textproduct: Portland

SYNOPSIS

Cooler but still above-normal temperatures with largely dry weather expected through much of the workweek beneath broad and persistent upper-level ridging. Chances for rain increase Friday into Saturday as the ridge deamplifies, but uncertainty in timing and coverage of precipitation continues.

DISCUSSION...Now through Sunday

Broad upper-level ridging spanning the Northeastern Pacific and western CONUS is favored to remain in place with a slight eastward tendency through the next week. An upper cutoff low over California which supported offshore flow resulting in record high temperatures on Sunday will exit eastward more quickly, favoring a return to more seasonable northerly to northwesterly flow aloft. Despite upper heights rising in the wake of the departing low, temperatures will actually trend cooler while also remaining seasonably warm thanks to this pattern change. Through midweek, renewed onshore and southerly flow will maintain fairly steady daytime high temperatures in the 60s along the coast and in high terrain and in the 70s to near 80 degrees within inland valleys, some 10 or so degrees above normal. This southerly surge will see marine overcast and periods of mist and drizzle along the coast and clouds reaching inland through coastal gaps each night, before retreating back toward the coast during the day. As such, largely dry and tranquil weather is expected for most.

By the end of the workweek, long-range ensembles depict a shortwave trough transiting the apex of the ridge, bringing cooler temperatures closer to seasonal norms as well as the most widespread chances for rainfall to the region. There is still uncertainty in the timing and distribution of precipitation, but the ensemble consensus favors rain occurring some time between Friday morning and Saturday afternoon, with the highest chances to the north and in areas of higher terrain and lower chances to the south and for valley locales. At this time, potential rainfall amounts look to be light, generally 0.1 inch or less.

Beyond this late-week system, ridging aloft will most likely rebuild, favoring a return to dry and warmer-than-normal conditions. There remains uncertainty, however, in the position, orientation, and amplitude of ridging, which could affect the sensible weather on the ground. -36

AVIATION

A southerly marine surge is ongoing with the latest satellite imagery depicting low stratus along the Oregon and southwest Washington coast, as well as reaching inland through gaps in the coastal terrain, particularly toward the southern Willamette Valley. This will result in persistent IFR/LIFR cigs for coastal terminals through much of the period. As diurnal mixing increases, cigs may lift slightly, with increasing chances of reaching MVFR levels to the north along the coast toward KAST, while to the south near KONP, the improvement will more likely be from LIFR to IFR. High-end IFR to MVFR vis of 2-5 SM is also currently being observed at coastal terminals south of KTMK, these restricted vis will tend to improve after 15-16z Mon. Meanwhile, gusty south-southwesterly winds of 8-12 kt with gusts to 20 kt are expected to continue through at least 21-24z Mon. Reinvigorated marine stratus late in the period, after 00-03z Tue, will yield a downward trend in vis/cigs to IFR/LIFR late in the period while winds ease to around 5 kt.

Inland, coverage of low stratus remains relatively low, with MVFR cigs limited to the southern Willamette Valley as of 12z Mon, however some additional inland expansion is expected through 15z Mon. While VFR conditions are most likely, chances for MVFR cigs peak around 15z Mon, reaching 20-40% at terminals along the Willamette and Lower Columbia Valleys. Low clouds will tend to mix out through the morning, largely clearing by 20-22z Mon. Winds out of the south to southeast at around 5 kt at Portland-area terminals and 5-10 kt along the Willamette Valley will veer out of the south to southwest at 5-10 kt through this afternoon, before easing below 5 kt out of the south to southeast after 00-06z Tue. Renewed marine stratus will again be most likely to yield MVFR cigs in the southern Willamette Valley including at KEUG after 06z Tue.

KPDX AND APPROACHES...VFR flying conditions favored to continue through the period. Marine stratus will continue to expand inland through 14-15z Mon, resulting in 20-40% chances for MVFR cigs this morning before tending to decrease in coverage as mixing increases. Largely clear skies expected by 20-22z Mon as light southeast flow veers out of the south at 5-10 kt this afternoon. Marine stratus may return late, after 06-09z Tue, as winds ease back out of the southeast tonight. -36

MARINE

A southerly surge is underway this morning, bringing gusty south winds up to 20 kt, marine overcast, and patchy mist and drizzle to the coastal waters, especially within 20-30 NM of shore. A diurnal tendency will persist through Wednesday, with increasing clouds, wind, and chances for drizzle or light rain at night before clouds recede and winds ease during the day. A lessening northwest swell will see seas fall from 6-8 ft today to 4-5 ft by Wednesday.

Winds look to turn northerly Wednesday through Thursday night with persistent seas of 5-7 ft. A system moving onshore Friday into Saturday will favor a return of southerly flow and increased chances for rain showers, before high pressure rebuilds later this weekend and northerly flow strengthens. Chances increase late Saturday into Sunday for northerly gusts of 20 kt, conditions which may be hazardous to small craft. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.