FXUS66 KPQR 260429 AFDPQR Area Forecast Discussion Weather Service Portland OR 919 PM PDT Sat May 25 2019 .SYNOPSIS...Low level onshore flow will keep temperatures near average with a risk for a shower or thunderstorm for inland locations through the first half of the week. && .SHORT TERM...Tonight through Tuesday...Water vapor satellite imagery this evening reveals a mid to upper level low pressure system dropping southward just off the Oregon coast. An area of mid level deformation led to widespread rain late this afternoon and early this evening across northwest Oregon and southwest Washington. The forcing driving this precipitation is quickly dropping southward with the low pressure and should result in a gradual decrease in rain from north to south across the area overnight. As the low pressure continues to drop southward towards California overnight, increasing easterly flow will develop above 925mb. This should result in a lot of the cloud cover departing the region as well. However, there may be enough northwesterly low level flow to keep at least some low clouds entrenched across portions of the area into Sunday morning. Any low clouds lingering into Sunday morning should dissipate rather quickly with daytime heating, though. With warmer temperatures aloft and increased sunshine, high temperatures should warm into the 70s for the Willamette and lower Columbia River valleys Sunday through Tuesday. The big forecast uncertainty revolves around when and where shower and thunderstorm chances are highest during this time period. At this point, it appears the Lane and Linn County Cascades appear to be the safest bet to have at least a thunderstorm or two develop Sunday afternoon per the latest round of model guidance. Pattern recognition suggests the threat should be at least as high across the south Washington Cascades, too. The problem with that idea is that an area of widespread showers over west-central Idaho associated with an area of mid level deformation and a weak 700mb low pressure will move northwestward and then eventually westward across Washington later tonight and Sunday. There may be enough residual cloud cover to limit solar insolation across the south Washington Cascades Sunday to inhibit convection. Nonetheless, some model scenarios weaken this feature before it arrives while others keep it farther north over the central Washington Cascades. As a result, the inherited thunderstorm forecast captures this uncertainty well so few changes were made to the southwest Washington thunder forecast. It should be noted that the thunderstorm threat for the Portland/Vancouver metro is very much dependent on whether or not thunderstorms develop over the south Washington Cascades Sunday afternoon and then subsequently spread southwestward towards the metro. Elsewhere, did expand the mention of thunder into the east slopes of the Coast Range late Sunday afternoon and early evening. This is primarily due to some models suggesting there may be enough instability for a surface based storm or two to pop late Sunday afternoon and evening. These same model dewpoint fields seem a bit high so am somewhat skeptical there will be enough low level moisture to make it happen, but cannot ignore the possibility either. While some models suggest this instability will be equally impressive across the central and southern Willamette Valley Sunday afternoon, any shower and thunderstorm development will likely be focused over the terrain initially given the light steering flow. It will take some time for outflows to converge and realize the potential for the center of the south and central Willamette Valley so have held off on the mention of thunder until the early evening hours for these areas. Otherwise, expect another round of showers and possibly a thunderstorm or two to develop near and across the Cascades on Monday and Tuesday, but pinpointing who will or will not see wet weather remains a low confidence forecast at best. /Neuman .LONG TERM...Tuesday night through Saturday...Ridge of high pressure remains in place over western Canada. The high is far enough north to allow for shortwave disturbances to slide south and clip our area with a few chances for showers, mainly in the Linn and Lane County Cascades. Otherwise, expect fairly nice weather for the rest of the forecast area through midweek, with marine clouds on the coast and partly cloudy skies inland. Temperatures will be near seasonal for this time of year, only a few degrees above normal inland. Next weekend a more zonal flow pattern sets up over the Pacific, but high pressure remains fairly dominant over the Pacific Northwest into next weekend. -McCoy && .AVIATION...Mostly VFR conditions this evening with light rain lingering. Expect rain to slowly taper off from north to south overnight with MVFR becoming more widespread as rain dissipates. VFR conditions should return Monday afternoon with convection over the Cascades. Convection, including the possibility of thunderstorms, will drift west into Sunday evening, encroaching on valley terminals. KPDX AND APPROACHES...Rain decreases overnight and expect CIG to drop to MVFR as rain ends. CIG rises to VFR Sunday afternoon, but convection, including thunderstorms, may move in from the east late Sunday afternoon and Sunday evening. Bowen && .MARINE...Seas are holding at 10 to 12 feet this afternoon, with the influence of a fresh swell holding conditions higher. Meanwhile, winds across the outer portions of the coastal waters continue to gust to 20-25 kt this afternoon. Have maintained the Small Craft Advisory for winds through 7 PM as a result. These stronger N-NW winds will likely keep seas above 10 ft into this evening, but gradually easing to 9 feet late. However, dominant periods will likely remain at 9 to 10 seconds, so opted to extend the advisory for hazardous seas a little longer, through 11 PM for the inner waters and 5 AM for the outer zones. Weaker gradients Sun night through early next week will generally result in wind speeds 15 kt or less. Seas eventually fall to around 5 ft Sun night or Mon as winds remain moderate Monday and Tuesday. && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory for hazardous seas until 5 AM PDT Sunday for Waters from Cape Shoalwater WA to Florence OR from 10 to 60 NM. Small Craft Advisory for hazardous seas until 11 PM PDT this evening for Coastal Waters from Cape Shoalwater WA to Florence OR out 10 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.