textproduct: Portland

SYNOPSIS

A low pressure system moving into northern California will maintain rain showers across southern parts of the forecast area through tonight, with snow confined to the highest Cascade peaks. Seasonably warm and dry conditions expected on Monday as high pressure moves over the region. The next frontal system arriving later Tuesday will bring more widespread valley rain and Cascade snow through Wednesday, followed by showers later Wed afternoon. Anomalously strong high pressure likely develops over the NE Pacific later this week, though the position of this ridge will determine if storms riding over the ridge will be able to impact our region. This uncertainty maintains chances for precipitation through the end of the week into next weekend.

DISCUSSION...Now through Saturday

Satellite and radar imagery this afternoon reveals a low pressure system off the northern California coast spreading clouds and light rain showers northward into parts of NW Oregon. The majority of rain showers across the forecast area are expected to continue across much of Lane County through this evening, while the northern periphery of the rain likely extends as far north as Linn, Benton, and Lincoln Counties. There is around a 30-40% (or less) chance of a few sprinkles from Salem northward, with most of SW Washington expected to remain dry through tonight. Highest rainfall amounts are expected over the central Oregon Cascades and foothills across Linn and Lane Counties due to orographic enhancements as the upper low pushes inland over northern California tonight. Mean 24-hour QPF amounts range from 0.50 to 1.0 inch, although NBM probs suggest there is around a 15- 35% chance for rainfall totals to exceed 1 inch. This could occur with any heavier showers that develop. Snowfall is not expected at Cascade passes as snow levels sitting at around 7000 ft are not expected to fall below 6000 ft tonight.

Weak, transient ridging moves across the Pacific NW on Monday returning most of the area to drier conditions. Slightly above average temperatures continue, although skies are likely to remain mostly cloudy to partly sunny. A few light showers will be possible (15-30% chance) at the coast and over higher terrain on Monday. By Tuesday, the upper ridge shifts east of the Cascades as moist southwest flow develops across the region. Precipitation chances will increase through the day from west to east, but any rain that does fall likely remains light through Tuesday afternoon.

More substantial rains arrives later Tuesday night into Wednesday as an upper level trough pushes a cold front across the region. Chances for exceeding 1 inch of precipitation through Wednesday night are highest across the terrain (65-95%), while there is around a 30-60% chance along the coast, and 5-25% chance through the inland valleys. No wind impacts expected, though could see southerly winds gusting up to 30-35 mph at the coast and up to 25 mph inland as the front pushes inland late Tue night/early Wed morning. By Wednesday afternoon, cooler air aloft associated with the upper trough may allow for some deeper showers to develop, but chances for thunderstorms remains capped at around 10-15%.

Snow levels are also expected to drop to around 4000 ft on Wednesday, bringing potential for advisory level snowfall to the Cascade passes. Chances for 6+ inches of snowfall in 24-hours at the Santiam and Willamette Passes is around 50%, with highest snowfall rates likely occurring Wednesday morning. Those planning to travel through the Cascades mid-week should check road conditions before leaving and carry an emergency supply kit.

Thursday to Saturday, there remains some uncertainty with the upper level pattern over the region, but the majority of ensemble members suggest anomalous, broad upper level ridging offshore over the northeast Pacific. The offshore placement of this ridge is not favorable for a return to dry and sunny weather. Most ensemble guidance suggests that moisture will ride the northern or eastern periphery of the ridge and track towards the Pacific Northwest, maintaining chances for precipitation through the end of the week. Though mean QPF amounts remain relatively light, there are a few scenarios where more substantial precipitation occurs, but chances for those is currently around 10-20%, highest to the north. DH/10

AVIATION

High level clouds continue to increase from the south as upper level low pressure shifts inland over northern California and southern Oregon. Scattered showers increase from south to north through the day but activity should remain south of KSLE/KONP. CIGs are expected to gradually lower through the day from south to north as well. Chances for MVFR CIGs in the southern and central Willamette Valley increase to 40-60% after 00z Monday and with potential in the northern Willamette Valley increasing after 06z Monday to 30-60%. Light offshore winds along the coast and variable inland less than 5 kt.

KPDX AND APPROACHES...Predominately VFR conditions through the TAF period with a 10-40% chance for MVFR or lower CIGs at any given hour after 12z Mon as the system from the south spreads lower CIGs northward. 10-15% chance for rain after 00z Mon. Variable winds around 5 kt or less. -19

MARINE

Benign conditions expected through Monday night. Surface high pressure offshore and a weak thermal trough along the coast will maintain northeasterly winds 5-10 kt across the waters today. Seas fall to 3-4 ft at 9-10 sec tonight into Monday. Winds turn more southerly on Monday ahead of the next system, but remain light. The next frontal system arrives on Tuesday, likely returning small craft conditions. There is a 60-80% chance for frequent and widespread southerly wind gusts of 22 kt or greater across the waters on Tuesday. Seas will also gradually build on Wednesday and Thursday as a westerly swell moves in. Chances for seas exceeding 10 ft are 30-50% during this time, with the highest chances beyond 10-20 NM offshore. -19/10

PQR WATCHES/WARNINGS/ADVISORIES

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