textproduct: Portland

UPDATED AVIATION DISCUSSION

SYNOPSIS

High pressure persists through Tuesday with temperatures warming each day. Moderate HeatRisk is expected along the I-5 Corridor north of Salem, including the Portland/Vancouver Metro, as temps potentially warm into the upper 80s to lower 90s. Temperatures cool again to near seasonable averages by Wednesday as high pressure is replaced with more zonal flow aloft. Chances for rain remain low (less than 10%) through at least Thursday, except for afternoon showers in the Cascades on Wednesday. Cooler conditions likely by next weekend as a low pressure system in the Gulf of Alaska brings increasing chances for rain later Friday into Saturday.

LONG TERM...Wednesday through Saturday night

A negatively tilted upper level trough is expected to swing across the Pacific NW on Wednesday, returning seasonable temperatures with mostly dry conditions. There remains high confidence that temperatures cool back into the 70s on Wednesday and persist through the end of the week. Models and their ensembles are in fairly good agreement that a low pressure system in the NE Pacific weakens as it moves toward the Gulf of Alaska on Wednesday. An associated frontal band well ahead of the surface low will approach the coast, but guidance continues to trend down with minimal, if any, precipitation spreading inland. In fact, NBM guidance generally shows less than a 10% chance of rain through Thursday. Except, there are around 20-30% chances for showers in the Cascades Wed afternoon and evening as the afore- mentioned trough lifts across the region. The timing of the trough could make things somewhat interesting east of the Cascades.

Uncertainty increases late in the week and into the weekend regarding the weakened low pressure system potentially dropping down from the Gulf of Alaska toward the Washington coast. In general, there are increasing chances for rain on Saturday, but very low chances for any impactful weather. Ensemble guidance suggests 24- hour rainfall accumulation could range anywhere from zero (10th and 25th percentiles) up to potentially exceeding 0.5 to 1.0 inch (closer to the 90th percentile). Despite the potential pattern shift, onshore flow is likely, which will usher in cooler marine air and thus lower temperatures. The overall spread between the 10th to 90th percentile (essentially the coolest and warmest scenario) ranges from around 61-73 deg F in the lowlands with little to no change along the coast. /DH

AVIATION

High pressure off the coastline begins to slowly build overhead the remainder of the night through Monday facilitating widespread VFR conditions. Winds aloft will gain a slight offshore component tonight while lightening, a trend we've already begun to see in surface observations across the region. While the bulk of any marine stratus/fog likely remains offshore come sunrise Monday, both KAST and KONP will have a ~20-25% chance for sub-MVFR conditions between 13-16z. Expect winds to increase again Monday afternoon with gusts around 20-25 knots along the central Oregon coast, otherwise lighter winds by comparison inland.

KPDX AND APPROACHES...Expect VFR conditions to persist beneath mostly clear skies through the TAF period. Winds generally remain out of N to NW. -99

MARINE

High pressure building offshore will continue to generate gusty north winds through much of Monday. A weak feature moving overhead through tonight will support accelerated winds today, before easing overnight and re-strengthening on Monday. Small Craft Advisories are now in effect across all coastal waters for gusts of up to 25-30 kt, with the strongest winds south of Cape Foulweather and weaker flow north of Cape Falcon. The Advisory north of Cape Falcon including the Columbia River Bar is expected to expire late this evening, while areas south of Cape Falcon will maintain hazardous conditions through late Monday evening. Weakening high pressure Monday night through Tuesday will see wind gusts across the waters ease below 20 kt. Seas continue at 4-8 ft at 9-10 seconds with a dominant WNW swell.

A pattern change midweek will see high pressure replaced by a trough offshore. Winds will shift southwesterly ahead of a decaying front Wednesday morning, then turn northwesterly again in its wake by Wednesday night. Rain shower chances will reach 30-50% beyond 30-40 NM Wednesday morning, but remain below 15-20% elsewhere as the weakening front moves onshore. Unsettled weather continues through the remainder of the week, with further rain possible next weekend as another front traverses the waters. A persistent westerly swell will support continued seas of 4-8 ft at 10-11 seconds. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ210. Small Craft Advisory until 2 AM PDT Monday for PZZ251-271. Small Craft Advisory until 11 PM PDT Monday for PZZ252-253-272- 273.