textproduct: Portland

UPDATE

Dense Fog Advisories have been issued along the north and central Oregon coast and south Washington coast until 10 AM today as regional observations and available cameras depict widespread visibility of 1/4 mile or less. Morning commuters should expect to encounter rapidly changing and sharply reduced roadway visibility, and should take actions to increase their visibility to others.

SYNOPSIS

Unseasonably warm and dry weather with weak offshore flow will persist through Thursday, before a trough brings a return to cooler temperatures with chances for light rain Friday into Saturday. Cool nights will bring chances for morning frost in rural valleys away from urban cores this weekend into early next week.

SHORT TERM...Now through Saturday Night

Ridging aloft will begin to weaken today ahead of an upper- level trough moving into the region from the Gulf of Alaska Friday into the weekend. Today will see conditions similar to recent days, namely pockets of morning fog along the coast and within sheltered valleys giving way to a mostly sunny afternoon. The offshore- oriented pressure gradient across the Cascades has already begun to weaken, with onshore flow expected to strengthen beginning tonight. Ahead of this pattern shift, another unseasonably warm day is expected inland with high temperatures some 10-15 degrees above normal in the upper 70s to mid 80s, while coastal areas maintain seasonable conditions. Despite the abnormal warmth inland, daily record high temperatures appear to be out of reach, with less than a 5% chance of tying existing records at climate sites across the region.

Cloud cover will begin to increase late this evening ahead of an an approaching upper-level trough which will move overhead Friday into Saturday. A leading upper shortwave and its associated weak surface cold front will move into the region on Friday. Rain likely begins Friday morning along the coast before reaching inland to the I-5 corridor by the afternoon to early evening. Rain accumulations will be favored along the north Oregon and south Washington coasts as well as the Cascades and their western slopes, while areas along the central Oregon coast and within the Willamette Valley are more likely to stay drier. Mean forecast rainfall totals during this period are 0.1-0.25 inches north of Lincoln City and in the Cascades and foothills north of US-20, with 0.1 inches or less elsewhere. As the trough moves inland, a second shortwave impulse moving south on its western flank on Saturday will see the trough elongate and eventually pinch off into a close upper low which will settle south over California by early next week. This feature will produce a second chance for light rain from Friday night into Saturday morning, with more widespread potential for up to 0.1 inches of additional rain. All told, the chances for 0.25 inches of rain in 48 hours reach 40-60% in the wettest areas from Tillamook County north on the coast and north of OR-22 in the Cascades and foothills. Elsewhere, those chances are 15-25% in the Lower Columbia and Cowlitz Valleys, and 5-15% within the Willamette Valley and along the coast south of Pacific City.

As the initial front moves inland Friday through Friday night, peak gusts out of the west in the Cascades, central Columbia Gorge, and Hood River Valley could reach 35-45 mph. Behind the boundary as the core of cooler air aloft moves overhead, temperatures will quickly fall back toward seasonal norms in the upper 50s and 60s Friday and Saturday. Overnight lows will similarly trend cooler, especially Saturday night. On Friday night, there is high confidence that higher elevation portions of the Upper Hood River Valley near Dee and Parkdale will see temperatures in the mid 30s and an elevated potential for frost. By Saturday night, the chances for a true freeze in the Upper Hood River Valley reach 35-65% in those same areas. There are additionally greater than 50% chances for temperatures in the mid 30s and frost on Saturday night in the Cascade foothills and north Oregon Coast Range Valleys, and 25-50% chances in the southern Willamette Valley and higher elevation portions of Clark and Cowlitz County lowlands in southwestern Washington. Frost Advisories and/or Freeze Watches may therefore be needed, and those with agricultural interests should continue to monitor the forecast. -36

LONG TERM...Sunday through Wednesday

Sunday into early next week, the aforementioned upper low separates from the mean mid- latitude flow and drifts south over California. Locally, transient upper-level ridging will replace the departing trough, leading to a drying and modest warming trend. Sunday morning will feature a near-repeat of Saturday morning with widespread chances for frost in sheltered valleys away from urban areas and chances for a freeze within the Upper Hood River Valley. High temperatures then trend a few degrees warmer into the mid to upper 60s across the region by Monday afternoon. There is increasing consensus among long-range ensemble members in additional upper-level troughing moving into the region Tuesday into Wednesday. At this point, low chances for rain are assessed during this period until confidence in the evolution of this feature increases. -36

AVIATION...Through 18Z Friday

VFR inland and a mix of VFR to LIFR along the coast due to marine stratus. Expect deteriorated conditions along the coast to mix out and improve to VFR by 19-20Z Thu. The only exception is KONP, which may only improve to MVFR through the afternoon. While VFR conditions are expected to continue at most inland terminals through most of the period, coastal terminal conditions will deteriorate to LIFR/IFR conditions around 01-04Z Fri, and possibly at KEUG around 06-12Z Fri. Guidance suggests around a 20-50% chance of LIFR conditions for these terminals, with the highest chances at KONP and lowest at KEUG. Near the end of the TAF period, rain will enter the region, bringing rain and possible MVFR conditions (30-50% chances) to inland terminals by 16-18Z Fri.

North to northwest winds will rise to 10-15 kt with gusts near 20 kt along the coast, and to 5-10 kt inland, before easing below 5 kt this evening.

KPDX AND APPROACHES...VFR with high cloud cover through most of the period. Diurnal northwest winds will rise to around 4 to 6 kt this afternoon, then ease this evening. Rain arrives at the terminal around 16Z Fri, bringing a 25-35% chance of MVFR CIGs after 11Z Fri. ~12

MARINE

High pressure at the surface will continue to linger over the northeastern Pacific through today facilitating peak afternoon/evening wind gusts of 20-25 kt. As a result, Small Craft Advisories remain in effect from 2 PM this afternoon until 11 PM this evening for the waters south of Cape Falcon. During this time period, anticipate largely wind-driven seas of 6-9 ft with an underlying west-northwest swell. Weakening high pressure aloft will continue to support areas of dense fog in the morning hours as cloud bases are compressed down to the surface today.

More active marine conditions are expected from Friday into the weekend as troughing over the Gulf of Alaska pushing southward generates a reinforcing northwest swell in addition to the arrival of a frontal boundary. Winds out of the northwest associated with a cold front will increase to around 20 kt on Friday, especially north of Cape Falcon, while seas peak at 8-12 ft Friday night into Saturday. By Sunday, winds ease and veer out of the north to north-northeast while seas fall back to 5-7 ft through early next week. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from midnight Friday night to 9 AM PDT Saturday for ORZ121.

WA...None. PZ...Dense Fog Advisory until noon PDT today for PZZ210-251>253.

Small Craft Advisory from 2 PM this afternoon to 11 PM PDT this evening for PZZ252-253-272-273.