textproduct: Portland

SYNOPSIS

A weakening frontal band is sliding slowly eastward through the forecast area this afternoon, producing widespread mid-level cloud cover and virga. Precipitation reaching the ground has been extremely limited, and dry conditions will prevail for most locations. After a brief interruption Wednesday night into Thursday with a weak front brushing the region, a broader transition toward a wetter, more active pattern takes shape over the weekend and into early next week.

LONG TERM...Friday through Tuesday Morning

The end of the workweek begins under a broad but low-amplitude ridge, keeping the primary storm track pointed into British Columbia on Friday, focusing the better rain chances over western Washington and brushing only the far northern portions of our region. Farther south, shower chances remain lower but not zero as weak disturbances pass along the periphery of the ridge.

Over the weekend, the ridge axis shifts inland and gradually breaks down as a series of Pacific waves drop southward. Ensemble guidance has grown more aligned in showing the storm track dipping south into Oregon by Sunday, introducing a steadily increasing probability of widespread rain. Confidence is higher for a more organized system late Sunday into Monday, which could deliver meaningful precipitation to the lowlands and accumulating snow to the Cascades. Cooling aloft within the approaching trough supports falling snow levels, and current probabilities show a 25-35% chance for at least 6 inches of new snow at the Cascade passes from late Sunday into early Tuesday morning. ~12

AVIATION

Tonight high pressure will maintain calm winds and mostly clear skies setting the stage for widespread fog and low cloud development resulting in degraded flight conditions through much of Tuesday morning. Currently as of ~05-06z, CIGs/VIS have already begun to lower across the central and southern Willamette valley with more localized pockets of fog elsewhere into the Portland Metro and in coast range/Cascade valleys. High confidence (70-90%)in the continuation of this trend towards IFR/LIFR conditions for most inland sites by 08-11z, including KPDX. The REFS which has captured fog development well thus far projects a ~70-80% chance for <0.5 mile VIS for all inland terminals excluding KTTD by 12z. It's this latter site (KTTD) and coastal sites outside of well protected valleys (KONP, and to some extent KAST) where forecast confidence is lower due to the development of easterly flow as the night goes on which would limit fog/low stratus formation where it can surface. Still, would give KTTD a 40-50% chance for IFR to LIFR conditions between 09-13z before those east winds become more prominent. It's also worth noting temperatures will be right around 32 degrees at KEUG by sunrise, meaning freezing fog is possible.

East flow and added mixing as the morning goes on will chip away at the fog/low stratus largely N to S through the Willamette Valley but will struggle to scour out the airmass KSLE to KEUG. Forecast confidence regarding the timing of any categorical improvement through Wednesday morning into the afternoon is low. That said, expect breezy east wind with gusts up 20-30 knots to develop at KTTD by 16-18z.

KPDX AND APPROACHES...Fog is currently attempting to develop around the terminal late this evening as confidence grows in a drop to IFR/LIFR conditions around 08-10z with a probabilities peaking at 75% for <0.5mile vis around 12-13z Wednesday. Confidence is still rather low as to exactly when fog and low stratus will scour out at the KPDX terminal, as the terminal will be near the edge of the stratus/fog deck. The end time depends solely on when easterly winds reach the KPDX terminal. Current guidance suggests the earliest this occurs would be around ~15z Wednesday but they tend to progress too quickly in these scenarios. Higher confidence in a return to VFR by 18-21z. -99

MARINE

Buoy observations from Tuesday afternoon showed seas ranging between 6 to 8 ft at 11 to 12 seconds, with light north to northeast winds around 10 kt. Winds are still expected to become easterly for a brief period of time Wednesday morning as offshore flow develops. East winds should generally stay at or under 10 kt, but may be locally stronger downwind of coastal gaps. Winds veer to the south and increase Wednesday afternoon as another frontal system approaches the waters, with wind gusts peaking between 25 and 30 kt Wednesday evening through Wednesday night as the front moves through. Small Craft Advisories have been issued to cover this threat.

Winds weaken significantly on Thursday behind the frontal passage, however a long period westerly swell will be rapidly increasing. Forecast seas have increased with today's forecast update, and are now expected to peak between 17-19 ft Thursday afternoon and evening with a dominant wave period of 14-15 seconds. There is a 10% chance seas will peak at or slightly above 20 ft. Given increasing confidence for steep and hazardous seas on Thursday, a Hazardous Seas Watch has been issued for the inner and outer waters late Thursday morning through Thursday night.

Seas are then expected to slowly subside to 10-13 ft Friday morning and afternoon, and then remain within that range through the upcoming weekend. Winds look to stay under 20 kt Friday through the weekend as well. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 7 PM Wednesday to 10 AM PST Thursday for PZZ251>253. Hazardous Seas Watch from Thursday morning through late Thursday night for PZZ251>253-271>273. Small Craft Advisory from 2 PM Wednesday to 10 AM PST Thursday for PZZ271>273.