359
FXUS66 KPQR 071845 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
353 AM PDT Tue May 7 2024

Updated aviation discussion

.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.

&&

.SHORT TERM...Now through Thursday...Scattered showers persist in
cool onshore flow across SW Washington and NW Oregon this morning.
Showers have been most numerous in the higher terrain, where
orographic westerly flow leads to upslope enhancement. Snow levels
have lowered to around 3500 feet, but overnight snow accumulations
have only amounted to an inch or two. Scattered showers will continue
to move onshore through this afternoon, but a strong capping
inversion near 9-10 kft should prevent them from growing tall enough
to produce lightning or much hail. Any showers will also have to work
against increasing subsidence as high pressure builds in from the
Pacific. Showers will end with the loss of solar heating, and that
will likely be the end of precipitation across our forecast area for
several days. The lingering cool air mass should keep temps mostly in
the mid to upper 50s for lowland highs today.

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. 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. Will issue a Frost
Advisory for tonight into Wednesday morning as temps are expected to
dip into the lower to mid 30s in the outlying inland areas. 00z HREF
guidance shows 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, so we
will exclude inner portions of the PDX metro from the Frost Advisory
as lows are expected to remain closer to 40 degrees there.

The chilly start will hobble the rise of temperatures early
Wednesday, 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 greater than a 60% chance of
reaching 80 degrees Thursday for the entire Willamette Valley, with
the chances exceeding 90% for the lowlands of the Portland metro
area.

With temperatures climbing into the 70s and 80s by Thursday, 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.  Weagle

.LONG TERM...Friday through Monday...High confidence in dry and
unseasonably warm weather as the latest WPC cluster analysis suggests
strongly positive 500 mb height anomalies through the weekend. 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. NBM prob guidance shows a 50-95% chance of reaching
90 deg F across the inner PDX metro and eastern Washington County,
with the highest values (greatest likelihood) focusing on Tualatin,
Tigard, Beaverton, and Aloha. In fact, NBM is beginning to show probs
of 95 deg F in the 10-30% range for that area. The wet ground and
lack of strong offshore flow should prevent these higher values, but
it would come at the cost of higher humidity due to evaporation
coming from the moist ground and vegetation. 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 weekend 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.  Weagle

&&

.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, though a small chance
(20-30%) of CIGS dropping to MVFR persists through 21Z Tuesday.
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. There
remains a 10-20% chance of CIGS dropping to MVFR briefly through
21Z. Chance 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...Breezy northwest winds and light scattered showers
continue through this afternoon with gusts to 20 kt. Seas around
9 to 10 ft at 12 seconds this morning 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. /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.

&&

$$

www.weather.gov/portland

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