486
FXUS66 KPQR 030435
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
935 PM PDT Wed Apr 2 2025

Updated aviation discussion.

&&

.SYNOPSIS...Showers persist today dissipating overnight as high
pressure builds. Clear days and mostly clear evenings will set
the stage for fog/frost development tonight and Thur night/Fri
morning. Increasing temperatures on Saturday will be above
normal. Frontal system arrives Sunday through early next week.

&&

.SHORT TERM...Satellite shows patchy skies associated with post
frontal showers. These showers have brought periods of heavy
rain and small hail to the region. A dry slot will be moving
inland later this evening which will cause skies to clear or
become partially clear. The clear skies combined with colder
air will promote fog development throughout the Willamette
Valley, and coastal valleys especially. There are some limiting
factors though, the first of which being cloud cover. If the
clouds are too dense, radiational cooling will not be enough to
reach the dewpoint. However, the amount of available moisture
from the previous day`s showers will provide the moisture needed
for saturation.

High pressure persists through Saturday with clearer skies each
day. The next weather factor to consider occurs Thursday night
into Friday morning. Overnight temperatures will dip into the
30s. This, combined with light winds and drier conditions, can
produce frost through the area. This would be the first frost of
the growing season so it is something we are watching closely.
In order to get frost we need a few ingredients: calm winds,
temperatures between 33-38 deg F, a drier atmosphere initially,
then temperatures reaching the dewpoint temperature (the
temperature an airmass has to cool to in order to become
saturated). Light easterly winds during the day will create the
decreased moisture and cooler air filtering in; winds are
expected to then go calm. Probabilistically there are a few
concerns. First: temperatures are sitting on the upper threshold
for frost formation closer to 37-40 deg F overnight. While the
10th percentile would be cold enough, any fluctuation to warmer
would not. In addition, the spread between the dew point
depression is around 3 degrees which may be just large enough to
prevent frost. Ultimately, it`s not a bad idea if you`re below
1500 ft in the valleys (coastal, inland, Cowlitz, and Gorge) to
prepare for patchy to widespread frost. Areas most at risk will
be rural areas and the southern Willamette Valley.

Once the sun rises, any frost that did form will melt off and
conditions will warm to a spring-like day. -Muessle


.LONG TERM...Saturday through Wednesday...Saturday will be
another springy day giving us a break before the next rain
arrives on Sunday. High pressure will usher in warmer air from
the south aloft, which will mix down causing highs to increase
into the upper 60s to lower 70s. Along the coast, will remain
seasonable, but sunnier than we have seen the last week. Inland
areas will be warmer than normal. Models still have some
discontinuity with the overall high temperature forecast. For
example, the NBM ranges from 70 deg F to nearly 76 deg F (the
90th percentile). The deterministic forecast though sat right
along the 25th percentile and in some areas even below this
value. Decided to incorporate and nudge to the 50th percentile
in the forecast in the Willamette Valley, Clark County, and the
lowlands of Cowlitz county. If easterly winds are elevated,
these temperatures may be less if close to the Cascades or the
Gorge.

Enjoy the sun as a frontal system will move in Sunday night with
a series of shortwaves through Wednesday. Overall troughing is
not necessarily well realized by models at this point, and
precipitation amounts will be low in comparison to what we
typically see this time of year. The coast though will be the
exception as the ECMWF is depicting anywhere from a quarter of
an inch in 24 hours, to some of the ensembles of greater than
1.25 inches. The GFS is less wet with much lower amounts (closer
to 0.25 inch). -Muessle

&&

.AVIATION...Current VFR conditions are expected to persist
through the period. Isolated rain showers continue to move
onshore within moist northwest flow at AST/ONP, with latest
high-resolution guidance suggesting additional light showers may
continue through the morning. While sct clouds at 3-4 kft are
anticipated, there remains a 30% chance of MVFR cigs at 2-3 kft
at coastal terminals overnight. Inland, dry conditions are
expected throughout the period. With modest clearing and calm to
light and variable winds expected, fog may develop near bodies
of water and within sheltered valleys overnight, most likely
around sunrise, 13-15Z Thu. There is a 10-20% chance of vis less
than 1SM within fog or mist from Portland-area terminals south
through the Willamette Valley. VFR conditions are otherwise
favored overnight, and are expected to be widespread by 16-18Z
Thu. Winds will increase to 5-10 kt out of the north to
northwest after 16-18Z Thu.

PDX AND APPROACHES...VFR conditions are expected throughout the
period. Potential fog overnight supports a 10-20% chance of IFR
vis, mostly likely around 13-15Z Thu, particularly immediately
along the Columbia River. Any restricted vis will lift by 15-17Z
Thu, with clearing skies and VFR conditions then prevailing
through the remainder of the period. Calm to light and variable
winds tonight will increase out of the northwest at around 5 kt
after sunrise, and then diminishing tomorrow evening. -Picard

&&

.MARINE...High pressure over the northeast Pacific and lower
pressure inland will maintain northerly to northwesterly winds
under 15 kt through the end of the week. The next front arrives
Saturday evening into Sunday and that will bring a return to
breezy southerly winds across all waters. As the front
approaches, there is a 15-30% probability for Gale-force
southerly wind gusts up to 40 kt on Sunday. Seas generally 3 to
6 ft through Sunday. As the front approaches the waters, seas
will build towards 10 to 12 ft as a fresh westerly swell pushes
into the waters.

Another strong ebb is expected Thursday morning; however, seas
with this ebb are forecast around 4 to 5 ft. -42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&

$$

www.weather.gov/portland

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