textproduct: Portland

SYNOPSIS

A building ridge of high pressure will lead to a warming trend through this weekend. Expect above average temperatures with dry conditions through the first week of October. Patchy morning fog at times across inland valleys will be possible, before offshore winds develop Saturday and Sunday. Otherwise, little in the way of weather impacts through the middle of next week.

DISCUSSION...Now through Tuesday Night

Current satellite imagery shows a few lingering clouds giving way to mostly sunny skies this afternoon as a broad ridge of high pressure develops over the region. Near average temperatures for the end of September will be followed by a warming trend through the weekend as we move into October. A weak weather system is expected to remain north of the area as it moves toward Vancouver Island Thursday and Friday. This feature will also keep a flatter ridge over the region with high clouds streaming across the area. 850 mb temperatures are also gradually warming a few degrees, which will in turn allow afternoon highs to warm into the mid to upper 70s across the interior lowlands through Friday.

Due to decent clearing during the overnight/morning hours, light winds, and a more prominent temperature inversion developing each night, patchy fog development is likely, especially in our more protected valley areas with a water source. Morning fog will greatly reduce visibility wherever present.

Expect temperatures to jump a few more degrees over the weekend as the upper level ridge amplifies over the Pacific NW along with mostly sunny skies each afternoon. Temperatures will likely warm into the upper 70s to lower 80s on Saturday before warming a few more degrees, into the lower to mid-80s on Sunday. Most areas are not expected to reach record high temps, but the record of 84 degrees on Sunday at PDX could be in jeopardy. Offshore flow could also bring slightly warmer temperatures to the coast as well with highs into the upper 60s to lower 70s.

Underneath this upper-level ridge pattern, ensembles indicate a near surface thermal trough develops along the coast on Friday through this weekend. This pattern is expected to cause winds to shift from the onshore (westerly) pattern the area has generally been observing over the past couple of weeks to offshore (east to northeasterly). At this time, pressure gradients do not look to be very high, peaking at a -3mb to -4mb difference between PDX and DLS, so winds aren't expected to be strong. Instead, expect to see a typical diurnal behavior of winds becoming easterly on the west side of terrain (mainly Cascades, potentially coast range peaks as well) in the overnight to morning hours and increasing to peak gusts up to 20-35 mph. Then, winds decrease over the terrain during the day and increase through the valley from the north in the afternoon hours with gusts up to 20 mph. The probability of wind gusts peaking above 30 mph is less than 10-25% everywhere except for through the western Columbia River Gorge and surrounding terrain at 50-65%, which is generally the windiest location during offshore flow. At least confidence is moderate to high regarding the transition back to onshore flow Sunday night into Monday which would bring an end to these drier and locally breezy conditions.

By Monday, the vast majority of ensemble members indicate the ridge axis shifts east of WA and OR, allowing for increased onshore flow. Temperatures should lower again into the 70s for the interior valleys, though still remaining somewhat above normal, with dry weather continuing through the middle of next week. DH/99

AVIATION...Through 00Z Friday

Currently widespread VFR with generally clear skies through the airspace. While most terminals will remain VFR, a few may see deteriorated conditions overnight. Specifically, KAST has a 15% chance of IFR and 10% of LIFR CIGs between 05-11Z Thu, KEUG has a 15% chance of IFR and 10% chance of LIFR CIGs between 09-18Z Thu, and KONP has a 10-30% chance of LIFR/IFR CIGs between 04-18Z Thu. Overall, very low confidence for deteriorated conditions at these terminals through the TAF period. Otherwise, expect north to northwesterly winds 5-10 kt through the afternoon, weakening by 04-06Z Thu.

KPDX AND APPROACHES...VFR conditions and northwesterly winds 3-6 kt continue through the period. ~12

MARINE

Calmer conditions over the waters continues today, with wave heights around 5 to 6 feet. A weak weather system out of the west will approach the northern waters late Thursday, temporarily shifting winds out of the west and increasing them slightly. Northerly winds then return Friday, likely persisting through the weekend into early next week. Otherwise, the west to northwesterly swell of 5 to 6 ft at 9 to 11 seconds today will gradually subside to around 3 to 4 ft Thursday and continue into the weekend. There is no other prominent swells of note at this time. ~12/99

PQR WATCHES/WARNINGS/ADVISORIES

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