Marine Weather and Tide Forecast for Portland, OR

SHORT TERM
Now through Friday Night...The potent cold front which crossed the region last night through this morning has exited the region to the south and east. Behind the front, scattered to numerous rain and mountain snow showers will linger into Thursday as a closed upper-level low passes overhead. With the cold core low aloft, any surface heating during sun breaks between showers will easily destabilize the column, generating perhaps 200-500 J/kg of SBCAPE. When combined with low freezing levels around 2500 ft, even a heavy shower could result in small hail reaching the surface. There is additionally a 15-30% chance of isolated thunderstorms this afternoon and evening, although limited instability will cap overall thunderstorm intensity. These chances will decrease after sunset, but may linger later into the evening as the coldest air within the upper low shifts overhead. Meanwhile in the Cascades above 2500 ft, snow showers will continue to produce accumulating snowfall.
Winter Storm Warnings remain in effect until 5 AM Thursday as another 9-12 inches of snow may fall, resulting in storm total accumulations of 1-2 feet at and above pass level.

As the upper low exits eastward and heights begin to rise aloft, rain showers will tend to dwindle in coverage tonight through Thursday and the cooler air mass will see temperatures fall to near-freezing across portions of the region. Despite morning low temperatures of 32-36 degrees across the lower elevations of NW Oregon and SW Washington, the combination of partly cloudy skies and persistent light winds tonight may inhibit widespread frost formation. Frost Advisories nonetheless remain in effect from 1-9 AM Thursday west of the Cascades with the exception of the immediate coast, Portland/Vancouver metro, and Columbia Gorge where temperatures will remain relatively warmer. East of the Cascade crest in the Upper Hood River Valley, temperatures will more likely fall into the upper 20s to low 30s, and a Freeze Warning has therefore been issued for 1-9 AM Thursday.
Agricultural interests and those with sensitive outdoor plants should plan to take protective action to prevent plant damage.
Conditions will be even more supportive of radiational cooling on Thursday night, however the slowly moderating air mass may counteract that tendency. A Freeze Watch has been issued from 1-9 AM Friday for the areas most likely to see temperatures fall below freezing, namely the Upper Hood River Valley and the Cascade foothills of Marion, Linn, and Lane Counties.

As upper-level ridging builds on Friday, drier and warmer weather is expected across the region, although there remains some uncertainty in the progression of yet another upper low over the Northeast Pacific. This low or open trough may move nearer to the region and bring clouds and rain, although confidence is relatively low and rainfall chances only reach 5-10% by late Friday night. -36


LONG TERM
Saturday through Wednesday...An upper-level low or open trough tracking from the Gulf of Alaska over the Northeast Pacific will be the driver of sensible weather this weekend and into early next week, and the forecast remains sensitive to its low-confidence 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. If it is 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. -36

AVIATION
Post frontal showers are expected to persist through the day with increasing chances for convection in the afternoon through at leas 01Z Thu. Highest probabilities lie along the Cascade foothills and to the north with around a 25-35% chance inland within the Willamette Valley. Have already observed thunder within the vicinity of some Willamette Valley terminals, but has yet to be widespread. Thunder is challenging here as often it may be one storm that pops up, while other times it can be a wide swath. In this case, because the flow is northwesterly (not as favorable) behind the previous system, would trend towards a more isolated thunderstorm scenario.
Overnight, showers will slowly dissipate and clear. Dropping temperatures mean that there is a slight chance for frost formation over exposed surfaces. If skies remain cloudy though, those chances decrease significantly. Another limiting factor will be that winds are just slightly above what we would like to see for more widespread frost formation.

KPDX AND APPROACHES...Combination of MVFR and VFR conditions with showers. Around a 30% chance of thunderstorms through 06Z Thu. Thunderstorms will trend towards an isolated scenario.
With passing storms, expect gusty outflow winds leading to LLWS type conditions. Note that atmospheric LLWS is not expected, but rather environment induced conditions. -27

MARINE
Post frontal showers will increase probability for lightning over the waters through around 1700 today. Small Craft Seas persist due to a westerly swell combined with a westerly wind wave. These small craft seas are hovering right around 9-10 ft at 8 seconds. Have extended the advisory through Thursday morning, though will see a steady decline in seas shifting from west to east. In addition, there will be a strong ebb early Thursday morning for the Columbia River Bar. The inner waters will see lowering seas as early as early Thursday morning. Winds will remain westerly through Friday morning, then shift to the north as a high pressure ridge sets up over the waters. This summer time pattern will bring increased winds to PZZ253 in the afternoon. While not quite a low- level jet, we could see a short burst of gusts up to 25 kt.

High pressure over the area will maintain more settled conditions through early next week. -27

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from 1 AM to 9 AM PDT Thursday for ORZ104>110- 114>118-123>125.

Freeze Warning from 1 AM to 9 AM PDT Thursday for ORZ121.

Freeze Watch from late Thursday night through Friday morning for ORZ121-124-125.

Winter Storm Warning until 5 AM PDT Thursday for ORZ126>128.

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

Winter Storm Warning until 5 AM PDT Thursday for WAZ211.

PZ...Small Craft Advisory until 5 AM PDT Thursday for PZZ210.

Small Craft Advisory until 5 PM PDT Thursday for PZZ251>253- 271>273.