textproduct: Portland

SYNOPSIS

Largely dry weather expected through midweek. Near normal temperatures today will warm above average Tuesday and Wednesday. Chances for wetter and cooler weather return late in the week with potential for a more robust system next weekend.

DISCUSSION...Now through Sunday

An upper level trough continues to move toward the PacNW, which has increased mid and upper level cloud cover across the region. This increased cloud cover has kept the surface insulated with temperatures ranging from the low 40s in the more sheltered valleys to mid to upper 40s elsewhere. Agreement among the ensembles remains very high in the trough elongating and pinching off into a closed upper low that drifts southward toward northern California through mid week. Precipitation chances remain low over land with the best chance over the Lane County Cascades at 15-20%. Cooler air associated with the trough will drop highs near normal today with the interior lowlands reaching the low to mid 60s and the coastal areas the mid to upper 50s.

Heading into Tuesday and Wednesday, the upper low will continue to drift toward California while heights rebound locally. This will keep conditions dry and will help temperatures jump back above normal into the upper 60s to low 70s across the interior lowlands. Overnight lows drop into the low 40s for most locations but the typical cool spots like the Upper Hood River Valley and portions of the coast range may drop into the upper 30s. Lows at this time could drop below 37 degrees briefly so not expecting widespread frost development so will hold off on Frost Advisories at this time.

The latter half of the week again trends toward cooler temperatures and modest chances for rain. Cluster analysis shows good agreement in troughing developing over the Gulf of Alaska and dropping southeast toward the PacNW Thursday into Friday. This pattern would favor a return to near normal temperatures and chances for light precipitation. Overnight lows Thursday and Friday night are expected to be much cooler with more widespread temperatures in the mid to upper 30s. If trends continues, will likely need Frost Advisories for portions of the lowlands, coast range valleys and other typical cool spots. Potential remains low for a more robust system to impact the PacNW this weekend as a trough drops out of the Gulf of Alaska. This system could be the first atmospheric river of the season, bringing substantial rain and possibly some gusty winds across the region. Uncertainty remains high at this time but details should become clearer in the coming days. -19

AVIATION...Through 06z Tuesday

A low pressure system dropping southward off the coast will spread mid and high level clouds across the region through 06z Tuesday. Expect light winds and predominantly VFR conditions across the area through this time period. It should be noted that around 40-50% of our high resolution weather models suggest a mixture of IFR and MVFR stratus will develop along the central coast of Oregon between 06-12z Monday before spreading eastward into the southern Willamette Valley towards 12-18z Monday. Confidence in this scenario is not high, but it does seem modestly plausible. Patches of stratus are just beginning to develop along the Cascade foothills, but these seem to be in the 4-7kft ASL range per ASOS observations. Will keep TAFS VFR for now for later tonight into Monday morning, but will be monitoring trends and additional guidance and may need to make amendments given the uncertainty. Also, dewpoint depressions are currently lowest at KAST, KONP and KEUG so fog concerns are highest at these airports if these sites continue to radiate out overnight.

KPDX AND APPROACHES...A low pressure system dropping southward off the coast will spread plenty of mid and high level clouds across the region through 06z Tuesday. Expect predominantly VFR conditions and light winds through this time period. There is a 20% chance for MVFR or high end IFR conditions to develop in the 12-18z Monday timeframe, but suspect a large band of mid and high clouds just beginning to move overhead will minimize fog and low cloud chances in the vicinity of KPDX later tonight.

MARINE

A continued west-northwesterly swell and weakening northerly winds will see seas of 8-9 ft today ease to 6-7 ft by early Monday morning. A system dropping southward well offshore will generate a stronger westerly swell arriving late Monday which will push seas to 8-11 ft, highest beyond 10 NM and south of Cape Falcon, where Small Craft Advisories have been hoisted from 5 PM PDT Monday through 5 AM PDT Tuesday before seas ease below 9-10 ft. Strengthening northerly flow in the wake of this passing system may support gusts peaking at 20-25 kt on Tuesday, potentially requiring an extension or expansion of the advisory if confidence in the coverage and duration of stronger winds increases. Conditions otherwise improve midweek as winds and seas ease, but a more active pattern looks to return late in the week. Increasing northwesterly winds and a northwesterly swell could push seas close to 10 ft again from Thursday into Friday, and a more potent weather system next weekend will also see increased chances for seas above 10 ft and gusty, potentially gale-force winds. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.

WA...None.

PZ...Small Craft Advisory from 2 AM to 5 PM PDT Tuesday for PZZ271.

Small Craft Advisory from 5 PM this afternoon to 5 PM PDT Tuesday for PZZ272-273.