Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
419 FXUS66 KPQR 222213 AFDPQR Area Forecast Discussion National Weather Service Portland OR 312 PM PDT Sun Sep 22 2024 .SYNOPSIS...An upper trough will move across the area tonight with high pressure quickly returning Monday. High pressure strenghtens on Tuesday with weak offshore flow through Wednesday morning. Will see quick warm-up Tuesday with inland highs 10 to 15 degrees above seasonal normals. A stronger trough and associated front moves inland Wednesday night through Thursday for more seasonable weather. && .SHORT TERM...Now through Tuesday night...Upper trough temporarily depressing an upper ridge today, with main effect is increasing marine clouds at the coast and porttion of the interior overnight. The high pressure returns Monday with the surface ridge laying over southwest WA, resulting in generally northerly winds over nw Oregon. Northerly winds gusting 15 to 20 mph in the Willamette Valley Monday afternoon. The high pressure aloft pumps up further Tuesday to bring unseasonably warm temperatures with inland high temperatures around 90 degrees. This is about 10-15 degrees above normal. The bulk of the warm air will be to the east of the Cascades but the thermal trough that will form along the coast will encourage a weak easterly offshore wind. By Tuesday afternoon the ridge axis shifts directly over central Oregon which will cause winds to shift to the south aloft. This will also induce a southwest marine push (surface high pressure to the southwest and lower pressure over the Columbia Basin) Tuesday night. This should bring a deeper marine layer inland by Wednesday morning. Probabilistically, the spread between the 10th-90th percentile is only around 2-3 degrees F via the NBM. The deterministic forecast is slightly higher than the NBM 50th percentile though. One factor that could impact this forecast is that aforementioned tropical system to the south. It is not uncommon that these systems are not well resolved by models and can "disrupt" them. If it weakens or shifts south, the amplification of this ridge may be less and thus, less of a southerly push to bring these unseasonably warm temperatures. -Muessle .LONG TERM...Wednesday through Sunday...A trough now over the Gulf of Alaska will sag over the PacNW Wed. This system is much more robust and is easily detected in all levels of the atmosphere. Looking at the jet stream at 250 mb (~35,000 ft), the forecast area sits in the right entrance region of the jet streak which is commonly associated with instability. This instability will support more rain on Wednesday. With the added instability, cooler air moving over the warm airmass at the surface, and the addition of moisture, chances for thunderstorms increase. Have capped this probability around 15-20% as the storms (generally over the north Willamette Valley into north Oregon and south Washington Cascades. Brewing in the northeast Pacific is a low pressure system which will move eastward on Thursday and Friday. This incoming low will cause the flow to become more zonal which can be observed in the jet stream motion. The forecast area sits on the cool side of the jet with a 140 kt jet streak just to the north along the Washington-Canadian border. The low will enhance precipitation on Thursday into Friday. Because this low is moving inland to the north, we currently are sitting on the southern portion of the front. But if that low shifts further south, we could experience more rain than what is currently forecast. Will mention that these incoming cooler temperatures at higher elevations plus incoming moisture equals one thing...termination dust! It is possible that the peaks of the volcanos could see a few snow flurries at times late this week which may accumulate up to 0.5 inch or so. That termination dust generally is the initial sign for an end to summer./mh -Muessle && .AVIATION...Weak upper level ridging has led to predominantly VFR conditions across most of northwest Oregon and southwest Washington this afternoon. A weak upper level trough will brush the Pacific Northwest, which will help marine clouds currently well offshore push onto the coast late this afternoon and evening. Models suggest the marine clouds will most likely come in at high end IFR to low end MVFR thresholds along the coast with probability for IFR conditions at any given hour peaking around 40-50% along the central Oregon coast and 50-60% for the north Oregon coast after 00-03z Monday. The probability these marine clouds reach the Portland metro taf sites between 10-14z Monday is now 10-20%, but does increase to 30-40% around 16-18z Monday. to 30-40%. The probability of these marine clouds reaching as far south as KEUG is below 10%. PDX APPROACHES...Weak upper level ridging and VFR conditions today will give way to weak upper level troughing overnight. This will allow marine clouds to push inland towards the Portland metro. At this point, there is a 10-20% chance that MVFR ceilings reach the terminal between 11-14z Monday with probabilities increasing to 30-40% between 16-18z Monday. There is a low probability (10% chance) that the marine clouds come in at high end IFR thresholds. && .MARINE...A weak front sliding across the waters will briefly turn the winds more southwesterly across the northern waters overnight. However, there is a high probability (>90% chance) winds remain below Small Craft Advisory thresholds. High pressure over the northeast Pacific and thermally induced lower pressure over the southern Oregon coast will re-establish itself and produce gusty northerly winds across the waters on Monday. There is a 90% chance that wind gusts climb into Small Craft Advsiroy thresholds of 21 kt or more across the waters off the central coast of Oregon. The main uncertainty lies around how far north these winds will extend, but it appears there is a low to moderate probability (20-40% chance) that they will reach northward to somewhere between Cape Foulweather and Cape Falcon. High pressure nudges closer to the coast Tuesday, which should result in northerly winds weakening substantially. An approaching front will turn the winds southerly late Tuesday night into Wednesday. There is a 50% chance that winds climb into the 20-25 kt range across the waters at least briefly ahead of the front with the highest probabilities (>75% chance) across the inner waters in the vicinity of the Columbia River. Uncertainty in forecast details grow Thursday into Friday. This is due to a developing surface low pressure that is most likely to track towards the north Vancouver Island/Haida Gwaii region during this time. Variations in the models on the strength and track of the low pressure are what are creating the uncertainty. The majority of models suggest the low pressure will remain weak enough or far enough away to only bring Small Craft Advisory level wind gusts to the waters. However, there is a 25% chance of Gale Force southerly winds of 35-40 kt spreading across the waters, at least the northern waters, and a 5-10% chance of storm force wind gusts of 50-55 kt at some point late Thursday or Friday if the stronger and closer track scenarios pan out. Given timing uncertainty is smoothing the potential peak in winds, the current forecast is still generally near to even slightly below Small Craft Advisory level winds. Would expect the official wind forecast to climb into at least Small Craft Advisory thresholds, if not higher, over the course of the next couple of days for the late Thursday and/or early Friday timeframe as confidence in the exact timing of peak winds grows. && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland