Sunrise 5:30 AM   Sunset 8:41 PM Moonrise 5:00 PM   Moonset 2:57 AM

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FXUS66 KPQR 201825 AFDPQR

Area Forecast Discussion...UPDATE National Weather Service Portland OR 1124 AM PDT Mon May 20 2024

Update to aviation discussion.

SYNOPSIS
Stratus across portions of the region this morning with low temperatures falling to the mid 30s for some areas in the southern Willamette Valley and upper Hood River Valley.
Dry and mostly sunny today. A weather system will bring more widespread but generally light rain across the region Tuesday.
Showers with a thunderstorms possible on Wednesday. Will maintain unsettled with near to slightly below average temperatures for rest of the week into the weekend.

SHORT TERM
Monday through Wednesday...Satellite imagery early Monday morning shows stratus forming along the western slopes and foothills of the Coast Range and Cascades. HRRR and sounding guidance indicates this stratus could spread up the Columbia River Valley as well as back build from the eastern Cascades into portions of the Willamette Valley, including into the Portland metro area. Stratus will dissipate through the morning hours and should be mostly clear by midday. Temperatures as of 2 AM remain fairly mild with 40s across the lowlands and the upper Hood River Valley and 30s in the higher elevations of the Coast Range and Cascades. Will still keep the Frost Advisory in place through 8 AM as dew point temperatures have fallen into the upper 30s in the lowlands, and HRRR indicates a 20-50% chance of temperatures falling to or below 35 degrees this morning for the southern Willamette Valley south of Salem.

For the rest of today, expect mostly clear skies and dry weather under a period of brief upper level ridging. Temperatures will be warmer than previous days, warming to the mid to upper 60s across the interior lowlands. However, probabilities of reaching 70 degrees for the Portland metro area have decreased to 30-45% based on latest NBM guidance, likely due to the lingering stratus. However, if stratus is not as thick as forecast or does not last as late into the afternoon, then would not be surprised if some metro locations reach the low 70s this afternoon.

Ensembles indicate the next frontal system is slated to move through NW Oregon and SW Washington Tuesday. This front and associated upper trough are moving in from the northwest, so it will bring some colder air along with it as well as a band of widespread stratiform rain across the region. The Willamette Valley will likely be rain shadowed due to the direction of the flow with higher precipitation amounts in SW Washington (including lowlands), the Coast Range, and Cascades. NBM indicates only a 20-40% probability of 0.25 inch or higher rain amounts in 24 hours in the central and southern Willamette Valley ending 5am Wednesday. These probabilities increase to 40-50% in the Portland/Vancouver metro area, and 75-90% in the lowlands north of Clark County. The Coast Range and Cascades north of a line from Tillamook to Government Camp are forecast to receive the highest QPF, anywhere from 0.75-1.25 inch. NBM indicates 50-70% probability of QPF greater than 0.75 inches for these areas through Tuesday night. Snow levels will remain above 5500-6000 feet through Tuesday, so snow at or below pass level is not expected. As the front will move through the region throughout the day, daytime temperatures in central Oregon may not fall as much as areas to the north. Expect lowland highs in the mid 50s to mid 60s, warmest in Lane County.

Ensemble guidance remains in good agreement (85% of WPC 500mb clusters) that a closed upper low tracking south along the BC coast Monday and Tuesday will continue the southern track along the northern Washington coast Tuesday night before moving southeast into eastern Oregon through the day Wednesday. This pattern will allow onshore flow to continue with another round of showers. Some uncertainty remains on exactly how deep into Oregon the low will track with some clusters hedging the low mostly along the Washington/Oregon border with others pushing it farther east into east central Oregon. However, this would only affect the amount of precipitation expected but not the overall impacts. Generally expecting light precipitation less than 0.15 inch in the lowlands with upslope flow producing 0.25-0.75 inch over the higher terrain. However, there could be widespread thunderstorm chances in the afternoon and evening hours depending on how deep the trough moves into Oregon as the cold core moves aloft. Heavier showers and small hail would be likely in this scenario. -HEC

LONG TERM
Thursday through Sunday...Thursday is looking mainly dry as brief ridging builds in the eastern Pacific.
However, ensembles are beginning to show the potential for a weak shortwave to form on the back end of the closed low on Thursday, which would keep light shower chances continuing, mainly for the higher elevations. WPC 500 mb clusters continue to indicate a general trough pattern Friday into the weekend, though specific details on timing and location of weather systems and any potential precipitation amounts are still very uncertain at this time. One aspect that does seem a little more certain is temperatures are expected to remain fairly mild through the weekend. NBM indicates a 75-95% chance that interior lowland temperatures will warm back into the 60s and remain in the 60s through the weekend. -HEC

AVIATION
Satellite shows stratocumulus increasing as it overturns from daytime heating. Generally low VFR to high end MVFR with cigs around 3000-4000 feet north of KSLE, but SCT-BKN conditions south of KSLE. Expect to cigs to lift and cloud coverage decrease some through the afternoon and evening hours.
But onshore flow continues ahead of next front for late Tuesday.
So overnight expect MVFR to IFR develop at the coast, while inland mainly mid-level cigs around 4000-6000 ft developing overnight.
HRRR indicating 40-70% chance for MVFR cigs 2000-3000 ft roughly 12-16Z Tue. The highest chances are over the north Willamette Valley and across the lower Columbia River, with lower chances and less coverage south.

Winds will be mostly light then pick up to 5-10 kt. Occasional gusts to 18-22 kt possible during the afternoon into the early evening along the coast before decreasing again overnight.

PDX AND APPROACHES...Low VFR with cigs just above 3000 feet as of 18Z. Cigs will continue to lift and dissipate through the afternoon and evening. With continued onshore flow expect low VFR to MVFR cigs to redevelop overnight. HRRR indicating around 70% chance for MVFR 12-16Z Tue. Winds will be northwesterly and around 5-10 kt although gusts around 15-20 knots will be possible around 00-04z. /mh -Schuldt

MARINE
Small Craft Advisory conditions will persist for the outer waters today with N-NW wind gusts near 20-22 knots to start before the concern shifts to steeper seas of ~7-8ft at 7-8 seconds. Fortunately, calmer conditions return tonight into early Tuesday morning, albeit briefly, before the next frontal passage arrives on Tuesday bringing rain and a burst of northwesterly winds. Confidence is high (70-80%) wind gusts reach into the 20-25 knot range for the outer waters north of Cape Foulweather Tuesday evening/night in the post-frontal environment – additional Small Craft Advisories may be needed. A northwesterly swell of 8 to 10 feet at 9-10 seconds then arrives Wednesday morning as winds begin to subside. Looking ahead, calmer winds/seas return Thursday followed by the potential for yet another weather disturbance moving into the region late Friday/Saturday continuing the rather progressive and active weather pattern. -Schuldt

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.
WA...None.
PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ271>273.

Ellsworth, Washington - IGNORE HEIGHTS, Tide feet

Vancouver, Washington - IGNORE HEIGHTS, Tide feet

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