textproduct: Portland

SYNOPSIS

Dry weather under higher pressure continues through late Saturday with another round of fog and frost development expected tonight into Saturday morning. A pattern shift early next week will favor a wet week for the upcoming holiday.

DISCUSSION...Friday afternoon through Thursday

Visible satellite imagery as of 2 PM PST Friday shows mostly clear skies across NW Oregon and SW Washington except for much of the central and southern Willamette valley where stratus lingers. Temperature observations under the stratus deck and over the higher terrain remain in the 40s, but elsewhere temperatures have risen into the low to mid 50s. Minimum temperatures this morning were in the 30s across the board except for 40s along the coast and 20s over the Cascades. Tonight into Saturday is expected to be mostly a rinse and repeat of the 24 hours previous. Model guidance has high confidence in widespread fog and low stratus forming in the inland valleys tonight, remaining until the late morning to early afternoon hours on Saturday. Expect cool overnight temperatures with frosty conditions warming into the upper 40s to low 50s for the high temperature.

A weather system slowly approaches the region Saturday causing precipitation chances to increase along the north Oregon and SW Washington coast through the day. As the frontal system reaches the coast, a widespread band of precipitation will spread inland late Saturday night through Sunday, turning to showers late Sunday and continuing through Monday afternoon. Rain amounts with this system will be on the lower side with 0.15-0.5 inches for the inland valleys, 0.5-1 inch along the coast and Coast Range, and 0.5-1.5 inches over the Cascades. Snow levels will remain above Cascade pass level through Sunday, then they will begin falling Sunday night behind the cold front, falling to 2500-3000 ft by Monday morning. However, as conditions will be showery by this point, limited snow accumulation is expected at pass level, generally less than 2 inches. Winds are not expected to increase much with this system with gusts up to 20-30 mph along the coast Sunday during the day and over the Cascades Sunday night, remaining less than 20 mph for inland valleys.

Temperatures Sunday night into Monday morning are once again expected to be on the cooler side despite overhead clouds due to colder air behind the frontal passage. Monday night into early Tuesday is expected to be dry with another night of colder temperatures in the 30s across the interior lowlands. By midday Tuesday, widespread precipitation chances increase again as ensemble members indicate the first in a series of back to back frontal systems could move through the region. Ensemble guidance indicates another frontal system will follow quickly behind Wednesday into Thursday, bringing widespread rain chances Tuesday through the Thanksgiving holiday. Timing details remain uncertain this far out, but limited impacts are expected with these systems. Probability for 3 inches of rain in the 72 hour period from 4 AM Tuesday to 4 AM Friday are less than 10% for inland lowlands, 10-20% chance south of Highway 20 in Oregon, and 20-45% chance north of Highway 20. Winds are not expected to be much of an impact either with a 20-40% chance of gusts exceeding 30 mph along the coast and 5-20% chance for inland lowlands Wednesday and Thursday. These will be warm Pacific weather systems, so snow levels will rise above pass level. Looking beyond Thursday, ensembles indicate yet another weather system could bring additional rain on Friday into Saturday. Keep an eye on the forecast if you have holiday travel plans! -03

AVIATION

The influence of high pressure aloft continues tonight with fog already in place across much of the Willamette Valley and portions of SW Washington leading to fairly prolific LIFR CIGs/VIS. These conditions will eventually progress into the Portland metro area the latter half of the night into the sunrise hours, primarily at KHIO and KPDX, with the latest 00z high resolution guidance giving those sites a ~70% chance for 0.5 mile VIS or less by 12z Sat. Tougher call at KTTD where much like last night weak east flow through the Gorge may spare them the worst of the impacts; 25-40% chance for IFR/LIFR conditions between ~13-17z. The most challenge aspect of the forecast, however, is midday Saturday through the afternoon in timing the dissipation of the fog. Areas from KUAO southward through the Willamette Valley will maintain LIFR conditions through at least 20-21Z but wouldn't be surprised if much like Friday a lingering low stratus layer fails to completely clear south of KSLE through KEUG. Winds stay light through the forecast period, generally less than 5-8 knots.

KPDX AND APPROACHES..MVFR conditions are in place late this evening but fog is lurking to the south and just to the WNW of the terminal. As the night goes on expected this fog layer to expand into PDX, most likely around ~08-09z Sat. That said, there is a layer of high clouds passing overhead which adds a slight bit more uncertainty to this timing than if it was otherwise clear - these high clouds have the potential to push LIFR CIGs/VIS back to 12z but given how close the fog layer already is to KPDX it won't take much growth to degrade conditions. Expecting conditions to improve around 18z Sat with a return to VFR conditions 19-20z although confidence in that exact timing is only moderate at this time. -99

MARINE

High pressure over the area is maintaining fairly stable conditions with minimal change through the next few days. Very few fluctuations in overall sea state and winds through Monday. Seas currently range from 8-11 ft at 14 seconds, but are unexpectedly easing below small craft advisory criteria for some buoys. However, in generally looking to see seas right around the threshold. Tonight, a fresh swell will surge in causing a brief increase in significant wave heights up to 16 ft at 15 seconds. Because of the short duration, have not issued a Hazardous Seas Warning but cannot rule out a more prolonged period if the background swell too increases. These seas will mainly impact the outer waters. By the evening, seas will return to a height of 10 ft at 14 seconds.

On Sunday a frontal system will make it's way over the waters causing winds to increase from the south. The low will move inland late Sunday into Monday and winds will shift to the northwest. When southerly, gusts up to 25 kt will be observed with a partially heightened wind wave. Seas fall below 10 ft on Monday. -27

BEACH HAZARDS

An energetic westerly swell is expected along the northwestern Oregon and southwestern Washington coasts through Monday. Seas 9-14 ft at 12-16 seconds are forecast through the the weekend. These energetic waves can run significantly farther up a beach than normal, including over rocks and jetties, creating potentially life-threatening conditions in the surf zone when beachgoers can be unexpectedly knocked off their feet and pulled out into cold ocean waters.

A Beach Hazards Statement has therefore been issued through 5 AM Monday. Caution should be used when in or near the water, and beachgoers with children should be especially watchful. Never turn your back on the ocean, and be sure to keep an eye on the waves. Those participating in razor clam digs should additionally exercise caution. -27/42

PQR WATCHES/WARNINGS/ADVISORIES

OR...Dense Fog Advisory until 11 AM PST Saturday for ORZ116>118. WA...None. PZ...Small Craft Advisory until 4 AM PST Monday for PZZ210-251>253- 271>273.