textproduct: Portland

SYNOPSIS

Scattered showers continue through this evening as low pressure remains overhead. Warmer and drier conditions return tomorrow as high pressure rebuilds. Wrap-around moisture will lead to chances for showers and a 15-20% chance for thunderstorms across the Lane County Cascades Wednesday afternoon. Thursday, increasing moisture will lead to more widespread chances for showers along the I-5 corridor with increasing chances for thunderstorms across the Cascades. Cooler temperatures return Friday with drier conditions this weekend. Uncertainty with how warm temperatures get early next week.

DISCUSSION...Today through Monday

Radar and satellite imagery as of early Tuesday afternoon depicts scattered showers and clouds as a broad upper-level low moves overhead and tracks southward toward California and Nevada. While most showers are currently over the Coast Range and Cascades, chances for showers continue across northwest Oregon and southwest Washington through this evening. Observations indicate limited instability; therefore, chances for isolated thunderstorms remain low (10-15%) through this evening. Any passing thunderstorms that do form may produce lightning, brief heavy downpours, erratic winds, and/or small hail.

Temperatures rebound Wednesday and Thursday as high pressure re- builds over the area and the aforementioned low drops down into the Great Basin. High temperatures are forecast 5-10 degrees above normal across interior valleys for this time of year, warmest from the Portland/Vancouver Metro northward. Chances for exceeding 80 degrees across the Portland/Vancouver Metro on Wednesday have increased to 55-75%, and 80-90% on Thursday. The rest of the interior valleys have a 30-50% chance for exceeding 80 degrees on Wednesday and Thursday. While Wednesday remains predominately dry and warm, will note that wrap-around moisture will result in a 30-50% chance for showers and a 10-20% chance for thunderstorms across the Lane County Cascades Wednesday evening. By Thursday, increasing wrap-around moisture will result in more widespread chances for showers across the Cascades and into the I-5 corridor. Increasing instability from south-southeast flow will also support a 20-40% chance for thunderstorms across the Cascades Thursday afternoon/evening, with the highest chances across the Lane County Cascades. If shower or thunderstorm coverage is more widespread throughout the day on Thursday, cloud cover could result in slightly cooler temperatures.

Friday into the weekend, temperatures cool down as ensemble guidance suggests another trough moving in from the northeast Pacific and lowering 500 mb heights. Chances for showers remain on Friday due to lingering wrap-around moisture from the Great Basin low, however, conditions dry out over the weekend due to limited moisture despite the troughing. Uncertainty in temperatures drastically increase for early next week, with ensemble guidance suggesting the possibility of an afternoon high temperature anywhere between 73 and 93 degrees on Monday for KPDX (coolest and warmest scenarios). Current guidance suggests a 15-30% chance for temperatures exceeding 90 degrees across the greater Portland/Vancouver Metro Area on Monday. For the rest of the Willamette Valley, southwest WA lowlands, and Upper Hood River Valley, chances for exceeding 90 degrees remain around 5-15%. The wide range of temperature values comes from uncertainty with the upper level pattern. The majority of ensemble guidance are in agreement with above-average 500 mb heights across the region on Monday due to potential ridging; however, there is uncertainty with the exact placement and magnitude of ridging which would determine how warm we get. -10

AVIATION

Mostly VFR conditions across the air space into the evening hours. Light showers remain possible mainly over the higher terrain and in the southern Willamette Valley into tonight. Chances around terminals are relatively low so have kept out of the forecast at this time. North/northwest winds at 8-14 kt with gusts to around 20 kt possible through 04z Wednesday.

Marine stratus is expected to develop this evening, approaching the coastal terminals between 02-06z Wednesday. Sub-VFR CIGs are expected to persist through 16-18z along the coast. Guidance suggests stratus building inland toward the Willamette Valley off the Cascade Foothills. MVFR CIGs are possible after 10z, mainly around the eastern metro and southern Willamette Valley.

KPDX AND APPROACHES...Mainly VFR conditions expected through the TAF period. Northerly winds 8-12 kt with a few gusts to around 20 kt possible this afternoon. Could see MVFR CIGs redevelop late tonight after 12-14z Wednesday. There is a 20-50% chance over the eastern metro. -19

MARINE

Westerly swell continues to move through the waters but the peak wave heights subsided earlier this morning. Seas are generally below 15 ft and will continue to drop closer to 10 ft overnight. Winds from the northwest at around 10 kt. Small Craft Advisories remain in effect for all zones due to steep seas. The Columbia River Bar will drop off at 11 PM tonight as seas subside. The Small Craft for the inner waters has been extended to 1 AM as and the outer waters extended through 5 PM Wednesday for steep seas and wind gusts up to 25 kt. Another northwesterly swell is expected to move into the water on Thursday, steepening seas across all marine zones again. Conditions are borderline for Hazardous Seas Warning or a Small Craft Advisory at this time. -19

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ210.

Small Craft Advisory until 2 AM PDT Wednesday for PZZ251>253.

Small Craft Advisory until 5 PM PDT Wednesday for PZZ271>273.