textproduct: Portland

SYNOPSIS

Dry conditions are favored across the inland valleys through the week with no noteworthy weather impacts going forward. High temperatures remain near or slightly above seasonable normals through Tuesday. However, we do have to watch for some light shower chances early next week, primarily in the Cascades and coast range. Drier and potentially warmer temperatures return Wednesday to Friday as high pressure builds overhead although forecast uncertainty increases mid to late week as well.

DISCUSSION...Today through Friday

Dry northerly flow presides across the region tonight through the remainder of the weekend with fairly normal late April conditions persisting. Similar to the previous night, we'll still need to keep an eye on low temperatures in regard to frost concerns, especially in the Upper Hood River Valley, where mostly clear skies, light winds, and a weak offshore gradient keeping a drier airmass in place likely facilitates efficient radiational cooling. High confidence temperatures bottom out in the low to mid 30s come sunrise Sunday morning in the area so a Frost Advisory is in effect again tonight. Otherwise, low temperatures are expected to be a couple degrees warmer across the broad for the rest of the region tonight helping to mitigate frost concerns elsewhere (like the southern Willamette valley).

The upper-level pattern turns more northwesterly Monday through Tuesday with weak upper-level shortwave troughs rotating into the Pacific Northwest from a parent low located over south-central Canada. These weak upper-level features combined with an influx of low to mid level moisture (surface to 800mb) likely increases our chances for light showers, mainly focused over terrain which can provide orographic lift. Chances for showers on each of these days peak around 5-10% along the I-5 corridor and 20-35% across the Cascades and Coast/Coast Range - this is a decrease from previous forecasts as guidance has trended drier. Due to the added cloud cover and onshore flow, daytime temperatures early next week cool down a few degrees compared to this weekend with highs in the mid 60s across interior valleys.

Wednesday to Friday, the majority of ensemble members suggest upper level ridging builds over the northeast Pacific and shift into the Pacific Northwest. Most ensemble members also show above-average 500 mb heights over the area, suggesting warmer temperatures by mid to late next week. There still remains some uncertainty on the exact progression, strength, and placement of the ridge axis, thus leading to a wide range of high temperatures. The last high end max temperatures (NBM 90th percentile) for Thursday and Friday next week are currently in the low 80s across interior valleys. Chances for exceeding 80 degrees on Thursday are still around 30-50% across the Portland/Vancouver Metro and 15-30% for the rest of the I-5 corridor. Meanwhile, low-end (NBM 10th percentile) max temperatures for Thursday and Friday are in the low to mid 60s. At least precipitation chances through this period remain rather low, 10-20% coast range/Cascades and 0-5% inland valleys into the end of the week. -99/10

AVIATION

VFR conditions prevail across the region through the TAF period with mostly clear skies and a few high clouds. North to northwesterly winds across the area, breezy along the coast and from KUAO southward with gusts up to 20 kt. Winds ease below 10 kts after 02-04z Sun. Guidance indicates a 50-70% chance for MVFR ceilings developing along the coast around and south of KONP from 08-20z Sun, and a 15-35% chance north of KONP, including KAST, from 00-08z Sun. Efficient cooling in the Upper Hood River Valley between 08-16z could lead to temperatures in the low to mid 30s, allowing for potential frost development over metal surfaces.

KPDX AND APPROACHES...VFR conditions with mostly clear skies today. Northerly winds around 6-8 kts, decreasing below 5 kts after 02-04z Sun. -03

MARINE

High pressure remains over the waters into Sunday, maintaining north to northwesterly winds. Winds shift more westerly on Monday into Tuesday morning as a weak front moves over the waters. By Tuesday evening, high pressure returns over the waters through the end of the week, returning northerly winds. Winds expected to remain under 20 kts this week. Seas of 5-7 ft persist through next week with a northwesterly swell. -03/10

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from 1 AM to 8 AM PDT Sunday for ORZ121. WA...None. PZ...None.