273 FXUS66 KPQR 161008 AFDPQR Area Forecast Discussion National Weather Service Portland OR 308 AM PDT Thu May 16 2024 .SYNOPSIS...A weak cool front sits off the coast and is slowly moving inland this afternoon. Some light showers on the leading edge before more persistent rainfall through Friday morning. Zonal flow through Saturday before another long wave trough arrives on Sunday. Higher confidence in troughing pattern with rain through mid-week. && .SHORT TERM...Now through Saturday Night...Satellite shows the pervasive marine stratus this morning with the cold front still sitting offshore. Upper level troughs are digging southward with the axis positioned right over the forecast area by early Friday morning. Today will be more of a transition day with stratus spilling over the Pacific Northwest, and onshore flow advecting low-level moisture into the region. The addition of the moisture near the surface will aid in setting up an environment where rain has a better chance of actually reaching the ground. Will see minimal, if any, rain today until late tonight into very early Friday morning when the trough really takes hold over the area. In the upper levels, the main energy of the trough hangs just to the north which coincides with the bulk of the precipitation falling over Washington state. On the convergent side of the trough we are going to see a jet max with winds around 100 kt set up right over Northwest Oregon north of Marion County. This jet will support rising motion and thus low pressure at the surface. The pressure gradient will cause winds to increase, especially in the Columbia River Gorge. The pressure gradient between Troutdale and the Dalles will range between 4-6 mb this afternoon so have increased westerly wind speeds within the Upper Hood River Valley and near Cascade Locks. Could see gustier conditions as well along the leading edge of the front as mixing will be amplified. On Friday, the combination of these features will provide enough energy to intensify the precipitation at the surface. One challenge will be the rain shadow possibility within the Willamette Valley. Confidence is high that rain will be heavier along the coast, Coast Range, and Cascades but some models (especially the NAM) are showing the signs of some rain shadowing east of the Coast Range. Because of this feature, precipitation will be even less in the interior lowlands. The NAM has a tendency to really capture these local effects well. Looking at the broad ensembles, the ECMWF is slightly drier than the GFS, and the HREF (combination of higher resolution models) is showing only a 10% chance for measurable precipitation around Portland. In contrast, the coast has a higher probability of seeing measurable rain. In Astoria, the 24 hour forecast ending at 4 PM Friday is showing around a 30% chance of 0.05-0.1 inch, and a 15% chance of up to 0.2 inch. The NBM in contrast is showing less than a 5% chance of QPF up to 0.2 inch. While this is more precipitation than we have seen over the last few days, it is still fairly light and more of a high PoP, low QPF scenario. By Saturday, zonal flow takes over with some residual rain present - especially along the coast. There is a shortwave trough passing through the mainly zonal flow aloft which will act as an encouraging feature for potentially more light rain once again. This remains a high PoP, low QPF pattern with rainfall totals being very localized.-Muessle .LONG TERM...Sunday through Wednesday...Minimal change in the forecast for Sunday and Monday as mild temperatures and a few sprinkles remain. Monday appears to be warmer than Sunday as the NBM shows ranges between 66-74 degrees F (the 25th-75th percentile) in Portland, 59-62 degree F along the coast, and 64-71 degrees F in Hood River. Any rain that does fall on these two days will be light and showery. Long term ensembles have narrowed in on a pattern finally where as yesterday there was still quite a bit of variation between them. On Tuesday, there is a trend towards troughing but the depth of that trough still remains unknown. Given that that is 6 days out, there is still time for it to shift. Have trended in the route of the grand ensemble which is showing moderate troughing, rain, northerly winds and moderate temperatures. The long-term forecast is unremarkable but there remains high uncertainty on Tuesday and Wednesday in regards to temperatures and precipitation accumulation. -Muessle && .AVIATION...Satellite and surface weather observations from 10z Thursday showed a marine stratus deck holding strong along the coast with widespread IFR cigs (including KONP and KAST). This stratus deck was also expanding inland through the Coast Range gaps, up the lower Columbia River and Cowlitz Valley. In addition, moist northwest flow in low levels of the atmosphere were aiding the development of a separate stratus deck over the south WA Cascade foothills and the mouth of the Columbia River Gorge. Still expecting this stratus deck to backbuild westward over the greater Portland/Vancouver metro, likely impacting KTTD, KPDX, KUAO, and KHIO with low-end MVFR cigs between 11-19z Thursday. Probabilities for MVFR cigs peak between 60-80% at the aforementioned terminals during that time, except at KHIO where probabilities peak around 40%. Probabilities are lowest from KSLE to KEUG at 20-30%, suggesting flight conditions will likely remain VFR the majority of the time at these terminals. If MVFR cigs are able to expand into these terminals, suspect it would only last a couple of hours before scattering out. PDX AND APPROACHES...VFR flight conditions will most likely give way to low-end MVFR cigs between 11-18z Thursday as a stratus deck near the mouth of the Columbia River Gorge backbuild westward towards the KPDX terminal. The probability for MVFR cigs peaks around 60% between 12-15z Thursday. Cannot completely rule out IFR cigs around 900-1000 feet (15-20% chance). Expect northwest surface winds to continue around 5-8 kt over the next 24 hours. -TK && .MARINE...A weak front will move over the coastal waters during the late morning and early afternoon hours on Thursday, bringing a shift from northerly winds to northwesterly winds Thursday afternoon through Thursday night. Northwest winds behind the front are still expected to increase to around 20-25 kt over the waters (90-95% chance of wind gusts in excess of 21 kt). As such, the Small Craft Advisory that was initially in effect for only the northern waters has been expanded to include the central and southern waters as well (beginning Thursday afternoon over the northern waters and Thursday evening over the central/southern waters). Beyond Thursday, the return of high pressure will result in a typical summertime northerly flow regime Friday through the upcoming weekend with wind gusts peaking in strength during the afternoon/evening hours each day. The strongest winds will occur over the southern waters where small craft advisory level wind gusts to 25 kt are likely to occur again. In regards to seas, buoy observations and the latest guidance continues to suggest significant wave heights remaining between 5 to 9 feet over the next several days as a primary northwest swell interacts with northerly wind waves. -TK && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory from 2 PM this afternoon to 8 PM PDT Friday for PZZ251-271. Small Craft Advisory from 7 PM this evening to 8 PM PDT Friday for PZZ252-253-272-273. Small Craft Advisory until 5 AM PDT early this morning for PZZ253-273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland