textproduct: Portland

SYNOPSIS

Dry conditions will persist through Friday aside from a chance of showers and thunderstorms over the Oregon Cascades, mainly on Thursday and Friday. A dry cold front will bring noticeably cooler temperatures today and Wednesday. Trending relatively warmer again Thursday into Friday with light offshore flow developing. Becoming cooler again over the upcoming weekend with increasing chances for showers.

LONG TERM...Saturday through Monday night

A pattern change is still expected this upcoming weekend, though details remain uncertain at this time. 500 mb cluster analysis shows that the majority of ensemble guidance indicates a trough originating from Alaska will approach the PacNW from the northwest early Saturday morning into Sunday, bringing the return of widespread precipitation chances. 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 1D Viewer highlights this well, suggesting inland high temperatures could range anywhere from the lower 50s to mid 60s Saturday through Monday per the 10th-90th percentile guidance.

Chances for showers are currently highest on Saturday and Sunday during the afternoon/evening hours, peaking between 40-60% west of the Cascades and 60-80% over the Cascades. Guidance continues to suggest rain amounts will most likely be on the lighter side if precipitation does occur. Probabilities for 48-hour rain totals below 0.25 inches from 5am Saturday through 5am Monday remain around 50-65%, except 25-35% over the Cascades. -23

AVIATION

Conditions are VFR across the inland terminals early this morning with low marine stratus draped across the coastline. However, satellite and surface weather observations from 09z Tuesday depicted a weak cold front progressing southward along the south WA coast, approaching KAST. This front will help scour out the marine stratus deck currently impacting the coast from north to south. The exact time low stratus will scatter out at KAST and KONP remains uncertain, however the latest trends suggest stratus will scatter out at KAST by 12z Tuesday and at KONP by 15-18z Tuesday.

Meanwhile, it is very likely (85-95% chance) VFR conditions will continue at KHIO, KPDX, KTTD, and KUAO through 12z Wednesday with a few thin high clouds. However, KSLE and KEUG have relatively higher chances for a period of MVFR cigs between 12-19z Tuesday as the weak front progressing southward. This is when KSLE has a 20-30% chance for cigs below 3000 ft at any given hour. KEUG has a 30-50% chance during that same time. Note cigs below 3000 ft are not reflected in the 12z TAFs for KSLE and KEUG as this is not the most likely outcome, however amendments may be needed.

KPDX AND APPROACHES...Expect VFR conditions to persist through 12z Wednesday with a few scattered high clouds. Chances for MVFR cigs below 3000 ft peak around 10-15% between 14-18z Tuesday. -23

MARINE

A weak cold front will progress southward across the coastal waters early Tuesday morning, bringing clearing skies and improving visibilities. No precipitation or significant wind shifts are expected with this frontal passage, as winds should remain out of the north to north-northwest around 15-20 kt. However, a fresh northwesterly swell is still on track to arrive behind this frontal passage late Tuesday morning into the afternoon. This will push seas up to 6 to 8 ft with a dominant wave period of 8 to 9 seconds, resulting in steeper waves that will be hazardous to small craft. Therefore, a marginal Small Craft Advisory remains in effect from 5 AM Tuesday through 5 AM Wednesday. Note brief wind gusts up to 25 kt may occur with the frontal passage.

Seas are still on track to decrease slightly to 5 to 7 ft on Wednesday and then remain within that range through the upcoming weekend, with winds likely staying under 20 kt. However, another round of marginal Small Craft Advisory conditions cannot be ruled out late in the week, especially on Thursday when there is a 25-50% chance for max wind gusts over 21 kt over the northern and southern waters and a 40-65% chance over the central waters. In addition, seas are forecast to become slightly steeper again. -23

PQR WATCHES/WARNINGS/ADVISORIES

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

WA...Frost Advisory from 11 PM this evening to 9 AM PDT Wednesday for WAZ202>205-208.

PZ...Small Craft Advisory from 5 AM early this morning to 5 AM PDT Wednesday for PZZ251>253-271>273.