textproduct: Portland

SYNOPSIS

A gradual cooling trend is underway across the Pacific Northwest as an upper-level trough develops offshore. Interior lowland highs will remain in the 80s today before dropping into the 70s later this week. Increasing onshore flow will bring a stronger push of marine air into the interior tonight into Wednesday. Cooler and more unsettled conditions are favored into the weekend, with periodic frontal passages bringing low precipitation chances.

DISCUSSION...Now through Monday

A broad trough over the northeast Pacific is beginning to shift eastward towards the Pacific Northwest, resulting in a trend towards modest cooling temperatures. Today is the only exception, with high temperatures generally in the mid to upper 80s across the interior lowlands and 60s along the coast. Onshore flow will become increasingly established this afternoon and evening, with winds turning westerly as a stronger marine push develops tonight. Marine air should spread inland through the overnight hours, while any lingering wildfire smoke is displaced east of the Cascades by Wednesday. Along with this marine push, fog may push into the coast as a result of fog formation over the ocean, bringing a potential reduction of visibility, primarily near the coastline.

The upper trough will continue progressing toward the Pacific Northwest on Wednesday, with the axis shifting farther northeast and bringing additional cooling. Increasing onshore influence should result in areas of low stratus across the interior lowlands Wednesday morning, which will limit afternoon warming. Highs are expected to reach the lower 80s inland, with cooler conditions along the coast.

The broader pattern remains favorable for continued troughing across the Pacific Northwest through the latter half of the week. Several smaller-scale disturbances are expected to rotate through the trough, reinforcing the cooler regime. Temperatures should continue trending downward, with the potential for additional marine pushes and increasing cloud cover. Precipitation chances remain relatively limited but are expected to increase somewhat as the pattern becomes more active. A weak frontal passage is possible Thursday, followed by another weak front Saturday, with precipitation chances generally around 10-20%. Neither system appears particularly wet at this point, though timing and strength will be refined as the week progresses.

By late weekend, ensemble guidance suggests the upper-level pattern begins to flatten, which could allow temperatures to rebound toward the 80s. However, ensemble guidance also suggests a broad trough may dig south into the PNW Monday into Tuesday, bringing a possibility of more rain. Confidence is lower during this portion of the forecast as the evolution of the trough and embedded disturbances remains somewhat uncertain. ~12

AVIATION...Through 06z Thursday

Inland penetration of low stratus will again yield lowered cigs across the airspace through tonight. Along the coast, MVFR cigs are already in place this evening, with increasing chances for IFR cigs, especially after 09-12z Wed. The highest confidence in IFR cigs is along the central Oregon coast (KONP), decreasing to the north. At terminals along the I-5 corridor, chances for MVFR cigs increase after 09-10z Wed, reaching 50-70% at terminals in the Lower Columbia Valley, Portland area, and southern Willamette Valley. Confidence is lower (30-50% chance of MVFR cigs) in the central Willamette Valley (KSLE, KUAO, KMMV) where cloud coverage may be lower. Low clouds will mix out by 18-19z Wed inland, yielding a return to VFR conditions, while more persistent low clouds at 2-3 kft may see MVFR conditions linger later on the coast. Light south winds this evening and tonight will build to 5-10 kt out of the southwest and veer more westerly through much of the day, before easing Wednesday evening. Strengthened onshore flow should keep smoke from fires in the Cascades and attendant vis restrictions to the east of area terminals.

KPDX AND APPROACHES...VFR conditions expected to prevail through 10-14z Wed, when low clouds reaching inland along the Columbia River will increase the chances for MVFR cigs, which reach 50-70% from 14-18z Wed before cloud cover decreases as low clouds mix out by the afternoon. Light northwest winds this evening will shift out of the south overnight, then build to around 5 kt and veer out of the southwest to west through much of Wednesday. -36

MARINE

A weak meso-low is moving up the north Oregon coast this afternoon bringing gusty south-southwest winds to the inner waters. Will maintain the Small Craft Advisory for gusts up to 25 kt for the inner waters north of Cape Falcon and the Columbia River Bar through 7 PM this evening. Otherwise, high pressure will build north along the coast tonight as low pressure well offshore weakens as it moves northeast toward Vancouver Island through Wednesday night. Southerly winds ease overnight before increasing again on Wednesday as a weak front approaches the outer waters. Another weak front approaches the inner waters Thursday morning, briefly increasing southerlies again, but in general, winds are expected to remain below 20 kt.

High pressure begins to rebuild over the waters later Thursday into Friday returning north to northwest winds across the coastal waters into the weekend. Predominately wind driven seas are expected to remain around 3 to 6 ft through the weekend. Somewhat more active pattern may return early next week. /DH

PQR WATCHES/WARNINGS/ADVISORIES

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