textproduct: Portland

SYNOPSIS

Showers increase today as a cold front approaches, then transitioning to widespread stratiform rain as the front moves through tonight into Wednesday. Through Wednesday, expect advisory-level snow at the Cascade passes and a 10-20% chance of thunderstorms across the region. Wednesday night into Thursday, rain will transition to post-frontal showers and snow levels will fall to around 2500 to 3000 ft. Late in the week, a strong ridge offshore develops, although precipitation chances continue as moisture rides over the region and into the Pacific Northwest. Additional precipitation chances continue through the weekend and likely into early next week.

DISCUSSION...Now through Monday

An upper-level ridge shifts east as an upper-level trough approaches, bringing increasing shower chances through today. Expect lowlands to stay mainly dry through the day, although light and scattered showers are possible. Chances for rain showers over the Coast Range and Cascades are greater due to orographic enhancement. Tonight, the cold front affiliated with the upper-level trough will pass through the area and bring widespread rain, then transition to post-frontal showers by Wednesday afternoon.

The cold front on Wednesday will bring colder air aloft and increase instability, resulting in the potential for thunderstorm development during Wednesday morning and afternoon (10-25% chance of thunderstorms). Guidance continues to show skinny CAPE profiles with CAPE values around 100 to 200 J/kg. Any passing thunderstorms may result in lightning, gusty winds, and small hail. Total precipitation amounts for Wednesday will vary locally due to the variable nature of rain showers. Chances for 0.5+ inch of rain over 24 hours (4 AM Wed to 4 AM Thu): 40-60% for the Coast and the I-5 corridor (Cowlitz to Lane County), and 70-90% for the Coast Range and Cascades. Winds will increase through Wednesday, with coastal gusts around 25 to 30 mph and inland gusts around 20 to 25 mph. Stronger localized terrain gusts possible with the frontal passage. Little to no impacts are expected from these breezy winds.

Wednesday will also consist of Cascade snow, with snow levels around 3500 to 4500 ft. Highest snowfall rates look to occur from 4 AM to 4 PM Wednesday, with a 5-15% chance of snowfall rates 1+ inch/hr. From 4 AM Wed to 4 AM Thursday, there is a 70-90% chance for 6+ inches of snow at Santiam and Willamette Passes, and a 50-60% chance at Government Camp. In response, a Winter Weather Advisory will be in effect for the Marion, Linn, and Lane County Cascades from 4 AM Wednesday to 4 AM Thursday. Areas north of Santiam Pass will see lower accumulations. Note, snow will likely be wet due to marginal temperatures. Travel impacts will be possible at both Santiam and Willamette Passes. Roads, and especially bridges and overpasses, will likely become slick and hazardous. Periods of moderate to heavy snowfall combined with low visibilities may create difficult driving conditions.

Early Thursday, showers will weaken but continue under northwest flow. Snow levels will fall to 2500 to 3500 ft but minimal additional snow accumulation is expected. Moving forward through Friday and into the weekend, a strong upper ridge will develop over the northeast Pacific. This ridge will not provide favorably dry conditions locally, as moisture from a shortwave trough is expected to ride over the ridge and into the region, continuing moderate chances of precipitation. Ensemble guidance continues to indicate non-impactful rain totals through the weekend. By Monday, the ridge flattens as a trough from British Columbia drops south, resulting in a zonal and marginally wet pattern. ~12

AVIATION

CIGs vary across the area with MVFR/IFR CIGs along the coast and VFR/MVFR CIGs inland. Cloud cover is expected improve to low-end VFR this afternoon for inland terminals before probabilities increase again for lower CIGs after 06z Wednesday. Probabilities for MVFR conditions vary from 20-60% so terminals may bounce between categories. Conditions along the coast may briefly improve to VFR this afternoon but should quickly drop back to sub-VFR CIGs after 00-03z Wednesday. A cold front approaching the region will bring an increase in shower activity this evening and a band of stratiform rain later tonight. Southerly to southwesterly winds increase today ahead of the next system, with gusts up to 20 kt along the coast.

KPDX AND APPROACHES...Low end VFR CIGs into the evening before CIGs drop below 3000 feet along with increasing shower activity after 00-03z Wednesday. Winds turn more southerly today and remain under 10 kt. -19

MARINE

A cold front approaches and passes through the region today with increasing southerly wind gusts to around 25 kt. A Small Craft Advisory remains in effect for all waters including the Columbia River Bar through 4 AM Wednesday gusty winds. Wind- driven seas today remain around 4-5 ft through tonight. Seas build to 8-10 feet at 10-12 seconds on Wednesday as a westerly swell moves in. Chances for seas exceeding 10 ft by Thursday are 40-70%, with the highest chances beyond 20 NM offshore. High confidence for calmer marine conditions behind this system at the end of the week and into the weekend, with seas falling below 10 feet and winds turning more westerly to northwesterly and remaining under 20 kt. -19/10

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory from 4 AM Wednesday to 4 AM PST Thursday for ORZ127-128.

WA...None.

PZ...Small Craft Advisory until 4 AM PST Wednesday for PZZ210-251>253- 271>273.