textproduct: Portland

SYNOPSIS

Dry conditions will persist through Friday aside from a chance of showers over the Oregon Cascades Wednesday evening through Friday to the south of Clackamas County. 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 relatively warmer again Thursday into Friday with light offshore flow developing. Shower chances return to northwest OR and southwest WA on Saturday and Sunday.

DISCUSSION...Now through Sunday night

Mostly clear skies and calm winds remain in place over the area Monday morning, aside from an area of low stratus that is beginning to push onto the central Oregon coast from Newport to Florence. Although high pressure is still on track to weaken a bit today with 850 mb temperatures cooling slightly, abundant sunshine will allow temperatures to warm well into the 70s once again this afternoon. The only exception is at the coast where the return of onshore flow will bring relatively cooler temperatures compared to Sunday. Several record high temperatures were either tied or broken on Saturday with observed highs in the mid to upper 70s (records were tied or broken at Portland, Eugene, Salem, Hillsboro, and Vancouver). A few more record highs may be tied or broken on Monday as forecast highs are once again in the mid to upper 70s. 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.

Beginning late Monday into Tuesday, a weak and dry cold front associated with a low pressure system moving east through British Columbia is slated to move over the region. Ahead of this, models are showing upper level flow becoming westerly with a very subtle upper shortwave associated with the front. This weak feature could bring just enough orographic lift for isolated showers over the southern Lane County Cascades Monday afternoon (15-20% chance), mainly along the crest. Higher 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 continues to depict 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. Beyond Wednesday, model ensemble guidance continues to show a brief warming trend Thursday through Friday due to shortwave ridging on the northeast periphery of an upper level low centered off the northern/central California coast. The latest iteration of the NBM has ticked up another degree or so, now suggesting highs between 69-72 degrees over the interior lowlands. There is a 25% chance highs will reach into the mid 70s both days. With the increasing signal for warmer temperatures late in the week, chances for morning frost remain limited mainly 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.

Mostly dry weather will persist through Friday except over the Oregon Cascades south of Clackamas County. Ensemble guidance indicates the potential for wrap-around moisture from the south associated with the aforementioned upper low spinning off of the northern California coast. This moisture could move far enough north to produce shower chances each day Wednesday through Friday, mainly during the afternoon and evening hours. An isolated thunderstorm or two cannot be ruled out either depending on the degree of instability in place, and probabilities for thunderstorms have increased to 10-15% over the Lane County Cascades on Thursday and Friday.

Widespread chances for showers finally return to the area next weekend, however exact timing and rain amounts remain uncertain. By next Saturday afternoon, the 500 mb cluster analysis of the GEFS, EPS, and GEPS now indicates three potential solutions. One solution shows an elongated weak upper low off the central California coast, resulting in southerly flow aloft over the Pacific Northwest (22% chance this solution verifies). This solution would likely bring at least some showers into the area as the southerly flow aloft would allow for increasing moisture and instability, and possibly even isolated thunderstorms (especially over the Cascades). The second solution (28% chance of verifying) shows two distinct closed lows, one off the WA/OR coast and one off the southern CA coast. This solution could also bring showers into the area, as long as the more northern upper low tracks overhead. The third solution (50% chance of verifying) suggests a closed low over the southern CA coast with some degree of weak troughing focused over British Columbia and/or northern WA, resulting in dry westerly flow aloft for northwest OR and southwest WA with near normal or even slightly above normal 500 mb heights. This would be the warmest and driest solution, albeit cooler than what is expected Thursday and Friday. This uncertainty in the overall pattern has resulted in a 20-40% chance of rain showers west of the Cascades and a 40-60% chance over the Cascades. Chances for showers increase by another 5-15% for all locations on Sunday. Even if showers do return on Saturday and/or Sunday, guidance suggests rain amounts will most likely be light. Probabilities for 48-hour rain totals below 0.25" from 5am Saturday through 5am Monday are around 70-75%, except 55-70% at the coast and 40-50% over the Cascades. -23/03

AVIATION

Predominately VFR under mostly clear skies will continue across much of the area as dry and relatively light westerly flow aloft persists through tonight. However, marine stratus along the coast will likely bring IFR to LIFR conditions at times today to coastal terminals. As of 17z, this stratus deck with CIGs around 100-300 ft, along with visibility reductions, has pushed back onshore at KONP. Southwest winds are expected to become westerly early this afternoon, and then turn northwest by late afternoon/early evening. This will likely cause some ebb and flow of the stratus deck near the terminal today. There is higher confidence in predominately IFR to LIFR stratus persisting after 02z Tuesday. Confidence remains lower in the extend of the stratus to the north which could impact KAST today. It is likely that IFR to lower MVFR stratus develops there between 02-08z Tuesday. That said, there is a 20-30% chance low stratus reaches KAST after 20z this afternoon. Meanwhile, inland terminals will remain mostly clear into Tuesday morning with light onshore winds developing this afternoon and evening.

KPDX AND APPROACHES...VFR expected to persist through Tuesday morning with few to scattered high clouds. Light winds expected to become northwest 5-9 kt after 22z. /DH

MARINE

A southerly wind reversal has developed over the central and southern waters early Monday morning. This will be short- lived, as a northerly flow regime will return by Monday afternoon and then persist through the remainder of the week. However, low stratus and dense fog is accompanying this southerly wind reversal over the central and southern waters. Guidance suggests areas of dense fog will continue over the central/southern waters through approximately 8 PM Monday. Therefore, a Dense Fog Advisory has been issued until then to cover this navigation hazard.

Seas should become steep enough for a period of marginal Small Craft Advisory conditions Tuesday through Tuesday night with wind gusts up to 20-25 kt and seas between 6 and 9 ft at 8 to 9 seconds. Seas and winds decrease slightly on Wednesday.

Seas are then expected to rebuild towards 7 to 9 ft towards the very end of the week in response to a fresh northwesterly swell that will impact the waters. Given a dominant wave period around 8 to 9 seconds, seas will become steep again. In addition, northerly winds will increase slightly with gusts topping out close to 20 kt. There is a 40-70% chance max wind gusts will exceed 21 kt on Friday over the outer waters and a 20-40% chance over the inner waters. Between the steeper seas and increasing chances for wind gusts over 21 kt, it is likely another round of marginal Small Craft Advisories will be warranted. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 5 AM Tuesday to 5 AM PDT Wednesday for PZZ251>253-271>273.

Dense Fog Advisory until 8 PM PDT this evening for PZZ252-253- 272-273.