Area Forecast Discussion

SYNOPSIS

After ample valley fog/low stratus to start Wednesday morning, a frontal boundary arriving Wednesday afternoon will kick of an active and wetter weather the remainder of the week and beyond. Still, these conditions will be mixed with temporary periods of dry weather in-between systems, and slightly warmer than average temperatures. At least weather related impacts over the next 5-7 days stay fairly limited overall.

DISCUSSION...Now Through Monday

Conditions trend completely dry and calm this afternoon into the evening hours as an upper- level low centered near the northern California/Oregon boarder progresses further eastward and a transient ridge of high pressure hurriedly builds overhead tonight. The eventual return of widespread clear skies and light winds likely allows for efficient radiational cooling as temperatures fall into the mid 30 to low 40s. Thanks to these conditions and ample lingering surface moisture, fog development increases into early Wednesday morning as the surface inversion deepens. Come the sunrise hours Wednesday morning, models like the REFS are very bullish on fog formation across our inland valleys areas, especially in the central/south Willamette Valley and in the more protected coast range valleys. If you encounter dense fog Wednesday morning, remember to keep your vehicle headlights on and slow down. Fog likely transitions to a low stratus layer by midday Wednesday before breaking up.

Afternoon temperatures Wednesday rise into the mid 50s to near 60 degrees as southerly winds increase ahead of our next approaching weather disturbance in the form upper level shortwave trough moving across the NE Pacific. This feature is slated to arrive later Wednesday afternoon at the coast, spreading inland by Wednesday evening. Chances for widespread rain peak Wednesday night at around 85-100%, though precipitation amounts remain relatively low. Mean QPF values ranging from around 0.10-0.30 inch while chances for exceeding 0.50 inch are highest along the coast (20-40%) and south WA/north OR Cascades and foothills (30-45%). A few lingering showers will be possible into Friday morning, though Thursday likely will see an extended dry period the latter half of the day with temperatures warming again a few degrees above normal.

Then on Friday, cluster analysis of 500 mb heights looks relatively similar with southwest flow aloft over the NE Pacific and weak upper level ridging nudging into the PacNW. But, there are subtle differences that will determine whether areas remain dry or wet weather returns to end the week. This still largely depends on the strength of the upper ridge and where the northern jet stream, which will likely be associated with atmospheric river moisture, will be directed towards. The latest clusters show the wetter solution is dominated by the European ENS (southern jet solution), while the GEFS (northern jet solution) is hedged toward a drier forecast for much of the forecast area by keeping much of the precipitation just to the north. So, still quite a bit of uncertainty in the forecast.

There remains decent confidence in the forecast over the weekend as ensembles are in good agreement depicting broad troughing over the Pacific pushing the upper level ridge farther east over the Rockies. Temperatures are likely to warm well into the 60s across the area on Saturday before a frontal system spreads rain across the Pacific Northwest late Saturday into Sunday. All that being said, no significant precipitation impacts are expected with any of these weather systems over the next 5 to 7 days, and snow levels hold around 5000-6000ft before lifting again this weekend. Keep in mind storm drains tend to get easily clogged this time of year. However, beyond day 7 into next week (11/11-11/15), a large number of ensemble systems are beginning to show a growing consensus for a potentially more impactful and active weather pattern. -99/DH

AVIATION...Through 06Z Thursday

Satellite and surface weather observations from 0530Z Wednesday depicted broken high clouds over the central and northern Willamette Valley into the Portland, resulting in broken to overcast skies at KSLE, KUAO, KHIO, KTTD, and KPDX with ceilings around 8000-9000 ft. If this cloud deck lingers through much of tonight, fog development will be limited. Confidence is high KTTD will remain VFR with no fog development due to a combination of the aforementioned cloud deck and continued breezy easterly winds around 10 kt with gusts up to around 20 kt. Confidence is relatively lower at KUAO, KSLE, KHIO, and KPDX; probabilities for surface visibilities below 1 SM at these TAF sites are around 20-40% between 12-17Z Wednesday, except near 50% at KHIO. KEUG also has a 20-40% chance for surface visibilities below 1 SM. Meanwhile, light offshore winds at KAST and KONP should be enough to prevent fog development at the coastal TAF sites.

Overall, VFR flight conditions are the most likely outcome through 22-00Z for coastal terminals and through 03-05Z for inland terminals. Chances for MVFR ceilings below 3000 ft increase rapidly towards 00Z at the coast and around 06Z inland as a front moves inland and brings a band of light stratiform rain with it. Rain will begin ahead of the frontal passage, most likely between 21-23Z at the coast and between 00-02Z inland. Breezy south winds are expected at the coast as the front moves inland, with gusts as high as 20-25 kt.

KPDX AND APPROACHES...Expect predominately VFR conditions to continue through 06Z Thursday. There is a 20-40% chance for surface visibilities below 1 SM between 12-17Z Wednesday, however confidence is not high enough to reflect this in the TAF. Light rain is expected to begin sometime between 00-02Z Thursday ahead of an incoming front. Chances for ceilings below 3000 ft increase at 06Z Thursday to around 40%. -23

MARINE

Light south winds continue over the coastal waters this evening as high pressure remains in place. A WNW swell continues to maintain seas around 8 to 9 ft at 13 to 14 seconds per the latest buoy observations. Seas are expected to decrease by one or two feet early Wednesday morning.

Another frontal system is then expected to bring gusty southerly winds and steep seas to the waters Wednesday afternoon. Small Craft Advisories have been issued for all the zones for varying times with the main focus between 10 AM through 10 PM Wednesday as the the front moves through the waters. Winds are expected to be strongest for waters north of Cape Falcon with gusts to around 30 possible. Waters south of Cape Falcon to Florence could experience gusts up to 25 kt. Seas 6 to 7 ft at 15 seconds expected.

The active pattern is expected to continue for the latter half of the week and into the weekend. A westerly swell is expected to enter the waters late Wednesday night into Thursday, pushing seas above 10 ft at 14 to 15 seconds which would be steep and dangerous to small craft. Headlines may need to be extended past their current 10 PM Wednesday expiration. Another front approaches late Thursday into Friday bringing another round of southerly wind gusts between 25-30 kt. There is a 20-40% chance for Gales for the waters north of Cape Falcon. Seas expected 11 to 14 ft at 15 to 17 seconds with a 10-20% chance seas exceed 20 ft. The hits keep on coming as another front could move through on Saturday with gusty winds and steep seas. -19/23

PQR WATCHES / WARNINGS / ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 1 PM to 10 PM PST Wednesday for PZZ210-251-252-272.

Small Craft Advisory from 1 PM to 7 PM PST Wednesday for PZZ253- 273.

Small Craft Advisory from 10 AM to 10 PM PST Wednesday for PZZ271.