textproduct: Portland

SYNOPSIS

A series of frontal storm systems bring will bring periods of high surf and sneaker waves to the coast and rain and breezy conditions areawide through at least early next week. Snow will begin to fall above 4000 ft in the Cascades late Sunday into Monday.

DISCUSSION

Water vapor satellite imagery shows an upper level pinching and cutting off the southern California coast. Meanwhile, a deep low pressure is spinning over the northeast Pacific and shifting east-southeastward towards the Pacific Northwest. An earlier WPC surface analysis estimated the surface low pressure at 960mb.

Ensembles are in good agreement this strong low pressure will push a front towards the coast late tonight before pushing into the I-5 corridor around daybreak. A strong wind field of 60-70 kt at 925mb will develop off the southern and central coast of Oregon before the front makes landfall. East to west pressure gradient forces should keep most of these winds off the coast, particularly off the north Oregon and south Washington coast.. However, the pressure gradients do open ever so slightly that some of this wind field could be realized at our most exposed locations along the central coast of Oregon. Right now estimating there is a 30% chance that our windiest sites meet High Wind Warning criteria with lower odds north of Tillamook County. As a result, will hold off on any high wind products, but will need to monitor trends and monitor buoy observations to see how well they are tracking with modeled wind fields. Either way, given the pressure gradients the strongest winds are unlikely to last more than 1-2 hours along any portion of the coast, which should reduce impacts. The bigger impact of this storm system will be another round of high surf along the coast late Friday into early Saturday. See the beach hazards section below for more information.

Models and their ensembles are in good agreement an embedded shortwave trough will swing northeastward into the area Friday afternoon into early evening, which should aid in creating a more organized band of showers spreading into the region around that time. Cooler sea surface temperatures near the coast may limit vigorous convection coming onto the coast, but wind fields will be supportive of some rotating storms farther offshore Friday.

The next storm system is currently south of the Aleutian Islands and will shift eastward over the next 48 hours. As it does so, another low pressure will deepen rapidly to around 960mb and move into the place of the current existing low pressure over the northeast Pacific. This will push another front eastward into the region Saturday night bringing another round of increased winds along the coast. It appears there is a 30% chance for a low end High Wind event along the coast late Saturday before a cooler and more showery weather regime takes hold. Snow levels will drop from above 6000 ft early Sunday to near 4000 ft by Monday morning. Given unmaintained mountain roads are free of snow and open, will likely need a Winter Weather product for the Cascades even if only a few inches of snow fall Sunday night into Monday.

There looks to be a brief lull in the wet and breezy weather Tuesday-ish before most global ensemble members suggest another storm system impacts the region around Wednesday of next week. /Neuman

AVIATION...Through 00Z Saturday

Dry, south to southwest flow aloft continues through this evening with high level clouds streaming across the area today. As of 23z Thu, LIFR stratus persists at KSLE. Elsewhere, predominately VFR conditions are being observed across the forecast area. There is around a 30-40% chance that IFR fog redevelops after 04z Friday, but an approaching frontal system is expected to bring increasing winds and cloud cover late tonight. Chances for rain also increase at the coast after 08z Friday, spreading inland by 10-12z Fri as a front approaches the region from the west. CIGs are expected to gradually lower with chances for MVFR increasing after 16-18z Friday. Scattered showers are expected increase across the area later Friday afternoon. South to southeast winds increase along the coast early Friday morning with gusts up to 20-30 kt.

KPDX AND APPROACHES...High clouds continue to stream northward overhead with VFR conditions expected through at least 17-18z Friday. Light east to southeast winds continue through this evening, before increasing late tonight into Friday morning. Light rain is expected after 10-12z Fri as a front approaches the region. Chances for MVFR CIGs also increase to around 20-30% after 17z Fri as showers increase across the area. /DH

MARINE

A 962 mb low spinning just south of the Gulf of Alaska will push an occluding front toward the coastal waters tonight. The surface low will gradually weaken as it drifts east, pushing up against the stalled frontal boundary over the outer waters. Southeast winds have already begun increasing this afternoon, and will continue to strengthen through tonight. The strongest winds are expected to persist across the outer coastal waters beyond 10 NM before the main front pushes across the waters late tonight into Friday morning. The Gale Warning remains in effect across the outer coastal waters from 4 PM this evening through 4 PM Friday, likely peaking between midnight and 6 AM with gusts up to 45 kt. There remains around a 30-50% chance for Storm Force wind gusts up to 50 kt, likely associated with the frontal passage. Breezy south winds are expected to persist through Friday afternoon before gradually diminishing Friday night.

