Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
579 FXUS66 KPQR 011027 AFDPQR Area Forecast Discussion National Weather Service Portland OR 327 AM PDT Mon Jul 1 2024 .SYNOPSIS...Offshore flow slowly decays as a strengthening ridge of high pressure develops over the Pacific. This pervasive high pressure will cause temperatures to increase significantly through the end of the week. && .SHORT TERM...Now through Wednesday...Area observations show dense fog forming along the central Oregon coast and the bowl portions of the southern Willamette Valley. Some lower stratus shifting inland through the Valleys, but thus far have not seen any real widespread issue other than along the coast. The shortwave trough that brought cloudy skies and drizzle over the last few days will continue to shift inland through the day. It will generally shift east of the Cascades by this late afternoon. Once that trough exits, high pressure will begin to slowly build in. We will see steady warming and clear skies over the next week or so, so if you like warm temperatures...this forecast is for you! The ridge will really amplify on Wednesday night. Overall sensible weather wise, not looking at anything overly impactful other than the fog this morning. Will note that with these dry conditions incoming, will see a steady decline of humidity, especially east of the Coast Range. Models have struggled with the areas that sit within the subsidence inversion around 3000 ft and above so have manually edited to lower the RH in those areas. -Muessle .LONG TERM...Thursday through Sunday...The longer term forecast is really where the weather is occurring...though it isn`t necessarily active weather. The main threat in the forecast, and where the most time has been spent, is in regards to temperatures. The synoptic pattern shows a strengthening ridge forming with a thermally induced trough forming right along the coastline. This ridge will continue to amplify through the weekend. But what does this mean for us? Models are showing a steading warming trend which will lead into several days of more intense heat. In order to capture an overall view will not necessarily dive into each day directly. Friday and Saturday look to be the warmest days of the coming week which coincides with our general trend of "summer starts on July 5th". The thermal trough will be quite deep at this point which will cause winds to be northeasterly at the upper levels, but more northerly at the lower elevations. They will be slightly elevated which will help mix down some of the warm air aloft. At 850 mb (5000 ft AGL) temperatures are forecast to range from 20-25 deg C (70-75 deg F). Even further aloft at 500 mb (18,000 ft AGL), temperatures range from -7 to -3 deg C (20-25 deg F). While these temperatures may not seem as impressive, given their elevation and what we typically see, they are trending on the warmer side. When we see this pattern, it confirms that we are looking at hotter than normal temperatures. In fact, if we look at the "Extreme Forecast Index", we are on the higher end of what is climatologically normal, with this being mainly the case from Salem southward. At this time, there is still a lack of confidence in just how warm those temperatures will be though. In Eugene, the global models are showing a fairly narrow spread in high temperatures for Friday and Saturday. The 25th-75th percentile (most likely range) shows only a 5 degree spread in the low 90s on Friday, and around a 10 degree spread on Saturday. The NBM on the other hand too has a 5 degree spread, but in the upper 90s to up to 105 degrees with little change on Saturday. So what does that mean forecast wise? At this point, confidence is still quite low in regards to whether we will see extreme heat. Based on the global models, there is a 30% chance of exceeding 100 degrees on Saturday, and the NBM is closer to 50%. There is high confidence that conditions will be hot...hotter than what we have seen thus far this year. But confidence is low in regards to whether or not we will see extreme heat. Luckily we will see some reprieve overnight as lows dip near 60 degrees in the areas that will see the highest daytime heating. -Muessle && .AVIATION...Onshore flow is allowing marine stratus at the coast, continuing through the rest of the night. Intermittent IFR/LIFR cigs expected, along with reduced visibilities at times. 80% chance of VFR conditions at the coast returning by 20z Mon. Monday night sees 50-80% chance of IFR conditions reforming by 03z Tue as onshore flow increases again. Inland, the rest of Sunday night sees around a 20-40% chance of some high-end MVFR cigs forming as stratus begins building off the Cascades, with better chances in the eastern Willamette Valley. KTTD sees higher chances, closer to 60% of MVFR or below, and 30-40% of IFR cigs. VFR conditions return by 18z with 90%+ confidence. PDX AND APPROACHES...Latter half of the night (12z-18z Mon) shows a 30-40% chance of high end MVFR cigs (2000-3000 ft), higher slightly east of the terminal, closer to KTTD. After 18z, 90%+ chance of VFR cigs returning. Flow remains generally light and northwesterly throughout the TAF period, between 5-7 kt throughout the period. -JLiu && .MARINE...Weak high pressure over the northeast Pacific with lower surface pressure over California and the Great Basin will more or less continue through the week. A weak front is in the process of passing through, but effects look fairly minimal, with light northwesterly winds through Monday morning. Threat of dense marine fog is possible for the southern waters, with visibilities of half a mile or lower possible through the rest of the night. This is expected to clear up shortly after sunrise. Afterwards, expect NNW winds across the waters as high pressure strengthens toward the middle of next week, with the strongest winds generally off the central coast of Oregon and lighter winds farther north. Winds will generally peak in the afternoon and evening hours each day. Small Craft Advisory remains on track for the central zones, but zones further to the north still look fairly marginal. Because as a thermal low develops along the northern California/southwest Oregon coast on Tuesday and strengthening/spreading north into Wednesday and thereafter. As a result will see an increase the pressure gradient along the coast and over the waters with a >90% chance that northerly wind gusts of 20-30 kt spread northward across the waters by mid week. This will in return develop steep wind driven seas of 5-8 ft with a dominant period of 7-8 seconds by late Tuesday or Wednesday. -JLiu && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Dense Fog Advisory until 9 AM PDT this morning for ORZ103. WA...None. PZ...Dense Fog Advisory until 9 AM PDT this morning for PZZ253-273. Small Craft Advisory from 5 PM this afternoon to 5 AM PDT Wednesday for PZZ253-273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland