textproduct: Portland

SYNOPSIS

A robust early-season low pressure system will bring continued showers across the region today in addition to a burst of gusty winds (mainly this morning). Wind Advisories are in effect for the central and northern Willamette Valley into southwest Washington through 11am. At least the rather progressive pattern ensures drier weather quickly returns by Saturday morning through the remainder of the weekend. Chances (20-45%) for rain increase again Tuesday and Wednesday along with forecast uncertainty.

DISCUSSION..Now through Thursday

This morning the core of an upper level low pressure system is rapidly approaching far northwest Oregon and southwest Washington with the region transitioning to a post-frontal airmass as the initial cold front which brought widespread stratiform rainfall shifted east of the Cascade crest-line. The immediate focus this morning regards gusty southwest to south winds associated with this system which are already being observed along the coast and in portions of the Willamette Valley. Confidence has increased surface low will come onshore around Pacific County, WA before swinging northeastward into the Puget Sound. This sets the stage for a period of time this morning when a decent surface pressure gradient orients itself perpendicular to I-5. While this alone can drive breezy winds, the gradient likely comes into alignment during a period of time when increasing instability near the surface has the potential to tap into the passing low level sting-jet wrapping around the backside of the low pressure center. The majority of the latest deterministic guidance depicts 925-850mb winds peaking near 40-50 knots as that N to S surface pressure gradient is overhead. So the ingredients your looking for when it comes to driving near Advisory level wind gust, and exceeding those projected by the baseline NBM, will be present. Exploring the latest CAMs add a notable amount of confidence in wind gusts reaching in the 35-50 mph range from the Central Willamette Valley northward into Southwest Washington. Expect your highest gusts in valley locations with a good south to north fetch and exposed ridgetops. Another area which will likely see some locally stronger winds is east of the crest within the Hood River Valley, downslope acceleration on east- facing slopes will see strong winds prolonged through Friday afternoon. Wind Advisories are in effect for the aforementioned areas. While these forecast winds may not be that impressive compared to some of our stronger winter-time storms, the early season nature of this system may favor exacerbated tree damage at lower wind speeds as full leaf cover acts to increase drag on tree limbs.

Otherwise, cooler air in place aloft near the low center will support sufficient instability to maintain rain showers at times today, and daytime surface heating will act to further increase instability and generate ~15-20% chances for thunder anywhere between the coast and the Cascade crest. An isolated cold core funnel also cannot be ruled out, most likely along the coast, but impacts at the ground from these features are unlikely. It's worth noting while limited instability will cap thunderstorm intensity, the availability of higher momentum air at low altitudes will make any thunderstorm or heavy rain shower capable of producing gusty outflow through the afternoon hours, which may also support local areas of wind damage. Rainfall amounts remain unlikely to cause impacts, with annual minimum streamflows able to handle runoff without flooding concerns.

Rain showers will end across most areas west of the Cascades this evening, and in the Cascades tonight. Progressive shortwave ridging from this point into Saturday will then favor rapid drying. Temperatures hold below average during this period, with afternoon highs only in the upper 50s to low 60s across the region today. Cooler air behind the frontal boundary will see overnight lows fall into the 40s by sunrise Saturday morning, perhaps to the upper 30s in the Hood River Valley. There is a 15-30% chance of low temperatures below 37 degrees in the Odell area, and 40-60% chances farther up the valley near Dee and Parkdale - heavily dependent on clearing skies. Frost Advisories may be needed in these areas if confidence in frost impacts increase.

The progressive pattern aloft will see upper-level ridging and surface high pressure support dry weather into early next week, although moderating temperatures are still likely to remain below seasonal norms with highs in the mid to upper 60s across the lowlands. Cool overnight lows may favor further chances for frost in the Upper Hood River Valley, with decreasing confidence after Saturday morning. The majority of long-range ensemble members favor troughing to return to the Pacific Northwest Tuesday into Wednesday of next week, although there remains much spread in its intensity and location. Chances for rainfall therefore increase to 20-40% during this period, generally higher to the north, along the coast, and within areas of terrain. -99/36

AVIATION...Through 12Z Friday

The region sits in a lull as far as precipitation is concerned as the main rainfall band shifts further east of the Cascades and rain showers begin to make their way onshore. Southerly flow aloft turns westerly later Friday morning as an upper level trough moves across the region, while a compact surface low near the coast drives gusty surface winds this morning. Current conditions are a mix of VFR to MVFR although we'll trend towards the latter through the morning hours. At the coast, probs for IFR peak between 12-18z at around 30-50%. Showers are expected to continue through this afternoon into the early evening. Best chance for thunderstorms (20%) appears to be near the north Oregon coast, with chances for afternoon thunderstorms generally 15%. South winds peak through 17-19z this morning with gusts up to 30-40 kt.

KPDX AND APPROACHES....Conditions likely remain a mix of VFR to MVFR this morning with lower CIGs/VIS present in showers throughout the day. Expect a return to dry weather tonight. South winds have already begin to increase, peaking around 30-35 knots 14-18z before decreasing then turning more westerly after 21z. Lighter westerly winds return tonight. -99/DH

MARINE

The center of a robust low pressure system is currently tracking into far northwest Oregon and southwest Washington this morning bringing a noticeable increase to winds and seas. Gale Warnings are current in effect for the inner and outer waters, including the Columbia River Bar, through the midday into the afternoon hours before they sequentially transition to Hazardous Seas Warnings into the late afternoon and evening. Winds and seas have already increased across the waters off the central Oregon coast with Buoy 50 recording gusts of 35-40 knots accompanied by steep 12 ft at 8 second seas. Expect similar conditions to shift further north over the next few hours. While winds begin to decrease late morning, expect wind-driven steep and choppy seas to peak around 13-16 ft, gradually subsiding late Friday afternoon into the evening. By Saturday morning, seas settle back to around 5-7ft at 8-9 seconds.

From this point our focus shift to northerly winds increasing over the weekend, strongest Saturday afternoon and evening when wind gusts increase to 20-25 kt, mainly over the outer waters. Back to rather benign weather conditions on Monday as winds stay below 20 knots and seas hold in the 5-8ft range before a slight increase in wave heights on Tuesday into Wednesday (7-10ft) due to the arrival of a WNW swell. -99

PQR WATCHES/WARNINGS/ADVISORIES

OR...Wind Advisory until 11 AM PDT this morning for ORZ108>115-123. Wind Advisory from 8 AM this morning to 8 PM PDT this evening for ORZ121. WA...Wind Advisory until 11 AM PDT this morning for WAZ203>208. PZ...Gale Warning until 2 PM PDT this afternoon for PZZ210-251-252- 271-272. Hazardous Seas Warning from 2 PM this afternoon to 8 PM PDT this evening for PZZ210-251-252-271-272. Gale Warning until 11 AM PDT this morning for PZZ253-273. Hazardous Seas Warning from 11 AM this morning to 5 PM PDT this afternoon for PZZ253-273.