Area Forecast Discussion
Issued by NWS Portland, OR

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000 FXUS66 KPQR 170510 AFDPQR Area Forecast Discussion National Weather Service Portland OR 910 PM PST Sat Nov 16 2019 Updated aviation discussion .SYNOPSIS...Patchy fog and low clouds will return tonight as the region will briefly be between systems. Another weak front on Sunday brings rain chances mainly affecting the northwestern half of the area. More widespread rain is expected late Monday through early Tuesday as a Pacific low pressure system tracks eastward across the Pac NW. Dry weather then returns Wednesday through the rest of the work week. && .SHORT TERM...Tonight through Tuesday: The upper short-wave has dug a little further south as was expected earlier. It`s continuing to bring sporadic measurable rain along a line from roughly Lincoln City to Saint Helens, Oregon and northward. Amounts north and west of that line are just a few hundredths while trace amounts are being noted within 20 miles just southeast. The affecting front will continue moving south-southeast through the early evening. Areas further south are mainly seeing a thin layer of the mid cloud deck leaving them susceptible for a fairly rapid return to fog and low clouds this evening. Light winds and modest radiational cooling will quickly set up with notable impacts from the fog/low clouds starting as early as 7 PM tonight. The upper ridge flattens Sunday as another shortwave rides along the northern side of the low. This does seem likely to bring rain across our northern 1/3rd to 1/2 of the CWA, especially Sunday afternoon as the trailing cold front drags inland. It`s still notable that the overall warmer air mass with 500 mb heights ranges from 573 to 582 dm. This brings snow levels nearly above the Cascade peaks. The mid and south Valley may end up staying rather dry, but potentially still cloudy from overnight fog/low stratus. Sunday night and Monday, the upper ridge flattens further as the downstream trough east of the Rockies weakens as well. This opens up the door for a fairly vigorous upper low to approach the coast. Models are in fairly good agreement with the position of the upper low with respect to each model time step between Sunday afternoon and Monday evening. The most interesting aspect of this upper low is the trajectory as it moves from west to east with minimal change in latitude, staying sandwiched between 45 and 50 degrees north. The GFS continues to show the strongest version of the low with surface central pressure now bottoming out at 994 mb while the ECMWF is the weakest at 1004 mb. Regardless, the leading edge of the rain bands will most likely arrive at the coast by Monday afternoon. The upper low moves inland Monday night, but a trailing vort max drops south along the western flank of the low Tuesday and forms a new low as it continues diving south along the coastline and into California. This will keep some chance of rain continuing through most of the day Tuesday. The differing details of the models will ultimately determine just how much rain fall and how long it will fall, but amounts do not seem to be all that impressive once the initial low passes by Monday night. Rain amounts through Tuesday continue to increase with successive model runs. The bulk of the rain arrives Monday afternoon through Tuesday morning. Amounts peak 1-1.5" for the coast/coast range, 0.30-0.70" for the interior low lands, and 1.00-1.50" for the Cascades with local 1.75-2.00" across the South Washington Cascades. /JBonk .LONG TERM...Tuesday night through Saturday...Operational models are in decent agreement that an upper ridge will set up across the Pacific NW Tuesday night and Wednesday with an upper low over California. The models indicate that a rex block will support dry weather for the Portland forecast area through the week into next weekend. The winds will n=be light enough Tuesday night for some radiation fog to form Wed morning in the interior valleys especially in areas that receive measurable rain with Tuesday`s front. The drier airmass late in the week will make fog development less likely despite the overnight winds remaining relatively light. Radiation cooling will likely result in near freezing temperatures Thursday through Saturday mornings. ~TJ && .AVIATION...Guidance continues to suggest that localized IFR fog will impact portions of the forecast area tonight, especially along the north coast and in wind sheltered river valleys in the interior away from the Columbia River Gorge. Elsewhere conditions look to be predominately VFR. A weak front will bring reduced flight conditions late tonight/early Sunday morning, with light rain possible along the north coast Sunday morning and northern half of the interior Sunday afternoon. A mixture of MVFR or lower cigs likely stick around through Sunday night for the forecast area. KPDX AND APPROACHES...Based on the latest trends and current dew point spreads suspect conditions will remain predominately VFR tonight under light flow. Will see an increasing chance for MVFR cigs Sunday morning, with light rain possible Sunday afternoon. MVFR or lower cigs likely stick around through Sunday night. /64 && .MARINE...Made no changes to the small craft advisory that is out for zone 270 as gusts to 25 kt remains likely for the northern section (north of Manzanita) out beyond 20 nm. Have also added a rough bar (Columbia Bar) small craft advisory for the the Sunday afternoon ebb, and the one for the strong ebb this evening still holds. The northern waters within 10 nm (PZZ250)could see a brief burst of gusts around 20-23 kt late Sunday morning and early Sunday afternoon, mainly north of Manzanita. Seas hovering around 10-12 ft,but below 10 ft for the nearshore waters through early next week. ENP wave model showing seas 15-18 ft Tue/Tue night, but this is dependent on a very good fetch directed to our waters that the GFS is currently showing. Other models are not quite in agreement. So have continued the previous forecast thinking and nudged the seas closer to the ECMWF solution, which is in good agreement with the NBM on significant wave heights. ~TJ && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Dense Fog Advisory until 10 AM PST this morning for Greater Portland Metro Area. WA...Dense Fog Advisory until 10 AM PST this morning for Greater Vancouver Area. PZ...Small Craft Advisory until 4 PM PST Sunday for Waters from Cape Shoalwater WA to Cascade Head OR from 10 to 60 NM. Small Craft Advisory from 5 PM this afternoon to 9 PM PST this evening for Columbia River Bar. && $$ 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.

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