textproduct: Portland

SYNOPSIS

Widespread rain and locally breezy to windy conditions continue this afternoon before we transition to showers for Wednesday. Mountain snowfall returns to the passes as well before all lingering precipitation tapers off early Thursday morning. High confidence in the return of dry weather accompanied by a warming trend late week into the weekend. Chances are increasing for another weather system late weekend into early next week, although the exact details of this event remain uncertain at this time.

DISCUSSION...Now through Monday

Current satellite and radar observations this afternoon show a cold-frontal boundary swinging through the region with widespread rainfall ongoing, although this will begin to change headed into the evening hours. Winds, which recently peaked along the coast, have been breezy to locally windy (gusts generally 40-55mph) with a few of the most exposed surface observations like Cape Disappointment and Clatsop Spit having seen gusts 55-65mph range. However, inland winds have been much lighter comparatively and likely remain that way - gusts continue in the 20-35 mph range for a few more hours before ramping down this evening. Overall this frontal feature remains rather progressive, quickly pushing to our east this evening followed by a transition to a showery, cooler, and slightly unsettled post-frontal airmass overnight.

For Wednesday our attention turns to a secondary upper-level shortwave feature expected to push into the Pacific Northwest during the midday and afternoon hours helping to further, albeit temporarily, enhance shower activity. Due to daytime heating and the colder airmass aloft, deterministic model soundings do indicate some weak instability (100-250j/kg MUCAPE) during the afternoon hours which if fully utilized could produce a weak thunderstorm with infrequent lightning, small hail, and locally gusty winds the main impacts. All in all, chances remain rather low for any given location (5-15%) to experience one of these weak "one-hit-wonders". Once we approach sunset and daytime heating wanes, any threat for this activity comes to an end.

For all the mountain snowfall enjoyers out there, snow levels finally drop below the Cascade passes tonight into early Wednesday morning once we've fully transitioned to the aforementioned post-frontal airmass. Unfortunately, the sad state of our snowpack won't receive much help as the showery nature of the precipitation from this point onward will limit our snowfall accumulation potential. The latest NBM probabilities of at least 6 inches of snow over the Cascades (both OR and SW WA) remains less than 5-15%, except for the highest peaks. Snow levels likely lower to 1500-2000 ft for a brief period Wednesday night into early Thursday morning as showers are ending, with a 5-10% chance of snow levels falling near valley floor in the southwest Washington lowlands. However, precipitation probabilities along with amounts are so low by this point that even if some wet snow does mix in, it would not accumulate or lead to any impacts, especially as road surfaces would be too warm.

On Thursday the shortwave moves east of the area with upper level flow becoming more zonal behind it. Generally zonal flow with slight ridging continues Friday into Saturday, bringing dry conditions with clearing skies. Expect a warming trend with daytime temperatures in the mid to upper 50s on Thursday, low to mid 60s Friday, and mid to upper 60s on Saturday/Sunday for the interior lowlands. Over the weekend hitting and/or exceeding 70 degrees remains within the realm of possibility but probabilities have decreased a few percentage points compared to yesterday. The latest NBM indicates a 20-50% chance for high temperatures to meet or exceed 70 degrees across the Portland metro area and Willamette Valley both Saturday and Sunday, with slightly higher probabilities (40-50%) south of Salem near Eugene.

Both deterministic and ensemble guidance still indicate a deep trough moving east over the Pacific and approaching the West Coast later on Sunday into Monday, bringing increasing chances of precipitation. However, there still remains much to be desired regarding the exact timing, amplitude, and thus impacts with this disturbance - model uncertainty remain high Sunday/Monday. This uncertainty continues to facilitate a significant NBM 25th-75th percentile temperature spread, showing highs ranging from the upper 50s to the mid 70s for the interior lowlands Sunday afternoon depending on how quickly the next system arrives. Should ensemble guidance trend slower with the deep trough offshore, Sunday may end up being the warmest day of the upcoming late week/weekend time period. Hopefully we'll see forecast solutions converge in the coming days. -99/03

AVIATION

The frontal system that brought widespread rain and MVFR conditions has all but exited the area leaving in it's wake moist air with VFR CIGs. Some areas of MVFR conditions remain this evening, and there will be pockets, especially in the southern Willamette Valley where the front hasn't fully exited yet. Will have to monitor for the presence of fog formation tonight with the lingering moisture from the rain. Luckily, conditions behind the front will mix, but it may not be significant enough. Cannot rule out areas of patchy fog in some of the rural areas and in the Cascade valleys with some models showing greater than a 20% chance. A break in the rain will be followed by a short wave trough that will bring around around a 20% chance of rain showers and thunderstorms in the afternoon. Thunderstorm chances are going to be concentrated inland and to the north. There are some components missing in the pattern to include PROB30s for TS in the TAFs, but cannot rule out small hail, breezy outflow winds, and maybe a rumble here or there after 21Z Wed.

KPDX AND APPROACHES...VFR conditions through the next 24 hours with isolated showers after 20Z Wed with a passing system. Around a 15-30% chance of IFR VIS in fog between 10-16Z Wed but if mixing remains those chances will be far lower and may remain in the vicinity of the terminal. 10-20% chance of TS between 18Z Wed and 00Z Thu. -27

MARINE

Southerly gales are expected to ease by 5 PM this evening as a cold front pushes inland. Breezy southwest winds will continue with gusts up to 25 kt across the coastal waters through Wednesday morning. Another low pressure system will push a weak cold front across the waters later Wed morning, followed by more westerly onshore flow easing through Wed night.

As of early Tuesday afternoon, combined seas at around 13 to 14 ft continue to build in response to the strong southerly winds, producing steep and choppy seas with a period of 8 to 9 seconds. Buoy 46089 is reporting seas of around 18 ft at 11 seconds, indicating the arrival of a fresh southwesterly swell. Local guidance suggests seas should build to around 17 ft at buoy 46029 later this afternoon. Seas then are expected to gradually subside tonight through Wednesday, though will likely remain elevated above 9 to 10 ft through Wed afternoon. Will maintain the Hazardous Seas Warning for the northern coastal waters through this evening, followed by Small Craft Advisories through Wednesday.

High pressure then builds over the waters on Thursday. Northerly winds return through Friday as a surface thermal trough strengthens along the coast. Seas generally remain around 7 to 8 ft. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Hazardous Seas Warning until 11 PM PDT this evening for PZZ210- 251-271.

Small Craft Advisory from 11 PM this evening to noon PDT Wednesday for PZZ210.

Small Craft Advisory from 11 PM this evening to 5 PM PDT Wednesday for PZZ251-271.

Small Craft Advisory until 5 PM PDT Wednesday for PZZ252-253-272- 273.