textproduct: Portland

SYNOPSIS

Dry weather persists going forward accompanied by a warming trend into the weekend as a ridge of high pressure builds overhead - well above normal temperatures by Sat/Sun. Expect patchy morning fog at times across inland valleys as well, although offshore winds develop Saturday and Sunday may limit the coverage of fog somewhat. Otherwise, little in the way of weather impacts through at least the middle of next week.

DISCUSSION...Now through Wednesday

Our dry streak continues for at least the next week as a above normal temperatures become a common theme to start October. At least for today however, a weak weather system is expected to brush the Pacific Northwest to the north near Vancouver Island helping to keep the ridge flatter than it otherwise would be while also streaming high clouds across the region from time to time. 850 mb temperatures warm several degrees compared to yesterday (Wednesday) which will in turn allow afternoon highs to warm into the mid to upper 70s across the interior lowlands today. The NBM even gives portions of the Willamette Valley a 10-30% chance to exceed 80 degrees, the first of many increasing chances to pass this threshold as we move onward into the weekend.

Expect temperatures to jump a few more degrees Friday and especially Saturday/Sunday as the upper level ridge amplifies over the Pacific NW along with mostly sunny skies each afternoon. Temperatures 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.

Throughout the end of the week into the weekend the combination of overnight clearing/longer nights, locally lighter winds, and a fairly prominent temperature inversion will facilitate fog development. But, the placement of valley fog likely shifts to some of our more protected valley areas over the weekend as the previously mentioned offshore flow develops - this is a point of notable forecast uncertainty. Should the overnight offshore flow/east winds end up weaker or decoupled from the boundary layer over the Willamette valley, fog end up fairly decent in coverage. The REFS and even the ECMWF points to this foggier scenario playing out at least on Saturday morning. Wherever present, fog will greatly reduce visibility and may potentially impact travel.

Underneath this upper-level ridge pattern over the next several days, 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 tops of the coast range as well) during the overnight to morning hours, increasing to peak gusts up to 20-35 mph. Then, as mixing gets underway and the inversion breaks, 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. Exploring the NBM, the probability of wind gusts peaking above 30 mph is less than 10-25% everywhere except in the western Columbia River Gorge and surrounding terrain at 40-55%, which is generally the windiest location during offshore easterly flow.

By Monday, the vast majority of ensemble members indicate the ridge axis shifts east of WA and OR, allowing the overall flow to turn back onshore while bringing an end to the locally breezier east winds. Temperatures lower again into the 70s to near 80 for the interior valleys Monday afternoon into the middle of the week, though we'll remain generally 5 to 10 above normal for early October. For better or worse, ensemble guidance is adamant in the continuation of dry weather through the middle of next week, potentially longer. -99/DH

AVIATION...Through 18Z Friday

High pressure over the region brings widespread VFR conditions with high clouds, expected to continue through at least 12Z Fri. After 12Z Friday, chances of deteriorated conditions increase throughout the airspace. Specifically, chances at KAST and KONP are around 20% for LIFR, 30% for IFR, and 40-50% for MVFR after 12Z Fri. As for inland terminals, chances are around 10-15% for LIFR, 20% for IFR, and 30% for MVFR after 12Z Fri. While such conditions will consist of CIGs and VIS, CIGs are more likely. Otherwise, light and variable winds will become north/northwesterly winds of 5-10 kt by this afternoon, strongest at KONP, KSLE, and KEUG.

KPDX AND APPROACHES...VFR with high clouds and north-northwesterly winds 3-6 kt through the TAF period. ~12

MARINE

We'll see the continuation of calmer weather today as a weak weather system brushes the waters to the north, temporarily shifting winds out of the west around Cape Falcon northward. Northerly winds then return across the entire waters Friday, likely persisting through the weekend into early next week. Wind gusts generally rise in the 10-20 knot range through this period. Otherwise, the west to northwesterly swell of ~6 ft at 9 to 11 seconds gradually subside to around 3 to 5 ft at 8 to 9 seconds by Friday into the weekend. At this time, there are no other prominent swells of note through at least the the middle of next week. -99/12

PQR WATCHES/WARNINGS/ADVISORIES

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