textproduct: Portland

SYNOPSIS

High pressure across the region maintains dry conditions into the middle of next week. With increasing easterly flow tonight into Friday, gusty winds likely develop through the Cascade gaps, and along the Columbia River Gorge into portions of the Portland metro. Expect overnight/morning low stratus and fog to remain limited to valley areas more protected from east winds like the southern Willamette Valley. Offshore winds decrease a touch the second half of the weekend with mostly clear skies, chilly mornings, and mild afternoons, albeit with fog increasing in coverage.

.DISCUSSION...Now through Wednesday night

An unusually strong area of high pressure will remain in place through at least Jan. 20th-21st continuing our prolonged stretch of dry weather. Satellite imagery this afternoon shows the last remnants of low stratus and fog slowly getting scoured out in the southerly Willamette Valley around Eugene with clear skies prevailing elsewhere. This is thanks in-part to the upper- level ridge having retrograded off the west coast which is inducing an easterly gradient across the Cascades and mixing the airmass near the surface. So going forward expect mainly clear skies across all of northwest Oregon and southwest Washington to continue through at least Tuesday of next week. The main exception will be the central and southern (mainly the latter) Willamette Valley, which will continue to see chances for fog and low stratus each night. Temperatures each day will be chilly during the morning with temperatures near or below freezing, and mild during the afternoon. However, temperatures will remain much warmer above 1000 ft, both overnight and during the day.

However while the easterly flow will lead to better clearing, it comes as a bit of a double-edged sword for those near the Cascade Gaps and Columbia River Gorge where east winds are already quickly increasing. Both deterministic and ensemble guidance are confident the surface pressure gradient between KPDX and KDLS (which can be used as a proxy for gap wind strength) peaks around 8-10mb on Friday which would likely produce winds between 40-55 mph in the Troutdale area and other portions of the far eastern Portland Metro. Anticipate even stronger gusts (55-65mph+) for our climatologically windiest spots like Corbett, Crown Point, Three Corner Rock, etc. Given the high confidence in wind gusts exceeding 45mph in the Troutdale/Gresham area, primarily along I-84, have opted to issue a Wind Advisory into Saturday afternoon for this forecast zone. Gusts around 30-45mph likely extend westward of I-205 as well. It's worth noting given a slight northerly component to the winds once they exit the Gorge opted to not add zone WA207 (Camas/Washougal/Green Mtn) to the Wind Advisory although localized gust around 45mph are certainly possible, especially in eastern Washougal. Models depict the gradient weakening later Saturday through Sunday which should reign back the extent of the strong winds, but it won't be until the middle to end of next week when it truly dissipates - we'll need the eventual pattern change for that.

Speaking of pattern changes, chances for rain finally return to the area late Wednesday into next Thursday, Jan. 21-22, with PoPs peaking between 25-50%. This is when most model guidance is showing a transition back to onshore flow and more seasonable temperatures. However, PoPs are far from 100% as there is still a handful of ensemble guidance showing very little to no precipitation at all through Jan. 23-26. In other words, it is not clear exactly when this extended dry stretch will end. Odds lean on it ending by Jan. 23rd, but only slightly. -99/23

AVIATION

High pressure over the area combined with with a thermal trough over the coast will drive a majority of the weather for the next 24 hours. Overall, a strong inversion is in place with warm and dry air aloft, and cooler and more moist air at the surface. Today has been a bit different than the previous two days in that the fog and LIFR stratus from the morning has eroded from most locations with the last hold out being within the southern Willamette Valley around KEUG and K77S. These LIFR conditions though are dissipating as seen in satellite. Overnight, stratus will fill back in throughout the areas with less wind. There is around a 50% chance of LIFR vis to reform around KEUG as early as 04Z Fri which is a lower probability from previous days. The kicker in whether it develops or not will be wind. Overall though, fog and low stratus is not as high of a threat as in previous nights.

Winds are beginning to pick up with easterly winds observed within the coastal gaps, and the Columbia River Gorge. Gusts up to 30 kt have been reported around KTTD this afternoon, and gusts closer to 20 kt along the coast. Through the Columbia River Gorge there is a 40-50% chance of gusts higher than 45 kt including KTTD. Peak wind speeds would occur after 04Z Fri, and peak Friday afternoon. Inland sites are northerly with wind speeds around 10 kt. Some gustier conditions are present within the central Willamette Valley around KUAO. Easterly winds are expected to persist through the next 24 hours, while the northerly winds will ease after 10Z Fri before amplifying once again in the afternoon. If the winds become slack overnight then there is a higher probability of IFR conditions.

KPDX AND APPROACHES...The main weather threat is wind today as the lower atmosphere has dried due to the influence of offshore flow. Winds at the terminal have yet to respond to the easterly winds, but approaches to the east are seeing gusts upwards of 30 kt. At around 3500 ft MSL, gusts up to 45 kt have been observed, with gusts around 35 kt at 600 ft MSL. Therefore, LLWS is present this afternoon. Through the late afternoon and evening those winds will mix down to the surface where gusts of 25 kt or greater are forecast. There is around a 20% chance of gusts of 40 kt or more, but confidence is incredibly low as we often struggle to see those types of speeds at KPDX. There may be a brief lull in wind through the morning before amplifying once again after 18Z Fri with the addition of daytime heating. -27

MARINE

High pressure over the area will maintain more settled conditions through the next week. The thermal through has built over the area and thus northerly winds have initiated. Speeds have been variable with gusts up to 25 kt observed earlier in the day, and now falling well below 20 kt. However, overnight the pressure gradient will increase and thus more persistent small craft advisory speeds are forecast. Due to the location of the ridge, the inner waters are less likely to experience gusts greater than 25 kt.

On Friday, winds will become easterly as the thermal trough moves slightly offshore. Expect winds around 10 to 20 kt with gusts up to 25 kt during that time, strongest downwind of gaps in the coastal terrain. Conditions have trended more marginal in the way of speeds with this latest forecast so confidence in the extent of these wind speeds is low. Winds remain easterly over the weekend, albeit much weaker in strength.

Seas should stay below 10 ft this afternoon through early next week, bringing a very quiet sea state for this time of year. -27/23

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory from 7 PM Saturday to 10 AM PST Tuesday for ORZ109-114>118. Wind Advisory from 9 PM this evening to 3 PM PST Saturday for ORZ112. WA...None. PZ...Small Craft Advisory from 7 PM this evening to 4 PM PST Friday for PZZ251. Small Craft Advisory until 4 AM PST Friday for PZZ252-253-272- 273. Small Craft Advisory until 4 PM PST Friday for PZZ271.