Seas have subsided slightly to around 11 to 14 ft this afternoon as a westerly swell persists. The sea state becomes more complex later this evening into Friday as a dominant fresh swell out of the southwest moves into the waters. Seas become very steep and hazardous, gradually building back into the mid to upper teens with a dominant period of around 10-11 seconds by Friday afternoon. A strong, long period westerly swell then pushes into the waters later Friday afternoon which is expected to build seas above 20 ft by 3-4 PM. Significant wave heights are likely (60-80% chance) to build into the mid-20s Friday night before slowly subsiding on Saturday. Seas likely fall back into the upper teens by late Sat afternoon, but there is a 20% chance that seas remain at 20 ft or higher through Sat night as the next frontal system arrives.

Another deep low pressure system quickly spins up in the NE Pacific on Saturday pushing another strong front toward the coastal waters Sat night. Another round of strong southerly Gales is expected, while there is also a 50-70% chance for Storm Force wind gusts exceeding 50 kt across the outer coastal waters. Have issued a Storm Watch from 4 PM Sat to 4 AM Sunday. Another westerly swell will likely build seas above 20 ft again on Sunday with the swell persisting into Monday. /DH

BEACH HAZARDS

A tidal anomaly from persistent southerly winds and warmer than average ocean temperatures will result in the total tide pushing 10.5 ft at the Garibaldi tidal gauge Friday afternoon. Historically this typically produces flooding along Fraser- Burton Road south of Tillamook along the Tillamook River. Elsewhere, lowland and agriculture flooding will occur, but it does not appear Highway 101 will be impacted in Pacific County where the Toke Point gauge is anticipated to top out near 11 ft.

More importantly, a large westerly swell will push towards the coast Friday afternoon and peak late Friday evening (most likely between 7-10 PM). This swell will take aim at the southern Oregon coast, but a swell of 25+ ft will still push onto the central and north Oregon coast while the swell will be closer to 20 ft along the south Washington coast. Given periods will be in the 17-19 second range, these waves will have a lot of mass and momentum than the typical Winter time storm swell. This will result in waves running significantly farther up on area beaches and jetties.

Or more simply put, the sneaker wave threat will be extreme. People should use extreme caution as 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. In addition, large wave runup may dislodge logs on area beaches and result in them rolling up and down area beaches, which have historically killed or seriously injured unsuspecting people, so there will be much more of a threat than just getting wet from a rogue wave.

The large westerly swell will gradually subside Saturday. However, another westerly swell that will be pushing 20+ ft with dominant periods of 16-17 seconds arrives Sunday afternoon and lasts into Monday. This will necessitate another High Surf product and result in another round of high surf conditions, beach erosion and sneaker waves during this time. /Neuman

PQR WATCHES/WARNINGS/ADVISORIES

OR...High Surf Warning from noon Friday to noon PST Saturday for ORZ101>103.

Coastal Flood Advisory from noon to 4 PM PST Friday for ORZ102.

WA...High Surf Advisory from noon Friday to noon PST Saturday for WAZ201.

PZ...Small Craft Advisory until 4 AM PST Friday for PZZ210.

Gale Warning from 4 AM to 7 PM PST Friday for PZZ210.

Hazardous Seas Warning from 7 PM Friday to 4 PM PST Saturday for PZZ210-251>253.

Small Craft Advisory until 1 AM PST Friday for PZZ251>253.

Gale Warning from 1 AM to 7 PM PST Friday for PZZ251>253.

Gale Warning until 4 PM PST Friday for PZZ271>273.

Hazardous Seas Warning from 4 PM Friday to 4 PM PST Saturday for PZZ271>273.

Storm Watch from Saturday afternoon through late Saturday night for PZZ271>273.