textproduct: Portland

SYNOPSIS

Cooler than average temperatures will continue tomorrow with chances with showers and isolated thunderstorms as a closed upper level low remains in place over the region. Trending drier Monday through Wednesday with temperatures gradually warming each day as low-level offshore flow develops. The forecast becomes more uncertain Thursday and Friday, with the potential for weather conditions to range anywhere from cool and showery to warm and dry depending on the evolution of the large scale weather pattern.

LONG TERM...Tuesday through Friday night

Temperatures will trend noticeably warmer Tuesday into Wednesday with high confidence for dry weather as low-level offshore flow begins to develop. On Tuesday, afternoon temperatures will warm into at least the mid to upper 70s, except lower 70s at the coast. Come Wednesday, widespread highs in the 80s are expected. Even the coast has a chance to reach 80 degrees or warmer on Wednesday if offshore winds last into the afternoon hours (1-15% chance, except a 30-50% chance for inland coastal communities such as Astoria and Tillamook).

Come Thursday and Friday, uncertainty increases substantially in regards to the large scale weather pattern and whether or not we will remain warm and dry or transition back to a cool and showery weather pattern. On Thursday, a cluster analysis for 500 mb heights and anomalies shows around 40% of the total ensemble space from the GEFS/ENS/GEPS produces weak upper level troughing over B.C., western WA and far northwest OR. The remaining 60% of the total ensemble space depicts some degree of ridging over the area. The current deterministic forecast aligns with the latter outcome, suggesting the continuation of dry weather with highs still in the 80s for inland valleys, albeit several degrees cooler than Wednesday. By Friday, clusters come into better agreement for upper level troughing (around 85% of the total ensemble space shows some degree of troughing over or near the area). However, there are significant differences in the exact amplitude and location of this trough, which will impact precipitation chances and temperatures. As such, temperature spread on the NBM 1D Viewer is very large both days, ranging from 70-90 degrees on Thursday and 65-85 degrees on Friday. Friday currently has a higher chance for conditions to trend cooler and wetter compared to Thursday, however the outcome is still uncertain. Anyone with outdoor plans late in the week should monitor the forecast for updates over the next several days. -23

AVIATION...Through 00z Monday

A low pressure system continues to meander overhead, bringing continued moist conditions with light winds and persistent low chances for rain showers. VFR conditions are nonetheless favored across the airspace. Showers are most likely in areas of terrain, particularly the Cascades, with 15-25% chances for an isolated thunderstorm which could produce locally heavy rain, gusty and erratic winds, and small hail through this evening. Terminal impacts from any light rain showers are unlikely, and would be very brief if they do occur. Otherwise, VFR cigs/vis continue with light winds of 5 kt or less across the area. Chances for MVFR cigs and additional rain showers increase after 06-12z Sun as low stratus coverage increases along the coast, lower Columbia and Cowlitz Valleys, and the southern Willamette Valley, and rain showers spread westward from the Cascades toward the I-5 corridor.

KPDX AND APPROACHES...VFR conditions expected through at least 12z Sun. Light northwest winds below 5 kt become variable after 06z Sun. Low chances, <10%, for rain showers persist through much of the day, but increase to 25-35% after 12z Sun. Low cloud coverage also increases overnight, with 15-25% chances of MVFR cigs after 12-15z Sun. -36

MARINE

Surface low pressure centered on the Oregon coast will continue to weaken as its parent upper-level low meanders overhead. Weak high pressure beginning to build offshore will see light northerly winds of less than 15 kt develop. Rather benign conditions then continue through at least the middle of next week with surface high pressure strengthening as the stagnant upper low ejects eastward, favoring stronger north winds. Seas continue at 3-6 ft with a dominant northwest swell at 8-10 seconds. -36

PQR WATCHES/WARNINGS/ADVISORIES

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