textproduct: Portland

SYNOPSIS

Rain and mountain snow showers will be on the decrease today as the overall pattern trends drier and warmer headed into the weekend. However, as this transition occurs, a lingering cooler airmass favors at least one more night of frost/freeze concerns tonight into Friday morning. Shower chances return Sunday into early next week although forecast uncertainty increases significantly from this point onward.

LONG TERM...Sunday through Wednesday

All eye are on the progression of an upper-level low or open trough tracking southward from the Gulf of Alaska over the Northeast Pacific early next week as it will be the driver of sensible weather going forward. The forecast and potential impacts remain extremely sensitive to this feature's evolution. An upper low located nearby will favor cloudy skies and rainfall chances, while a position well to the west would see continued warm and dry weather. The current forecast as far as temperatures is concerned is somewhere in-between. If the upper-level low located off the northern CA or southern OR coast, the pattern could allow sufficient moisture to move northward on its eastern flank to support convection along the Cascades early next week. To help highlight this forecast uncertainty, the projected high temperature spread in the NBM ensemble ranges from 83 degrees (90th percentile) to 57 degrees (10th percentile) on Monday, a 26 degree spread. Tt's not often we see that much uncertainty on day 4-5 of the forecast. At least there is a decent consensus this upper-level low or trough feature kicks eastward by the middle of the week leaving us in a more typical onshore flow pattern. That said, the exact transition and thus potential impacts remain nebulous at this time. -99/36

AVIATION

At 18z Thursday, satellite and surface weather observations showed scattered to broken cumulus clouds across northwest OR and southwest WA with isolated light showers/sprinkles over the Cascade foothills and Coast Range. Cigs were generally VFR, aside from brief periods of high-end MVFR cigs at KTTD, KUAO, and KSLE. Expect this trend to continue today before clouds decrease in coverage this evening into tonight. Given the clearing skies and calm winds in place tonight, widespread frost is expected both inland and at the coast. One exception is at KAST and KPDX where temperatures will most likely stay above 36 degrees, thus limiting the potential for frost development. Note there is a 45% chance for fog and/or low stratus below 500 feet at KHIO beginning around 15z Friday, and a 30-40% chance at KPDX.

KPDX AND APPROACHES...Expect mainly VFR conditions today and tonight with light and variable winds around 5 kt or less. Between 15-18z Friday, there is a 30-40% chance for low stratus with cigs below 1000 ft. Confidence in this stratus deck developing over the terminal is currently not high enough to reflect in the TAF. -23

MARINE

Broad northwesterly flow continues today with lingering showers decreasing through the day. We'll see Small Craft Seas at 8-11 feet at 9-10 seconds persist as well due to a fresh WNW swell combined with a WNW wind wave. In addition, there will also be a strong ebb this morning for the Columbia River Bar helping to keep waves steep. Expect wave heights to slowly wane through the day across both the inner and outer waters. Winds will remain W-WNW through Friday morning, then shift to the north as a high pressure ridge sets up overhead. This summer-like weather pattern is fairly short-lived however, switching southerly on Saturday and increasing across the outer waters. Guidance suggest a 40-60% chance for gusts above 21+ knots across the outer waters Saturday evening/night. Fairly benign conditions continue during the early to middle portion of next week with wave heights holding in the 4 to 7 feet. -99/27

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from 1 AM to 9 AM PDT Friday for ORZ102>110- 112>115.

Freeze Warning from 1 AM to 9 AM PDT Friday for ORZ116>119-121- 123>125.

WA...Frost Advisory from 1 AM to 9 AM PDT Friday for WAZ203>205-207- 208.

PZ...Small Craft Advisory until 5 PM PDT this afternoon for PZZ251>253-271>273.