Area Forecast Discussion
Issued by NWS Portland, OR

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000
FXUS66 KPQR 232129
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
229 PM PDT Sat Mar 23 2024

.SYNOPSIS...Showery conditions continue today and Sunday as a
low pressure system sits offshore of northwest Oregon and
southwest Washington. Snow levels will be around pass level this
weekend but impacts will be minimal. Cool air aloft may lead to
a couple thunderstorms this afternoon and evening. A shortwave
trough will bring another round of steady rain and non-impactful
Cascade snow on Monday. Strong low pressure systems will develop
over the NE Pacific beginning Wednesday, likely leading to strong
winds, heavy rain, and the potential for high surf and/or coastal
flooding mid to late next week.

&&

.SHORT TERM...Saturday afternoon through Monday...A 997 mb
closed low pressure system remains situated west of the Oregon
coast this afternoon. Another low pressure system deepening in
the far NE Pacific south of Alaska has been slowly pushing this
closed low SE closer to the Oregon coast throughout the morning
and early afternoon hours. This progression will continue as the
closed low makes landfall tonight along the central/southern
Oregon coastline and continues moving southeast into N
California/Nevada tomorrow morning. Showers will continue
pushing NE through central Oregon and will continue moving N/NE
through NW Oregon and SW Washington through the afternoon and
evening hours. The cold core aloft of this low will enhance
instability across NW Oregon and SW Washington as it pushes
onshore. CAM soundings indicate 100-300 J/kg of CAPE could build
this afternoon and evening along and behind this band of
showers, which could produce stronger cells with heavy rain and
small hail/graupel and even a few thunderstorms. If cloud cover
remains fairly thick, this could limit convective potential and
therefore the thunderstorm potential. Most locations will see
lighter showers through tomorrow morning with only up to an
additional 0.10-0.25 inch of rain. However, locations that see
stronger convective cells could see locally higher rain amounts
up to 0.4-0.5 inch.

Conditions remain showery with periods of widespread precipitation
tomorrow and Monday as the majority of ensemble members from the 12z
WPC cluster analyses suggest cool, northwesterly flow aloft. Rain
amounts will not be significant with NBM suggesting only a 10-20%
probability of 0.5 inch of rain from 5 am Sunday to 5 am Tuesday for
most of the lowlands. For the Coast Range and Cascades, those
probabilities increase to 40-60% for the same time period with a 10-
30% probability of 1 inch of rain.

Snow is expected each day through Monday, though though amounts are
forecast to be relatively low at pass levels due to the lighter and
spotty nature of precipitation. Snow levels remain above 4000-4500
feet through tomorrow, temporarily lowering down to around 3500-4000
feet tonight into Monday morning. Only a dusting up to 3 inches of
snow is forecast at Cascade passes through tomorrow night, though
elevations above 5500 feet could see up to 5-8 inches. Snow levels
then fall on Monday to around 3000-3500 feet, with snow amounts at
passes up to 5 inches or lower. -HEC

.LONG TERM...Tuesday through Friday...Tuesday remains showery as
southwest flow aloft slowly pushes east as a weather system
approaches from the Pacific. The second half of next week looks more
active as a strong jet stream appears poised to push a series of
fronts across the Pacific Northwest beginning Wednesday. 12z WPC
cluster analyses are in good agreement of a deep upper level trough
approaching the region. Strong winds and periods of moderate to
heavy rain will be possible at times along with higher snow amounts
in the Cascades. With the fast jet stream, the timing and exact
track of any particular frontal system remains uncertain, but the
message here is that the confidence is high that more active weather
arrives mid to late week. Fortunately, with rivers levels starting
off low, there is a less than 5-10% chance of coastal rivers,
Willamette mainstem rivers, and Willamette tributaries rising above
action stage.

Marine impacts are discussed in the marine discussion below, but the
rapid cyclogenesis and track of the resulting lows appear poised to
support a dynamic fetch for wave generation in the north-central and
NE Pacific as the lows strengthen and race E-NE. With limited wave
data, it is difficult to come up with probabilistic numbers for the
potential of coastal flooding and/or high surf, but the pattern
appears to suggests a considerable likelihood of both as early as
Wednesday. If you have plans at the coast mid to late next week,
keep an eye out on the latest forecast and WWA headlines.
-Alviz/HEC

&&

.AVIATION...A low pressure system offshore will gradually weaken
as it moves southeast toward the south Oregon coast. Southerly
flow aloft today becomes light and variable as the upper low moves
across the area tonight. Predominately VFR expected as generally
light showers persist through this evening. Heavier showers may
bring reduced visibility or MVFR cigs around 2500 ft. Then, more
clearing likely as showers diminish and the low drops south.
Chances for MVFR to LIFR begin to increase tonight after 03Z
Sunday through Sunday morning, anywhere from 20-60%. Modest
instability lingers across the area this afternoon, maintaining a
10-20% chance of thunderstorms through 03Z.

* Note: The observations out of KONP are generally limited to wind
  and pressure at this time. Periodically cloud cover, vis, and
  present wx are reported but are unreliable.

PDX AND APPROACHES...Predominately VFR expected as, through
heavier showers may bring brief periods of MVFR conditions. A
10-15% chance of thunderstorms continues this afternoon. Showers
likely diminish overnight with some clearing possible, which could
lead to another round of fog or low stratus. Chances for MVFR
increase to around 40-50% after 06Z Sunday, while chances for IFR
increase to around 10-20%. Then, rain likely returns after 12Z
Sunday from the northwest. /DH

&&

.MARINE...Surface analysis shows a 997 mb low approximately 100 nm
offshore of Newport as of early Saturday afternoon, with buoy
observations reporting southerly winds of 15-20 kt with gusts to
25 kt across the central waters. A southerly fresh swell continues
to produce steep seas of 9 to 10 ft at around 10 seconds through
this afternoon, possibly into early evening. Will maintain current
Small Craft Advisory headlines. The low will slowly move southeast
tonight, pushing inland along the southern Oregon coast early
Sunday morning, bringing about a transition to lighter northwest
winds and subsiding seas for Sunday. Relatively quiet conditions
will persist through Monday as winds turn southwesterly ahead of a
weak front that will move across the waters on Monday. Seas are
likely to hover around 5 to 7 ft through Monday night.

Conditions likely remain fairly benign into Tuesday as transient
high pressure moves across the waters. But, a strong low pressure
system is expected to develop in the NE Pacific and bring more
active weather to the coastal waters late Tuesday through late
next week. There is high probability that the initial front will
bring strong southerly Gales across the coastal waters, possibly
late Tue night to Wed morning. The strong, sub-980 mb low pressure
will likely move near Vancouver Island and weaken, but could
continue to produce a dynamic fetch, which tends to pile up seas.
Guidance continues to suggest seas likely build into the upper
teens on Thursday, with the possibility of seas building to
around 20 feet. /DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 5 PM PDT this afternoon for
     PZZ251>253-271.
     Small Craft Advisory until 8 PM PDT this evening for PZZ272-
     273.
&&

$$

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