textproduct: Portland

SYNOPSIS

Largely dry weather through Thursday night aside from a weak front bringing light rain Wednesday night into early Thursday. The pattern shifts wetter by the end of the workweek as a more potent system could bring more widespread lowland rain and mountain snow late this weekend or early next week.

LONG TERM...Friday through Monday

While another upper low dives southward into southern California, very low amplitude ridging will be centered over the Pacific Northwest. This will place the mid-latitude jet and resultant storm track pointed into Vancouver Island. As shortwave impulses approach the coast, this will keep persistent chances of rain in western Washington with lower chances extending south into western Oregon. Ensemble guidance is in fairly good agreement that the storm track will sag southward as the upper ridge axis shifts inland through the weekend, bringing increased chances for precipitation to the region especially through Sunday and Monday. As snow levels fall due to cold air within any troughing, impactful mountain snows remain possible by next Monday, with a 35-45% chance for 6" of snow at the Cascade passes through 4 AM Tuesday. -36

AVIATION

High pressure will maintain calm winds across the area throughout the TAF period, except around 15-18z Wednesday when easterly winds will develop at KTTD and KPDX, beginning at KTTD first and KPDX last. High clouds this morning will push east of the Cascades between 00-03z Wednesday, while cloud cover clears out west of the Cascades. This will set the stage for fog and low stratus to develop across much of the Willamette Valley between 06-12z Wednesday, likely impacting all inland terminals. The exception is at KTTD where easterly winds should prevent fog from developing. KPDX will likely be near the edge of the fog/low stratus deck, but suspect visibilities at KPDX will fall to 1 SM or lower by 09-10z Wednesday before fog scours out by 17-18z Wednesday as east winds reach the terminal. There is a 50-75% chance for dense fog reducing surface visibilities to 1/4 SM at KEUG, KSLE, and KUAO. Probabilities are lower at KHIO at 10%, but feel these probabilities are misleadingly low given the clear skies and calm winds in place this evening and with KHIO being a fog-prone TAF site. Also worth noting is temperatures will be right around 32 degrees at KEUG when fog develops, meaning freezing fog is possible.

KPDX AND APPROACHES...VFR flight conditions will persist through approximately 09z Wednesday. Clearing skies after 00z Wednesday with calm winds will favor fog and low stratus development after 09z Wednesday, with surface visibilities likely falling to 1 SM or lower. There is a 30% chance visibilities will fall as low as 1/4 SM. Confidence is rather low regarding the exact time fog and low stratus will scour out at the KPDX terminal, as the terminal will be near the edge of the stratus/fog deck and the end time will depend on when easterly winds reach the KPDX terminal. Latest guidance suggests east winds will finally reach the KPDX terminal around 17-18z Wednesday, which is when fog and low clouds are expected to scour out. -23

MARINE

A brief reprieve from active weather is expected Tuesday and Wednesday as seas subside to 6-8 ft at 12-13 seconds. Winds will also ease while remaining out of the northwest through Tuesday evening, becoming northeasterly Tuesday night and then easterly for a brief period of time Wednesday morning as offshore flow develops. East winds should generally stay under 10 kt, but may be locally stronger downwind of coastal gaps.

Winds veer to the south on Wednesday as another frontal system approaches the waters, then increase with gusts up to 30 kt through Wednesday night, with the strongest gusts likely to remain beyond 10 NM offshore. Behind the passing front, winds become northwesterly and weaken through Thursday. A building westerly swell moving into the waters will most likely see seas become steep and hazardous by Thursday afternoon with wave heights of 14-16 ft and a dominant wave period of 14-15 seconds. While this is the most likely outcome, there remains notable uncertainty: there is a 10% chance that waves only reach 10-13 ft, and there is equally a 10% chance seas reach 18-19 ft by Thursday afternoon. Seas are then expected to slowly subside to 10-13 ft on Friday, then persisting through the weekend. -36/42

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.