Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
188 FXUS66 KPQR 211816 AFDPQR Area Forecast Discussion...Updated Aviation National Weather Service Portland OR 1116 AM PDT Tue May 21 2024 .SYNOPSIS...Widespread rain returns later this morning, spreading through the region through the day, which could lead to a wet evening commute for most areas. Scattered showers tonight into Wednesday with a thunderstorm chance over the Cascade foothills. Dry and warmer Thursday before becoming unsettled for Friday into the weekend. && .SHORT TERM...Tuesday through Thursday...Satellite imagery around 1 AM Tuesday morning shows mid-level clouds beginning to push into SW Washington and NW Oregon from the northwest ahead of an incoming frontal system. The onset of the widespread stratiform rain band associated with this frontal system has slowed significantly from previous guidance. Comparing latest hi-res surface pressure guidance to Monday`s 12z runs, the amplitude of the surface low has become wider in current run, which in turn slows the progression of incoming clouds and rainfall. Rain is now not expected to reach the far NW corner of the region (generally Pacific County and far northern Oregon coast) until around 8-10 am, slowly spreading southwest into the Portland/Vancouver metro area by early afternoon, and reaching Lane County around late afternoon/early evening. Still expecting to see quite a bit of rain shadowing in the western Willamette Valley stretching north into western Columbia County (generally east of the Coast Range and west of cities such as Portland and Salem) due to the northwesterly wind trajectory. Also, hi-res guidance is beginning to indicate the strength of the precipitation may fizzle as it moves south, so the lowlands and Coast Range of Lane County may not see as much rain, either. HREF indicates only a 5-20% chance that these areas could receive 0.1 inch of rain through 5 AM Wednesday. Elsewhere in the Willamette Valley has 60-90% probability of reaching 0.25 inch of rain for the same time period, increasing to 90+% for the lowlands north of Portland, the coast, Coast Range, and Cascades. The westerly flow will allow for enhanced upslope flow over the mountains with these areas receiving anywhere from 0.75-1.5 inches of rain. Snow levels will remain above 6000-7000 feet ahead of the front, and with the majority of precipitation also occurring ahead of or along the front, little to no mountain snow is expected. Additionally, daytime temperatures are not expected to fall as much for locations south of a line from southern Tillamook County to southern Clackamas county. Interior lowlands north of here will see temperatures in the upper 50s to low 60s. South of here, temperatures will remain in the mid to upper 60s with even a 20-40% chance of reaching 70 degrees around and south of Eugene. Rain turns showery this evening, becoming isolated showers for areas west of the Cascades overnight. Quickly behind this front will be an upper closed low moving south from western British Columbia into western Washington by 5 AM Wednesday then pushing southeast through northeast Oregon through the day. Latest ensemble guidance indicates the core of this low will just miss our forecast area to the north and east, as will the majority of the forcing for widespread precipitation. With westerly flow continuing, the majority of precipitation will be over the Cascades due to upslope flow, with lighter amounts over the Coast Range. Showers will be very isolated over the lowlands. Although colder air aloft will funnel in behind today`s front and Wednesday`s closed low, there doesn`t seem to be quite enough instability building for thunderstorm chances except over the western Cascade foothills on Wednesday. However, stronger showers are possible with the limited instability elsewhere with the potential for small hail. The high Cascades will also receive some snow with the colder air, though snow levels are expected to remain mainly above mountain passes. On Thursday, the closed low moves east over the northern Rockies with guidance continuing to show a weak shortwave developing on the backside of the low over the PacNW as ridging builds over the eastern Pacific. Temperatures will warm back into the upper 60s for the interior lowlands under sunny skies. No rain is expected with this wave, but it will cause winds aloft and at the surface to turn north to northwesterly for a brief period Thursday. Winds are expected to remain relatively light with the only impact to potentially help enhance the warming temperatures. -HEC .LONG TERM...Friday through Monday...An unsettled weather pattern is expected to continue through at least Saturday. WPC`s 500 mb cluster analysis indicates a week shortwave could bring light showers Friday followed by a an upper level closed low bringing slightly more showers to the region Saturday. Specific details on timing and amounts of rain continue to remain uncertain, though temperatures are expected to remain near or just below seasonal normals. The pattern becomes a bit more uncertain Sunday on whether troughing will still around to continue showers. By Monday, dry conditions are expected. However, about 60% of the 500 mb clusters indicate strong ridging building right over the Pacific NW on Monday, about 27% of the clusters indicate this ridging building more east over the Intermountain West, and 15% of the clusters indicate a weak trough approaching the Pacific NW. This brings significant uncertainty in temperatures with NBM 25th to 75th percentiles for inland valleys ranging anywhere from upper 60s to low 80s. -HEC && .AVIATION...The first bits of rain from the next front are beginning to move in, with rain showers starting to begin at northern areas as of 18z Tue. KAST is already seeing IFR cigs, with ceilings having dropped below 1000 ft, though overnight fog is starting to rise. By 22z Tue to 00Z Wed, the front pushes in and the rest of the area should drop to high end MVFR cigs (70% confidence). Coastal areas see at least a 80% chance for IFR/LIFR conditions at that time. Southern terminals may remain low-end VFR, as precipitation mostly passes to the north and southern terminals may not be affected. Between 06-10z Wed, conditions transition to post-frontal, and most terminals will trend back to VFR, with the valley transitioning faster. By 09z Wed, even the coast only sees 30-40% chance of MVFR or lower ceilings. West/southwesterly winds will also begin to pick up around that time, with gusts for most areas up to 20 kts (25-30 kt at KAST). PDX AND APPROACHES...VFR conditions look to degrade to MVFR around 23z Tue (80% confidence in MVFR or lower cigs). Winds gradually turn W as the front passes through, though top gusts should only be around 10-12 kt. Conditions trend back to VFR around 08z, with less than 10% chance of MVFR conditions continuing past that time. Winds will pick up slightly, with top gusts up to closer to 15-16 kt from the west at that time. /JLiu && .MARINE...The morning begins in a brief lull between weather disturbances before the next frontal system arrives as the day progresses bringing rain and Small Craft Advisory Winds ahead and behind the front. Confidence is very high (80-90%) wind gusts reach into the 20-25 knot range for the outer waters north of Cape Foulweather later Tuesday morning spreading southward to PZZ273 and persisting at times through Tuesday night and Wednesday. With the persistent northwest fetch in the post-frontal environment, also expected seas to become fairly steep Tuesday night into Wednesday with greatest impacts to PZZ251 as seas rise to 10 to 13 ft with a period 9 to 10 seconds. As we move into Thursday, calmer winds/seas return for a day followed by yet another weather disturbance moving into the region from the NNW late Friday/Saturday continuing the rather progressive and active weather pattern. Seas likely rise to 7-10 feet at 8-9 seconds on Saturday before decreasing Sunday into early next week - wave steepness may again be a concern Saturday. -Schuldt/mh && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until 5 PM PDT Wednesday for PZZ210. Small Craft Advisory until 5 PM PDT Wednesday for PZZ251-252-271- 272. Small Craft Advisory from 8 PM this evening to 5 PM PDT Wednesday for PZZ273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland