textproduct: Portland

SYNOPSIS

A progressive weather pattern with prevailing onshore flow continues through the weekend. Weak cold front on Saturday ushers in cooler temperatures, chances for light rain, and locally gusty west winds over the weekend. Wind related impacts are highest in the central Columbia Gorge. Trailing this frontal system, a broad pattern change favoring warmer and drier weather is expected by the middle of next week with a period of northerly to northeasterly winds as well. Elevated temperatures on Tuesday before cooling down the remainder of the week.

LONG TERM...Monday through Friday

While Sunday will see little change, the same cannot be said for Monday and Tuesday. On Monday there continues to be growing confidence in a large scale ridging pattern building over the Pacific Northwest. While there continue to be ensemble models that favor a trough or zonal flow, nearly all of the 500 mb clusters show at least some form of a ridge forming. This ridge will lead to warming temperatures and lower relative humidity on both Monday and Tuesday. A feature we continue to dive into is the thermal trough and whether or not that is going to develop on Monday. Based on surface analysis plots, there appears to be a thermal trough forming along the coast which moves inland through the afternoon on Monday. This trough would cause northerly winds to amplify in the lower elevations, and increase easterly winds at 850 mb (around 5000 ft). These easterly winds will bring in drier air from east of the Cascades and the downsloping action will increase drying in the lower elevations and in the Willamette Valley. Combine this with daytime heating and a deep layer of dry air, we will see relative humidity drop within the interior lowlands on Monday. In addition, the thermal trough will narrow the isobars in the area and thus increase the pressure driven northerly wind. Therefore, could see gusts up to 25 mph within the Willamette Valley, Clark County lowlands, and along the coast. In the southern Willamette Valley around Eugene there is a higher probability of seeing these wind speeds than in the north. This is likely due to more of a channeling component and more of a stark north-south pressure gradient. These dry and windy conditions may lead to increased fire weather concerns.

While this is still the favored pattern right now, it is not the only possibility. There still remains the potential for zonal or flow and more of a zonal flow which would cause humidity to increase above critical levels and winds would be more westerly and thus not nearly as strong. Currently, the GFS ensembles show a much weaker pattern on Monday which would favor gusts closer to 15 mph while the ECMWF is closer to 25-30 mph.

The other impactful weather component is the heat that will arrive on Tuesday. Temperatures continue to stay steady with most areas seeing at least a 75-80% chance of meeting of exceeding 90 degrees F. Moderate HeatRisk remains for much of the Willamette Valley and the lower I-5 corridor. Mid to late week it'll all come down to the exact pattern amplification and longitudinal placement of the ridge over the far eastern Pacific. In the warmer solutions (favored by GFS and more GEFS members), stronger inland ridging and the resulting coastal thermal trough may be sufficient to drive decent offshore flow and potential gusty north to northeast winds along the coast and within the Willamette Valley. The EPS keeps this ridge flattened comparatively but still shows a signal for north to northeast winds, albeit weaker. At least the bulk of deterministic and ensemble guidance depict a return to W-NW winds by late Thursday signaling moderating conditions. -27/99

AVIATION...Through 00Z Sunday

Currently, widespread VFR conditions with clear skies and coastal stratus hanging away from the coast. Inland terminals will continue to see VFR conditions through the TAF period (with just a 20-30% chance of MVFR CIGs between 12-19Z Sat), while coastal terminals will see conditions deteriorate towards MVFR/IFR/LIFR by 02-04Z Sat as low stratus pushes into the coast. Guidance currently shows a 70-90% chance of MVFR conditions, 30-50% chance of IFR, and 20-30% chance of LIFR between 02-18Z Sat. Beyond 18Z Sat, coastal terminals may stay MVFR (30-60% chance of MVFR CIGs). Otherwise, expect winds around 5-10 kt through the period, northwesterly through the afternoon, then shifting southwesterly by 03-09Z Sat for most terminals. Winds will then gradually shift more westerly through late Saturday.

KPDX AND APPROACHES...Clear skies and VFR conditions will continue through the period, with northwest winds generally 5-10 kts shifting to southwesterly around 12Z Sat. Additionally, there is a 20% chance of MVFR CIGs between 12-19Z Sat. -12

MARINE

Fairly benign conditions continue across the waters today as winds remain out of the west to northwest. On Saturday, a frontal system approaching the waters brings increasing winds, mainly for the outer waters and the inner waters north of Cape Falcon. This front will bring chances for light rain Saturday morning, likely dissipating during the afternoon hours.

Westerly flow behind the frost will then shift northerly by Sunday as surface high pressure builds offshore. Chances for maximum wind gusts of at least 21 kt are around 80-90% from Sunday afternoon/evening through at least Tuesday evening - highest probabilities in the outer waters. Additionally, there is a chance for a period of marginal Gale Force northerly winds late Monday afternoon (20-40% chance south of Cape Foulweather, mainly the inner coastal waters). Wave heights through this time period remain around 5-7 ft at 7-9 seconds, except for up to 8-9 ft in the outer waters south of Cape Foulweather OR.-12/99

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.