textproduct: Portland

SYNOPSIS

The arrival of a warm-frontal system this afternoon kicks off a fairly wet stretch of weather for NW Oregon and SW Washington through Thursday. At least snow-levels climbing to 6,000-8,500 feet will help to mitigate any significant travel concerns across the Cascade Passes. Conditions gradually trend drier Friday into Saturday with lingering precipitation largely pinned to higher terrain features - snow levels drop to the passes by late Friday and Saturday. Looking ahead, guidance continues to show a cooling trend taking hold by early next week, however, beyond the potential for our coldest low temperatures of the season additional impacts appear minimal through Tuesday at this time.

DISCUSSION...Now through Tuesday

This afternoon a warm frontal system has taken aim at the region by steering a stream of atmospheric moisture into western Oregon and Southwest Washington. Currently surface observations show above freezing temperatures across the the Cascades passes helping to keep travel impacts largely in check before snow levels have a chance to jump to 6,000-8,000 ft later this evening - highest relative snow levels south near Willamette pass. As we progress through tonight, this elongated band of rainfall oriented WSW to ENE is expected to slowly shift southward with the majority of deterministic and ensemble guidance depicting the axis of heaviest rainfall south of the Portland/Vancouver metro by sunrise Wednesday. That said, moisture begins to slowly creep northward through the day Wednesday into Wednesday evening thanks to a developing secondary low pressure system over the eastern Pacific. Between high resolution guidance and global models there is still some lingering uncertainty as to just how quickly this S to N transition takes place - if precipitation stalls and get hung up over a section of Western Oregon and Southwest Washington longer than anticipated some of the more responsive rivers, primarily in the coast range and Willapa Hills, may be more impacted. However, the chances for minor river flooding appear low (<15%) at this time. Confidence in the QPF forecast through Wednesday night is only moderate due to this previously mentioned uncertainty.

Turning our attention to the developing low pressure system off the coast Wednesday night into Thursday, the latest 12z ensemble runs for the EPS, GEFS, and GEPS (including their deterministic counterparts) have continued project the core of the low moving into Washington/Oregon coastline. There's a bit of N to S variability on where exactly where along the coast the low will move inland, but the majority of the ensemble members suggest this will be somewhere between central Washington and northern Oregon. This is further south when compared to yesterdays 12z model runs in addition to a slight delay in arrival time. Ultimately, there's not much change in the overall impacts with this system except for potentially slightly increased winds depending on exactly where the low sets up. Current probability for 30+ mph winds are 10-25% for interior lowlands and 50-70% along the coast, mainly Astoria southward. Probability for 40+ mph winds drop to less than 5% across the interior lowlands and 25-30% along the coast. Rain amounts for a localized area will likely fluctuate based on the exact low track too, but not expecting a significant amount of rain on top of accumulation from the preceding warm-frontal system overhead. From Thursday through Thursday night expect another 0.15-0.25 inch for the central and southern Willamette Valley, 0.3-0.75 inch for the northern Willamette Valley into the SW Washington lowlands, 0.5-1.25 inches along the coast and Coast Range, and 0.5-2 inches along the Cascades (highest SW WA).

Then Friday into the weekend and beyond, model uncertainty increases further as both deterministic and ensemble systems struggle to resolve the placement of the upper-level longwave pattern across the eastern Pacific and western CONUS. On Saturday in particular guidance is split as to whether another shortwave trough near British Columbia dives southward into the Pacific Northwest, or if high pressure ridge over the eastern Pacific strengthens enough to keep this feature well to our east. As of 12z, around 65-70% of GEFS/EPS ensemble members, and the ECMWF AIFS favor a drier solution Saturday while the other 30-35% of ensemble members and deterministic ECMWF would facilitate a period of shower activity which would almost certainly bring light snowfall to the passes. At the very least all models solutions do show the development of broad northerly flow which favors a cooling trend into Sunday and Monday, although the vast majority of guidance (80%+) now keep the coldest airmass near the Rockies. The NBM probabilities for subfreezing low temperatures across the lowlands are still rather good, generally 45-75%, by Monday morning which will give us a shot for the lowest temperatures of the season thus far. Come Tuesday models do hint at a potential shortwave sliding south-southeastward into the Pacific Northwest which would increase showery precipitation chances again. All in all, the overall model spread is moderate to large during this early next week period with impacts beyond a decrease in temperatures staying near climatological norms for late November/early December. Still, it'll be worth keep an eye on the forecast as we go through this week, especially if you have holiday travel plans. -99/03

AVIATION

At 22z Tuesday, widespread light stratiform rain was being observed across all of southwest WA and northwest OR. Cigs were mainly in the VFR range above 5000 ft, however surface visibilities were beginning to lower to around 4 SM is southwest WA and at KHIO where rain is becoming slightly heavier. Expect surface visibilities to lower to 3-6 SM at all terminals later Tuesday evening as rain becomes slightly heavier from north to south.

In addition to lowering surface visibilities, cigs will gradually lower between 03-12z Wednesday from north to south, with probabilities for cigs below 3000 ft increasing to 70-90% and probabilities for cigs below 1000 ft increasing to 10-30%, except 90% at the coast where IFR cigs are very likely to occur. Overall, inland terminals will trend towards predominately MVFR while coastal terminals trend towards predominately IFR. Expect very little improvement through the day on Wednesday as widespread stratiform rain continues and cigs hold fairly steady.

KPDX AND APPROACHES...VFR cigs/visibilities will likely continue through at least 00z Wednesday before cigs lower to MVFR thresholds or lower Wednesday evening as rain becomes slightly heavier. Probabilities for cigs below 3000 ft rapidly increase to 80% at 03z Wednesday. Probabilities for cigs below 1000 ft increase to 30% at that time as well. Once cigs fall to IFR or MVFR thresholds, expect very little to no improvement through 00z Wednesday as persistent stratiform rain continues. -23

MARINE

South winds up to around 30 kt will continue through Tuesday evening ahead of a cold front that will move over the coastal waters Tuesday night. Seas will rise to 9-11 ft at 12 seconds by Tuesday night, continuing through early Wednesday morning before falling to around 7-8 ft after sunrise. A Small Craft Advisory remains in effect through 4 AM Wednesday for all marine zones, except through 1 AM Wednesday for the Columbia River Bar, to cover the gusty southerly winds through Tuesday evening and for the period of steeper seas to follow.

Another frontal system approaches the waters sometime Wednesday night into Thursday, though there are still uncertainties in exact timing. Another round of increased southerly wind gusts up to at least 30 kt is expected with this system. There's a 45-60% chance for maximum wind gusts to peak around 35 kt for all marine zones, though 3 hourly probabilities drop to 10-20%, indicating widespread gale force wind gusts are unlikely to occur. Seas will most likely peak near 14-16 ft late Thursday with a 5-10% chance seas peak as high as 20 ft. This is due to the uncertainty regarding the exact strength and track of the closed surface low and the frontal system accompanying the low. Regardless of the outcome for this system, winds and seas will lower significantly on Friday. Winds and seas over the upcoming weekend are currently forecast to stay below small craft advisory thresholds with a period of offshore flow. -23/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 1 AM PST Wednesday for PZZ210. Small Craft Advisory until 4 AM PST Wednesday for PZZ251-252- 271>273. Small Craft Advisory until 4 AM PST Wednesday for PZZ253.