textproduct: Portland

SYNOPSIS

The passage of a weather system, that started overnight and will continue through today will result in a very a noticeable pattern change as cooler and slightly wetter conditions develop. Light showers expected to continue through the daytime hours. Afterwards, expect drier conditions to be interlaced with periods of showers through the end of the week. Then another low from the Gulf of Alaska increases precipitation chances (30-70%) for the start the weekend while maintaining near to slightly below normal temperatures for May.

DISCUSSION...Today through Tuesday

A shortwave trough moving through the region is responsible for the added cloud cover and slightly cooler daytime temperatures. While this shortwave brought some lightning and thunder to the region last night, what can be expected through late this afternoon is a more stable pattern, with light precipitation expected across the CWA through tonight. Total precipitation amounts range from around 0.10-0.25 inches along the coast and coast range, 0.05-0.20 inches within the Willamette Valley and 0.20-0.40 inches for the Cascades. The added cloud cover, westerly flow, and cooler airmass likely pushes high temperatures down into upper 50s to mid 60s for much of the area. Looking towards Thursday and Friday, a more zonal flow pattern in the upper levels of the atmosphere will help to keep temperatures near or below normal for May. Within the upper level flow, perturbations can be observed and these will keep very light showers in the forecast for the Coast, Cascades and northern Willamette Valley through Friday, with little to no accumulation expected.

Late Friday/early Saturday most deterministic and ensemble guidance shows another, more robust shortwave trough dropping down from the Gulf of Alaska into the Pac NW. This system will likely increases chances for precipitation across the CWA and continue to cooling trend into mainly the 50s to near 60s by Saturday afternoon. Looking towards the start of next week, daytime temperatures are expected to warm back into the 60s and 70s as there is moderate confidence in a ridge of high pressure beginning to build back overhead by early next week. /42-99

AVIATION

A shortwave trough moving into the region this morning will bring generally VFR conditions across the airspace, along with intermittent pockets of MVFR/IFR conditions. In addition to the variable flight conditions expect showery precipitation through the TAF period. Showers will take a southwest to northeast track over the airspace, with the majority of precipitation expected to be out of the airspace around 00Z Thursday.

Winds increase out of the southwest behind the front. Look for the wind increase starting around 12Z Wednesday for southern inland TAF locations, reaching KSLE around 15Z Wednesday, reaching KPDX around 16Z Wednesday. Coastal locations should see these southwesterly winds starting around 12Z Wednesday. Expect southwesterly gusts up to 25 kt along the coast starting around 14Z-16Z Wednesday and starting around 15Z-17Z Wednesday for inland locations. Gusts should start to subside around 00Z Thursday.

KPDX AND APPROACHES...VFR conditions expected to continue with intermittent MVFR conditions as precipitation continues to stream into the region from the southwest through around 00Z Thursday. Winds increase out of the southwest behind the front, reaching KPDX around 16Z Wednesday. Expect southwesterly gusts up to 25 kt starting around 18Z Wednesday. /42

MARINE

Relatively benign conditions expected through the week. Southerly winds early this morning will develop into westerly wind by sunrise. Afterwards, predominately westerly winds through the end of the week, with northwesterly winds expected by the weekend, mainly under 10 kts.

An upper level trough and associated front moves through the waters on today, which will result in a 35-50% chance of isolated small craft gusts up to 25 kt across all waters. This will mainly be when the front is passing between 4 AM-12 PM today. Not expecting gusts to last long enough over any specific area to issue a small craft advisory at this time, though we will continue to monitor conditions. Winds remain westerly but decrease once again tonight. Seas generally below 8 ft around 10-12 seconds through the majority of week, but could see seas close to 10 ft by Friday/Saturday. /42

PQR WATCHES/WARNINGS/ADVISORIES

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