textproduct: Portland

SYNOPSIS

Persistent onshore flow will bring near-average temperatures with varying degrees of morning clouds and afternoon sunshine for much of the next week. A series of weak fronts will bring notable chances for light rain and drizzle to portions of the Washington and north Oregon coasts today and this weekend, while Friday is likely to be the warmest day.

DISCUSSION...Now through Monday

The large-scale pattern will remain fairly steady through the next week with the region wedged between upper-level troughing over the Gulf of Alaska and upper-level ridging over the Rockies and Central Plains. In general, the weather locally will be typical for the mid-summer: morning clouds followed by increasing afternoon sunshine, seasonable temperatures, and largely dry conditions, but with some variations as the relative strengths of these two dominant features change.

Today, an upper shortwave passing to the north will tilt the scales in favor of upper troughing, sending front toward the coast resulting in slightly below normal temperatures across the region, namely afternoon highs in the low to mid 60s at the coast and upper 70s to low 80s within inland valleys, as well as chances for light rain or drizzle along the northern Oregon and southern Washington coasts and within coastal terrain. Chances for accumulating rainfall (0.01 inches or more) are less than 10% in Lincoln and Lane Counties, 20-40% in Tillamook and Clatsop Counties, and 50-80% in Pacific County. Inland, chances are lower, only 5-25% from the Portland metro north into the Lower Columbia and Cowlitz Valleys.

Wednesday through Friday, the upper-level ridge will strengthen and retrograde westward into the Great Basin, increasing heights aloft locally and tipping the balance toward warmer and drier weather. Continued onshore flow will limit the likelihood of hazardous heat, but except above-normal high temperatures most likely in the upper 60s to low 70s along the coast and in the mid to upper 80s within inland valleys by Friday. These temperatures result in only Minor HeatRisk across the region, with the chances for Moderate HeatRisk of around 5% along most of the I-5 corridor except 15-45% in the Portland/Vancouver metro, highest in the Tualatin Valley.

By the weekend, there is higher uncertainty in the evolution of the upper-level pattern, but many ensemble members depict more pronounced troughing returning to the Pacific Northwest, favoring cooler temperatures and further chances for rainfall. At this time, rain chances peak on Saturday along the coast and especially from Lincoln City northward. -36

AVIATION...Through 18Z Wednesday

At 18Z Tuesday, satellite and surface weather observations depicted mostly clear skies over the Oregon Cascades and Willamette Valley. A narrow band of clouds with VFR ceilings around 7000 ft remained in place over KPDX amd KTTD, however this cloud deck is expected to scatter out by the early afternoon. Meanwhile, a decaying cold front was beginning to push inland along the south Washington and north Oregon coast. This front will bring a brief period of light rain to KAST between 18-19Z Tuesday. The remaining TAF sites should stay dry. MVFR ceilings at KONP are already beginning to scatter out, and MVFR ceilings are expected to scatter out at KAST by 19-20Z Tuesday.

The main uncertainty with the aviation forecast is exact ceiling heights late tonight into tomorrow morning, as well as whether or not cloud cover will be predominately scattered or broken. The latest guidance currently suggests a broken cloud deck will attempt developing over the north Oregon Cascades and portions of the Willamette Valley and Portland metro between 06-12Z Wednesday. However, probabilities currently favor low-end VFR ceilings with this potential stratus deck. Probabilities for MVFR ceilings below 3000 ft peak between 10-16Z Wednesday between 15-35% at all TAF sites, except 45-55% at KAST.

KPDX AND APPROACHES...Scattered clouds with ceilings above 7000 ft are expected today, resulting in VFR conditions. VFR conditions will most likely remain in place through 18Z Wednesday, however there is a 15% chance for MVFR ceilings below 3000 ft between 10-16Z Wednesday. Light and variable winds around 5 kt are expected to become northwesterly later this afternoon, with gusts up to approximately 15 kt by the late afternoon with an incoming sea breeze. -23

MARINE

Seas of 4-5 ft per buoy observations are expected to continue with a dominant west to northwest swell at 8-9 seconds and a secondary long-period south-southwesterly swell persisting through much of the week. Light southerly winds early this morning will build and veer out of the north to northwest by this evening behind a weak frontal boundary. Maximum gusts will most likely remain in the 15-20 kt range, however intermittent or isolated gusts to 25 kt may occur during the frontal passage, most likely to the north of Cape Falcon. More typical summertime northerlies will then continue through the remainder of the workweek.

Another weak front looks to traverse the waters early this weekend, after which northwesterly flow will strengthen and veer out of north into early next week as high pressure builds offshore. Confidence in wind gusts of at least 20-25 kt is moderate to high on Sunday, increasing from around 60% west of the mouth of the Columbia to greater than 90% offshore of Florence. Those chances are even higher on Monday, some 90-95% or higher across the coastal waters. The chances for gale-force gusts on Monday are low but not negligible, generally 10-20% south of Cape Foulweather as well as within 10 NM of the coast from Cape Falcon to Cape Foulweather. -36

PQR WATCHES/WARNINGS/ADVISORIES

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