textproduct: Portland

SYNOPSIS

The region remains under the influence of a ridge of high pressure this morning with additional low clouds and limited clearing across the inland valleys. Mostly dry conditions likely persist through Tuesday as a weak frontal boundary grazes the coast. Then, an active and wetter weather pattern returns later Wednesday through the remainder of the week.

DISCUSSION...Now Through Sunday

The upper level ridge axis currently over the West Coast will shift east of the Cascades later this morning. Satellite imagery early this morning shows much of the inland valleys socked in under low stratus, while mid to high level clouds are beginning to lift northeastward into NW Oregon. Forecast model soundings suggest the deep stratus layer should become more shallow through the morning hours into early afternoon. Clearing may be a struggle across the interior lowlands this afternoon, which presents the highest uncertainty in the short term forecast, as high temperatures could range anywhere from the upper 40s to lower 60s depending on cloud coverage.

Dry conditions will likely persist through Tuesday as a low pressure system off the northern California coast moves east, remaining south of the forecast area. At least the weak troughing overhead and cooling temperatures in the mid-levels by Tuesday afternoon should weaken the temperature inversion and help our daytime lingering low cloud/fog situation. It's also worth quickly noting the placement of this low pressure system likely facilitates a period of locally breezy/gusty east winds through the Cascade gaps and western Columbia River Gorge, peaking Tuesday morning. High pressure briefly builds again Tuesday night into Wed morning as a transient upper level ridge moves across the region.

On Wednesday, a shortwave trough moving across the NE Pacific quickly pushes a frontal system toward the coast later in the day, increasing precipitation chances across the area through Wed night. Most ensembles indicate this system will be weakening with just light amounts of rainfall, most likely 0.1-0.30" coast range, <0.10" inland valleys. Cluster analysis of 500 mb heights are in relatively good agreement heading into the latter part of the week with zonal to southwest flow aloft allowing for a somewhat active pattern over the NE Pacific to approach the coast. By the weekend, the ridge axis will likely build east of the Cascades. There is still quite a bit of uncertainty in regards to exact rainfall coverage and amounts, but there is continued potential for multiple rounds of systems moving across the area into next weekend. At this point, no significant precipitation impacts are expected with any of these weather systems, and snow levels hold around 5000-6000ft. DH/99

AVIATION...Through 12Z Tuesday

Southwest flow aloft as an upper level ridge of high pressure shifts east of the area today will bring increasing mid to high level clouds. As of 11z, satellite imagery shows much of the interior lowlands entrenched in low stratus with tops of around 1500-2000 ft. Surface observations indicate CIGs of around 500-1000 ft, except around 300 ft in the southern Willamette Valley near KEUG. Expect the IFR to low-end MVFR stratus to persist through at least 17-19z this morning with guidance suggesting the stratus layer continues to lower and become more shallow. There is high confidence (80-90% chance) that conditions trend toward VFR by 21z this afternoon with mid- level clouds persisting through the evening. Winds remain relatively light, except for easterly winds through KTTD increasing to around 10-12 kt.

KPDX AND APPROACHES...MVFR CIGs of around 1000 ft continue as of 11z, but are expected to lower to IFR after 12z this morning. Conditions likely begin to improve around 17-19z as stratus breaks with increasing mid to high level clouds this afternoon. VFR conditions are likely by 21z Monday. Light east winds around 3-6 kt are expected through much of the day. /DH

MARINE

Light offshore east to southeast winds are expected today as a low pressure system remains well offshore. An incoming long period west-northwest swell will continue to build seas across the coastal waters today. Buoy observations early this morning indicate seas have risen to around 7 to 9 ft at 19 seconds. Expect wave heights to peak at around 10 to 11 ft beyond 10 NM offshore through Tuesday morning. Will maintain the Small Craft Advisory for the outer coastal waters currently in place. This long period swell will also result in a moderate threat for sneaker waves at the coast today through Tuesday. Caution should be used at beaches when in or near the water.

Winds become light and variable on Tuesday while seas gradually subside back to 7 to 9 ft. Winds turn more southerly by Wednesday ahead of an approaching frontal system. This system is expected to be relatively weak, with around a 50-60% chance for wind gusts up to 25 kt north of Cape Falcon. By the end of the week, model guidance hints at a stronger system entering the waters, resulting in a 40-50% chance for maximum wind gusts exceeding 33 kt north of Cape Falcon on Friday. In addition, another long period westerly swell will likely build seas into the mid-teens with a 15-25% chance for significant wave heights exceeding 20 ft. DH/10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 7 AM this morning to 7 AM PST Tuesday for PZZ271>273.