textproduct: Portland

SYNOPSIS

Isolated to scattered rain showers continue through today, with an isolated thunderstorm possible north of US-26. Low chances for low-elevation snow along the Cowlitz Valley early Tuesday morning with travel impacts unlikely. A warmer and wetter pattern midweek will bring widespread rain, with the highest rainfall totals most likely north of US-20, Uncertainty in the placement of heavier rainfall increases this weekend.

LONG TERM...Thursday through Sunday

Transitioning into later in the week, precipitation will continue with warm air lingering over the area. The bulk of the precipitation will stream to the north with the jet stream. Heavier rain potential remains high on Thursday into Friday where the mean 24-hr precipitation totals ending Friday morning are around range from 2.0-2.5" of precipitation in the south Washington Cascades and 1.5-1.9" along the north Oregon Coast Range. Inland within the Willamette Valley there is anywhere from 1.0-1.25" in Portland to 0.2-0.4" in Eugene. If the atmospheric river that sits over Washington shifts southward, we could see more impactful rain - especially along our more susceptible Coast Range and Willipa Hills rivers.

On Friday, some models are attempting to shift the Atmospheric River further south which puts us in a borderline wet vs dry scenario. In general, the ECMWF favors a wetter solution overall on Thursday night into Friday. It is not clear yet whether the heavier rain will end Thursday night, Friday, Friday night, or Saturday. As such, there is still a very large degree of model spread in regards to total rain amounts. Depending on how quickly precipitation falls over a given watershed or urban area and the duration of precipitation, these higher end solutions could result in some river flooding and urban flooding (however this would represent the worse case scenario with a 10% chance of occurring; HEFS guidance for river flooding backs this up and shows probabilities under 10%, and under 5% for slower- responding rivers). Lower to middle end QPF solutions would result in no hydro concerns at all. As of right now, the most likely outcome is for minimal to no flooding concerns, however this system will need to be monitored closely over the coming days.

Heading into next weekend, uncertainty increases regarding exact temperatures and precipitation amounts. 500 mb heights show two potential scenarios with half of the clusters favoring a wetter solution, and half a drier outcome. The same rings true for a warm vs cooler scenario. An example of this difference lies in Salem where the 10th to 90th percentiles for maximum temperature on Saturday show a range of 49 deg F to 61 deg F...an 11 degree spread. There is less of a discrepancy as you move further north. Sunday though brings even higher uncertainty with the 10th to 90th percentile in Portland ranging from 45 deg F to 61 deg F. In Eugene, the maximum temperature spread ranges from 48 deg F to 69 deg F; nearly a 20 degree difference. This helps show just how much uncertainty lies in the long term forecast. Ultimately, it comes to how strong the high in the Pacific builds, the jet tracking north or south, and just how much of the warm air we will see. -27

AVIATION

VFR flying conditions likely (75% or higher chances) to continue at terminals throughout the period. Light rain showers, with highest coverage along the coast and north of KSLE, may briefly restrict vis/cigs to MVFR, but confidence in timing of impacts is too low to include in TAFs. An isolated thunderstorm is also possible to the north of KTMK-KPDX, but terminal impacts are unlikely (<10% chance). Tonight after 06z Tue, there are low chances for low-elevation snow from KKLS northward along the Cowlitz Valley, although light snowfall rates and minimal surface accumulations will limit surface impacts. West winds at 5-10 kt continue through this evening and into tonight, turning out of the south to southwest at 5 kt or less after 06-12z Tue.

KPDX AND APPROACHES...VFR flying conditions likely (80% chance) to persist through the period. Light rain showers may briefly bring MVFR cigs/vis, especially through this afternoon and evening before shower coverage becomes more isolated after 06z Tue. There is a less than 10% chance of an isolated thunderstorm during this same time period, but terminal impacts are very unlikely. West winds of 5-10 kt turn out of the south to southwest at 5 kt or less after 06z Tue. -36

MARINE

Conditions remain elevated in regards to wind as a frontal system in the Gulf of Alaska continues to drop southward. Overall, winds have bounced around quite a bit with gusts ranging from 17-25 kt. Seas have remained steady at 7-9 ft at 13 seconds. This morning we will see a brief lull in winds but seas will quickly build. Combined seas of 9-11 ft are expected this afternoon and will spread to encompass all of the waters. Have expanded the Small Craft Advisory to account for the building seas in the southern and inner waters. The Columbia River Bar has a low probability of being impacted until Tuesday as the ebb reaches 10 ft in the morning.

A secondary low pressure system and accompanying warm front will skirt over Vancouver Island on Wednesday increasing the north-south pressure gradient. A southerly wind reversal is expected and followed by a substantial increase in both sustained winds and gusts. Confidence has decreased slightly in Gale Force Wind potential Wednesday afternoon and evening with the NBM projecting a 30-40% chance for hourly wind gusts to exceed 34 knots during this period. The main time frame of concern looks to be Wednesday afternoon. The overall coverage remains uncertain at this time as it looks to mainly impact the central waters of PZZ252/272. Have decided against issuing a Gale Watch at this time, but there is a very high probability that one may be issued in the coming forecasts.

Seas are likely to respond as well, rising up into the 14-17 ft range at 10-11 seconds which would lead to a Hazardous Seas state. At least a coastal jet is not forecast, but wind gust speeds will hover right around 34-40 kt. Due to stronger winds aloft, they could mix down and areas around the Columbia River Bar have around a 10% chance of wind gusts of 50 kt or greater. Conditions gradually settle down Thursday through the end of the week but the overall pattern looks to remains active. -27/99

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 5 PM PDT Tuesday for ORZ126.

WA...Winter Weather Advisory until 5 PM PDT Tuesday for WAZ211.

Winter Storm Watch from Tuesday afternoon through Wednesday afternoon for WAZ211.

PZ...Small Craft Advisory from 8 AM Tuesday to 8 AM PDT Wednesday for PZZ210.

Small Craft Advisory from 8 PM this evening to 8 AM PDT Wednesday for PZZ251>253-273.

Small Craft Advisory until 8 AM PDT Wednesday for PZZ271-272.