linger into Thursday morning. Scattered showers will linger into
Thursday especially over the north through the afternoon. The upper
level trough over western Canada will shear apart and sag south
Thursday evening...perhaps keeping a few showers around, then
trending drier into Friday as the flow aloft becomes more zonal.
Long term /Saturday through Tuesday/...from previous
discussion...over the last several days, model guidance had been
hinting at a fairly amplified ridge building into the Pacific
northwest rather quickly to start the weekend. Latest trends now
argue to delay this pattern change. Weak ridging will still likely
build into the area on Saturday, allowing for a dry day, but better
amplification is now seen for Sunday. The ridge is expected to
develop as the central/eastern US ridge retrogrades westwards and
builds into The Rockies/4 corners area. This would then shift the
larger scale troughing offshore.
This ridge will then essentially act as a block in the synoptic
scale pattern and is expected to hold thru the middle of next
week. This would keep western Washington dry with temperatures
returning back to the upper 70s to lower 80s. The trough may then
try to build back towards the area thereafter, but guidance not
returning any pops at this time.
Aviation...ceilings will fill back in overnight with lower cigs
likely late tonight ahead of an approaching frontal system. Ceilings
will remain fairly low through Wednesday with most areas remaining
MVFR with areas of IFR are likely with heavier rain. The mountains
will be obscured. Southwest flow aloft will increase into Wednesday
and surface winds will be breezy southerlies with the front.
Ksea...ceilings will fill back in overnight and lower to MVFR levels
2500 to 3000 feet. A front will bring rain to the terminal by late
morning. Abundant moisture will likely keep ceilings at MVFR
levels through the day.
Marine...a front will reach western Washington on Wednesday and
south winds will pick up with that front with Small Craft Advisory
strength winds likely most waters. There will be westerlies in the
Strait of Juan de Fuca this evening, and then westerly gales are
likely Wednesday night behind the front. Onshore flow Thursday will
ease Friday and then the winds over the weekend should more typical
summertime diurnal sea-breezes.
Hydrology...the daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
Pz...Small Craft Advisory from 3 am Wednesday to 6 am PDT Thursday
for coastal waters from Cape Flattery to James Island 10 to
60 nm-coastal waters from Cape Flattery to James Island out
10 nm-coastal waters from James Island to Point Grenville
10 to 60 nm-coastal waters from James Island to Point
Grenville out 10 nm-coastal waters from Point Grenville to
Cape Shoalwater 10 to 60 nm-coastal waters from Point
Grenville to Cape Shoalwater out 10 nm.
Gale watch from Wednesday evening through late Wednesday night
for central U.S. Waters Strait of Juan de Fuca-East
Entrance U.S. Waters Strait of Juan de Fuca.
Small Craft Advisory from 6 PM this evening to 7 PM PDT
Wednesday for East Entrance U.S. Waters Strait of Juan de
Small Craft Advisory from 3 am to 11 PM PDT Wednesday for
northern inland waters including the San Juan Islands.
Small Craft Advisory from 6 PM this evening to 2 am PDT
Wednesday for central U.S. Waters Strait of Juan de Fuca.
Small Craft Advisory from 5 am Wednesday to 2 am PDT Thursday
for Admiralty Inlet.
Small Craft Advisory from 5 am to 7 PM PDT Wednesday for Puget
Sound and Hood Canal.