textproduct: Portland

SYNOPSIS

After one final afternoon of summer-like conditions today, an unusually strong frontal system by early September standards takes aim at the Pacific Northwest for the middle of the week. In juxtaposition to the last few weeks/months the Wednesday into Thursday time-frame features ample widespread rainfall, locally breezy/gusty winds, and a 15-30% chance for pop-up thunderstorms (mainly Thursday). Time to clear those rain gutters and dust off the ole' rain jacket. Conditions trend slightly drier by Friday into the weekend but cooler than normal temperatures and shower chances persist. Those eager for fall- like weather rejoice!

LONG TERM...Wednesday through Sunday

It appears we'll be entering Meteorological Fall (Sept. 1st onward) with a bang as a strong frontal system tees up the Pacific Northwest for Wednesday. Deterministic models and their respective ensembles continue to show a much deeper upper-level low pressure dropping down out of the Gulf of Alaska and into the Pacific Northwest Wednesday morning and afternoon ushering in widespread moderate rainfall, cooler temperatures, and locally breezier/gustier southerly winds. Following the frontal passage, there'll also be a 15-30% chance for thunderstorms. At this point it's almost a sure bet (85-95%+) rainfall amounts exceed 1 inch from the coast to the crests of the Cascades between 5 AM Wednesday to 5 AM Friday. Chances for exceeding 2 inches during this same timeframe have increased with the latest run of the NBM and sit around 40-70% for the coast and our higher terrain features - 5-20% chance along the I-5 corridor. For many locations the amount of rainfall forecast over the course of just a few days will get close to or exceed our monthly climatological average for September which really emphasizes the anomalous nature of this system. It's much more akin to a front you'd see in late October or November minus the colder temperatures at the surface.

As mentioned above, southerly winds likely increase as the frontal boundary moves onshore Wednesday allowing for a period of breezy conditions, especially along the coast, although exactly how strong wind may get is a point of uncertainty. Models soundings from the NAM, GFS, ECMWF, etc. all point to a low level jet feature just ahead and along the cold front with 850mb winds of 55-65 knots approximately 4,000-5,000ft off the surface from the coast into the I-5 corridor. It's just a matter of whether there is a forcing mechanism to mix these winds closer to the surface. Given the current model sounding profiles which don't appear readily conducive to to mix these high winds to a lower elevation, it would take a modest convective element right along the cold front to push these strong winds to the surface - would give it a 10-20% chance at the moment. Otherwise, winds should be largely pressure gradient/synoptic driven and based on the EPS ensemble and NBM, confidence is moderate for southerly wind gusts in the 35-50 mph range across the coast, highest beaches/headlands, and gusts between 20-35 mph along I-5 corridor. While these wind gusts may not seem particularly strong inland in the context of what we normally experience annually, the early season nature of this event may present exacerbated impacts. At least winds quickly decrease behind the frontal boundary Wednesday evening.

Thursday through Saturday/Sunday the core of the upper-level low remains over the Pacific Northwest with a convective post- frontal environment lingering over the CWA. Given the quick intrusion of cooler air aloft and decent daytime heating this time of year increasing atmospheric instability is favored along with showers and chances (15-30%) for thunderstorms focused Thursday afternoon. Current vertical shear values in the post- frontal airmass do not support storm organization (weaker pop- up cell storm mode) however given the placement of the surface low pressure Wednesday evening through Thursday an isolated waterspout and/or cold air funnel cloud or two can't be ruled out with any stronger convection. Any passing thunderstorms may produce lightning, heavy rain, erratic winds, and small hail. Chances for showers likely linger Friday through the weekend as the upper-level low weakens, transitions to a trough, then stalls along the coastline. Temperatures remain below normal as well. -99/42

AVIATION...Through 06z Wednesday

VFR flying conditions largely continue across the airspace late Tuesday evening, although a pattern shift is causing a southerly surge of moisture push northward along the coast and inland overnight, bringing widespread cig restrictions. South- southwesterly winds and bkn- ovc low clouds have spread along the coast as of 04z Tue. The wind shift is pushing inland to just south of the Portland metro area with northwest flow in place elsewhere. Mainly MVFR cigs will continue along the coast, with moderate chances for IFR cigs at any given hour through 15-18z Tue. Inland, MVFR cigs at 1.5-2 kft are favored (70-90% chances) after 08-12z Tue, earliest in the southern Willamette Valley and latest in the Portland area and Lower Columbia/Cowlitz Valleys. Diurnal mixing will see cigs trend higher inland with VFR favored to return by 18-21z Tue. Winds will shift out of the south to southwest as the push arrives, reaching 5-10 kt by Tuesday morning. Speeds will build further to 10-15 kt with gusts to around 20 kt along the coast by midday. Additionally, there's a 15-30% chance of scattered rain showers along the coast after 12z Tue, though the probability is too low at any given terminal to include in the TAFs.

KPDX AND APPROACHES...VFR conditions expected into tonight. Northwest winds become south to southwest overnight, and a marine push brings 70-90% chances of MVFR cigs near 2 kft after 10-12z Tue. Conditions return to VFR by 18-21z Tue as diurnal mixing lifts cigs to 4-6 kft. -03/36

MARINE

Upper-level ridging continues to shift inland while being replaced by an upper-level low over the Northeast Pacific. Breezy north to northwest winds persist over the waters this afternoon, with gusts to 25 kt most widespread within 10 NM of the southern WA and far northern OR coasts including near the mouth of the Columbia River. A Small Craft Advisory remains in effect in these areas through 8 PM PDT until gusty winds ease this evening. Meanwhile, largely wind-drived seas continue at 4-6 ft.

A potent frontal system looks to traverse the waters midweek, with winds backing out of the south to southwest by early Tuesday morning ahead of its arrival. Winds will then build through Wednesday afternoon, reaching 30-35 kt. Gusts as high as 40-45 kt are most likely from 8 AM through 2 PM PDT Wednesday across the majority of the waters, and from 11 AM through 5 PM PDT Wednesday within 10 NM of the coast. There is a 20-40% chance of isolated storm-force gusts within 20 NM of the coast. Seas are expected to reach 10-13 ft during this period, with only up to a 5-10% chance that significant wave heights exceed 15 ft. Behind this system, there are 10-20% chances for thunderstorms across the waters on Thursday. Winds and seas will then steadily ease through the remainder of the week and into the weekend. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Gale Watch from Tuesday evening through Wednesday evening for PZZ210-251>253-271>273.