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

A cold frontal boundary sits just off of the coast producing showers over much of the area. This front has been slow to move and therefore the impacts have been a bit slower to develop than previously thought. Skies are VFR and will stay that way for a few more hours until the cool air moves in behind the front. As the cooler air settles, will see widespread MVFR CIGs with rain showers. Snow will fall along the Cascades. The major forecast component for the next 24-hrs is the potential for thunderstorms with the incoming airmass. We are seeing signatures that would fall in line with a thunderstorm forecast. The timing and placement of the storms is difficult, but there is a higher probability in the southern Willamette Valley around KEUG due to orographic lift over the terrain. Cannot rule them out around KSLE but the probability is less than 25%. Therefore, have included a PROB30 for thunderstorms =at KEUG after 16Z Wed but some models are suggesting this potential persists until 00Z Thu.

Another component for those attempting to fly along the coast will be the strong southerly winds along coastal terminals. The probability for gusts greater than 25 kt decreases after 12Z Wed where it hovers around 40-50% chance through 20Z Wed.

KPDX AND APPROACHES...CIGs remain VFR this evening but will slowly fall to MVFR over the next couple of hours. Confidence in the exact timing is lower due to nature of the incoming system, however a scattered deck is beginning to form which signifies that this transition may be sooner than forecast. There is less than a 20% chance for thunderstorms today so have omitted from the TAF at this time. However, if the system shifts further north, cannot rule out a stray storm or two. -27

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.