946
FXUS66 KPQR 072147
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
247 PM PDT Tue May 7 2024

.SYNOPSIS...One more day of cool and unsettled weather today,
then high pressure will bring a substantial drying and warming
trend that will last through the end of the week. Before the
warming trend, lingering cool air and clear skies will likely
lead to frost in some of the outlying valleys, but urban centers
should remain a few degrees above freezing. Record warm
temperatures are possible Friday and/or Saturday, but rivers and
lakes will remain dangerously cold.

&&

.DISCUSSION...Tuesday Afternoon through Monday...Radar imagery
shows scattered showers continuing across NW Oregon and SW
Washington Tuesday afternoon as a shortwave trough moves east of
the region. Snow levels have risen to around 4000-4500 feet this
afternoon, though webcams indicate snow is light and scattered
enough that little to no snow is sticking to roads. Showers
will end from west to east this evening as high pressure begins
rapidly building east just off the coast.

With high pressure building in quickly, clear/calm conditions will
lead to excellent radiational cooling within a chilly air mass
lingering over the Pac NW tonight into Wednesday morning.
Nights are getting short, but it appears temps will have the
opportunity to cool off enough for frost in the outlying valleys
and possibly the suburbs. 12z HREF and NBM guidance continues
to show anywhere from a 40-80% chance of temps 35 deg F or
cooler for most of the Willamette Valley excluding the PDX
metro, whereas the suburbs generally have a 20-40% chance of
temps 35 deg F or cooler. The inner Portland metro only has a
10-25% chance. A Frost Advisory remains in effect for lowland
areas excluding the Portland Metro area north through the
Washington I-5 corridor tonight through 8 AM Wednesday.

There is high confidence amongst ensemble guidance in a sharp
warming trend and dry weather returning Wednesday into the
weekend, with temperatures peaking Friday and Saturday. The
chilly start on Wednesday will initially hobble the rise of
temperatures, but strong May sunshine will eventually mix out
any valley inversions and temps should climb well into the 60s
(and possibly to 70 degrees) Wednesday afternoon. Low-level
offshore flow will ramp up Wednesday night into Thursday, and
with 850 mb temps +10 to +12 deg C by Thursday afternoon, it
appears very likely inland areas and possibly even the coastal
valleys will reach 80 degrees Thursday. Latest NBM probabilistic
data show 60-85% chance of reaching 80 degrees Thursday for the
entire Willamette Valley, with the chances exceeding 90% for
the lowlands of the Portland metro area.

Latest WPC cluster analysis remains confident in strong ridging
building Friday into Saturday. All clusters suggest the ridge
will eventually be worn down by a strengthening Pacific jet
stream moving from the north- central Pacific into the NE
Pacific, but most guidance holds on to the ridge long enough to
keep Monday dry except perhaps for some coastal drizzle.

Deterministic models are depicting a fairly typical evolution for our
anticipated warm spell: Offshore flow Thursday/Friday leading to
unseasonably warm temperatures all the way to the coast, thermal low
pressure shifting inland Saturday bringing cooler onshore flow to the
coast and coastal valleys while inland valleys remain near 90
degrees, then more widespread cooling Sunday as thermal low pressure
focuses on the Columbia Basin and pulls onshore flow all the way to
the Cascades. The marine layer will likely remain too shallow for
much in the way of morning cloudiness inland through Sunday, but the
upper ridge may weaken enough for some low clouds to push into the
inland valleys for a few hours Monday morning.

As for the magnitude of the warmth, the inner Portland metro appears
the most likely to reach 90 degrees Fri/Sat based on NBM
probabilistic guidance, with 80s a near certainty for all other
inland valleys. Latest NBM prob guidance shows a 70-95% chance
of reaching 90 deg F across the inner PDX metro and eastern
Washington County as well as along the I-5 corridor in
Washington south of Longview. South into the central and
southern Willamette Valleys, these probabilities drop to 5-20%.
For now our forecast is around 90 degrees for the Portland metro
Friday and Saturday, with mid to upper 80s for the remainder of
the interior lowlands.

The unseasonably warm temperatures will no doubt cause people
to flock to the rivers, lakes, and ocean beaches of SW
Washington and NW Oregon. It is important to remember that
rivers and lakes remain dangerously cold with water temperatures
mainly in the 40s. Such temperatures can easily cause cold
water shock for those without proper protective equipment. The
bright sun and warm temperatures can make the cold water look
refreshing, but the consequences of jumping in without the
proper equipment could be deadly. Be sure to bring a life vest
and be extremely cautious around area rivers. -Weagle/HEC

&&

.AVIATION...Northwest flow aloft continues today as high pressure
ridge begins to build across the region. This will cause scattered
light rain showers to gradually dissipate by this evening. Expect
predominately VFR conditions today. Northwest winds at the
surface begin to shift northerly later today through tonight.
Guidance then suggests stratus may rebuild by early Wed morning
near the Cascade foothills with around a 30-50% chance of MVFR
cigs creeping into the terminals through the Willamette Valley
between 12-18Z Wed.

PDX AND APPROACHES...Predominately VFR expected with light
scattered showers eventually dissipating by this evening. Chances
for MVFR stratus creeping into the terminal increase after 12Z
Wednesday to around 30-50%, but whatever does form should burn off
quickly with increasing sun through Wed morning. Northwest winds
increase to around 7-9 kt this afternoon. /DH

&&

.MARINE...Northwest winds 10-15 kt gradually shifts north
overnight. Seas around 9 to 10 ft at 12 seconds are expected to
persist through early this evening, barely meeting Small Craft
Advisory criteria. Afterwards, high pressure begins to build
across the waters, bringing more tranquil weather. Winds turn
northerly as high pressure persists offshore and the thermal
trough strengthens along the CA/OR coast. This summer like pattern
will bring breezy north winds with gusts to 30 kt on Wednesday.
Have issued a Small Craft Advisory for this from Wed afternoon
through Wed night. Winds ease somewhat Thursday morning, before
pressure gradients strengthen again through Thu evening. /DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Frost Advisory from midnight tonight to 8 AM PDT Wednesday for
     ORZ104-105-109-114>118-121-123>125.

WA...Frost Advisory from midnight tonight to 8 AM PDT Wednesday for
     WAZ205-208.

PZ...Small Craft Advisory from 3 AM to 8 AM PDT Wednesday for PZZ210.

     Small Craft Advisory until 8 PM PDT this evening for PZZ251>253-
     271>273.

     Small Craft Advisory from 2 PM Wednesday to 2 AM PDT Thursday
     for PZZ251-271.

     Small Craft Advisory from 11 AM Wednesday to 5 AM PDT Thursday
     for PZZ252-253-272-273.

&&

$$

www.weather.gov/portland

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