textproduct: Portland

SYNOPSIS

Isolated showers taper off this morning. Otherwise, dry weather returns today, continuing through the weekend, along with temperatures in the 60s Friday into the weekend. Rain is expected to return sometime early next week, though the exact details remain uncertain at this time.

DISCUSSION...Thursday through Wednesday

Radar imagery early Thursday morning shows very isolated showers continuing across NW OR and SW WA as the upper level shortwave moves into eastern OR and ID. These showers should taper off by around 5-8 AM. Outside of these showers, satellite imagery indicates that skies are mostly clear. This is allowing for radiative cooling, which combined with the colder airmass behind today's cold front, is causing a much cooler temperatures early this morning. Current observations show temperatures across the interior lowlands and the coast have already fallen to the mid to upper 30s, and low temperatures this morning are forecast to drop to the low to mid 30s with pockets of upper 20s in rural areas. Widespread frost is likely, which could cause impacts to sensitive outdoor vegetation. Additionally, this pattern could lead to a period of low stratus or fog development around the sunrise hours due to lingering moisture from the recent rains. Motorists should be cautious of sudden changes in visibilities, especially in more rural areas.

Zonal flow will take shape over the PacNW today behind the shortwave, and ensembles are in good agreement that this pattern will continue through at least Saturday with increasing chances into Sunday. The colder airmass behind yesterday's cold front will keep high temperatures today in the mid to upper 50s across the interior lowlands, but daytime temperatures will warm quickly into the 60s for Friday into Sunday. Clear skies will allow for a similar overnight pattern as today with chances of frost and fog. However, these chances decrease each day as overnight temperatures increase slightly each night and surface conditions become more dry. A weak thermal trough will develop over the Oregon coast this afternoon continuing through Friday afternoon. This will tighten surface pressure gradients, causing weak offshore flow along the western Cascades and Coast Range, especially tonight into Friday morning.

Ensemble guidance is coming into better agreement in pushing back the next round of precipitation into late Sunday at the earliest with better chances sometime on Monday. Ensemble members in latest runs have better have resolved the upper level pattern for Sunday into Monday, indicating that instead of one long trough moving through the eastern Pacific, there will be two separate features: a closed low moving southeast through the central Pacific and a low off of western Canada pushing south through the northeastern Pacific. The majority of ensemble members suggest these will meet over the eastern Pacific late Sunday and continue moving east towards the West Coast into Monday. This meeting is where uncertainty increases as some members essentially indicate the two features will merge into one longwave trough while other members keep them mainly separate and moving east at different speeds. The dynamics can be very different with these solutions, but either way, guidance shows the portion that will affect NW OR and SW WA weakening as it reaches the coast late Sunday/Monday, leading to not much precipitation. Another feature is evident Tuesday into Wednesday that could lead to higher chances of another round of precipitation, though details are still uncertain. -03

AVIATION

Predominately VFR conditions expected over the next 24 hours as upper level flow becomes zonal with westerly winds aloft. Mostly clear skies and calm winds will support temperatures in the low to mid 30s through 14-16z Thu, leading to potential frost development over metal surfaces. Though frost is more likely due to cold temperatures, can't rule out fog or low stratus development either (5-15% chance) after 10-12Z Thu, especially for our typical foggy sites like KHIO and KEUG. Any lowered flight conditions will improve to VFR by 16-18z Thu. Winds become northerly after 18Z Thu which will promote increased winds around 8-12 kts along the coastal terminals and those from KUAO south, decreasing by 03-06z Fri.

KPDX AND APPROACHES...VFR conditions through the TAF period. Clear skies and calm winds will support frost development over metal surfaces through 16z Thu. Light and variable winds become northerly after 18-20z Thu, though remaining below 10 kts. -03

MARINE

High pressure is building over the waters with winds turning northerly throughout today. A surface thermal trough then develops tonight into Friday morning, tightening pressure gradients and strengthening these northerly winds. Breezy northerly winds initially remain south of Cape Falcon and beyond 20 NM offshore, however by Friday afternoon there is a 80-90% chance for widespread northerly wind gusts of 21 kt or greater across all waters south of Cape Falcon. For now, a Small Craft Advisory remains for the waters south of Cape Foulweather and beyond 10 NM offshore between 5 PM Thu through 5 AM Fri, but this advisory will likely be extended in areal coverage and duration with upcoming forecast packages. The thermal trough breaks down Saturday with high pressure continuing over the waters through the weekend. North winds will continue through the weekend, just not as gusty. Seas will generally remain between 7-9 ft at 10-11 sec through the weekend. -03/10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 5 PM this afternoon to 5 PM PDT Friday for PZZ272-273.