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

Dry conditions and above average temperatures will be the story to start October. Ridging is currently centered over the Great Basin with a weak short wave passing over Vancouver Island is keeping the ridge suppressed over the PacNW. Upper level cloud cover can be seen streaming in overhead along westerly flow will continue through much of the day. Temperatures today are expected to top out 4-6 degrees warmer than yesterday (Wednesday), topping out in the mid to upper 70s across the interior lowlands. Chances to reach 80 remain around 15-30% for the Willamette Valley with higher chances of 40-50% across the Portland Metro.

Tomorrow is expected to be very similar to today with patchy fog possible tonight and stratus developing along the coast. Highs are expected to top out in the mid to upper 70s with lower probabilities of reaching 80 as 850 mb temperatures across the area drop a degree or two.

Heading into the weekend, the upper level ridge is expected to amplify into the PacNW, supporting a noticeable warm-up both days. 850 mb temperatures are expected to jump into the 14-20 C range on Saturday and between 17-22 C on Sunday. This is expected to translate to Saturday highs in the upper 70s to low 80s across the interior lowlands and mid 60s to low 70s along the coast. Sunday highs jump up into the low to mid 80s inland while coastal areas see similar temperatures to Saturday. Most areas are not expected to reach record high temps, but the relatively weak 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 over the next several days, ensembles continue to support the development of a surface thermal trough along the coast on Friday through this weekend. This is expected to shift winds from onshore (westerly) to offshore (easterly) 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.

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.

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. -19/99

AVIATION...Through 00Z Saturday

Building high pressure over the region will continue to maintain widespread VFR conditions with high clouds through at least 11Z Fri. After 11Z 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 11Z Fri. As for inland terminals, chances are around 10-15% for LIFR, 20% for IFR, and 25% for MVFR after 11Z Fri. While such conditions will consist of CIGs and VIS, CIGs are more likely. Conditions will begin to improve by 17-18Z Fri. Otherwise, expect north to northwesterly winds of 5-10 kt through 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

Expect calmer weather today as a weak weather system brushes the waters to the north. Winds will temporarily shift out of the west around Cape Falcon northward this evening. Northerly winds then return across the entire waters Friday through the weekend into early next week, bringing wind gusts around 10 to 20 knot. Otherwise, the west to northwesterly swell of 4 to 6 ft at 9 to 11 seconds will gradually subside to around 3 to 5 ft at 8 to 9 seconds by Friday and persist through the weekend. At this time, there are no other prominent swells of note through at least the the middle of next week. ~12

PQR WATCHES/WARNINGS/ADVISORIES

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