textproduct: Portland

SYNOPSIS

Dry weather continues through Monday with temperatures near to slightly below seasonal average. A weakening frontal boundary will bring the next chance for rain Tuesday into early Wednesday, with the highest probabilities along the coast, Coast Range, and northern Cascades while most inland valleys remain dry. Dry weather is favored again for the latter half of the week.

DISCUSSION...Now through Saturday

High pressure remains in control across the region this afternoon, maintaining generally quiet weather through Monday. A weak upper-level low is settling southward toward California while low-level flow gradually shifts from weak offshore flow today toward a light onshore component Monday. Temperatures will remain seasonably cool, with inland valley highs generally in the upper 60s to possibly 70 on Monday.

Clear skies and light winds tonight will again promote efficient radiational cooling, particularly in sheltered valleys. The Upper Hood River Valley has the best potential for isolated frost, with temperatures potentially falling into the mid to upper 30s. However, the probability of temperatures dropping below 36 degrees remains relatively low at around 10%. With temperatures around 36 degrees this morning around Parkdale, definitely will need to keep an eye on temperatures tonight into Monday morning. Patchy morning fog is another possibility where low-level moisture, light winds, and strong overnight cooling overlap. The central and southern Willamette Valley, Coast Range valleys, and Cascade valleys are among the more favored locations. With that said, there is lower confidence for fog tonight into Monday morning compared to previous mornings as guidance has trended towards lower chances (10-20%). Any fog that does form should remain localized and dissipate by late morning Monday.

The next meaningful change arrives Tuesday as an upper-level trough and weakening frontal boundary move into the Pacific Northwest. Precipitation chances increase substantially along the north Oregon Coast and Coast Range into southwest Washington, with probabilities around 60-90% Tuesday afternoon before gradually decreasing Tuesday night into Wednesday morning. Some rain will likely extend into the northern Oregon Cascades as well. Farther inland, confidence in measurable rainfall remains much lower, with only around 5-25% of ensemble members indicating more than a trace in the Portland metro and Willamette Valley through Wednesday morning. However, just to the north, there is a 45-70% chance of at least measurable rainfall from Battle Ground to Longview. Most inland valley locations should therefore remain dry, particularly farther south.

The front weakens as it moves through the region, with dry conditions expected to return by midday Wednesday and continue through Thursday. Ensemble guidance remains split regarding the upper-level pattern late next week, with roughly 35% of members favoring weak troughing, 35% weak ridging, and 30% a more zonal flow. The troughing scenario would bring cooler temperatures and perhaps a few light coastal showers Thursday, while the ridging and zonal solutions generally favor dry weather. Regardless of the eventual result, the current range of outcomes suggests relatively limited weather-related impacts through the end of the week. ~12/99

AVIATION...Through 00Z Tuesday

VFR conditions prevail through the TAF period with mostly clear skies. The only exception is a 30-50% chance of marine stratus along the far N OR and SW WA coast, including KAST, between 12-19z Mon. North winds continue, up to 10-15 kt along the coast with occasional gusts up to 20-25 kts by 19-21z Sun, remaining generally less than 10 kts inland. Winds decrease by 06z Mon and remain light through the end of the TAF period.

KPDX AND APPROACHES...VFR conditions are anticipated through the TAF period. North to northwest winds around 8-10 kt, decreasing after 04-06z Mon and remaining generally less than 5 kts through the rest of the period. -03

MARINE

Expect northerly winds to persist across the coastal waters today thanks to lingering high pressure near the surface helping to facilitate gusts up to 20-25 kts at times through this evening. Small Craft Advisories remain in effect through 11 PM PT Sunday. Wave heights will remain around 4-6 ft through tonight.

Then the focus shifts to a frontal boundary approaching the waters on Monday, shifting winds more westerly during the daytime hours then eventually southerly Monday evening. This frontal feature finally moves through the waters the second half of Tuesday. There are high chances (70-90%) of wind gusts exceeding 21 kts (Small Craft Advisory criteria) late Monday into Tuesday for the coastal waters north of Cape Foulweather, with gusts most likely peaking around 25-30 kts. Lighter northerly winds return Tuesday night. A longer period west to northwesterly swell of 5-8 ft at 12-16 seconds will also move into the coastal waters late Monday through Wednesday, resulting in a moderate sneaker wave threat and increasing significant wave heights into the 6-9 ft range. While seas gradually fall into Thursday, models hint at the arrival of another WNW swell of similar magnitude around Friday. -03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 8 PM PDT this evening for PZZ210.

Small Craft Advisory until 11 PM PDT this evening for PZZ251>253- 271>273.