textproduct: Portland

SYNOPSIS

Dry and warmer conditions persist through Thursday with weak onshore flow continuing through Wednesday. Cooler temperatures return for Friday through the weekend. Rain returns to the area on Saturday with shower chances continuing over the Cascades on Sunday. Rather benign, dry weather returns early next week.

DISCUSSION...Wednesday morning through Tuesday

Surface observations and satellite imagery indicate clear skies across the majority of NW OR and SW WA early Wednesday morning with localized fog developing in sheltered valleys. High pressure over the southern US will briefly spread north into the PacNW today, producing another sunny and dry day with high temperatures warming a few degrees from yesterday for most locations, including the coast where offshore flow (winds from the east) through midday will likely enhance warming. Expect highs in the mid to upper 80s for the interior lowlands, 70s to low 80s for the terrain, and upper 60s to mid 70s along the coast.

Ensemble guidance is now in very good agreement on the weather pattern through the weekend. A weak trough will deepen into the PacNW late Wednesday into Thursday, pushing south the high pressure. Expect high temperatures to lower anywhere from 3-10 degrees from today's highs, though dry weather remains.

A stronger closed low pressure system develops along the western Canada border late Thursday and moves south along the coast into the PacNW through Saturday, then pushing inland to the east Sunday. This weather system will bring a round of rain to the area through the day on Saturday with shower chances continuing over the Cascades through Sunday afternoon. There's a 50-70% chance of locations west of the Cascades receiving at least 0.25 inch of rain with a 65-85% chance for the Cascades. For chances of at least 0.5 inch of rain, those lower to 15-30% west of the Cascades and 40-65% for the Cascades. Not a soaker by any means, but a nice round of steady rain to break up the warm and dry conditions. One note is the Hood River Valley may be rain shadowed due to the westerly nature of this weather system, with only a 25-40% chance of receiving 0.25 inch of rain. This weather system will also bring even cooler temperatures on Saturday with highs forecast in the low to mid 60s across the interior lowlands and coast with 40s to 50s over the terrain. Temperatures are forecast to warm roughly 5 degrees for most inland locations on Sunday, though will still be on the cooler side of normal (which is the upper 70s for the interior lowlands in mid September).

As the main weather system moves to the east early next week, rather benign weather takes hold of the area Monday and Tuesday. Dry conditions are expected to return, though uncertainty remains in temperatures as some ensemble members suggest weak longwave troughing settles in place over the PacNW, which would keep temperatures on the cooler than normal side, while others indicate more zonal flow or slight ridging, which would allow temperatures to rise to near or a bit higher than normal. Whichever pattern wins out, no impacts are expected at this time. -03

AVIATION...Through 12z Thursday

High pressure over the region will continue to support widespread VFR conditions across the airspace beneath largely clear skies. Light offshore flow into Wednesday morning will limit chances for low cigs to around 20% along the coast, but if east winds are lighter than expected, lowered cigs will be more likely. Favored east-southeast winds along Youngs Bay at KAST may also result in restricted vis within fog/mist overnight despite the offshore orientation, however confidence in occurrence is low. North to northwest winds build to 5-10 kt inland and 8-12 kt with gusts to 20 kt along the coast by 18-20z Wed with high confidence in widespread VFR conditions.

Onshore flow returns along the coast with the northwest winds Wednesday afternoon and will continue through Wednesday night, even as winds decrease after 03z Thu. This will allow for the return of marine stratus along the coast, developing along the coast after 06-09z Thu.

KPDX AND APPROACHES...VFR conditions expected through the period. Diurnal northwest winds at 3-5 kt overnight will build to 6-8 kt by 18-20z Wed, decreasing again by 03-06z Thu. High cloud coverage increases after 06z Thu. -03/36

MARINE

High pressure offshore will maintain predominately northerly winds across the coastal waters through the week. The pressure gradient over the waters is slowly decreasing, and gusts up to 25 kts are expected to continue for the outer coastal waters through 6 AM Wednesday. Breezy north winds continue through Thursday, but should remain 20 kt or less. Seas have built to around 6 to 7 ft Wednesday morning with a dominant northwest swell at 14 to 15 seconds behind the stronger northerly winds from yesterday. Seas remains around 6 to 7 ft through Thursday with period lowering to 10-11 seconds by this evening. A weak low pressure system will move through the waters Thursday night, briefly elevating seas to 7 to 8 ft at 10 seconds Thursday afternoon and night as winds increase slightly ahead of the system. Another weaker is expected to approach the coastal waters from the north late Friday into Saturday, though seas are forecast to remain around 5 to 6 ft and wind gusts below 20 kts with this system. -03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 6 AM PDT early this morning for PZZ272-273.