textproduct: Portland

SYNOPSIS

Somewhat calmer conditions are anticipated today as the region sits in-between weather disturbances. This'll change over weekend however as broad upper-level trough draped across the eastern Pacific steers a few frontal systems our way into early next week in addition to inducing a period of breezy/gusty east winds through the Cascade gaps. Model guidance hints at the broader trough feature off the coast dissipating around the middle of next week with modest forecast confidence in the continuation of a progressive large scale pattern for the end of the forecast period (Day 6-7).

DISCUSSION...Now through Thursday night

Current radar, satellite, and surface observations show fairly calm and dry conditions across the region early this morning albeit with ares of fog and cooler temperatures in the upper 20s to mid 30s from the coast through the inland valleys. This is brief respite is all thanks to a transient upper-level ridge of high pressure passing overhead keeping any lingering shower activity confined to the coast, coast range, and Cascades as we head through today. At least for the morning commute today (Friday), keep in mind any remaining moisture/fog may freeze approaching sunrise resulting in low visibility and/or locally slick conditions- especially on elevated surfaces like bridges/overpasses across dips and swales in the local terrain.

Then starting on Saturday ensemble and deterministic guidance are in excellent agreement showing the ridge shifting to our east while a broad upper-level low amplifies over the eastern Pacific. As our aforementioned ridge moves eastward temperatures aloft expected to warm pushing snow levels to 3500-4500 ft by Saturday, then 4000-6000 ft on Sunday (highest central OR Cascades). Also of note, offshore gradients are expected to strengthen Saturday and Sunday as low pressure systems moves into the eastern Pacific. Most EPS ensemble members and the GFS are currently forecasting a -7 to -8 mb pressure gradient from KDLS to KPDX come late Saturday night and if this does occur would drive easterly gusts up to 40-50+ mph within the western part of the Columbia River Gorge. Across the eastern Portland/Vancouver metro area confidence is moderate for gusts between 30-40 mph extending into portions of the I-205 corridor. While the expected pressure gradient is fairly impressive, it's worth mentioning the airmass on the east side of the Cascades isn't particularly cold (this a factor which would otherwise help to even further enhance Gorge winds) thus keeping the strongest winds somewhat muted due to the lack of the added temperature gradient. Other Cascade gaps locations should anticipate breezy/windy conditions as well although headed to the coast and coast range winds likely gain a southerly component. Models are in good agreement the offshore gradient noticeably decreases on Sunday into Sunday night.

Regarding precipitation this weekend into early next week the first disturbance flung into the Pacific Northwest by the broad trough to our east will be in the form of a weak warm-frontal boundary on Saturday followed by a second stronger frontal system on Sunday into at least Monday. Given the high confidence in IVT values greater than 250 kg/ms for an extended period of time (>24hrs) accompanying the latter disturbance, it can be classified as a weak atmospheric river, but at this time it doesn't appear particularly impactful for any one region. That said, there is a decent uncertainty in regards to to precipitation amounts and location of the heaviest rainfall Sunday into early next week. Given the largely SSE to NNE orientation of the moisture plume large scale orographic enhancement won't be particularly favorable outside portions of the coast range (would want more W to E orientation) so it'll likely come down to frontal dynamics/placement. Precipitation rates will also likely be enhanced by a secondary shortwave feature and accompanying cold-front ejecting into the Pacific Northwest late Monday into Tuesday. Currently, the highest probabilities of enhanced rainfall are pointed at portions of southern Oregon and California but we'll need to keep an eye on the coast range. Depending on maximum precipitation amounts as well as snow melt, there could be minor hydrological concerns as the start of next week approaches - the HEFS gives a 5-10% for our flashier coast range river to reach minor flood stage. Nevertheless, we will continue to monitor conditions as they develop.

Headed into Wednesday through the end of the week ensembles favor either weak long-wave ridging or zonal flow aloft keeping the best chances for lingering shower activity pinned to our north, at least initially. However, around 75% of ensemble members from the EPS, GEFS, and GEPS depict the upper-level flow pattern buckling enough for increased chances for precipitation late Thursday into Friday. At least confidence is moderate to high conditions during this mid to late week period remain fairly close to climatological norms for late February. -99

AVIATION

Generally VFR/MVFR flight conditions across the airspace, with isolated pockets of IFR within the Willamette Valley. Through 20Z-23Z Friday, expect conditions to improve towards predominantly VFR, with those conditions the persisting through the majority of the TAF period. Winds generally light and southerly, then becoming more easterly around 22Z Fri through 00Z Sat as an offshore pressure gradient sets up. This offshore gradient combined with increasing high clouds will help to maintain VFR conditions across the airspace through at least 10Z-14Z Saturday. A weak frontal band towards the end of the TAF period looks to bring a mixture of VFR/MVFR conditions across the airspace as well as precipitation in the form of rain. This will also result in winds along the coast taking on a south/southeasterly direction with gusts around 25 kt for areas around KONP. This will also aid in increasing the easterly winds around the western end and the mouth of the Columbia River Gorge with gusts around 25 kt. These gusts are expected to impact KTTD starting around 13Z Sat, but will likely (75-85% chance) not impact KPDX.

KPDX AND APPROACHES...Current MVFR conditions should improve to VFR through 20Z Fri. Afterwards, VFR conditions expected to persist with light, southerly winds through at least 10Z Saturday. A weak frontal band towards the end of the TAF period looks to bring a mixture of VFR/MVFR conditions across the airspace as well as precipitation in the form of rain. Around 13Z Sat, easterly winds from the mouth of the Columbia River Gorge are expected to increase, but these increasing winds will likely (75-85% chance) not impact KPDX. /42

MARINE

Today into tonight expecting largely benign conditions by February standards with the sea state driven by a slowly decreasing WNW swell. However, this changes on Saturday thanks to a strong low pressure system amplifying over the eastern Pacific causing winds to switch southerly and increase considerably. A Gale Watch in effect for the outer waters starting early Saturday morning continuing through Sunday night into Sunday morning. The challenge will be the orientation of the frontal system and associated low. In it's current position, there is a band of potential (30-40%) storm force winds just to our south and west, with a slice moving into the outer waters of PZZ273 west of 30NM. Chances of these storm force winds becoming more widespread is around 30-40% within the southern waters. The northern outer waters appear to escape these speeds and should remain firmly in Gale Force wind criteria. Another consideration is the impact these winds will have on seas. Wave heights will be predominately wind wave driven coupled with a fresh southerly swell in addition to a weak 4 ft at 13 second northwesterly background swell all helping to amplify seas. While winds won't be as strong over the inner waters, a Hazardous Seas Watch is in effect including for the Columbia River bar, given the potential seas state. Winds and seas gradually decrease Sunday into early next week with significant wave heights expected to drop below 10 ft on Tuesday. -99/27

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Hazardous Seas Watch from late tonight through Sunday morning for PZZ210-251>253.

Gale Watch from late tonight through Sunday morning for PZZ271>273.