textproduct: Portland

SYNOPSIS

A weak cold front moving through the region today ushers in cooler temperatures, chances for light rain, and locally gusty west winds. Wind related impacts are highest in the central Columbia Gorge. The below normal temperatures won't last long however. Warmer and drier conditions are expected by the middle of next week with a period of northerly to northeasterly winds Mon/Tue as well. Elevated temperatures peak on Tuesday before gradually moderating towards seasonable norms the remainder of the week.

LONG TERM...Monday through Friday Night

Early next week the proverbial weather pendulum begins to rapidly swing the other direction. On Monday confidence continues to grow for a large scale ridging pattern building just off the coast of the Pacific Northwest. While there continue to be a small handful of 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 - lowest on the latter day.

The one weather feature we continue to closely watch is the near surface thermal trough early next week which this time of year helps to dictate our fire weather concerns. Typically it builds overhead from southwestern Oregon (dry northerly winds when this occurs) then it modulates diurnally back-and-forth from the coast/coast range (AM) towards I-5 and/or the Cascades (PM) helping to influence the magnitude and placement of offshore flow. Currently it forms along the coast then moves inland through the afternoon on Monday as it builds northward from southwestern Oregon. This progression 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. 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/fetch component and a tighter north-south pressure gradient. These dry and windy conditions may lead to elevated fire weather concerns as they overlap mid/late afternoon into the early evening on Monday. That said, confidence is not yet high enough to pull the trigger on any fire-weather related watches/warnings at this time - we'll see how the model guidance continues to evolve the next few forecast cycles.

Temperatures the peak on on Tuesday 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. The majority of of deterministic and ensemble guidance depict a return to W-NW winds by late Thursday and Friday signaling moderating conditions. -99/27

AVIATION...Through 18Z Sunday

Lingering MVFR CIGs inland and IFR/MVFR conditions along the coast continue to dissipate, likely improving back to VFR by 21-23Z Sat. The passing frontal system will continue to maintain a potential for light rain until 21Z Sat, primarily along the coast. Additionally, expect south to southwesterly winds around 5-10 kt this morning to shift more westerly by 21-23Z Sat, then shifting to northwesterly around 00Z Sun and remaining so for the rest of the TAF period.

KPDX AND APPROACHES...MVFR conditions may linger until 21Z Sat, otherwise expect VFR conditions through the TAF period, with only a 10-20% chance of MVFR CIGs redeveloping after 09-12Z Sun. Southerly to southwesterly winds around 5-10 kt turn more west- northwesterly around 21-23Z Sat and remain northwesterly through the remainder of the period. May see some light rain showers before 21Z Sat. ~12

MARINE

A frontal systems will traverse the waters this morning slightly increasing winds and bringing decent chances for light rain/mist. At least any chances for precipitation decrease in the afternoon hours. Westerly flow behind the frost will then shifts northerly by Sunday as surface high pressure builds offshore. Chances for maximum wind gusts of at least 21 kt are around 85-95% from Sunday afternoon/evening through at least Tuesday night with the highest relative probabilities across the outer waters. Given this high confidence in Small Craft conditions, a Small Craft advisory was issued for the inner and outer waters, including the Columbia River Bar, starting Monday mid/late afternoon. Once we get in to the middle of the week these Small Craft concerns primarily shift to the outer waters but likely persist through the end of the week. Wave heights through the forecast period remain around 5-8 ft at 7-9 seconds. -99

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 5 PM Sunday to 2 AM PDT Monday for PZZ210. Small Craft Advisory from 5 PM Sunday to 5 AM PDT Tuesday for PZZ251-252-271-272. Small Craft Advisory from 2 PM Sunday to 5 AM PDT Tuesday for PZZ253-273.