Funnel Cloud in Oregon, June 19, 2023

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
943 AM PDT Mon Jun 19 2023

Updated aviation discussion

.SYNOPSIS...A cold upper trough is impacting the Pacific NW,
bringing cool temperatures, as well as rain, mountain snow, and
isolated thunderstorm chances through Tuesday. Snow levels are
unseasonably low, with light accumulations down to the Cascade
passes at times through Tuesday morning. Drier weather is then
expected as temperatures gradually climb back to seasonal normals
Wednesday with additional warming into late week.

&&

.SHORT TERM...Today through Wednesday...Water vapor satellite
imagery early Monday morning reveals a broad upper level low over
western Washington slowly dropping south. Deterministic models show
this upper low moving across northwest Oregon today, bringing in
slightly colder air aloft this afternoon and evening. Synoptically,
today's pattern will be different from yesterday's as the strong
upper level jet is forced south and east of the area. Showers will
continue today as onshore flow will persist in the lower levels.

Shower activity has mostly diminished across the area this morning,
but is expected to increase again this afternoon into early evening.
Model sounding forecasts show an increase in MLCAPE values from
yesterday, around 200-400 J/kg, but much less shear. In fact, both
the HREF and NBM are showing areas of 30-40% chance of thunderstorms
occurring in the southern Willamette Valley into the Lane County
Cascade foothills. Other locations within the Valley will have
around a 15-25% chance of thunderstorms later today. Rainfall
amounts today will again vary due to the showery nature of the
precipitation, with around a 50% chance of most locations within the
Willamette Valley seeing 0.20-0.30 inch of rainfall. A few locations
may see rainfall amounts again reaching the 0.75-1.00 inch 24-hour
totals (5-10% chance), especially along the west slopes of the
Cascade foothills. At higher elevations in the Cascades, snow will
continue above 4500-5000 ft, but accumulations are expected to
remain light.

The upper low then shifts east of the Cascades tonight into Tuesday
while weak surface low pressure drops south across the coastal
waters. Showers become more stratiform in nature as a surface front
pushes into the region. Expect one more day of unseasonably cool and
damp weather on Tuesday. Snow levels will gradually rise to above
6000 ft by Tuesday evening with a few lingering showers quickly
dissipating Tuesday night. Temperatures are very likely to rebound
to near normal values on Wednesday as we dry out and morning clouds
give way to mostly sunny skies. /DH

.LONG TERM...Thursday through Sunday...Ensemble guidance indicates
the main upper trough moves northeast into Canada, but a weak
troughy pattern remains across the Pacific NW. Deterministic models
show various depictions of upper level cutoff lows near California
Thursday through the weekend. This pattern will keep occasional
showers and thunderstorm chances, mainly over the Cascades, into the
weekend. NBM guidance suggests temperatures will warm back into the
upper 70s to lower 80s, while precipitation chances (away from the
Cascades) remain below 20%. /DH

&&

.AVIATION...Low pressure sits over the coastline which is
enhancing the already present showers. Generally VFR through the
whole forecast area. As these showers move inland from the coast
they will cause lower ceilings and reduced visibility due to
rainfall. While it is not trending nearly as strong, probabilities
are around 30% for thunderstorms through the Willamette Valley.
PROB30 for these thunderstorms remains in tact although they will
be scattered. One slight shift with newer model data is that there
is instability aloft which may create more robust thunderstorms
with small hail Meteorological wind shear is not as likely so have
not included in the TAFs, although it may be experienced around
2000 ft. In thunderstorms though gusty winds and shear is expected
with small hail.

For detailed Pac NW aviation weather information, go online to:
http://weather.gov/zse

KPDX AND APPROACHES...Predominantly VFR cigs above FL040 in
passing showers through the TAF period. Some lower level scattered
clouds around 1500 ft but not expecting CIG development.
Thunderstorm chances increase to around 30% after 18z Mon and last
through Mon evening. Any thunderstorms will be scattered and may
not impact the terminal at all but possibly approaches. -Muessle
&&

.MARINE...Winds to increase late this afternoon and
tonight as a low develops near Vancouver Island, then drops toward
the north Oregon coastal waters late tonight into Tuesday. Some
models indicate Small Craft Advisories reaching PZZ271,272, 251
and 252 tonight into Tuesday morning, and possibly affecting
PZZ273, 253 during the day Tuesday. May actually see a bit of a
coastal jet Tuesday. Current NAM, GFS, & ECMWF in better agreement
with this feature for Tuesday than for this afternoon and tonight.

Expect a choppy wind driven seas to build with southerly waves of
4 to 6 ft around 5 to 6 sec. This coupled with an approaching
northwesterly swell of 6 ft at 9 to 10 sec will create confused
seas until the winds diminish Tuesday evening. Seas will subside
behind the system to around 4 to 6 ft and persist through the
middle of the week.

Winds and seas ease later Tuesday into Wednesday, then high
pressure rebuilds toward the coastal waters Thursday increasing
the pressure gradient against the thermal trough on the south
Oregon coast. Will see northerly winds increase, likely to bring
Small Craft Winds and choppy seas late Wednesday into Thursday.
Current ensemble guidance for Thursday shows greater than 75
percent of the ensemble members have gusts greater than 21 kts,
but no members shows gusts greater than 34 kt.

Winds weaken later in the week as a weak low pressure system
approaches the waters.

The new moon will produce stronger tidal currents during the
early morning ebb cycle through Tuesday morning. Those moving in
and out of harbors and crossing coastal bars should use caution.
                                                            -MH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 11 PM this evening to 8 AM PDT Tuesday
     for Coastal waters from Cape Shoalwater WA to Cape Falcon
     OR out 60 NM.

     Small Craft Advisory from 2 AM to 8 AM PDT Tuesday for Coastal
     waters from Cape Falcon OR to Cape Foulweather OR out 60 NM.

&&

www.weather.gov/portland

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