Area Forecast Discussion

SYNOPSIS

Seasonably warm and dry weather this weekend will trend toward more typical early fall weather through the workweek as temperatures cool and a weather system approaches from the Gulf of Alaska. Uncertainty in the position of this system leads to low chances for rain beyond midweek.

SHORT TERM...Now through Tuesday Night

Guidance is in strong agreement that a broad but low-amplitude ridge will steadily shift eastward through the middle of next week while being replaced by an upper-level trough digging southward over the Gulf of Alaska. After peaking on Sunday, upper-level heights and mid-level temperatures will begin to trend downward on Monday, leading to a general cooling trend across the region. At the surface, temperatures will reach some 5-10 degrees above normal inland Sunday afternoon, namely the upper 70s to low 80s, while the coast remains near normal in the 60s. As onshore flow strengthens Monday through Wednesday, inland temperatures will steadily cool be a few degrees each day. Otherwise, longer nights and light winds will favor periods of early morning fog in sheltered valleys, and a typical fall pattern of low clouds pushing inland overnight and clearing through the day.

.LONG TERM...Wednesday through Saturday...Model consensus decreases beyond the middle of the week as uncertainty in the position of an upper-level trough dropping southward from the Gulf of Alaska increases. This uncertainty is reflected in the increased spread in possible high temperatures as well as low chances of rain from late Wednesday through Friday, but in general, a trough located closer to the coast will favor rain and cooler temperatures while a more distant trough will instead favor drier weather and near-normal temperatures.

Mean forecast highs fall to the upper 60s inland and low 60s along the coast by Friday, some 5 degrees below normal. However the 80% confidence interval (the difference between the 10th and 90th percentiles of the forecast distribution) nearly doubles in width from 2 degrees F on the coast and 4-6 degrees F inland on Wednesday to 4 degrees F on the coast and 6-10 degrees F inland on Friday. In general, Rainfall chances will also increase as the trough approaches. There is good agreement in the location of an initial passing shortwave on Wednesday, with rainfall chances of only 10-15% along the south Washington and far north Oregon coasts with higher chances farther north on the Olympic Peninsula. Thursday into Friday, about one quarter of ensemble members favor the coolest and wettest scenario, resulting in fairly widespread 15-25% chances for rain to end the workweek. -36

AVIATION...Through 18Z Sunday

Satellite imagery and terminal observations as of late Saturday morning depicts widespread stratus across northwest Oregon, bringing IFR/MVFR CIGs to most terminals. Some of this stratus is beginning to dissipate along the Cascade foothills. Based on these surface observations and PIREPs, expect stratus to completely dissipate across the Willamette Valley and coast by 20-21z Sat as daytime heating progresses. VFR conditions with mostly clear skies prevail for inland terminals through late this evening, while coastal terminals keep brief clearing this afternoon. There is high confidence (>80% chance) that IFR/MVFR marine stratus returns to the coast after 03-05z Sun and prevails through the morning. Between 12-18z Sun, there is also a 30-40% chance at any given hour for MVFR stratus returning to Valley terminals. Northwesterly winds generally under 10 kt today.

KPDX AND APPROACHES...MVFR stratus through 20z Sat. Afterwards, VFR conditions prevail through late tonight. 20-40% chance for MVFR stratus returning 12-18z Sun. Northwesterly winds around 5 kt or less. -10

MARINE

High pressure over the waters will maintain northerly winds through this weekend. Northerly winds strengthen this afternoon and evening with gusts up to 15 kt, strongest over the waters south of Cape Falcon. Northerly winds peak in strength Sunday afternoon and evening when there is a 50-90% chance for maximum wind gusts of 21 kt or slightly stronger across the waters, with the highest chances across the inner waters out to 10 NM offshore. Given wind gusts of this magnitude are not expected to be frequent or widespread, Small Craft Advisories are not anticipated at this time. Significant wave heights will remain around 3 to 5 ft through the weekend, mainly driven by a persistent northwesterly swell with a dominant wave period between 7 to 8 seconds.

Northerly winds are expected to linger Monday through Wednesday, with wind gusts generally staying under 20 kt. A long period west-northwesterly swell is set to arrive either Monday afternoon or evening, pushing seas up by one to two feet. However, seas will not be particularly steep as the dominant wave period with this swell will be around 11-13 seconds. -10/23

PQR WATCHES / WARNINGS / ADVISORIES

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