textproduct: Portland

SYNOPSIS

High pressure maintains dry weather through Thursday with nightly chances for fog and frost in interior valleys and gusty east winds through the Columbia River Gorge. There is moderate to high confidence in a return to cooler and wetter weather Thursday night into early next week.

LONG TERM...Saturday through Wednesday

The pattern remains active through the weekend as the upper level low pinches off and moves toward central CA, leaving OR in the saddle point between upper level disturbances. As such, post-frontal precipitation chances will persist through the weekend, but will trend lower as the cut-off low develops. Weaker support for ascent and the scattered nature of showery precipitation both contribute to a lowering of potential precipitation totals through the period. Snow levels falling to 3000-4000 ft within the upper trough will allow for accumulating snow at the Cascade passes, with 25-45% chances of six inches or more falling from Friday through Sunday afternoons, and 10% or less chances for one foot of snow.

A reinforcing trough is expected to drop south from northern AK, bringing much colder air aloft to the region early next week. Morning lows fall to near or below freezing each of Sunday, Monday, and Tuesday mornings. Particularly on Monday and Tuesday, there is a 10-25% chance that snow levels will fall to 1000 ft or lower, which could see snow fall across the Coast Range, Cascade foothills, and possibly even to the valley floor in the early morning hours. 850 mb temperatures in the -3 to -6 C range are expected which is marginal for snow to the valley floor within onshore flow. If temperatures are cooler, closer to -8 to -10 C, chances will increase. At this time, the NBM shows no chance for snow in the valley but precipitation meteograms show a ~5% chance for snow throughout the Willamette Valley. Chances are a bit higher in the West Hills of the Portland Metro at around 10-20% for 0.10 inches Tuesday morning according to the NBM. Any snow that does accumulate would quickly melt as marginal morning temperatures warm above freezing each day, thereby limiting potential impacts. Will continue to monitor the pattern for major shifts in the pattern next week. -19/36

AVIATION

Clear skies and light offshore winds will maintain VFR flight conditions across the area through 06z Thursday. High clouds will begin pushing inland from west to east after 06z Thursday, however VFR conditions will most likely continue as cigs will be above 15-20 kft. The main exception to that is at KEUG where fog is likely to return by 06-08z Thursday (50-60% chance), resulting in LIFR flight conditions from approximately 08-19z before fog lifts and brings a return to VFR conditions for Thursday afternoon.

Elsewhere, frost is favored over fog tonight as temperatures dip into the mid 30s. Admittedly, there is a 15-20% chance of fog resulting in LIFR/IFR visibilities at KSLE, KUAO, KPDX, and KHIO, however this is not reflected in the TAFs as this is not the most likely outcome. Probabilities for fog are less than a 10% at KAST, KONP, and KTTD. Note easterly winds will continue through at least 12z Thursday at KTTD with gusts up to 20 kt. After 12z, guidance suggests winds will become light and variable less than 5 kt at KTTD as offshore flow weakens and begins to transition to a light onshore flow regime.

KPDX AND APPROACHES...VFR conditions will most likely continue through 00z Thursday. Clear skies will give way to increasing high clouds after 06z Thursday, with cigs above 15-20 kft. There is a 20% chance for fog development tonight, however frost is the most likely outcome as temperatures dip into the mid 30s again. -23

MARINE

Northerly winds will continue easing this evening into tonight, falling below 15-20 kt. Winds subside further to below 10 kt by mid to late Thursday morning before shifting out of the south Thursday night into Friday ahead of an approaching frontal system. This frontal system will be very weak, resulting in only a slight increase in winds up to 10-15 kt.

Despite weak winds, seas will rise dramatically late Thursday night into Friday morning as a westerly long-period swell arrives. Still expecting seas to peak somewhere between 14-16 ft by early Friday morning, however there is a 1-10% chance seas peak as high as 16-19 ft. Note probabilities are highest over the northern outer waters and lowest over the southern waters. Seas look to ease to 9-10 ft by Saturday morning and then remain within that range through the weekend with a return of offshore winds. -23

BEACH HAZARDS

A high threat for sneaker waves is likely Friday through Sunday evening due to a long period westerly swell. Waves can run up significantly farther on a beach than normal, including over rocks and jetties. Sneaker waves can suddenly knock people off of their feet and quickly pull them into the frigid ocean which may lead to serious injury or drowning. Caution should be used when in or near the water, especially those who will be razor clamming. Keep children and pets away from the surf zone. Keep off of jetties, rocks and logs near the surf zone. If you see someone swept into the sea do not swim in after them. Call 911 and keep an eye on them until help arrives. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None.

WA...None.

PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ272- 273.