textproduct: Portland

SYNOPSIS

A series of weather systems are expected to bring widespread wet conditions to much the region on Tuesday through Thursday with rising snow-levels helping to mitigate any significant travel concerns over the Oregon Cascades passes leading into Thanksgiving. That said, guidance continues to show a trend towards a cooler and somewhat drier weather pattern late weekend into early next week. Due to high model uncertainty beyond Friday, the exact impact of this pattern change is rather nebulous, however, the potential for our coldest temperatures of the season are likely within reach by Monday.

LONG TERM...Friday through Monday

Late week into the weekend and beyond model uncertainty increases significantly as both deterministic and ensemble systems struggle to resolve the placement of the upper-level longwave pattern. Guidance is split as to whether another shortwave trough off the coast of British Columbia dives southward into the Pacific Northwest for Saturday, or if a high pressure ridge over the eastern Pacific weakens and shunts this feature well to our east. At the very least all models show the development of broad northerly flow over the weekend which favors a cooling trend by Sunday and Monday. Exploring the Ensemble cluster analysis for Monday in particular reveals roughly 35-40% of ensemble members between the EPS, GEFS, and GEPS favor a scenario in which a trough digs southward just east of the Cascades allowing for a colder airmass to seep into the region through the Columbia Gorge and Cascade Gaps punching temperatures well below normal (overnight lows in the mid 20s to near 30 across the lowlands). On the flip side, the other 60-65% of ensemble members keep the amplifying trough near the rockies with a more predominate ridge feature overhead resulting in less of an easterly wind influence. In any case the NBM probability for subfreezing low temperatures across the lowlands is decent (generally 45-75%) by Monday morning which'll give us a fighting shot for the lowest temperatures of the season thus far. For reference our lowest overnight minimum temperatures from Oct through Nov are as follows: Portland 36F, Salem 32F, Hillsboro 32F, and Eugene 30F. All in all, the overall model spread is large during this period with any impacts beyond the decrease in temperatures tough to nail down - forecast confidence is low Friday onward. Definitely keep an eye on the forecast as we go through this week, especially if you have holiday travel plans over the weekend. -99

AVIATION

Satellite and surface weather observations from early to mid afternoon show cigs and visibilities have improved to MVFR/VFR thresholds at all terminals. Still expecting areas of fog and low clouds to redevelop over the Willamette Valley this evening into tonight. By 08-09z Tuesday, probabilities for surface visibilities below 1 SM and/or LIFR/IFR cigs increase to 45-65% for all inland terminals, except 10% at KTTD due to the development of light east winds from the Columbia River Gorge. Meanwhile, the coast should remain VFR due to the development of downsloping offshore flow at the coast. Fog and LIFR/IFR cigs will likely begin gradually lifting towards 13-16z Tuesday as widespread stratiform rain begins pushing into the region from the west. Once rain begins Tuesday morning, expect rain to persist through the day at all terminals with cigs trending towards a mix of MVFR/VFR thresholds, except IFR/MVFR at the coast and in southwest Washington.

KPDX AND APPROACHES...Expect mainly VFR flight conditions to continue through approximately 08z Tuesday. Fog and/or low stratus with LIFR to IFR cigs is then expected to develop thereafter (65% chance). By 14-15z Tuesday, fog will likely lift with cigs gradually trending towards MVFR or even low-end VFR thresholds as persistent rain develops. Once rain begins Tuesday morning, expect rain to persist through the day. Cigs and surface visibilities trend lower again around 00z Wednesday onward with a 60% chance for cigs below 2000-3000 ft. -23

MARINE

Buoy observations from early Monday morning showed seas ranging between 7 to 8 ft with a dominant wave period around 13 to 14 seconds. Winds were out of the south to southwest at 5 to 15 kt across the coastal waters, but will become more westerly while weakening later Monday afternoon into the early evening. By late Monday evening, light offshore develops, bringing a transition to easterly winds over the waters. Offshore flow will be short-lived, as winds will veer to the south early to mid Tuesday morning ahead of the next frontal passage.

The aforementioned front will move over the waters Tuesday afternoon through early Wednesday morning, bringing southerly wind gusts up to 30 kt with a 15-30% chance for gale force wind gusts up to 35 kt over the northern and central waters, except around a 50% chance over northern outer waters beyond 20-30 NM offshore. Not expecting gale force wind gusts to be frequent enough or widespread enough to justify the issuance of a Gale Warning. Therefore, a Small Craft Advisory has been issued instead.

A stronger frontal system arrives Wednesday night into Thursday, bringing southerly wind gusts up to at least 30 kt with a 45-55% chance for gale force wind gusts over 34 kt for all marine zones. Seas should peak near at least 14-15 ft on Thursday. There is a 10% chance seas will peak around 16-19 ft, with a 1-5% chance seas will reach 20-22 ft. This is due to uncertainty regarding the exact strength and track of a closed surface low and the frontal system accompanying the low. Regardless of the outcome for this system, it appears winds and seas will lower significantly on Friday, especially late in the day. Winds and seas over the upcoming weekend are currently forecast to stay below small craft advisory thresholds with a period of offshore flow. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 10 AM Tuesday to 1 AM PST Wednesday for PZZ210. Small Craft Advisory from 10 AM Tuesday to 10 AM PST Wednesday for PZZ251-252-273. Small Craft Advisory from 4 PM Tuesday to 10 AM PST Wednesday for PZZ253. Small Craft Advisory from 4 AM Tuesday to 10 AM PST Wednesday for PZZ271-272.