textproduct: Portland

SYNOPSIS

Dry conditions will persist through Friday aside from a chance of showers over the Oregon Cascades Wednesday through Friday. High pressure will maintain well above normal temperatures for this time of year through Monday. Onshore flow returns Tuesday and Wednesday, bringing relatively cooler temperatures. Trending a bit warmer again Thursday into Friday with light offshore flow developing. Shower chances return to most of NW OR and SW WA on Saturday.

DISCUSSION...Sunday through Saturday night

Satellite imagery early Sunday afternoon shows a thin layer of high clouds moving over NW OR and SW WA as high pressure and low-level offshore flow remains in place. Easterly winds remain a bit breezy through the western Columbia River Gorge into the Troutdale, Washougal, and Camas areas with gusts up to 15-25 mph. Expect easterly winds to ease late in the day as low-level offshore flow weakens. Despite the high clouds in place today, temperatures are still on track to warm well into the mid to upper 70s across the interior lowlands and upper 60s to lower 70s at the coast. As of noon, many locations have already passed the 70 degrees mark. Most locations within the Willamette Valley and greater Portland/Vancouver metro have anywhere from a 10-30% chance of reaching 80 degrees or warmer this afternoon. Although high pressure weakens a bit tomorrow with 850 mb temperatures cooling slightly, high clouds will be dissipating. Given a full day of sun, Monday is on track to be just as warm as Sunday with similar probabilities of reaching 80 degrees. The only exception is at the coast where the return of onshore flow will bring relatively cooler temperatures compared to Sunday with highs near 60 degrees. A few record high temperatures may be tied or broken both today and Monday. It is not recommended to cool off in local lakes and rivers as water temperatures are currently frigid enough to result in cold water shock, which can become life threatening.

A weak and dry cold front associated with a low pressure system moving east through British Columbia is slated to move over the region late Monday into Tuesday. Ahead of this, models are showing upper level flow shifts westerly and a very subtle upper shortwave associated with the front could cause enough orographic lift for a 15-20% of isolated showers over the southern Lane County Cascades Monday afternoon. However, better chances will be east of the Cascade crest. Temperatures then trend cooler Tuesday and Wednesday behind the front with dry northwest flow aloft and relatively stronger low-level onshore flow. The NBM depicts low model spread for temperatures both days, suggesting high temperatures will likely wind up in the low to mid 60s, except 55-60 degrees at the coast.

Mostly dry weather will persist through Friday except for over the Oregon Cascades south of Clackamas County. Ensemble guidance indicates the potential for wrap-around moisture from the south associated with a low pressure system spinning off of the northern California coast. This moisture could move far enough north to produce shower chances each day, mainly in the late morning through early evening hours. Potential frost chances are now mainly limited to Wednesday morning. Probabilities for low temperatures of 36 degrees or colder range between 10-25% over the interior lowlands and inland coastal communities, except 1-5% in the Portland metro to the east of the West Hills, and 50-80% in the Upper Hood River Valley from Odell to Parkdale. Anyone with sensitive outdoor vegetation that is susceptible to frost damage should stay up- to-date on the temperature forecast over the coming days and monitor for potential Frost Advisories.

Model guidance continues the brief warming trend Thursday through Friday due to potential shortwave ridging on the northeast periphery of the aforementioned upper low. As such, the deterministic NBM is suggesting highs in the upper 60s to low 70s over the interior lowlands both days. NBM indicates a 40-60% chance of high temperatures of 70 degrees or higher each day, except for a 65-80% chance for the Portland metro area. This weak ridging is the main reason frost chances are decreasing for Thursday and Friday mornings as it will also cause slightly warmer overnight temperatures, generally in the 40s.

By next Saturday, the 500 mb cluster analysis of the GEFS, EPS, and GEPS indicates two potential solutions in the pattern with about a 50/50 chance for each. One solution keeps the pattern similar to Thursday and Friday except with a weak trough approaching the West Coast from the Pacific. This solution keeps the area mostly dry with shower chances in the Cascades, though temperatures are more likely to begin cooling towards normal with the approach of a trough. The other solution suggests a deeper trough impacting the PacNW and bringing increased rain chances across the entire region sometime on Saturday. This uncertainty has resulted in a 20-35% chance of rain showers west of the Cascades and a 35-60% chance over the Cascades. Even if showers do return Saturday, guidance suggests rain amounts will be light with an 80-90% chance total rain amounts will be less than 0.25 inch west of the Cascades and a 50-70% chance over the Cascades. -03/23

AVIATION

Widespread VFR flight conditions continue across the majority of the airspace through Sunday night with scattered to broken high clouds above FL250. Breezy easterly winds continue at KTTD with gusts up to 25 kt, but should weaken around 03Z-06Z Monday. Otherwise, winds remain variable less than 10 kt.

A southerly wind reversal along the coast will bring a surge of clouds to coastal locations after 06Z Monday. Expect IFR/LIFR conditions along the coast starting at KONP around 07Z Monday and at KAST around 10Z Monday. Given the persistent southerly winds expected, minimal if any improvement along the coast is expected from through 18Z Monday.

KPDX AND APPROACHES...Expect VFR flight conditions to continue with scattered to broken high clouds above FL250. Variable winds less than 10 kt. /42

MARINE

The high pressure system, which has been driving the current conditions across all waters will starts to weaken this evening. As it weakens, this will result in a southerly wind shift across all waters as well as a surge of marine stratus along the coast. This pattern change is expected to be relatively short lived as the aforementioned the upper level high becomes displaced by a pair of lows, that are moving south and east through the fist part of the week. This will bring a return of northerly winds by Monday afternoon and these northerly winds are expected to persist through the remainder of the week. Seas throughout the majority of the week will remain around 4 to 7 feet with a persistent westerly swell.

Seas are expected to build slightly towards 7 to 9 ft towards the very end of the week in response to a fresh northwesterly swell that will impact the waters. This could (30-50% chance) result in another round of marginal Small Craft Advisory conditions. /42

PQR WATCHES/WARNINGS/ADVISORIES

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