textproduct: Portland

SYNOPSIS

A transient upper level shortwave ridge will bring warmer temperatures today with highs in the low to mid 70s, except 60s at the coast. Chances for showers and thunderstorms increase Thursday afternoon and evening over the central Oregon Cascades and foothills as southerly to southeasterly flow aloft develops. A few stray showers or storms may drift into the southern/central Willamette Valley and central OR Coast Range/coast late Thursday afternoon into the evening hours. Chances for showers and thunderstorms increase for all locations in southwest WA and northwest OR Friday into Saturday. Becoming cooler Sunday through early next week with continued chances for showers.

LONG TERM...Sunday through Wednesday night

The long term forecast is highlighted by a cool and showery onshore flow pattern with slightly below normal to near normal temperatures for this time of year. Starting off with Sunday, ensembles are in good agreement a closed low pressure system will slowly track over northern California and far southern Oregon. Wrap-around moisture on the northern side of the low will favor relatively high chances for rain showers, especially during the afternoon when probabilities for precipitation peak between 60-90%, highest over the mountains. It appears most of the showery activity on Sunday will be light, and chances for thunder are under 10%. Sunday is also on track to be the coolest day of the week with high temperatures most likely in the 50s.

On Monday, the GEFS/ENS/GEPS cluster analysis suggests the aforementioned low pressure system will progress eastward, bringing a transition to northwesterly flow aloft over Oregon and Washington. It appears a weak shortwave trough embedded within the northwesterly flow will move through Monday, maintaining cool conditions with chances for light showers over all of northwest OR and southwest WA (25-50% chance, except 65-85% in the Cascades).

Chances for precipitation then decrease Monday night into Tuesday as upper level flow becomes westerly and strengthens ahead of an incoming Pacific frontal system. Although there are some model timing differences, ensembles are in good agreement this system will bring widespread rain to the area beginning sometime between late Tuesday and Wednesday morning. There is also uncertainty regarding total rain amounts with this mid-week system. The NBM currently suggests there is a 30-60% chance for at least 0.25 inches of rain between 5pm Tuesday and 5pm Wednesday over southwest WA and northwest OR, except 50-75% in the Coast Range and Cascades. Probabilities for at least 0.5 inches of rain decrease to 15-30%, except 30-60% in the mountains. Chances for over 1 inch of rain fall below 10% in the lowlands and below 30-40% in the mountains, suggesting flooding is unlikely to occur. -23

AVIATION

High pressure remains in place over the area Thursday morning, bringing clear skies and calm winds with widespread VFR flight conditions. Expect these conditions to continue through Thursday morning, aside from potential exceptions at KAST and KONP. Similar to yesterday morning, these TAF sites may see a very brief period of shallow fog between 12-16z. The probability for fog to develop at both terminals during that time is only around 10-15%. This is not reflected in the TAFs given it is not the most likely outcome, however amendments will become warranted if satellite and surface weather observations later this morning suggest fog is developing.

Scattered mid to high level clouds will begin to increase from south to north over northwest OR and southwest WA between 22z Thursday and 12z Friday as upper level flow becomes southerly. This flow pattern will also bring increasing chances for scattered rain showers and isolated thunderstorms late Thursday afternoon into the evening hours, mainly to the south of KAST, KHIO, KPDX, KTTD, and KUAO. KONP and KEUG has the highest chance for showers, peaking around 30-50%. The highest chance for showers is between 00-06z Friday at KEUG and 06-12z Friday at KONP. Probabilities for thunderstorms at KEUG peak near 10%, and near 5% at KONP. Therefore, have decided not to include a PROB30 group for thunderstorms as probabilities at each TAF site are not high enough. It appears most thunderstorm activity will occur to the south and east of KEUG, focused over southwest OR and the Lane County Cascades. Nevertheless, locations that do see stronger showers or thunderstorms will have the potential for brief moderate to heavy rain, small hail, and gusty and erratic outflow wind gusts up to 20-35 kt.

KPDX AND APPROACHES...Expect VFR flight conditions to continue through 12z Friday. Clear skies should give way to scattered mid to high level clouds between 00-12z Friday. Light and variable winds around 3 kt or less Thursday morning should become northeasterly around 4-7 kt Thursday afternoon. -23

MARINE

Tranquil conditions are expected to persist over the coastal waters through early next week with winds out of the north to west and seas around 5 to 7 ft. It is unlikely Small Craft Advisory conditions will occur over waters at any point in time through Monday as winds are expected to remain below 20 kt, aside from one potential minor exception. Between 4-9 PM PDT Thursday, marginal small craft advisory level wind gusts as high as 21-22 kt may occur over portions of the northern and central outer waters. Although probabilities for max wind gusts to peak around this range are fairly high at 50-70% over the southern half of PZZ271 and the northern half of PZZ272, it appears wind gusts of this magnitude would only occur for a 1-3 hour period at any given location. Given seas are only expected to range between 5 and 6 ft at that time, have decided not to issue a Small Craft Advisory as conditions appear too marginal and short-lived to justify a headline. -23

PQR WATCHES/WARNINGS/ADVISORIES

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