textproduct: Portland

SYNOPSIS

Light scattered showers continue in far western WA and northwestern OR through most of the day. A slow- moving cold front will move southeastward across southwest WA and northwest OR late Friday afternoon through Friday night, spreading light showers southeast across the region. Isolated post- frontal showers Saturday morning will diminish Saturday afternoon as surface high pressure builds into the area. This pattern will then hold steady for a few days, bringing cooler and drier conditions Saturday afternoon through Monday. A stronger Pacific cold front arrives sometime on Tuesday, bringing widespread rain and breezy winds to the area. Snow levels fall behind the front late Wednesday, however the bulk of the precipitation associated with this system should be finished by the time snow levels fall below the Cascade passes. Minimal snow accumulation and travel impacts expected at the passes.

DISCUSSION...Friday through Thursday

Radar imagery early Friday morning shows scattered showers mainly in SW Washington and along the north near and north of Astoria. Latest guidance has slowed the progression of the approaching cold front through SW WA and NW OR today, so showers are expected to remain in this general area through most of the day, finally spreading southeastward through NW OR late this afternoon through early Saturday morning. Rain amounts will be quite light overall, with some locations in the interior lowlands picking up only a trace to a few hundredths of an inch. Meanwhile, rain amounts will be higher across southwest WA, the Cascades, the north OR coast, and north OR Coast Range, up to 0.25-0.75 inch.

There is very little change in the forecast through the rest of the week. Models and their ensembles remain in agreement for surface high pressure and zonal flow aloft behind the frontal passage for the Saturday through Monday time frame. This will bring drier weather and relatively cooler temperatures with highs close to average for this time of year (mainly in the mid to upper 50s). Winds will also be light, especially during the overnight hours each day. The light winds will be in conjunction with clearing skies Saturday afternoon into Saturday night, resulting in an ideal set-up for radiational cooling. As such, forecast low temperatures remain coldest Saturday night with widespread lows in the 30s. Areas of frost will likely develop as a result. Sunday night into Monday morning will be chilly once again, though low temperatures will be a few degrees warmer in the mid to upper 30s. Those with sensitive outdoor plants should take precautions to protect vegetation from frost. Note that NWS Portland will not begin issuing Frost Advisories and Freeze Watches/Warnings for agricultural purposes until April 1st when the growing season begins to ramp up more and impacts from frost and freeze conditions become more significant.

Confidence remains high for widespread rain and breezy winds Tuesday into Wednesday as a stronger Pacific cold front moves inland. Model ensemble guidance continues to show uncertainty on timing and exact precipitation amounts. However, confidence is high that river flooding is not expected, even in the wettest possible scenario. This front could arrive as early as late Monday night or as late as Tuesday night, with the most likely arrival time being sometime between Tuesday afternoon and Tuesday evening. As a result of these timing differences, the NBM 10-90th percentile for snow levels continues to show large spread on Tuesday, ranging from 4500-9000 ft. Confidence is high that snow levels will drop considerably behind the cold frontal passage, and continue dropping into Wednesday night and Thursday morning. This is when even the NBM 90th percentile suggests snow levels will be below the level of the Cascade passes, while the 10th percentile drops to 500-1500 feet. That being said, ensemble guidance for QPF clearly shows precipitation beginning to end by that time, meaning conditions will trend colder and drier with little accumulating snow and travel impacts at pass level. NBMv5.0 probabilities for 6 inches of snow or more in a 48-hour period ending at 5am PDT Thursday continue to back this expectation up, as chances are only around 5-10% at pass level and near 0% below pass level. Note that lowland snow is NOT expected given conditions will be drying out by the time temperatures become colder.

By Thursday, the weather system is expected to move out of the area with upper level flow becoming more zonal. Expecting dry conditions with clearing skies by midday Thursday, which could allow for temperatures to warm into the upper 50s. -23/03

AVIATION

A surface front slowly approaches the region from the northwest today, making landfall along the coast between 18z Fri - 00z Sat and moving southeast across the region after 00z Sat. Light and occasional showers continue around and north of KAST before shower chances slowly spread south along the coast and east into NW WA after 12-15z Fri as the front gets closer to the coast. Showers will then spread southeast across the region along and ahead of the front after 00z Sat, mainly clearing by 12-15z Sat. Mid to low level clouds are expected to spread ahead of the front this morning, lowering coastal cigs to at least MVFR by 12-15z Fri north of KTMK, spreading south by 15-18z. There's a 70-90% chance of IFR ceilings along the coast through around 00-03z Sat with chances decreasing after the front passes. MVFR chances increase for inland terminals after 14-18z Fri, as well, with chances decreasing significantly after the front passes through. Winds will be generally southerly less than 10 kts, except for coastal sites which could see occasional gusts to 20 kts, shifting westerly by 21-23z Fri as the front approaches then northerly by 09-12z Sat after the front passes.

KPDX AND APPROACHES...VFR conditions through early Friday morning, then chances of MVFR ceilings increase to 60-80% after 16-18z Fri. Ceilings improve to VFR after 00-03z Sat as light showers move through the area. Generally southerly winds less than 10 kts become westerly by 21-23z Fri and then northwesterly by 01-04z Sat. -03

MARINE

Low pressure offshore will maintain breezy southerly winds through early Friday morning, mainly for the waters north of Cape Falcon including the Columbia River Bar. The Small Craft Advisory for these waters remain in effect through 5 AM Friday for southerly wind gusts up to 25 kt. An exception is the Columbia River Bar, where the Small Craft Advisory continues through 8 AM Friday due to a strong ebb current around 6 AM which will build seas to 8-9 ft along the Main Channel of the Bar. General seas for the rest of the waters are forecast around 6-8 ft at 10-11 sec.

Surface high pressure re-builds this weekend, turning winds more northerly and remaining under 20 kt. Seas also gradually fall to 5-7 ft at 11 sec by late Saturday night into Sunday. Benign marine conditions continue through Monday before another, more robust weather system returns Tuesday into Wednesday. This system will bring a return of breezy southerly winds across all waters. Current guidance suggests high confidence (70-90% chance) for widespread and frequent small craft wind gusts of 21 kt or greater, and a 10-30% chance for widespread and frequent Gale force wind gusts of 34 kt or greater on Tuesday. Chances for isolated Gale force wind gusts are around 50-75%. Seas on Tuesday and Wednesday are also expected to build to around 13-15 ft in response to increasing wind waves along with a building westerly swell. There's a 25-50% chance seas could exceed 15 ft with a 5-15% chance for seas exceeding 20 ft. Chances increase from south to north, peaking in zones PZZ271-272. -10/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 8 AM PDT this morning for PZZ210.

Small Craft Advisory until 5 AM PDT early this morning for PZZ251-271.