textproduct: Portland

SYNOPSIS

Ridging aloft will maintain largely warm and dry conditions through the next week. The only exception will be a weak system arriving Friday into Saturday, which will bring briefly cooler temperatures but only minimal chances for even light rainfall.

DISCUSSION...Now through Tuesday

A narrow upper-level ridge just off the West Coast will broaden through the workweek, driving generally persistent weather conditions across the region. Diurnally-varying marine cloud cover will reach inland overnight into the morning each day, yielding mist or drizzle in areas near the coast, before retreating back to the coast or offshore through the day. There remains uncertainty in the timing of daytime clearing which could greatly modulate high temperatures; the longer clouds persist through the morning, the cooler afternoon highs will be. Even in the reasonable coolest scenario, temperatures should reach near seasonal norms, and will more likely be at least a few degrees above normal each day. Diurnal west winds could also become breezy along the central Columbia River Gorge, with gusts to 25-30 mph in the afternoon and evening through the end of the workweek.

Friday into Friday night, an upper-level shortwave trough will track over the top of the ridge. This feature has trended weaker in recent guidance, with impacts generally limited to enhanced cloud cover and temperatures a few degrees cooler. Rain chances are now minimal across the region, generally 5-15% along the coast and in coastal terrain from Tillamook northward, and in the Cascades north of US-26, peaking Friday afternoon and evening. To the south and at lower elevations inland, chances for even a hundredth of an inch of rain are now 5% or less.

Resurgent upper-level ridging will see the warm and dry weather then continue this weekend into next week, including seasonably warm temperatures in the 60s along the coast and in higher terrain and 70s to around 80 degrees within inland valleys beneath clearing skies on Mother's Day. -36

AVIATION

Latest satellite imagery depicts an increasing inland extent of marine stratus this morning. North of KSLE, stratus has already reached the Cascade crest, while to the south, the Willamette Valley continues to steadily fill with stratus. While coastal and Lower Columbia Valley terminals have maintained MVFR cigs around 1500 ft through much of the overnight period, cloud bases farther inland have stubbornly remained near 3500 ft. All guidance steadfastly supports MVFR cigs at all inland terminals by 12-15z Wed, but observational trends so far do not support this. Have therefore opted to maintain low-end VFR cigs at 3-4 kft at inland terminals, but MVFR cigs cannot be ruled out. With low confidence, the chances of MVFR at inland terminals is estimated to be 30-45% through 20-22z Wed when increased mixing should tend to lift cloud bases to 4-5 kft and yield decreasing sky cover. Calm to light winds out of the west to northwest this morning will build to around 5 kt by 20-21z Wed, then ease below 5 kt again late in the period, after 03-06z Thu.

There is higher confidence that cigs will remain MVFR or lower along the coast, however as inland, guidance favors IFR cigs through this morning despite a lack of observational evidence. Have therefore also adjusted cigs higher in the forecast and maintained MVFR cigs of 1-2 kft throughout, with the chance of IFR cigs through 21-23z Wed estimated at 20-35%. Chances increase late in the period for IFR cigs, but confidence is low at this time. Northwest winds will build to 5-10 kt by 18-21z Wed, before easing to around 5 kt late in the period, after 03-06z Thu.

KPDX AND APPROACHES...Low-end VFR cigs at 3-4 kft expected to continue through 20-22z Wed, although chances for MVFR cigs are conservatively estimated at 30-45%. By the early afternoon, increased mixing should act to lift cloud bases above 4 kft and to reduce cloud coverage, but the amount of clearing is uncertain. Diurnal northwest winds will increase to around 5 kt by 20-22z Wed, then ease below 5 kt again late in the period, after 06z Thu. -36

MARINE

Strengthening high pressure offshore will see northwest winds intensify and turn more northerly through today, while a broader onshore flow regime will favor continued marine overcast which thickens and lowers overnight before lifting and backing offshore each day through this week. These stronger winds by this afternoon along with a building northwest swell will push seas up to 7-9 ft by tonight, but a dominant swell period of 11-13 seconds should keep seas from becoming steep enough to present a hazard to small craft. Isolated gusts may exceed 20 kt this afternoon into this evening beyond 30 NM.

As northerly flow weakens through Thursday and turns southerly on Friday as a weak system moves onshore, seas will ease back to 4-7 ft. Southerly gusts may briefly approach 20 kt on Friday afternoon before weakening again overnight. Chances for rain showers on Friday have fallen to 5-15%.

High pressure will restrengthen offshore this weekend into early next week, supporting a reintensification of northerly winds. Chances for widespread gusts of 20-30 kt increases daily from Saturday through Tuesday, while seas continue at 4-8 ft. -36

PQR WATCHES/WARNINGS/ADVISORIES

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