Area Forecast Discussion
Issued by NWS Portland, OR

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000 FXUS66 KPQR 072102 AFDPQR Area Forecast Discussion National Weather Service Portland OR 200 PM PDT Tue Apr 7 2020 .SYNOPSIS...High pressure will continue to strengthen over the Pacific Northwest through Wednesday. Onshore low-level flow today will transition to light offshore flow late tonight and Wednesday. The offshore flow pattern is expected to persist through Thursday morning and then switch back to onshore late in the day. A gradual cooling trend begins Friday as onshore flow strengthens. The Eastern Pacific upper ridge retreats slightly west over the weekend as an upper trough slide south through the Northern Rockies. High pressure nudges closer to the coast early next week for dry and seasonal conditions. && .SHORT TERM...Tonight through Friday...Water vapor satellite imagery revealed a deep upper low off the southern California coast and a high pressure ridge centered along 140W. Meanwhile, the northern branch of the jet stream could be seen over Washington and the Northern Rockies. Early afternoon GOES-17 visible imagery showed the marine stratus field over the south Washington and far north Oregon coast diminishing. Low-level northeast flow could be seen across southwest Oregon with the marine stratus being pushed further out to sea. At 19Z the KTTD-KDLS gradient was 4.7 mb, slightly stronger than the NAM forecast. Gusts to 30 mph have been noted in the Hood River area early this afternoon. The 12Z NAM forecast a peak KTTD-KDLS gradient of 4.3 mb early this afternoon. The end result will likely be closer to 5 mb. The 12Z model runs are in good agreement showing the onshore low-level weakening fairly quickly this evening and transitioning to light offshore flow by 12Z Wed. Not anticipating much, if any, fog due to the switch to offshore flow. The 12Z HREF hints at some patchy low cloud in the Willamette Valley late tonight. The 12Z HiRes-ARW boundary layer condensation pressure deficit valid 12Z Wed appears to be rather marginal for any low cloud development. The offshore gradient, specifically KTTD-KDLS, is expected to peak around -3.5 mb early Wed afternoon. Local forecast rule of thumb would indicate peak gusts 30-35 mph in the windiest spots at the west end of the Gorge and 20-25 mph for KTTD. Model forecast 850 mb temps warm from 4C in SW Washington and 6-8C in Lane County today to 9C in the north to 11-12C over the Lane County Cascades Wed. Several locations likely to hit the 70-degree mark Wed. A thermally-induced surface trough resides over the inland valleys Wed night through mid-afternoon Thu. The NAM suggests a switch to light onshore flow for the coast and coastal valleys by mid-afternoon Thu. Thus, SW Washington coastal areas and locations west of the Oregon Coast Range will likely see some cooling Thu afternoon. However, areas east of the Coast Range and also in the SW Washington interior will be as warm or even a degree or two warmer Thu. A weak short-wave is expected to slide across southern British Columbia Thu evening. This will enhance the onshore low-level flow and increase the likelihood of low clouds pushing up the Columbia. Expect about 4-8 degrees of cooling Fri as 850 mb temps fall by 4-6C. The stronger onshore flow Fri will bring another round of breezy to windy conditions to the Central Gorge. Weishaar .LONG TERM...Friday night through Tuesday...The 12Z operational runs and ensembles show the eastern Pacific ridge retrograding slightly west Fri night and Sat as an upper level trough gets carved out over the southern British Columbia and the Northern Rockies. The GEFS 500 mb mean height forecast valid 00Z Sun is in good agreement with its control run. The ECMWF ensembles indicating strong upper level troughing over eastern Washington and Oregon. All in all, this should translate into a deepening marine layer Fri night and Sat. Any precip would most likely be drizzle from the deeper marine layer. Will maintain 20-30 percent POPS late Fri night through Sat. The eastern Pacific ridge begins to re-amplify Sat night, with generally dry north flow aloft Sun. The operational GFS and ECMWF show surface thermal trough development along the coast Sat night and Sun. A high percentage of the ECMWF ensemble members indicate no precip Sat night and Sun. The GEFS and GEFS mean valid Sun afternoon show ridging closer to the coast with the large upper trough displaced more to the east over the Upper Midwest. Dry and seasonal conditions look to hold through into early next week. Models and their ensembles become more out of phase around the middle of next week. There is reasonable confidence that a change to cooler and unsettled conditions occurs during the latter half of next week. Weishaar && .AVIATION...High pressure across the area is keeping conditions calm and clear. Increasing northerly flow is bringing gusts up to 25 kt to the central Oregon coast. KAST could have some high MVFR/low VFR cigs between 2500 to 3500 ft redevelop around 12Z Wednesday. Otherwise, VFR conditions over the next 24 hours along with northerly winds. PDX AND APPROACHES...VFR conditions over the next 24 hours. Winds will be generally light and from the north. /42 && .MARINE...A thermally induced trough extended into the central Oregon waters this afternoon bring small craft winds to zones PZZ275 and PZZ255. This thermal gradient will bring northerly winds to small craft levels with gusts to 25 kt within the central Oregon waters, and a chance that some isolated gusts to 25 kt could be observed in the southern parts of zones PZZ270 and PZZ250 west of 10 NM. This thermal gradient is expected to intensify along the coast through Wednesday, which will bring small craft advisory winds with gust up to 25 kt to PZZ270 and PZZ250. Gusts up to 30 kt will be possible in zones PZZ275 and PZZ255 at this time too. These small craft advisories will probably be extended further, but have not done that as guidance is suggesting that zones PZZ270 and PZZ250 could have a significant period of time without small craft gusts. Seas will remain around 6 to 8 feet with a dominate period between 15 to 18 seconds through the middle of the week. North/northwesterly swell is in the 3 to 5 foot range with the seas being wind wave dominate. Wednesday night and into Thursday, winds begin to ramp up and could increase seas to near 9 ft with a 10 to 13 second period. /42 && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory from 11 AM to 11 PM PDT Wednesday for coastal waters from Cape Shoalwater WA to Cascade Head OR out 60 NM. Small Craft Advisory until 11 PM PDT Wednesday 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 nm offshore. This area is commonly referred to as the forecast area.

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