textproduct: Portland

SYNOPSIS

Dry conditions will persist through Friday aside from a chance of showers over the Oregon Cascades Wednesday afternoon through Friday. Above normal temperatures continue today before 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...Monday afternoon through Sunday night

It will be another warm spring day across much of NW OR and SW WA. As of 1 PM, most of the interior lowlands have reached at least 70 degrees with locations in the southern Willamette Valley and adjacent foothills already showing observations in the mid 70s. Temperatures are on track to peak in the upper 70s to low 80s for the interior lowlands. However, flow both in the upper levels and at the surface has turned onshore/westerly today. This is bringing in cooler, moister marine air to locations west of the Coast Range. The coast will be much cooler than yesterday with high temperatures in the 50s to 60s. Additionally, the westerly flow is introducing a little more moisture in the mid levels. This along with a very subtle shortwave along the upper level flow could bring just enough orographic lift, which combined with instability from surface heating, could produce isolated shower chances (15-20%) over the southern Lane County Cascades this afternoon mainly along the crest. Better chances will be east of the Cascade crest.

A weak and dry cold front associated with a low pressure system moving east through British Columbia is slated to move over the region tonight into tomorrow morning with temperatures trending cooler for Tuesday and Wednesday. There is high confidence that high temperatures will 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. Expecting inland temperatures to peak in the upper 60s to low 70s as there's a 50-75% chance of temperatures exceeding 70 degrees each day, except 80-95% chance for the Portland metro area. Probabilities have decreased to 10-15% chance for temperatures to reach 75 degrees, except for a 40-60% chance for the Portland metro. 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 outside of the Portland metro area, and 50-80% in the Upper Hood River Valley from Odell to Parkdale and the northern OR Coast Range. 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. Ensemble guidance indicates the potential for wrap-around moisture from the south associated with the aforementioned upper low spinning off of the northern/central 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. Chances increase and spread farther north along the Oregon Cascades each day. An isolated thunderstorm or two cannot be ruled out either depending on the degree of instability in place, mainly Thursday and Friday.

A pattern change is expected for the weekend, though details remain uncertain at this time. Generally, 500 mb cluster analysis suggest that the majority of ensemble members indicate a trough originating from Alaska will approach the PacNW from the northwest late Friday into Sunday, bringing the return of widespread precipitation chances. However, there are a number of differences in the timing, depth, and intensity of this trough as well as how it interacts with the low pressure system along the low pressure system along the California coast. These differences impact the overall forecast, though one aspect the ensembles agree on is there are low chances for any high impact weather. The latest NBM guidance continues to indicate shower chances increasing Saturday morning along the coast and Cascades, spreading to the interior lowlands later Saturday, then decreasing Sunday evening. Guidance continues to suggest rain amounts will most likely be on the lighter side. Probabilities for 48-hour rain totals below 0.25 inches from 5am Saturday through 5am Monday are around 50-65%, except 55-70% at the coast and 30-45% over the Cascades. -03/23

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 21z, this stratus deck with CIGs around 100-300 ft, along with visibility reductions to 1SM, persists at KONP. Northwest winds have arrived, likely maintaining 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 extent 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

High pressure rebuilding over the waters today will return northerly winds later this afternoon through Tuesday. Still expect dense fog will continue over the central and southern waters through 8 PM this evening, so will maintain the Dense Fog Advisory for this hazard.

With increasing northerlies and associated wind chop, seas are expected to become steep enough for a period of marginal Small Craft Advisory conditions Tuesday through Tuesday night. Winds will likely gust up to 20-25 kt while seas build to around 6 to 9 ft at 8-9 seconds. Winds and seas decrease slightly on Wednesday.

Breezy northerly winds continue through Friday, generally remaining around 15 kt or less with gusts topping out close to 20 kt. Seas may rebuild again to around 7 to 8 ft with a dominant period of around 8-9 seconds, becoming marginally steep and choppy. -DH/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.