textproduct: Portland

SYNOPSIS

Unsettled conditions continue through today as broad southerly flow and breaks in cloud cover support scattered showers and thunderstorms, especially across the Oregon Cascades and adjacent foothills. Thunderstorm chances return Saturday afternoon and evening, with the greatest coverage over the central Oregon Cascades and foothills. Cooler conditions develop Sunday into early next week as flow turns more westerly to northwesterly. Another system may bring widespread precipitation and Cascade snow by the middle of next week.

LONG TERM...Monday through Thursday

Ensemble guidance indicates a transition to broad west-northwest to northwest flow aloft early next week. A weak embedded shortwave is expected to move through Monday, maintaining a 50-90% chance of showers, highest across the Cascades. Temperatures remain near to slightly below normal.

Shower activity decreases Monday night into Tuesday as a transient ridge briefly builds overhead. Attention then turns to the next Pacific frontal system expected between late Tuesday and Wednesday. While timing differences remain, confidence is increasing in a return to widespread precipitation during this period.

Cooling temperatures associated with this system will lower snow levels to near or below Cascade pass elevations, raising the potential for travel impacts. Current probabilistic guidance indicates a 60-80% chance of 6+ inches of snowfall at the Cascade passes over 48 hours (between Tuesday and Thursday afternoon). As for 12+ inches of snowfall, there is a 40-60% chance during the aforementioned time. In addition, cooler and potentially clearer conditions behind the system may introduce renewed frost concerns across portions of the interior valleys by mid to late week. Current guidance suggests a 30-60% chance of frost (less than 36 F) in the Upper Hood River Valley Tuesday night into Wednesday morning. As for Wednesday night into Thursday morning, guidance suggests a 30-70% chance of widespread frost across most inland locations. Thursday morning may be much colder, with a 5-30% chance of freezing temperatures across most inland locations, except for a 50-80% chance in the Upper Hood River Valley (chances from Odell to Parkdale). ~12

AVIATION

Generally VFR conditions are in place with mid to high clouds slowly clearing and moving from south to north across the region. with mid to high level clouds slowly moving from south to north across the airspace. Current radar observations are showing showers starting to develop around the OR/CA borer and moving northward. These showers are likely to start impacting areas near KEUG around 21Z-23Z Friday and will continue to develop as they move northward. Around 22Z Friday trough 03Z Saturday showers and thunderstorms are likely along and east of the I-5 corridor through around 06Z Saturday. Given decent model consistency, have added PROB30 groups for thunderstorms for KEUG, KSLE, KUAO and KHIO as those sites currently have the highest chance to see activity.

Confidence remains relatively lower (15-25% chance) for other inland terminals, but the highest chance for thunderstorms at these locations is from around 00Z Saturday through 04Z Saturday. Thunderstorm chances across the airspace rapidly decrease after 06Z Saturday.

It should be noted locations that do see stronger showers or thunderstorms will have the potential for lightning, small hail, moderate to heavy rain, and gusty/erratic outflow wind gusts up to 45 knots. This activity would also result in intermittent MVFR/IFR conditions if it moves directly overhead.

After 06Z Saturday, conditions start to settle and will result in lowered flight conditions with lingering showers across the airspace. This will bring IFR/LIFR conditions to the coast and VFR/MVFR conditions for inland locations. These conditions are expected to persist through the end of the TAF period.

KPDX AND APPROACHES...Expect predominately VFR flight conditions through the TAF period with mid to high cloud cover at times. Locally, shower (50-70%) and thunderstorm (15-25%) chances ramp up around 00Z Saturday before diminishing around 06Z Saturday. Stronger showers or thunderstorms will have the potential for lightning, small hail, moderate to heavy rain, and gusty and erratic outflow wind particularly at the onset of activity in the late afternoon hours if it occurs. After 06Z Saturday, conditions will start to settle with a mixture of VFR/MVFR conditions that will persist through the end of the TAF period. /42

MARINE

Overall conditions remain rather benign through the weekend as seas remain in the 4 to 7 foot range with winds below 20 kt. The next several days will see south/southwest winds slowly become more north/northwest by Saturday and these winds are expected to persist through at least Monday night. Looking towards the middle of next week (Tuesday/Wednesday) a decently robust frontal boundary will likely (40-60% chance) result in Small Craft Advisory wind gusts and seas around 7 to 10 feet.

It's worth noting that there is a 5-10% chance of thunderstorms this afternoon and Saturday afternoon across all waters from the southeast. Any thunderstorms that do develop could produce lightning, hail, gusty and erratic winds as well as a less than 2% chance for a funnel cloud or waterspout. /42-99

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.