Area Forecast Discussion

SYNOPSIS

High pressure overhead is keeping northwest Oregon and southwest Washington warm and dry today. Out over the Pacific, a storm system is organizing and is expected to become a slow-moving closed low. This system is expected to bring more cloud cover on Monday, then a better chance for cooler, wetter weather by the middle of the week. Confidence is increasing that showers become more likely late Monday into Tuesday as the system begins to spread moisture inland. Wednesday still looks like the coolest and most showery day overall.

SHORT TERM...Now through Monday

Another day of warm and dry conditions expected as upper level ridging remains in control over the region. A cut-off low will be positioned well off the coast, supporting areas of surface low pressure and associated fronts offshore. However, this low will support upper level cloud cover through the day. Temperatures are expected to reach around 10 degrees above average inland with highs topping out in the low to mid 70s. Along the coast, expect temperatures 5 degrees above average topping out in the low to mid 60s. cloud cover may thin out enough over the northern Willamette Valley, mainly the Portland/Vancouver metro, for some locations to hit 80 degrees. Probabilities remain around 30-60% for Portland/Vancouver and the surrounding communities. Overnight lows will again be on the mild side with upper 40s to low 50s expected.

Monday will see a shift during the latter half of the day as the upper level low begins to slowly shift eastward toward the coast. This will bring increasing cloud cover and the increasing PoPs from south to north through the day. Temperatures will be similar to Sunday, just a tad cooler along the coast and in the central and southern Willamette Valley. Temperatures top out in the upper 50s to low 60s along the coast and upper 60s to low 70s in the Willamette Valley. Temperatures around the Portland/Vancouver metro and in the western Columbia River Gorge will likely see the mid 70s with a 20-40% chance for isolated areas reaching 80 degrees. Chances for light rain creep northward through the day, entering the southern portion of the forecast area late Monday morning and reaching the metro during the evening. -19

LONG TERM...Monday Night through Saturday

Ensembles show good agreement in the upper low making a larger move eastward Monday night, bringing a better surge of moisture into the region. Precipitation remains light with the highest totals through early Tuesday morning expected along the coast and southern Willamette Valley where up to 0.15 inches could fall. Elsewhere, totals less than 0.10 inches and likely some places remaining dry.

Confidence is higher Tuesday and Tuesday night as the upper level low moves inland and begins to transition into an open wave trough. Shower activity is expected to continue more persistently over the area. There is some uncertainty heading into Wednesday with the model ensembles differing on how quickly the trough moves eastward along with some smaller scale features that could enhance precipitation over the area. Some models show an area of low pressure over vancouver Island dropping southeast over the area on Wednesday which could help enhance precipitation totals. While rain amounts remain uncertain, expect around 0.25 to 0.75 inches in the lowlands and around 1 to 1.75 inches in the Coast Range and Cascades from Tuesday morning through Wednesday evening. Temperatures both Tuesday and Wednesday top out in the upper 50s to low 60s.

Conditions Thursday and Friday tend drier and warmer with temperatures moderating back into the low to mid 60s on Thursday and possibly low 70s on Friday. Shower activity tapers off through Thursday, possibly lingering over the Cascades through Thursday night. Uncertainty increases again heading into next weekend. -19

AVIATION

High pressure will maintain widespread VFR conditions through the 18Z TAF period with high clouds in place. Cigs are generally above 10-15 kft. Winds will remain light and variable around 5 kt or less for most of the TAF period, except at KTTD where breezy easterly winds continue with wind gusts upwards of 30 kt. That said, the pressure gradient over the Columbia River Gorge is expected to weaken around and after 00Z Monday, which will bring decreasing wind speeds to KTTD.

KPDX AND APPROACHES...VFR conditions will continue through 18Z Monday with cigs above 10-15 kft through the TAF period. Although easterly winds were initially expected to develop at the KPDX terminal by 17-18Z Sunday, excess morning cloud cover has delayed daytime heating, thus preventing breezy easterly winds aloft around 1.5 kft from mixing down to the surface. However, some thinning of high clouds is likely over the next few hours. This will most likely allow temperatures to heat up enough for easterly winds aloft to finally mix down to the KPDX terminal, sustained somewhere between 6-11 kt. Overall confidence in the exact timing and duration of easterly winds is low. It appears easterly winds will not last long if they do materialize. -23

MARINE

Benign conditions continue through the weekend. Southerly to southwesterly winds between 5-15 kt continue today with seas under 5 ft. Winds will weaken and become east to southeast by early Sunday morning. Winds then remain light through early next week with seas increasing by a few feet, but staying under 10 ft.

An area of low pressure sitting off the coast will move inland during the middle of next week. This will bring onshore flow, showers, and increasing winds and seas. However, uncertainty remains regarding exact wind speeds and wave heights. The placement of the low will determine what conditions materialize across the waters. The current forecast favors a period of increasing west to northwest winds late Tuesday into Wednesday with wind gusts reaching 20-25 kt or stronger by Wednesday afternoon. In fact, there is a 50-75% chance for max wind gusts over 25 kt. The highest probabilities are beyond 30 NM offshore. Seas are expected to reach at least 10-12 ft by late Wednesday, however there is a 40-70% chance seas will peak over 12 ft over the northern and central outer waters. There is a 1-5% chance seas will peak as high as 15 ft, which represents the worst case scenario.

Very strong ebb currents will continue across the Columbia River Bar each morning through Tuesday. With seas expected to build slightly by early next week, a steep ebb chop may necessitate Small Craft Advisories in the hours around peak flow each day. -23

PQR WATCHES / WARNINGS / ADVISORIES

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