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, bringing clear skies and calm winds with widespread VFR flight conditions. Expect these conditions to continue through at least 20Z-22Z Thursday as scattered mid to high level clouds will begin to increase from south to north as upper level flow becomes southerly.

This flow pattern will result in increasing chances for showers as well as isolated thunderstorms and will likely result in generally VFR flight conditions with intermittent MVFR conditions that will be associated with any showers. Look for these showers to start around 20Z Thursday with a 10-20% of thunderstorms south and east of KEUG starting around 22Z Thursday through 03Z Friday. KONP will also see an increase in showers chances through the same time period, but only a 5-10% chance for thunderstorms at this time. Currently, have decided to not include a PROB30 group for thunderstorms as probabilities at each TAF site are not high enough. For locations other than KONP and KEUG, there is generally a 10-25% chance for showers starting around 22Z Thursday through the remainder of the TAF period. It should be noted that 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 35 kt.

KPDX AND APPROACHES...Expect predominately VFR flight conditions to continue through the TAF period. Clear skies should give way to scattered mid to high level clouds between 00-12z Friday. Light and variable winds around 5 kt or less should become more southerly as the day progresses around 4-7 kt. /42

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.