579
FXUS66 KPQR 011027
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
327 AM PDT Mon Jul 1 2024

.SYNOPSIS...Offshore flow slowly decays as a strengthening ridge
of high pressure develops over the Pacific. This pervasive high
pressure will cause temperatures to increase significantly
through the end of the week.

&&

.SHORT TERM...Now through Wednesday...Area observations show
dense fog forming along the central Oregon coast and the bowl
portions of the southern Willamette Valley. Some lower stratus
shifting inland through the Valleys, but thus far have not seen
any real widespread issue other than along the coast.

The shortwave trough that brought cloudy skies and drizzle over
the last few days will continue to shift inland through the
day. It will generally shift east of the Cascades by this late
afternoon. Once that trough exits, high pressure will begin to
slowly build in. We will see steady warming and clear skies over
the next week or so, so if you like warm temperatures...this
forecast is for you! The ridge will really amplify on Wednesday
night. Overall sensible weather wise, not looking at anything
overly impactful other than the fog this morning.

Will note that with these dry conditions incoming, will see a
steady decline of humidity, especially east of the Coast Range.
Models have struggled with the areas that sit within the
subsidence inversion around 3000 ft and above so have manually
edited to lower the RH in those areas. -Muessle



.LONG TERM...Thursday through Sunday...The longer term forecast
is really where the weather is occurring...though it isn`t
necessarily active weather. The main threat in the forecast, and
where the most time has been spent, is in regards to
temperatures. The synoptic pattern shows a strengthening ridge
forming with a thermally induced trough forming right along the
coastline. This ridge will continue to amplify through the
weekend. But what does this mean for us? Models are showing a
steading warming trend which will lead into several days of more
intense heat. In order to capture an overall view will not
necessarily dive into each day directly.

Friday and Saturday look to be the warmest days of the coming
week which coincides with our general trend of "summer starts
on July 5th". The thermal trough will be quite deep at this
point which will cause winds to be northeasterly at the upper
levels, but more northerly at the lower elevations. They will be
slightly elevated which will help mix down some of the warm air
aloft. At 850 mb (5000 ft AGL) temperatures are forecast to
range from 20-25 deg C (70-75 deg F). Even further aloft at 500
mb (18,000 ft AGL), temperatures range from -7 to -3 deg C
(20-25 deg F). While these temperatures may not seem as
impressive, given their elevation and what we typically see,
they are trending on the warmer side. When we see this pattern,
it confirms that we are looking at hotter than normal
temperatures. In fact, if we look at the "Extreme Forecast
Index", we are on the higher end of what is climatologically
normal, with this being mainly the case from Salem southward.

At this time, there is still a lack of confidence in just how
warm those temperatures will be though. In Eugene, the global
models are showing a fairly narrow spread in high temperatures
for Friday and Saturday. The 25th-75th percentile (most likely
range) shows only a 5 degree spread in the low 90s on Friday,
and around a 10 degree spread on Saturday. The NBM on the other
hand too has a 5 degree spread, but in the upper 90s to up to
105 degrees with little change on Saturday. So what does that
mean forecast wise? At this point, confidence is still quite low
in regards to whether we will see extreme heat. Based on the
global models, there is a 30% chance of exceeding 100 degrees on
Saturday, and the NBM is closer to 50%. There is high confidence
that conditions will be hot...hotter than what we have seen thus
far this year. But confidence is low in regards to whether or
not we will see extreme heat. Luckily we will see some reprieve
overnight as lows dip near 60 degrees in the areas that will see
the highest daytime heating. -Muessle



&&

.AVIATION...Onshore flow is allowing marine stratus at the coast,
continuing through the rest of the night. Intermittent IFR/LIFR
cigs expected, along with reduced visibilities at times. 80%
chance of VFR conditions at the coast returning by 20z Mon. Monday
night sees 50-80% chance of IFR conditions reforming by 03z Tue
as onshore flow increases again.

Inland, the rest of Sunday night sees around a 20-40% chance of
some high-end MVFR cigs forming as stratus begins building off the
Cascades, with better chances in the eastern Willamette Valley.
KTTD sees higher chances, closer to 60% of MVFR or below, and
30-40% of IFR cigs. VFR conditions return by 18z with 90%+
confidence.

PDX AND APPROACHES...Latter half of the night (12z-18z Mon) shows
a 30-40% chance of high end MVFR cigs (2000-3000 ft), higher
slightly east of the terminal, closer to KTTD. After 18z, 90%+
chance of VFR cigs returning. Flow remains generally light and
northwesterly throughout the TAF period, between 5-7 kt
throughout the period. -JLiu

&&

.MARINE...Weak high pressure over the northeast Pacific with lower
surface pressure over California and the Great Basin will more or
less continue through the week. A weak front is in the process of
passing through, but effects look fairly minimal, with light
northwesterly winds through Monday morning. Threat of dense marine
fog is possible for the southern waters, with visibilities of
half a mile or lower possible through the rest of the night.
This is expected to clear up shortly after sunrise. Afterwards,
expect NNW winds across the waters as high pressure strengthens
toward the middle of next week, with the strongest winds
generally off the central coast of Oregon and lighter winds
farther north. Winds will generally peak in the afternoon and
evening hours each day. Small Craft Advisory remains on track
for the central zones, but zones further to the north still look
fairly marginal. Because as a thermal low develops along the
northern California/southwest Oregon coast on Tuesday and
strengthening/spreading north into Wednesday and thereafter. As
a result will see an increase the pressure gradient along the
coast and over the waters with a >90% chance that northerly wind
gusts of 20-30 kt spread northward across the waters by mid
week. This will in return develop steep wind driven seas of 5-8
ft with a dominant period of 7-8 seconds by late Tuesday or
Wednesday. -JLiu

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Dense Fog Advisory until 9 AM PDT this morning for ORZ103.
WA...None.
PZ...Dense Fog Advisory until 9 AM PDT this morning for PZZ253-273.
     Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
     Wednesday for PZZ253-273.
&&

$$

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