textproduct: Portland

SYNOPSIS

Mostly dry conditions expected today, followed by inland fog and low clouds to start Wednesday. An active and wetter weather pattern returns later Wednesday through the end of the week and beyond, although will be mixed with periods of dry weather and slightly warmer than average temperatures. Weather related impacts over the next 5-7 days remain fairly limited.

DISCUSSION...Now Through Monday

A low pressure system off the northern California coast this morning will push east as the upper level low moves inland this afternoon. On the northwest periphery of this system, mid-level clouds with mainly virga is streaming north across the forecast area into southwest WA. Trace amounts of rain has been observed early this morning. As the upper low moves inland over northern CA, and inverted upper trough will shift across NW Oregon, dragging the mid level clouds east of the Cascades by this evening, with skies clearing tonight. If skies can mostly clear out before sunset, a few hours of sunshine may allow late afternoon temperatures to warm into the lower 60s, allowing for a pleasant early November day. Also of note are the breezy east winds out of the Columbia River Gorge where offshore pressure gradients are peaking this morning with gusts to around 20-30 mph across eastern portions of the Portland Metro area.

A transient upper level ridge then moves east across the region later tonight into early Wednesday. Clear skies and light winds should allow for radiational cooling as temperatures fall into the mid to upper 30s. Chances for fog development also increase into early Wed morning as the surface inversion deepens. Temperatures again warm on Wednesday into the upper 50s to near 60 degrees as southerly winds increase. Mostly dry conditions are likely to continue through Wed afternoon. But, an upper level shortwave trough moving across the NE Pacific will quickly push a frontal system onshore later Wed afternoon at the coast, spreading inland by Wed evening. Chances for widespread rain peak Wed evening through Wed 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 (30-50%). A few lingering showers will be possible into Friday morning, though Thursday likely will see an extended dry period with temperatures warming again a few degrees above normal.

Late Thursday into 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 later this week. Largely this will depend 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 cluster showing the wetter solution is dominated by the European ENS while the GEFS is trending more toward a drier solution for much of the forecast area, keeping much of the precipitation to the north. So, still quite a bit of uncertainty in the forecast.

There is actually higher confidence in the forecast over the weekend as ensembles are in better agreement depicting broad troughing over the Pacific pushing the upper level ridge farther east over the Rockies. Temperatures are likely to warm into the 60s across the area on Saturday before a frontal system spreads rain across the area late Saturday into Sunday. In fact, there is at least a 50% chance for 24-hr rainfall amounts in excess of 0.25 inch across the forecast area.

At this point, 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. However, beyond day 7 into next week (11/10-11/14), almost all ensemble systems are beginning to show a growing consensus for a potentially much more impactful and active weather pattern. /DH

AVIATION...Through 12Z Wednesday

Moist, southerly flow aloft this morning as an upper level low off the northern CA coast moves eastward. Mid-level clouds continue to stream northward with very light rain or virga. Predominately VFR conditions are being observed, except for some reduced VIS around 4-5 SM at KEUG and KUAO. Expect these VFR conditions to continue through this evening as the mid-level clouds scatter out shift east later this afternoon. Clearing skies and building high pressure late tonight into Wed morning will bring increasing chances for fog development after 09z Wed, especially at KEUG and KHIO. Light winds at most terminals except for breezy east winds at KTTD with gusts up to 22 kt through this morning.

KPDX AND APPROACHES...VFR conditions under a mid-level cloud deck scattering out later this afternoon. Increasing chances for fog or low stratus (20-30%) by 10-12z Wed. Light and variable winds around 5 kt or less expected. /DH

MARINE

Light offshore winds this morning become more variable as high pressure settles over the area. A WNW swell continues to maintain seas at around 9 to 10 ft at 15 to 17 seconds at buoy 29 and 50, while seas at buoy 89 have recently built to 11 to 12 ft. Guidance suggests this sea conditions will likely persist through this afternoon so have extended the Small Craft Advisory for the outer coastal waters beyond 10 NM through 4 PM.

Southerly winds are expected to increase on Wednesday ahead of an approaching frontal system. There is still some uncertainty regarding the strength of the southerlies, but there is at least a 70% chance for wind gusts exceeding 21 kt north of Cape Falcon by Wed afternoon. Winds shift westerly behind the frontal passage Wed night before southerly winds return later Thursday.

Active weather continues as a series of potentially stronger frontal systems move through the waters later Thursday into Friday, and again over the weekend. With these systems come increased chances for gale force wind gusts exceeding 33 kt from around 30-70%, highest on Saturday. Incoming westerly swells are also expected to build seas into at least the mid-teens on Friday with around a 10-20% chance for significant wave heights exceeding 20 ft. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ271>273.