textproduct: Portland

SYNOPSIS

Frost chances Wednesday morning across much of the region. Dry conditions will persist through Friday aside from a chance of showers and thunderstorms over the Oregon Cascades, mainly on Thursday and Friday. Cool temperatures today before another brief warming trend Wednesday into Friday. Becoming cooler again over the upcoming weekend with increasing chances for showers.

DISCUSSION...Tuesday afternoon through Monday

Visible satellite imagery early Tuesday afternoon shows the stratus deck that formed across much of the interior has mostly dissipated with a few high clouds streaming over NW OR and SW WA. Temperatures are much cooler today than the previous few days, with observations at 1 pm indicating temperatures east of the coast anywhere from 10-20 degrees cooler than 24 hours ago. Temperatures are expected to peak in the low to mid 60s across the interior and 50s along the coast. Surface level flow will become less westerly and more northerly by tonight, which will limit any onshore moisture push and keep skies mostly clear. Radiative cooling will allow for chilly overnight and early morning temperatures. Many locations across the area could see frost develop Wednesday morning, especially in more rural areas. A Frost Advisory has been issued for all of SW WA and the Willamette Valley outside of the central Portland/Vancouver metro, the Tualatin Valley, north Oregon Coast Range, and the Upper Hood River Valley from late tonight through 9 AM Wednesday for temperatures falling as low as 33-36 degrees. Anyone with sensitive outdoor vegetation that is susceptible to frost damage should take necessary precautions today.

A large upper level cut-off low slowly approaches the northern California coast Wednesday through Friday. Ensemble guidance continues to indicate weak ridging over NW OR and SW WA on the northeastern periphery of the low Wednesday into Friday. This will bring mostly clear skies and a warming trend over the area. Expect to see temperatures across the interior lowlands increase to the upper 60s Wednesday and low to mid 70s Thursday and Friday. There's now a 25-50% chance of the Willamette Valley reaching 70 degrees on Wednesday, with higher chances of 35-55% for the Portland metro area and 50-75% chance for the Lane County lowlands. On Thursday, probabilities for temperatures reaching 75 degrees are around 20-30% across the majority of the Willamette Valley, increasing to 60-80% for the Portland metro area. Probabilities of 75 degrees have lowered for Friday as ensemble guidance indicates the low pressure system will begin moving south just off of the California coast, with ridging beginning to break down and potentially more cloud cover spreading over the area.

In regards to precipitation, expect the ongoing dry spell to persist through Friday, aside from the Oregon Cascades where there are chances for showers and thunderstorms Thursday and Friday afternoon and evenings, especially to the south of Mount Hood. This is because models and their ensembles continue to show a period of increasing southerly to southeasterly flow aloft on the north side of the upper low pressure system to our south. This synoptic- scale setup will favor increasing moisture and instability, though some uncertainty remains in regards to how moist and unstable the thermodynamic profile will become. The NBM 10th-90th percentile for SBCAPE values over the Oregon Cascades and foothills ranges from near 0 J/kg all the way up to 400-900 J/kg, with mean values around 100-350 J/kg. Given what looks to be a weak wind shear profile, severe thunderstorms are unlikely to occur, however isolated stronger storms cannot be completely ruled out as suggested by the NBM 90th percentile for SBCAPE. For now, NBM PoPs and thunder probabilities seem reasonable given the ongoing uncertainty involved. PoPs peak near 50-70% Thursday afternoon/evening and near 60-90% Friday afternoon/evening, suggesting showery activity is likely. The thunderstorm potential is relatively lower, with thunder probabilities peaking around 15-25%. Note if upper level flow does become southeasterly with convection developing over the Cascades, a few stray cells would have the potential to drift into the Willamette Valley, Coast Range, and/or Portland metro.

A pattern change is still expected this upcoming weekend, with the majority (75-80%) of ensemble members indicating widespread showers will return Saturday into Monday. 500 mb cluster analysis shows that a trough originating from Alaska will approach the PacNW from the northwest late Friday into the weekend. However, there are a number of differences in the timing and amplitude of this trough as well as how it interacts with the pre-existing low pressure system along the California coast. These differences impact the overall forecast, though one aspect that the ensembles agree on is temperatures will be trending noticeably cooler with low chances for any high impact weather. For example, afternoon high temperatures in the 70s late in the week will cool into either the 50s or 60s beginning Saturday. The NBM 10th-90th percentile spread indicates inland high temperatures could range anywhere from the lower 50s to mid 60s Saturday through Monday.

Chances for showers are currently highest on Saturday and Sunday, peaking between 40-60% west of the Cascades and 60-80% over the Cascades, falling below 30% for Monday. Guidance continues to suggest rain amounts will most likely be on the lighter side if precipitation does occur, though QPF has raised a touch from previous forecasts. NBM indicates a 40-60% chance of at least 0.25 inch of rain west of the Cascades and a 70-80% chance over the Cascades between 5 AM Saturday and 5 AM Monday. -03/23

AVIATION

VFR conditions very likely (greater than 90% chance) to continue into tonight across the region beneath sct high clouds at 25 kft. There are 30-50% chances for marine stratus to yield MVFR cigs at coastal terminals after 09-12z Wed, before any low clouds erode by 18-21z Wed. Diurnal north to northwest winds around 5 kt inland and 8-12 kt along the coast will ease by 06z Wed, then increase again after 15-18z Wed. Clear skies and light winds will allow for areas of frost to develop at most inland terminals tonight.

KPDX AND APPROACHES...VFR conditions very likely (greater than 90% chance) to continue through tonight, with low (10-20% chance) of MVFR cigs if low stratus redevelops tonight. Diurnal northwest winds continue around 3-6 kt, highest in the afternoon to early evening and lowest overnight. -36

MARINE

Seas of only 5-8 ft remain steep enough with average periods of 5-6 seconds to support continued Small Craft Advisories across the waters through 5 AM Wednesday. Northerly winds of 10-15 kt may gust as high as 20 kt this afternoon and evening. Little in the way of active weather is then expected through the workweek other than passing rain showers on the northern flank of an area of low pressure off the northern California coast by Friday. This feature to the south may support further Small Craft Advisory conditions as there is a 30-60% chance northerly to northwesterly wind gusts exceed 25 kt on Thursday. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from 11 PM this evening to 9 AM PDT Wednesday for ORZ104-106-108-109-114>118-121.

WA...Frost Advisory from 11 PM this evening to 9 AM PDT Wednesday for WAZ202>206-208.

PZ...Small Craft Advisory until 5 AM PDT Wednesday for PZZ251>253- 271>273.