FXUS66 KSEW 121039

Area Forecast Discussion
National Weather Service Seattle WA
339 AM PDT Sat Jun 12 2021

.SYNOPSIS...Unsettled weather pattern will dominate Northwest US weather
through early next week as a series of systems will bring periods
of widespread rain to the region. Heavy rain will be possible at
times this weekend as sub-tropical moisture encroaches upon the
region. Showers and potential thunderstorms will persist through
Tuesday before a warming and drying trend takes hold from mid week


continue across portions of Western Washington at this hour. The
latest water vapor satellite imagery shows an area of dry mid-
level air starting to encroach upon the region from the southwest
which will cause shower activity to start to wane as reach
daybreak and progress through the morning. This is consistent with
the latest HREF trends that suggest a fairly nice Saturday with a
mix of sun and clouds into this afternoon. Expect temperatures to
rise into the low to mid 70s as a result.

Later this afternoon and evening, the long anticipated arrival of
the long fetch of rich sub-tropical moisture will arrive in
Western Washington. Expect increasing clouds late this afternoon
with rain quickly spreading across the southwest interior and
Olympic Peninsula this evening. Rain will spread across the entire
area by Saturday night and will persist through late Sunday night
as the moisture band slowly traverses eastward. Given that mean
flow will be southerly, banding is likely. Strong difluence aloft
on top of a potential record setting moist airmass with dewpoints
rising into the low 60s and precipital water values 2 to 3
standard deviations above normal for June (1.25" - 1.5") will set
the stage for a very wet 24 hour period. Rainfall rates will be
abnormally heavy at times - potentially up to 1"/hour rates at
times. This will yield hazardous travel conditions as visibility
will be poor at times and water will pond on roadways. Urban
flooding will also be possible, especially in poor drainage areas.
If you have travel plans Saturday night and Sunday, be sure to
plan for plenty of travel time and use your low beam headlights
when driving through heavy rain. The heaviest rainfall will
likely fall in the southern Puget Sound region southward into the
Chehalis Valley, where 24 hour rainfall totals will likely range
from 0.5 to 1". Isolated spots of 1"+ are not out of the question,
particularly where the heaviest banding sets up. The heaviest
rainfall will likely fall from the south Puget Sound region
southward into the Southwest Interior (Lewis/Thurston Counties and
south), where 24 hour rainfall totals will likely range from 0.5
to 1". Isolated spots of 1"+ are not out of the question,
particularly where the heaviest banding sets up. At this time, the
higher probabilities for heavy banded rainfall reside in the
aforementioned areas.

While heavy rain will be the greatest threat on Sunday, it is
important to note several limiting factors with respect to
thunderstorm potential - 1) Widespread cloud cover across the region
will prevent adequate destabilization, 2) the atmospheric will be
saturated through the column which is not conducive for adequate
cooling aloft. That said, a rumble of thunder wont be out of the
question in far southeastern portions of the area (southern
Cascades) late Sunday as a subtle upper level disturbance
traverses the region. That said, the highest chances for
thunderstorm activity Sunday evening will remain east of the
Cascades where the environment wont be modified by persistent
heavy rainfall and cloud cover.

.LONG TERM /MONDAY THROUGH FRIDAY/...Areas of efficient showers
will persist through the day Monday with periods of heavy rain
possible. PW`s will begin to decrease late Monday into Tuesday as
slightly drier air filters into the region once again. This will
cause shower coverage to decrease late Monday afternoon,
potentially allowing some sun to peak through the clouds.

Operational model guidance continues to indicate the large upper low
off of the coast becoming negatively tilted late Monday into Tuesday
before ejecting a shortwave trough across the region. While this
remains a constant - there remains a fair amount of uncertainty in
the forecast that will have an effect on thunderstorm coverage and
strength. The placement of the shortwave will be important - if the
energy is too far south and east, the potential for widespread
thunderstorms will decrease rather significantly for western
Washington. The timing is important as well - upper level energy
arriving a little later in the day on Tuesday will allow for the
atmosphere to destabilize, allowing for enough CAPE to support
thunderstorms Tuesday afternoon and evening. That said, the
potential for thunderstorms remains for Tuesday and will continue
to monitor the important fine details in the model guidance
leading up to the event.

Wednesday will mark the beginning of warming and drying conditions
for Western Washington as the ridge axis over the Great Basin shifts
westward. Expect to see temperatures rising back into the mid to
upper 70s, potentially approaching 80 degrees to close out the week.



.AVIATION...Southwesterly flow aloft with frontal system stalling
over the area Sunday. Ceilings 4000 to 6000 feet this morning
becoming aoa 10000 feet this afternoon. Ceilings lowering to
around 2000 feet with local visibility 3-5sm in moderate rain 06z-
12z Sunday with the onset of rain.

KSEA...Ceilings near 4000 feet this morning becoming aoa 10000
feet this afternoon. Ceilings lowering to near 4000 feet around
06z tonight with light rain. Ceilings continuing to lower
overnight down to around 2000 feet with visibility 3-5sm in rain
by 12z Sunday.



.MARINE...Another frontal system will move through the
coastal waters tonight then stall over the inland waters Sunday.
The front will dissipate Sunday night. Weak high pressure will
gradually build into the region early next week for lighter winds
across area waters.

Small craft advisory winds over the coastal waters later tonight
through Sunday morning. Possible small craft advisory winds over
the northern portion of the inland waters Sunday.



.HYDROLOGY...Minor urban flooding and ponding of water on roadways
will be likely Sunday as persistent moderate heavy rain is
expected. Rainfall rates will reach or exceed 1"/hr at times, with
areas from the South Puget Sound southward through Thurston and
Lewis Counties likely seeing the highest rainfall totals. 24 hour
rainfall totals of 0.5 to 1" is expected, with isolated areas over
1" possible (most likely in the aforementioned areas). River
flooding is not expected with this system.


PZ...Small Craft Advisory from 2 AM to 2 PM PDT Sunday 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.




NWS SEW Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather