Area Forecast Discussion (AFD)
Issued by Portland, OR (PQR)
000 FXUS66 KPQR 190453 AAA AFDPQR Area Forecast Discussion Weather Service Portland OR 953 PM PDT Tue Jun 18 2019 Aviation discussion updated. .SYNOPSIS...The pattern of areas of morning clouds and some afternoon sun continues through Wednesday, with seasonable temperatures. Thursday brings the coolest day of the week along with a chance for showers as an upper trough of low pressure moves across the Pacific Northwest. Dry weather and seasonable temperatures return Friday and Saturday. && .SHORT TERM...Tonight through Friday...Upper heights will continue to gradually lower tonight and Wednesday as another weak short wave moves through in the northwest flow aloft. This will continue the pattern of late night and morning clouds along the coast and in the northern interior. Daytime temperatures will cool to a bit below normal as 850 mb temperatures drop another 5 degrees Celsius. Wednesday night through Thursday night a pair of more substantial shortwaves carves out an upper trough of low pressure over the Pacific Northwest. Moisture deepens some with this system, although models still generally confined it to below 10K feet at its peak Thursday morning. The moist layer is marginally unstable, so given the dynamics associated with the shortwaves expect to see some showers with this system. While most of the forecast area will see some chance for showers, the best chances will be over the west facing slopes of the Cascades and Coast Range where orographic flow will enhance lift. Temperatures will be at their coolest of the week, with snow levels likely dropping down to the highers passes for the day before the summer solstice. Any accumulations would be likely a dusting at best, and would melt quickly during the day due to some of the strongest solar energy of the year. Drier weather returns Friday as the upper trough moves east and ridging builds in from the west. Surface flow to remain onshore, but becomes more northerly with the surface ridge extending inland into Washington. This should result in a return to a pattern of morning clouds and afternoon clearing for the interior, with daytime temperatures returning to closer to normal after a cool morning. .LONG TERM...Friday night through Tuesday...The beginning of summer will not seem like it as a cool and somewhat unsettled pattern dominates the forecast area. A weak 500 mb ridge is forecast to be over the area Sat. However, onshore low-level flow will maintain the pattern of late night/morning low clouds and afternoon clearing. Max temps Sat will be close to normal. The long-wave pattern begins to show signs of change Sat night and Sun. A fairly deep 500 mb low is forecast to be near Haida Gwaii 12Z Sun. Expect a deeper marine layer Sat night and Sun, with the potential for some light precip in SW Washington and also the north Oregon coast Sun morning. By Mon the large upper level trough will be over the NE Pac. The 12Z operational GFS shows the upper low along 46N 130W at 00Z Tue, while the 12Z ECMWF is more to the west. The ECMWF ensemble mean suggests higher-than-average confidence with the upper low out near 135W. The GFS ensembles show decent continuity, but the mean 500 mb low position appears to be closer to 130W, similar to the 12Z operational run. All in all, the latter half of the extended period will feature below normal daytime temperatures. In fact, the CPC 6- 10 day temp/precip progs indicate well-above average probability of below normal temps. CPC also suggests above-average confidence for above normal precip, albeit at a slightly lower confidence level than temps. The WPC forecast valid Mon and Tue favors a GEFS/ ECMWF ensemble mean. The deep upper trough eventually swings into Western Washington and NW Oregon Tue night and Wed. Looking beyond day 7, the GFS and ECMWF continue the unsettled pattern the latter half of next week. Weishaar && .AVIATION...A disturbance is forecast to move across NW Washington late tonight, which may disturb the marine layer enough to limit MVFR or lower conditions, even along the coast. Still expect MVFR to continue along the S Washington and N Oregon coasts, but coastal areas south of KTMK may hold on to VFR much of the night. MVFR CIGs forecast to develop in the SW Washington interior overnight and extend up the Columbia River to KPDX around sunrise, but CIG may also end up being low VFR. KPDX AND APPROACHES...VFR conditions to prevail through at least 12Z Wed. Another upper level disturbance moves across NW Washington late tonight, which may induce mixing and make morning CIGs low VFR rather than MVFR. Highest chance for MVFR would be on east approaches. Bowen && .MARINE...Surface high pressure will reside over the NE Pacific through the week, while thermal low pressure remains over the northern California and southern Oregon coast. This will result in persistent northerly winds. The winds will be the strongest this afternoon through late evening, with gusts to 25 kt expected over PZZ255/PZZ275. Wind speeds will be lighter from Wed through Fri as an upper level trough moves in and weakens the NE Pacific surface high. Gusts to 20 kt are expected during the afternoon and evening hours each day, but do not expect that additional advisories will be required. Will need to watch the area south of Heceta Head late Wed afternoon for possible 25 kt gusts. Wind speeds drop off even more this weekend and early next week as an upper level low settles over the NE Pac. Seas mostly 4 to 6 ft for the next few days, although areas of PZZ255 and PZZ275 likely to see brief periods of 6-8 ft at times due to contributions from fresh swell. Weishaar && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until midnight PDT tonight for Coastal Waters from Cascade Head OR to Florence OR out 60 NM. && $$ Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland This discussion is for Northwest Oregon and Southwest Washington from the Cascade crest to 60 nautical miles offshore. This area is commonly referred to as the forecast area.