571
FXUS66 KSEW 141105
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
405 AM PDT Fri Jun 14 2024

.SYNOPSIS...Upper level low off the British Columbia coast will
continue to move southeast ending up over Western Washington
Saturday. Weak front spinning out of the low moving through later
this morning and early this afternoon. Low remaining in the
vicinity Sunday before moving east Monday. Weak upper level trough
over Western Washington Tuesday and Wednesday. Upper level ridge
will try to build into British Columbia Thursday.

&&

.SHORT TERM /TODAY THROUGH SUNDAY/...Satellite imagery shows upper
level low made good progress Thursday moving out of the Gulf of
Alaska and is now west of Haida Gwaii. Weak front spinning out of
the low off the coast. Doppler radar picking up the leading edge
of the precipitation associated with the front just reaching the
north coast at 4 am/11z. Skies still mostly clear over the
southern portion of the area with mostly cloudy skies north.
Temperatures at 4 am were in the mid 40s to mid 50s.

Weak front reaching the coast later this morning moving through
the interior late morning/early afternoon. Showers associated
with the front along the north coast will spread inland over
mainly the northern portion of the area this morning into the
early afternoon. The front is pretty narrow so expect the duration
of the shower activity to only be a few hours at best.
Temperatures aloft cooling during the day with the upper level low
continuing to move towards Western Washington. This combined with
increasing cloud cover will keep highs in the upper 50s to mid
60s.


Upper level over Vancouver Island tonight moving over Western
Washington Saturday. Air mass becoming unstable with lifted
indexes 0 to -3 and lapse rates plus 7 or 8C. Models have the 500
mb temperatures by 00z Sunday in the minus 26c to -28C range.
Little in the way of daytime heating Saturday but convective
temperatures are only in the mid to upper 50s. Showers developing
tonight as the low approaches with showers and a chance of
thunderstorms Saturday for the entire area. Snow levels as low as
4000 feet so we could see some snow, will not really accumulate,
on the higher passes, Washington, Chinook, Cayuse, that are open
this time of year. Lows tonight in the mid to upper 40s. Highs
Saturday in the mid 50s to lower 60s, a good 10 degrees below
normal.

Low weakening Saturday night with the low drifting into Eastern
Washington Sunday. Shower activity decreasing during the day
Sunday. Chance for thunderstorms also decreasing Saturday night
into Sunday. While lapse rates remain near plus 7C, lifted indexes
go positive and the flow aloft becomes northerly on the back side
of the low. It still going to be a cool day for Father`s Day with
highs only a couple of degrees either side of 60.

.LONG TERM /MONDAY THROUGH THURSDAY/...Extended model solutions
not showing as much consistency as earlier in the week. The
general idea is northerly flow aloft on the back side of the low
Monday with a very weak trough carving out over the area Tuesday
and Wednesday then a weak ridge trying to build into British
Columbia late in the week. The theme of all this is weak. Weak
onshore flow in the lower levels, weak trough, weak ridge. Air
mass remaining slightly unstable for most of the period. With none
of the features being able to take over the pattern will keep a
chance of showers for almost the entire period. Ensemble
solutions leaning towards the ridge winning out in the latter
part of next week for some dry and warmer weather. Slow warming
trend with highs Monday in the lower to mid 60s warming to the mid
60s to mid 70s by Thursday. Felton

&&

.AVIATION...VFR. As a front progresses inland this morning,
conditions along the coast will deteriorate to MVFR/localized IFR in
low clouds and showers. A line of showers will move inland
throughout the day with a 20% to 30% chance of thunderstorms in
stronger showers that develop. Gusty outflow winds are possible near
thunderstorms. Cloud cover will persist through Friday evening ahead
of another round of showers and thunderstorms on Saturday, with
forecast models hinting at convergence zone showers later on.
Surface winds will remain generally out of the S/SW throughout the
day 12 kt or less.

KSEA...VFR this morning with increasing clouds ahead of an
approaching front. Showers will pass the terminal between 18z/Fri-
03z/Sat, with brief MVFR conditions possible in heavier shower
activity. Thunder is possible after 18z (25% chance) but confidence
is not high enough to add to TAF. Thunderstorms look more favorable
near KSEA on Saturday. S/SW surface winds roughly 8 to 12 kt
persisting throughout the day.

15

&&

.MARINE...Although a weak frontal system will move across area
waters throughout the day today, conditions will be fairly quiet for
the next few days. The primary concern will be any thunderstorms
that develop, with roughly a 20% to 30% chance of thunder activity
in stronger showers that develop each afternoon through the weekend.
Heading into early next week, surface ridging offshore and lower
pressure inland is expected to produce varying degrees of onshore
flow. Seas are favored to remain between 4 to 6 feet through the
period, with winds likely staying below SCA criteria throughout.

15


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$