Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 271020
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
320 AM PDT Thu Jun 27 2019

.SYNOPSIS...A large closed area of low pressure will carry the
threat for showers and isolated thunderstorms on Thursday. A few
of these storms could produce locally heavy rain, gusty winds, and
small hail. Another round of showers and potential thunder is
expected Friday as the low moves into Washington. The low will
weaken for the weekend with showers mainly confined to the
mountains. Unsettled weather with lingering troughing will
continue into next week.

&&

.SHORT TERM /TODAY THROUGH SATURDAY/...10z water vapor imagery
early this morning reveals a robust closed area of low pressure
centered roughly along the north-central Oregon coast with
numerous vorticity perturbations rotating about the center.
Several of these perturbations have been affecting the weather
across western Washington thru the overnight hours. One of these
perturbations is located roughing along & east of the northern
Cascade crest, which has allowed a broad shield of precipitation
to blossom across northern counties, roughly from northern
Snohomish County westward into Island and Clallam Counties and
northward. Meanwhile, the second area of vorticity weakened as it
moved north towards the Sound, essentially ending the broad area
of precip across Lewis County earlier in the evening. Right now,
main vort max associated with center of low pressure is allowing
for showers to migrate northward from Oregon into the southern
portion of the CWA from Grays Harbor County into Thurston and
Lewis County.

Going forward this morning, and using current observations and
environmental conditions along with some hi-res models for
guidance, the broad area of moderate to locally heavier rainfall
across northern Counties will continue to expand as it moves
westward into British Columbia. Therefore, expect steady rainfall
if traveling early from northern Snohomish northward with lighter
showers from Kitsap County towards Seattle north. By the bulk of
the morning commute, think heaviest and steadiest rain will be
pushing west across Vancouver Island, southern BC, and into the
northern Olympic Peninsula, with scattered showers Kitsap to King
County northward. Aside from a few stray showers across the South,
this area may actually catch a break. Best chance for isolated
lightning appears to be across southern counties, indicated by a
few hundred J/Kg (at best) MUCAPE.

The main forecast concern today will be the threat for
thunderstorms across the majority of the CWA. The closed mid/upper
level low responsible for most of the synoptic lift will not make
much forward progress today, looking at best to be slightly
further north along the northern OR coast by day`s end.
Nonetheless, it will supply large scale lift and is expected to
eject a shortwave northward this afternoon from southern counties
towards the Sound. This should help spawn the development of
showers and isolated thunderstorms across the area. Daytime
heating, although likely shunted somewhat by cloud cover should
work in tandem with cooler air aloft from the upper low to support
500-1000J/KG CAPE by afternoon. Convection is confirmed in the
higher-res guidance as well. Thunderstorm activity is expected to
be isolated and single cell in nature given meager shear values
along and modest instability, though some cells may collide along
outflow boundaries and smear together at times. Locally heavy
rain, gusty winds, and small hail appear to be the main threats
with any thunderstorm that does materialize. SPC general thunder
risk looks good for the local area today. Main caveats for more
widespread thunder will be lack of instability from cloud cover
this morning as well as possible stabilization from last night`s
convection across southern CWA.

Activity should wane gradually overnight tonight and the threat
for thunder will decrease, given mainly loss of solar radiation,
though mid level temps will still be quite cool. Closed area of
low pressure will finally begin to move into Washington. By late
Friday morning, closed low is expected to be centered over western
Washington. This should generate another round of precipitation
Friday afternoon, especially across the Cascades. With cooler
temps continuing aloft and decent forcing for ascent, will need to
monitor thunderstorm potential again.

Troughing from this mid/upper level area of low pressure looks to
linger all the way into Saturday, though noticeably weaker than
the previous few days. Saturday looks drier across the lowlands,
but could see showers and maybe isolated thunderstorms develop in
the afternoon across the Cascades and Olympics.

Kovacik

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Western Washington can`t
seem to shake the troughing pattern through the weekend, as both
deterministic and ensemble solutions show the pattern sticking
with us through the weekend. Sunday does not look terribly wet.
Similar to Saturday the lowlands look mainly dry with a few
showers possible across the higher terrain. Monday will follow
suit, although a shortwave looks to lift northward across western
Washington from Oregon, allowing for the development of showers
across the area, at this time from Puget Sound eastward along
with the higher terrain across the Olympics. Unsettled weather
with the chance for showers will linger thru the remainder of the
long term. Do not see a promising pattern change of any kind even
looking right outside the forecast period.

Kovacik

&&

.AVIATION....An upper level low will continue to linger across
the Pacific Northwest today. As of 230 AM, radar shows heavier
showers pushing westward across the northern interior, while
lighter showers move northward across the southern Sound. Current
conditions across the terminals vary-from VFR to MVFR across most
of the Sound to IFR for terminals in the vicinity of heavier
shower activity. Expect overall conditions to trend towards MVFR
as shower activity increases over the next few hours. IFR
conditions will remain possible in any areas of heavier shower
activity. Winds at the surface have become more southwesterly over
the past few hours, while flow aloft has shifted from
southeasterly to more southerly/southwesterly. Another round of
showers and thunderstorms will be possible across the mountains
and lowlands this afternoon and evening.

SEA...Current conditions MVFR. Conditions should remain MVFR
through the early morning hours. Additional showers will likely develop
this morning and could push northward across the terminal at
times. Southwest surface winds 10-15 knots. Another round of
showers and a slight chance of thunderstorms will be possible this
afternoon and evening. 14

&&

.MARINE...An upper level low will linger over the Pacific Northwest
for the next few days. This will maintain onshore flow across western
Washington today. Small craft westerlies will be likely along the central
and eastern Strait of Juan de Fuca this afternoon and evening, while
other waters will remain below advisory criteria. 14

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT early this morning for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 2 PM this afternoon to 2 AM PDT Friday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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