textproduct: Portland

SYNOPSIS

Dry weather and near normal high temperatures through Monday with morning low temperatures in the 30s, producing widespread frost. A weather system brings widespread rain and breezy winds Tuesday into Wednesday along with light Cascades snow Tuesday night into Wednesday. No snow or flooding impacts expected. Dry weather returns Thursday into Friday.

DISCUSSION...Sunday through Saturday

Mostly clear skies are being observed on satellite imagery across NW OR and SW WA early Sunday morning under building surface high pressure and zonal flow aloft. A weak cold front will graze far NW OR and SW WA coast later this morning with mid- level clouds and a 15-25% chance of scattered showers through around 8-10 AM. Temperature observations at 2 AM show most locations across the interior lowlands have fallen to the mid to upper 30s, except for low 40s in the Portland metro area. Temperatures will continue dropping over the next few hours with low temperatures at most locations reaching the low 30s. Expecting widespread frost with light winds and mostly clear skies this morning. Patchy fog is also a possibility for areas that remain above 34 degrees. This pattern continues into Monday with daytime temperatures peaking in the upper 50s and another round of cooler low temperatures Monday morning. However, low temperatures will be slightly higher than this morning, in the mid to upper 30s, leading to less widespread frost development. Note that NWS Portland will not begin issuing Frost Advisories and Freeze Watches/Warnings for agricultural purposes until April 1st as this is when the growing season begins to ramp up and impacts from frost and freeze conditions become more significant.

Expect increasing cloud cover through the day on Monday as the next weather system approaches the PacNW. Ensemble guidance remains in excellent agreement that a surface closed low pressure system will move northeast into British Columbia with a trailing cold front moving east into WA and OR Tuesday into Wednesday. This cold front will bring widespread rain and breezy winds to NW OR and SW WA. There is some uncertainty on how long rain will last into Wednesday night or Thursday morning as some ensemble members indicate moisture will continue into Thursday morning while others cut off moisture by late Wednesday night. The terrain and lowland locations along and north of the Columbia River have better chances for showers continuing into Thursday morning, though QPF amounts are on the low end by this time. Overall, although this system will bring widespread rain, impacts are expected to be limited. 48 hour QPF ending at 5 AM PDT Thursday is around 0.25-0.75 inches for the interior lowlands, 0.5-1.5 inch for the coast, Coast Range, and Willapa Hills, 0.5-1.5 inch for the north Oregon Cascades, and 0.75-2 inches for the SW Washington Cascades. Flooding impacts are not expected from this system.

In terms of Cascade snow, the heaviest precipitation is expected to fall ahead of and along the cold front on Tuesday when snow levels are generally higher than pass level. Snow levels will fall quickly after the front passes late Tuesday into Wednesday to around 3500-4500 ft by Wednesday morning. However, the lowest snow levels will be in SW WA Cascades with snow levels rising moving south into the central OR Cascades. As showers will continue into Wednesday, there is the potential for some accumulating snow at pass level, though due to the current sun angle along with warmer road temperatures, it might be difficult for much accumulation in scattered showers. Snow levels will continue falling through Wednesday, especially Wednesday night, falling to around 1500-3000 ft by Thursday morning. The 10th percentile (only 10% chance of snow levels this low) does indicate snow levels could fall as low as 500-1500 ft by Thursday morning. However, if precipitation really is decreasing by this time, then there will be limited precipitation to even cause snow. Only a few ensemble members suggest any kind of accumulating snow in the interior lowlands north of Portland, and generally less than an inch. NBM probabilities for 6 inches of snow or more in a 48-hour period ending at 5 AM PDT Thursday continue to back this expectation up, as chances are only around 5% at Willamette and Santiam Passes, and 10-20% along Highway 26 near Government Camp.

Breezy southerly winds are also expected with the frontal passage on Tuesday. Strongest winds will be along the coast, Coast Range, and Willapa Hills where there is a 60-80% chance of maximum wind gusts exceeding 40 mph. Inland, this probability drops to only a 10-20% chance, highest in the central Willamette Valley south of the Portland metro area to the Salem area. Chances for maximum wind gusts exceeding 30 mph for inland areas increase to around 50-75%, so most likely scenario is peak wind gusts of 30-35 mph for inland locations and 40-45 mph along the coast as the front passes.

By Thursday afternoon into Friday, 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 on Thursday and into the low to mid 60s for Friday and Saturday for the interior lowlands. Saturday is expected to be the peak day of the warming trend, and NBM indicates a 20-30% chance of temperatures reaching 70 degrees across the Willamette Valley. -03

AVIATION

Dry, westerly flow aloft continues to bring VFR conditions with FEW/SCT high clouds through the TAF period. Will begin to see more BKN high clouds after 12z Mon ahead of the next system. Winds remain northerly today around 5-10 kt for most of the Willamette Valley, except 10-15 kt at KEUG. Tightening pressure gradients will also increase northerly to northwesterly winds along the coast to 10-15 kt with gusts up to 25 kt between 21z Sat and 03- 05z Sun, then weakening overnight. Cloud breaks and light winds tonight may result in Monday morning temperatures dropping to the low to mid 30s across the Willamette Valley, leading to potential frost development over metal surfaces between 09-16z Sun.

KPDX AND APPROACHES...VFR conditions through the TAF period. Northwesterly winds between 5-10 kt this afternoon, weakening below 5 kt overnight. Potential for frost development over metal surfaces between 09-16z Sun. -10

MARINE

High pressure over the waters will maintain northerly winds through Monday. The next very strong ebb will occur around 730 AM Sunday, so a Small Craft Advisory has been issued for the the Columbia River Bar between 5-10 AM Sunday as the ebb builds seas to around 8 ft along the Main Channel of the Bar.

A thermal trough develops along the southern Oregon coast Sunday afternoon through early Monday morning, tightening pressure gradients across the waters south of Cape Falcon. This will lead to breezier northerly winds with gusts up to 25 kt. A Small Craft Advisory remains in effect for all waters south of Cape Falcon from 2 PM Sunday through at least 5 AM Monday. Northerly winds may last longer for the outer waters beyond 10 NM offshore, so the Small Craft Advisory for these zones will go through 8 AM Monday. Seas fall to 5-6 ft tonight, re-building to 8-9 ft at 11 sec as wind waves increase from the breezy northerly winds.

The next 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 (90%+ chance) for widespread and frequent small craft wind gusts of 21 kt or greater, and a 35-55% 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 75-90%, with the highest chances beyond 20 NM offshore. Latest guidance suggests the strongest winds occur between 8 AM to 5 PM Tuesday. Seas on Tuesday and Wednesday are also expected to build to around 13-16 ft in response to increasing wind waves along with an increasing westerly swell. There's a 30-60% chance seas could exceed 15 ft with a 5-15% chance for seas exceeding 20 ft. Chances increase from south to north, peaking across the waters north of Cape Falcon and mainly beyond 40 NM offshore. -10/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 2 PM this afternoon to 5 AM PDT Monday for PZZ252-253. Small Craft Advisory from 2 PM this afternoon to 8 AM PDT Monday for PZZ272-273.