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000
FXUS66 KPQR 072102
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
200 PM PDT Tue Apr 7 2020

.SYNOPSIS...High pressure will continue to strengthen over the
Pacific Northwest through Wednesday. Onshore low-level flow today
will transition to light offshore flow late tonight and Wednesday.
The offshore flow pattern is expected to persist through Thursday
morning and then switch back to onshore late in the day. A gradual
cooling trend begins Friday as onshore flow strengthens. The Eastern
Pacific upper ridge retreats slightly west over the weekend as an
upper trough slide south through the Northern Rockies. High pressure
nudges closer to the coast early next week for dry and seasonal
conditions.

&&

.SHORT TERM...Tonight through Friday...Water vapor satellite imagery
revealed a deep upper low off the southern California coast and a
high pressure ridge centered along 140W. Meanwhile, the northern
branch of the jet stream could be seen over Washington and the
Northern Rockies. Early afternoon GOES-17 visible imagery showed the
marine stratus field over the south Washington and far north Oregon
coast diminishing. Low-level northeast flow could be seen across
southwest Oregon with the marine stratus being pushed further out to
sea.

At 19Z the KTTD-KDLS gradient was 4.7 mb, slightly stronger than the
NAM forecast. Gusts to 30 mph have been noted in the Hood River area
early this afternoon. The 12Z NAM forecast a peak KTTD-KDLS gradient
of 4.3 mb early this afternoon. The end result will likely be closer
to 5 mb. The 12Z model runs are in good agreement showing the onshore
low-level weakening fairly quickly this evening and transitioning to
light offshore flow by 12Z Wed. Not anticipating much, if any, fog
due to the switch to offshore flow. The 12Z HREF hints at some patchy
low cloud in the Willamette Valley late tonight. The 12Z HiRes-ARW
boundary layer condensation pressure deficit valid 12Z Wed appears to
be rather marginal for any low cloud development.

The offshore gradient, specifically KTTD-KDLS, is expected to peak
around -3.5 mb early Wed afternoon. Local forecast rule of thumb
would indicate peak gusts 30-35 mph in the windiest spots at the west
end of the Gorge and 20-25 mph for KTTD. Model forecast 850 mb temps
warm from 4C in SW Washington and 6-8C in Lane County today to 9C in
the north to 11-12C over the Lane County Cascades Wed. Several
locations likely to hit the 70-degree mark Wed. A thermally-induced
surface trough resides over the inland valleys Wed night through
mid-afternoon Thu. The NAM suggests a switch to light onshore flow
for the coast and coastal valleys by mid-afternoon Thu. Thus, SW
Washington coastal areas and locations west of the Oregon Coast Range
will likely see some cooling Thu afternoon. However, areas east of
the Coast Range and also in the SW Washington interior will be as
warm or even a degree or two warmer Thu.

A weak short-wave is expected to slide across southern British
Columbia Thu evening. This will enhance the onshore low-level flow
and increase the likelihood of low clouds pushing up the Columbia.
Expect about 4-8 degrees of cooling Fri as 850 mb temps fall by 4-6C.
The stronger onshore flow Fri will bring another round of breezy to
windy conditions to the Central Gorge. Weishaar

.LONG TERM...Friday night through Tuesday...The 12Z operational runs
and ensembles show the eastern Pacific ridge retrograding slightly
west Fri night and Sat as an upper level trough gets carved out over
the southern British Columbia and the Northern Rockies. The GEFS 500
mb mean height forecast valid 00Z Sun is in good agreement with its
control run. The ECMWF ensembles indicating strong upper level
troughing over eastern Washington and Oregon. All in all, this should
translate into a deepening marine layer Fri night and Sat. Any precip
would most likely be drizzle from the deeper marine layer. Will
maintain 20-30 percent POPS late Fri night through Sat.

The eastern Pacific ridge begins to re-amplify Sat night, with
generally dry north flow aloft Sun. The operational GFS and ECMWF
show surface thermal trough development along the coast Sat night and
Sun. A high percentage of the ECMWF ensemble members indicate no
precip Sat night and Sun. The GEFS and GEFS mean valid Sun afternoon
show ridging closer to the coast with the large upper trough
displaced more to the east over the Upper Midwest. Dry and seasonal
conditions look to hold through into early next week. Models and
their ensembles become more out of phase around the middle of next
week. There is reasonable confidence that a change to cooler and
unsettled conditions occurs during the latter half of next week.
Weishaar

&&

.AVIATION...High pressure across the area is keeping conditions
calm and clear. Increasing northerly flow is bringing gusts up to
25 kt to the central Oregon coast. KAST could have some high
MVFR/low VFR cigs between 2500 to 3500 ft redevelop around 12Z
Wednesday. Otherwise, VFR conditions over the next 24 hours along
with northerly winds.

PDX AND APPROACHES...VFR conditions over the next 24 hours.
Winds will be generally light and from the north. /42

&&

.MARINE...A thermally induced trough extended into the central
Oregon waters this afternoon bring small craft winds to zones
PZZ275 and PZZ255. This thermal gradient will bring northerly
winds to small craft levels with gusts to 25 kt within the
central Oregon waters, and a chance that some isolated gusts to
25 kt could be observed in the southern parts of zones PZZ270 and
PZZ250 west of 10 NM. This thermal gradient is expected to
intensify along the coast through Wednesday, which will bring
small craft advisory winds with gust up to 25 kt to PZZ270 and
PZZ250. Gusts up to 30 kt will be possible in zones PZZ275 and
PZZ255 at this time too.

These small craft advisories will probably be extended further,
but have not done that as guidance is suggesting that zones
PZZ270 and PZZ250 could have a significant period of time without
small craft gusts.

Seas will remain around 6 to 8 feet with a dominate period
between 15 to 18 seconds through the middle of the week.
North/northwesterly swell is in the 3 to 5 foot range with the
seas being wind wave dominate.

Wednesday night and into Thursday, winds begin to ramp up and
could increase seas to near 9 ft with a 10 to 13 second period.
/42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 11 AM to 11 PM PDT Wednesday for
     coastal waters from Cape Shoalwater WA to Cascade Head OR
     out 60 NM.

     Small Craft Advisory until 11 PM PDT Wednesday for coastal
     waters from Cascade Head OR to Florence OR out 60 NM.

&&

$$

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This discussion is for Northwest oregon and Southwest Washington
from the Cascade crest to 60 nm offshore. This area is commonly
referred to as the forecast area.