textproduct: Portland

SYNOPSIS

A cool and unsettled weekend is ahead as an upper-level low moves across the Pacific Northwest. Widespread rain and showers will develop Saturday and continue into Sunday, with temperatures remaining below seasonal average. A much warmer and drier pattern then develops early next week as high pressure builds offshore.

DISCUSSION...Now through Thursday

Today will be a relatively quiet day between weather systems, but temperatures will remain cool compared to yesterday. Increasing clouds ahead of the next system will limit afternoon warming, with highs reaching the upper 60s to low 70s across the interior and low to mid 60s near the coast. Some interior valleys could also see patchy fog or low clouds this morning where skies remain clear, especially in areas with light winds and higher surface moisture.

The next system will become the main weather story this weekend. An upper-level low will deepen offshore tonight before moving across the region Saturday. Recent guidance continues to suggest that rain will reach the coast Saturday morning and spread inland through the afternoon. Rain and showers will continue Saturday night, followed by additional showers through most of Sunday. Confidence is high that most of the forecast area will see measurable precipitation, with most ensemble members supporting rain somewhere across the region.

Saturday through Sunday, total rainfall amounts should generally remain on the lighter side, with around 0.10-0.50 inch expected for most locations. The Cascades will likely receive more, with the southwest Washington and north Oregon Cascades currently favored for the highest totals around 0.50-1.00 inch (highest values in the southwest Washington Cascades). Showers will become more scattered Sunday as the system moves east, although the higher terrain will continue to have the best chance for stratiform rain.

There is also a slight chance for thunderstorms (15-25% chance) Saturday as the cold air associated with the upper-level low moves overhead. How much instability develops will depend partly on the timing of the system. If the coldest air arrives later in the day, after daytime heating has weakened, thunderstorm chances will be lower. An isolated thunderstorm is still possible, however. Cold-core funnel clouds could also develop near the upper low. These are generally short lived and often remain above the ground, but a funnel reaching the surface cannot be completely ruled out.

The weekend system will also bring another noticeable drop in temperatures. Highs Saturday and Sunday will generally be in the low to upper 60s across the interior. Conditions should gradually improve Sunday night as the low moves east and shower activity decreases.

A much warmer pattern is then expected to develop early next week as high pressure strengthens over the eastern Pacific. Some lingering troughing could keep Tuesday from becoming completely clear and warm, but the overall trend is toward drier weather and rising temperatures. Interior highs should climb into the mid to upper 70s by Tuesday.

By Wednesday, ensemble guidance favors the offshore ridge moving closer to the Pacific Northwest, with about a 75% chance of this scenario. If the ridge continues to build as expected, temperatures could reach the low to mid 80s across the interior lowlands. The other 25% of the guidance keeps some troughing near the region and would result in cooler temperatures. Increasing easterly flow is also possible by midweek as the ridge strengthens. Current guidance suggests the strongest winds from Wednesday night into Thursday morning, with sustained winds of 15-20 mph and gusts 25-30 mph (isolated gusts up 35 mph). With rising temperatures and lowering relative humidity Tuesday through Thursday and increased winds on Wednesday, this will definitely be something to monitor as the warmer pattern develops next week. ~12

AVIATION...Through 12Z Saturday

Expect primarily VFR conditions through the TAF period. The only exception is from now until 17Z Fri, where the potential for stratus formation brings a 20-30% chance of MVFR CIGs at all terminals (VIS included for coastal terminals). Conditions should improve back to VFR by 17-18Z Fri. Otherwise, expect light and variable winds early this morning to become 5-8 kt west to northwest winds in the afternoon.

KPDX AND APPROACHES...Currently high end MVFR CIGs (with guidance suggesting a 20-30% chance of MVFR CIGs), which will likely improve back to VFR by 17-18Z as low stratus mixes out. Beyond 17-18Z Fri, expect VFR conditions through the TAF period. Light and variable winds will become 4-6 kt northwest winds in the afternoon.~12

MARINE

Northerly winds continue across the waters today. A weak system is expected to approach the coastal waters from the north tonight into Saturday. This will result in a southerly wind shift along with seas subsiding towards 4 to 6 ft and gusts below 20 kts. A more active pattern is expected to return late Monday into Tuesday. This system looks to bring northerly winds back into the forecast with gusts approaching 20 kt. Seas are also expected to build towards 5 to 7 ft. ~12/42

PQR WATCHES/WARNINGS/ADVISORIES

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