textproduct: Portland

SYNOPSIS

Showers will increase in coverage Monday morning and afternoon behind a cold frontal passage. Although showers continue Monday night into Tuesday, overall coverage and intensity will be on the decrease. Temperatures aloft will be trending cooler Sunday night through Tuesday morning, pushing snow levels down to around 1000-1500 ft late Sunday night into Monday morning and then to around 500 ft Monday night into Tuesday morning. This will result in some accumulating snow for elevations above 500-1000 ft with travel impacts expected, especially over the Cascade passes. Little to no impacts are expected below 500 ft where precipitation will fall mainly in the form of rain or a rain/snow mix. Becoming wet and breezy Wednesday through Friday with the arrival of a moderate atmospheric river, however river flooding is unlikely at this time (90-95% chance river flooding will NOT occur).

LONG TERM...Tuesday night through Saturday night

The long term forecast is highlighted by wet and breezy conditions Wednesday through Friday with the arrival of a moderate atmospheric river, however river flooding is unlikely at this time (90-95% chance river flooding will NOT occur). Forecast details become more uncertain over the weekend, especially in regards to precipitation amounts and exact temperatures.

Beginning with the Tuesday night and Wednesday timeframe, models and their ensembles are in good agreement for widespread rain to begin over western WA and much of northwest OR with NBM PoPs increasing to around 90% or higher. This rain will be with the arrival of an atmosphere river embedded within zonal flow aloft, which looks to hit northwest OR hardest before slowly sagging southward as a moderate atmospheric river (IVT values between 500-750 kg/m/s per the GEFS and EPS) for southwest WA and northwest OR. While model ensemble guidance is in fairly good agreement regarding the strength of this system, agreement is not as great regarding the exact duration rain will last. In other words, 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. For example, the NBM 10th-90th percentile for 72-hr QPF amounts ending at 5am Saturday ranges from under one inch in the lowlands and under 2 inches in the mountains to 3-4 inches in the lowlands and 4-7 inches in the mountains. 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. For example, the NBM 10th-90th percentile for 24-hr QPF amounts from 5am Saturday to 5am Sunday range from no precipitation at all to around 1 inch, except to around 1.5-3 inches in the mountains. NBM 6-hourly PoPs gradually decrease during that time as well, lowering to near 40-50%. In addition, NBM spread for max temperatures ranges from the upper 40s to near 60 degrees. This is mainly due to uncertainty regarding how amplified a ridge over the western CONUS will be over the weekend. In fact, WPC's 500 mb EOF patterns suggest nearly 60% of model variance can be explained by this, and around 26% can be explained by the exact position of the ridge axis. If the ridge winds up centered over western WA/OR and is amplified enough, we will trend warmer and drier with spring-like temperatures. If the ridge flattens, we will trend cooler and wetter than average with more zonal flow aloft. -23

AVIATION

Predominately VFR flight conditions with occasional MVFR conditions for most terminals through 10Z Monday. This will be the result of a lull in shower activity for the majority of the airspace with the exception being along the coast north of KTMK as showers will continue to linger 10Z Monday. Otherwise, the lull in showers will cease of the remainder of the airspace starting around 10Z-15Z Monday. Then, showers will persist through the majority of the TAF period, across the airspace and will result in a 30-50% chance for MVFR conditions. Winds generally remain below 10 knots but a few gusts out of the west/northwest up to 20 knots will be possible along the coast after 18Z Monday.

KPDX AND APPROACHES...VFR conditions through much of Monday. Another front around 08Z-10Z Monday will bring a 30-50% chance for MVFR conditions near the terminal through the remainder of the TAF period. Northwest winds less than 10 kt expected. -42

MARINE

Late this afternoon a weak frontal boundary continues to move across the coast waters with winds quickly turning northwesterly in its wake. These winds combined with a northwesterly swell and low pressure system in the Gulf of Alaska push seas in the 9-11 ft at 12 seconds tonight into Monday. Expect the greatest impact over the northern outer waters where a Small Craft Advisory is in effect through Tuesday afternoon.

Headed into the middle of the week, 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 is rising in Gale Force Wind potential Wednesday afternoon and evening with the NBM projecting a 40-60% chance for hourly wind gusts to exceed 34 knots during this period. Seas likely 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 remains active moving forward.-99

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory from 5 AM early this morning to 5 PM PDT Tuesday for ORZ126. WA...Winter Weather Advisory from 5 AM early this morning to 5 PM PDT Tuesday for WAZ211. Winter Storm Watch from Tuesday afternoon through Wednesday afternoon for WAZ211. PZ...Small Craft Advisory until 5 PM PDT Tuesday for PZZ271-272.