Forecast Discussion for SEW NWS Office
000
FXUS66 KSEW 312204
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
304 PM PDT Sun May 31 2020

.SYNOPSIS...Benign weather through at least mid week before a
pattern change. A weak PSCZ will keep showers near the Sound
through this afternoon, before a dry period follows through the
first half of Tuesday. A weak cold front may bring some showers to
northern parts of the area Tuesday evening and Tuesday night,
before another dry period follows through Thursday. Larger scale
troughing will then bring unsettled and cooler weather to the area
Friday through at least the weekend.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Much quieter conditions
across western Washington this afternoon as compared to 24 hours
ago. A synoptic analysis of the mid and upper level pattern
continues to yield large scale troughing across the majority of
the far North Pacific that extends to west coast of the US, with a
high amplitude ridge across the middle of the country. At the
surface, low pressure lingers across eastern Washington, with high
pressure nosing from the Pacific into western Washington.
Downstream, a seasonably strong area of high pressure is centered
near the Great Lakes, with influence extending out to most of the
country east of the Rockies.

This afternoon across the Pacific Northwest, a shortwave trough is
seen departing the US from the northern Rockies into southern
Alberta, with which across Washington is being replaced by a very
subtle, weak shortwave ridge. This is allowing for mid level
height rises and it combined with increasing high pressure
influence in the lower levels, is allowing for some cloud cover
across the area to make an attempt to break up a bit. A notable
exception to this is around the Kitsap/southern Snohomish/northern
King vicinity where strong overnight push of westerly wind down
the Strait of Juan de Fuca aided in the development of a PSCZ.
This zone has been stubborn to diminish today, but recent MRMS and
KATX radar trends show promise that shower activity may finish by
late afternoon/early evening. 12 hour rainfall totals in this
vicinity suggest rainfall amounts just shy of half an inch. No
impacts are expected with this zone over the next few hours as it
gradually erodes, there will just be some nuisance hit or miss
rain showers.

Overnight tonight, the subtle upper level shortwave ridge will
slide east as a weak shortwave trough moves into the coastal
zones. This particular wave does not contain much moisture with
it, and with high pressure influence continuing at the surface, do
not expect precip during its passage through the day Monday.
Perhaps it could bring some mid and high clouds with it, but there
still appears a much higher chance to see some sun tomorrow
compared to today. Then, following the passage of this shortwave,
quasi-zonal flow will take it place into Tuesday. The first half
of Tuesday looks dry with cloud cover increasing quickly through
the morning hours. A weak shortwave trough, carrying a weak sfc
frontal boundary with it, will traverse through the flow and bring
some showers to at least the northern half of the CWA the second
half of Tuesday and into Tuesday night. Coverage and amounts do
not look impressive. Zonal flow will then reestablish itself
across the region of the frontal passage, with tighter gradients
suggestive of stronger upper level flow. Seeing numerous
perturbations in the H5 layer pass through or very near the local
area, but model guidance is not enthusiastic about there being any
rain chances within this setup. This will change for the long
term forecast...

Kovacik

.LONG TERM /THURSDAY THROUGH SUNDAY/...An unsettled, zonal flow
pattern will carry us into the long term forecast period and
conditions still appear to remain overall dry for most locations.
By Friday, however, a strong area of closed low pressure will
sink deeper into the Gulf of Alaska, turning the mid and upper
level flow across the Pacific Northwest more SW. Shower activity
may increase to some degree on Friday, with chances ramping up
overnight Friday into Saturday as the Gulf of Alaska low sets
sight on the Pacific Northwest and begins to sink further south.
The closed low will drop south through the weekend, centering
itself offshore the WA/OR Coasts. Numerous smaller scale systems
will rotate right into the local area through the weekend,
providing a focus for unsettled weather and what appears to be the
chance for periodic showers. In addition to the persistent
unsettled, wetter conditions, temperatures will also remain
somewhat cool, with highs generally in the low to mid 60s.

Kovacik

&&

.AVIATION...An upper level trough will continue to linger over the
area this afternoon and tonight. The flow aloft will be westerly
with low level onshore flow. The air mass is generally stable and
is expected to remain so.

Cigs have lifted for the most part with most W WA locations showing
VFR conditions. Some exceptions still linger around the Sound...most
notably BFI and PAE...who are still reporting MVFR cigs. Cigs still
expected to continue to gradually lift a little bit more...but not
too much as plenty of low level moisture is still in place. That
being said...cigs over the area still expected to fall in VFR
criteria overnight with most locations generally seeing cloud bases
of 4-6k ft. Some isolated spots that are more prone to lower cigs
might dip down into MVFR territory...but confidence is not there for
inclusion for TAFs at this time.

KSEA...VFR conditions expected to persist. Cigs around 5000 feet
will likely redevelop later tonight. Southerly wind 8-12 knots will
become northerly 4-8 knots this evening. 18

&&

.MARINE...Onshore flow will ease today in the wake of a vigorous
weather system. Small craft advisory conditions remain in place over
the central and eastern Strait of Juan de Fuca. These conditions are
still expected to ease by late tonight and as such...will leave
inherited headlines in place for the afternoon forecast package.

Onshore flow will continue for the rest of the week with high
pressure offshore and lower pressure inland. Small craft advisory
strength west winds are likely most evenings in the central and
eastern Strait in this pattern. Monday night looks to be the
strongest instance of these. Should model data remain consistent...a
headline for this time period may be warranted.

A weak frontal system will brush the area later Tuesday. The main
impact from this system will be briefly to weaken the onshore
flow. Schneider

&&

.HYDROLOGY...The recent rainfall has actually caused river levels to
rise in both the Skagit River and the White River. Current
observations show that the Skagit at Concrete has crested and is on
its way down and the Skagit at Mt Vernon is close to cresting and
this crest is forecast to be below flood stage. Similarly...the
White River at R Street has crested well below flood stage and is on
its way down as well. With generally dry conditions expected beyond
today...any potential flood risk is minimal and the need for any
additional hydro discussion rendered moot.  18

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until midnight PDT tonight for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

NWS SEW Office Area Forecast Discussion