Area Forecast Discussion
Issued by NWS Portland, OR

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000 FXUS66 KPQR 242139 AFDPQR Area Forecast Discussion Weather Service Portland OR 239 PM PDT Fri May 24 2019 .SYNOPSIS...Cool and showery weather for today and Saturday as a couple of upper level lows move through the Pac NW. By Saturday, colder air will bring snow levels down to the higher Cascade passes. No significant accumulations are expected but those planning recreation for the holiday weekend will be impacted. Warmer weather is expected Sun and Mon as ridging over the NE Pac builds to the region. However, there will be potential for Cascades thunderstorms on Sun and Mon. && .SHORT TERM...Tonight through Monday...A closed upper level low pressure system centered over central Washington has been rotating a vort max onshore over our CWA today. This feature is driving strong onshore flow, which has resulted in a rather cool and cloudy day. High temps look like they will fall about 10-15 degrees cooler than yesterday across the interior lowlands, generally topping out in the low to mid 60s. In addition to the clouds, periodic light showers or drizzle have been occurring. The stratus from this morning is becoming more of a stratocumulus layer this afternoon with a few sunbreaks, and some weak low-level instability is beginning to produce some slightly more vigorous showers over the past hour. Expect that we will continue to see some heavier showers developing through early evening, but it would be a surprise if any of them were able to produce thunder. This first upper level low will drop south through central Oregon and down into far SE Oregon tonight. Expect shower activity to gradually decrease as the night goes on, with stratus settling back in to the lowlands. However, another closed low will arrive quickly on the heels of today`s system. This second low is currently along the Alaska panhandle, but it will dive sharply south over the next 24 hrs, and is modeled to ride right along the Washington and Oregon coast during the day Sat and Sat evening. This track will produce solid onshore flow through the low and mid-levels, with plenty of moisture and upslope flow into the Cascades. Light showers will begin to increase in the mid to late morning, and then it appears that it will be a pretty wet afternoon and evening across much of the region. The fcst models are suggesting that the heaviest shower activity will be focused over the Cascades and foothills, and potentially the eastern portions of the Willamette Valley. Sat looks to be another cool day, with high temps running 5 to 10 degrees cooler than normal in the upper 50s and low 60s. Snow levels will also fall down close to the higher Cascades passes, but at this time of year expect little to no accumulation on the actual road surfaces. The upper level low will continue to push south to the northern California coast by late Sat evening or during the Sat overnight hours. Showers will drop off pretty quickly from north to south going into early Sun morning behind its departure. Expect a mostly dry Sun, with clouds breaking up fairly quickly as the low level flow turns more northerly. Sun also looks to be quite a bit warmer, with 850 mb temps jumping from 0-2 deg C on Sat to 7-9 deg C on Sun. This should support highs back into the 70s for the lowlands. It should be a pretty good day for outdoor activities. However, one thing that will bear watching is the potential for thunderstorms. With the aforementioned upper low moving into central California, we will see mid level flow turn to the E-NE. The models are also showing some pretty decent instability over the Cascades. Think chances are pretty good that we see some thunderstorms develop along the crest and east-slopes of the Cascades later Sun afternoon. The direction of the steering flow would suggest that any storms that do develop would have the potential of drifting into the Cascades foothills and potentially even into eastern portions of the valley during the late afternoon and evening hours. Mon looks to be pretty close to a repeat of Sun. Temps should again be several degrees above normal with quite a bit of sunshine. We will need to watch for potential Cascades thunderstorms again. However, with the mid-level flow turning more northerly, the threat would appear to be mostly along the crest. Pyle .LONG TERM...Monday night through Friday...The broad upper-level ridge that has been positioned off the West Coast finally shifts inland into British Columbia on Monday. Because it will be so far north, this will allow for a shortwave trough to slide just to our south on Monday night into Tuesday. This brings showers and perhaps a few evening thunderstorms to the Cascades. Chance for showers comes to an end after midnight Monday night. Another shortwave passes to our south on Tuesday, but there is only a slight chance of some wrap-around precipitation to clip our Lane County Cascades Tuesday afternoon and evening. During the latter half of next week the forecast becomes more difficult as the models don`t have a great handle on the pattern until Friday when there`s a fairly good signal that a zonal flow pattern is setting up for next weekend. -McCoy && .AVIATION...Marine layer gradually over-turning early this afternoon. This has resulted in more VFR inland, although cigs were in the 035-040 range. Astoria profiler early this afternoon suggests the top of the marine layer is to at least FL045. Visible satellite imagery clearly shows clouds extending to the Cascade crest. Moderate onshore low-level flow continues through tonight. Coastal areas likely to see an MVFR to IFR mix through Sat morning. Inland areas will have predominant VFR through about 06Z and then fall back into MVFR overnight. The next low pressure system dropping south along the British Columbia coast early Sat will reach the forecast area by the afternoon. Expect increasing MVFR, especially over the Cascade foothills where up-slope low- level flow will result in orographic lifting. KPDX AND APPROACHES...Low-end VFR at the terminal as of 20Z. Deep marine layer and moderate onshore flow (KTTD- KDLS gradient nearly 4 mb) will continue through tonight. Expect cigs to remain 035-040 through the evening, but lower to MVFR again overnight. Weishaar && .MARINE...NW-N wind gusting to 27 kt at buoy 46089 and 25 kt at buoy 46029 at 19Z. Wind speeds will remain 15 to 25 kt this afternoon through the evening, with the strongest wind over the outer waters. Wind speeds gradually diminish late this evening and overnight, but small craft advisory level gusts will continue over the outer waters. The 12Z model runs show marginal small craft advisory level boundary layer wind speeds over the far outer waters Sat. The NAM is slightly stronger than the GFS. Current advisory for PZZ270 runs through 18Z Sat. It is more likely that gusts to 25 kt will be more persistent over PZZ275 Sat afternoon. Wind becomes more northerly Sat night and weakens below advisory thresholds. However, a surface thermal trough along the south Oregon coast is forecast to strengthen Sun, which will result in 20-25 kt gusts for PZZ275 and possibly into PZZ255 Sun afternoon. Weaker gradients Sun afternoon through early next week will result in wind speeds 15 kt or less. Wave heights continue to build today due to the breezy NW wind behind a departing short-wave disturbance. This will be primarily fresh swell. Seas look to peak around 14 ft over the northwest part of PZZ270 today. Latest guidance shows seas reaching 10 ft in PZZ255 this evening and tonight and will hoist a small craft advisory for that zone in the afternoon forecast. Seas eventually fall to around 5 ft Sun night or Mon. Weishaar && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory for winds until 11 AM PDT Saturday for Waters from Cape Shoalwater WA to Florence OR from 10 to 60 NM. Small Craft Advisory for hazardous seas until 5 PM PDT Saturday for Coastal Waters from Cape Shoalwater WA to Cascade Head OR out 60 NM.Waters from Cascade Head to Florence OR from 10 to 60 NM. Small Craft Advisory for winds until 11 PM PDT this evening for Coastal waters from Cape Shoalwater WA to Cascade Head OR out 10 NM. Small Craft Advisory for hazardous seas from 11 PM this evening to 2 PM PDT Saturday for Coastal waters from Cascade Head to Florence OR out 10 NM. && $$ Interact with us via social media: This discussion is for Northwest Oregon and Southwest Washington from the Cascade crest to 60 nautical miles offshore. 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