Funnel Cloud in Oregon, May 12, 2025

Area Forecast Discussion
National Weather Service Portland OR
250 PM PDT Mon May 12 2025

.SYNOPSIS...
A broad low pressure system is moving through southwest
Washington and northwest Oregon, bringing cooler temperatures
and scattered rain showers. Expect daytime highs in the 50s
along the coast and 60s inland through at least Tuesday.
There's a slight chance of thunderstorms from this afternoon
through early evening, mainly inland. A brief period of warm
and dry conditions are expected Wednesday with light
precipitation returning later in the week.

&&

.SHORT TERM...Now through Wednesday Night...A stretch of cool
and unsettled weather is underway across southwest Washington
and northwest Oregon, thanks to a broad low pressure system
moving through the region. Many areas are seeing off and on
showers today, and there is a slight chance for isolated
thunderstorms this afternoon and evening, especially inland.
While conditions of the atmosphere are not entirely favorable
for frequent thunderstorms (long skinny CAPE), there is a
15-30% chance for enough cool air aloft and modest atmospheric
instability to support a few lightning-producing cells. Any
thunderstorms that do develop are expected to be brief and not
particularly intense, but a few may produce very small hail
(less than a quarter inch in size) and could create slick spots
on roads or sidewalks for short periods.

Rainfall totals today will differ greatly over the area as
showers have been stalling, weakening, and then redeveloping
over nearby locations. In general, expect totals around 0.10 to
0.30 inches in the valleys and along the coast, with locally
higher amounts up to 0.60 inches possible in the Cascades.

By Tuesday, showers will start to fade as the system shifts off
to the southeast. A weak ridge of high pressure will begin to
build in behind it, helping to dry things out slightly and bring
more stability. A few lingering light showers could still pop up
Tuesday, especially in the interior valleys and Cascades, but
accumulations should be minimum - less than 0.05 inches in most
lower elevation areas and up to 0.20 inches in the mountains.

Wednesday will mark a noticeable break in the wet pattern. With
the ridge in place, expect drier conditions, a bit more sun, and
a slight bump in temperatures.

.LONG TERM...Thursday through Sunday...Another system is
expected to swing through Thursday, bringing renewed chances for
light precipitation across the region. However, temperatures
should stay slightly above average for this time of year,
especially inland. Forecast models suggest that as we head
toward next week, high pressure may start to build over the
eastern Pacific and push into the western US, favoring a shift
toward drier and more stable conditions.

For now, expect coastal highs to stay in the 50s and inland
highs to rise into the 60s through the latter part of the week
and into the weekend.
~Hall

&&

.AVIATION...An upper level low over the region will continue to
bring scattered showers and isolated thunderstorms to northwest OR
and southwest WA through 00-03z Tuesday before convection
diminishes after 03-06z Tuesday. Until then, any stronger shower
or thunderstorm that does develop will have the potential to
produce brief heavy downpours, small hail around the size of peas
or smaller, and brief IFR to MVFR cigs and/or visibilities. Away
from stronger showers and storms, conditions will generally remain
VFR through 09-12z Tuesday. After 12z Tuesday, chances for MVFR
cigs increase to 80-90% at the coast and 40-60% inland. Note
thunderstorm probabilities are too low to justify a PROB30 group
at any given TAF site.

PDX AND APPROACHES...Off and on rain showers will continue through
00-03z Tuesday before diminishing thereafter. There is also a
15-20% chance of thunderstorms until then. Any stronger shower or
thunderstorm that moves over the terminal will have the potential
to produce brief heavy downpours, small hail around the size of peas
or smaller, and brief IFR to MVFR cigs and/or visibilities. Away
from convection, conditions will generally remain VFR until
approximately 09-12z Tuesday when MVFR cigs below 3000 ft will
likely develop (40-60% chance). -TK

&&

.MARINE...Northwest winds will increase over the coastal waters
this evening into tonight as low pressure moves inland. Expect
gusts up to 20-25 kt over the outer waters and up to 15-20 kt over
the inner waters. In addition, a northwesterly swell will increase
to 8 to 10 ft at 10 seconds by Tuesday morning. Given the
increasing seas and winds, have issued a Small Craft Advisory for
the outer waters from this evening through Tuesday night and for
the inner waters from Tuesday afternoon through Tuesday evening.
Have also issued a marginal Small Craft Advisory for the Columbia
River Bar to cover a strong ebb current Tuesday morning and
another ebb current Tuesday evening. Expect seas as high as 9-10
ft during both ebb currents.

Winds and seas gradually fall Wednesday through Thursday as high
pressure builds in, bringing weaker northwest winds and seas below
6-8 ft. -TK

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 3 AM to 11 PM PDT Tuesday for PZZ210.

     Small Craft Advisory from 11 AM to 11 PM PDT Tuesday for
     PZZ251>253.

     Small Craft Advisory until 5 AM PDT Wednesday for PZZ271>273.
&&

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