textproduct: Portland

SYNOPSIS

Calmer conditions are expected through the weekend into next week with a few largely non-impactful features worth noting. Today through Friday, lingering shower chances (20-45%) remain largely confined to southwest Washington. Our focus shifts to central Oregon Saturday into Sunday as a weather system moving into northern California brushes the region. There's moderate confidence that precipitation chances hold generally to Salem southward. Completely dry weather then returns early next week, albeit only temporarily as a weak weather system brings the return of widespread precipitation late Tuesday into Wednesday.

DISCUSSION...Thursday afternoon through Wednesday

Visible satellite imagery early Wednesday afternoon shows a thin cirrus deck streaming over NW OR and scattered mid-level clouds over SW WA. Going through today and Friday, the upper level flow remains fairly zonal keeping any light shower chances limited to SW WA. Otherwise, conditions stay dry for the rest of the area with high temperatures in the low to mid 50s for the lowlands, low temperatures in the low to mid 30s, and the potential for foggy and/or frosty overnight conditions.

Both deterministic and ensemble guidance remain in sync with a cut-off low pressure system moving inland from the west over northern California during the late Saturday through early Monday morning time period. There still remains a decent amount of uncertainty on how this system will impact NW OR dependent on the north/south track of the low. If it approaches the coast closer to the CA/OR border, precipitation chances could spread as far north as the Salem area, and possibly to the south Portland metro area, with high precipitation chances in Lane County. However, if it approaches the coast closer to central CA, then our forecast area could remain completely dry. Looking at the 12z ensemble members from the GEFS, EPS and GEPS, only around 15-30% of members indicate the low will be north enough for light precipitation around or just north of Salem with around 20-30% of ensemble members keep rain just Lane County southward. The remaining 40-50% of ensemble members are somewhere in- between. In any case, not anticipating much rain and therefore impacts from any precipitation from this system, and snow levels around 6000 ft will keep any snowfall concerns at Cascade passes at bay. The temperature forecast will also be impacted by the track of this low. The farther north it tracks, more clouds will push north into N OR and SW Washington, limiting daytime warming and nighttime cooling. A southern track will allow for clear skies, warmer daytime temperatures, and colder overnight temperatures near to below freezing in the interior lowlands.

Transient ridging returns to the PacNW for Monday and Tuesday, leading to a return of completely dry weather and increasing daytime temperatures. By Tuesday, daytime highs are expected to peak in the upper 50s to low 60s. These will be great days to squeeze in outdoor activities before rain returns on Wednesday as a weak weather system pushes into the PacNW. Some timing uncertainty continues on when precipitation will begin, but most ensemble guidance (~75%) favors the dry weather coming to an end by Tuesday night or early Wednesday morning. At this point, no wind impacts are expected with this system. However, snow could return to the Cascades as snow levels could drop to around 3500-4500 feet on Wednesday. Latest NBM guidance indicates a 40-60% chance of 6 inches of snow in the 48 hour period from 4 AM Tuesday through 4 AM Thursday, where most snow would fall late Tuesday into Wednesday. -03/99

AVIATION

As of 22Z Thursday, high cirrus clouds move eastward over the airspace (mostly south of KUAO), with VFR conditions throughout expected to persist through at least 06Z Friday. Tonight, there is potential for fog or low stratus around KSLE and KEUG after 09Z Friday (40-50% chance for LIFR conditions). For all other terminals, there is a 20-40% chance for MVFR conditions after 06-09Z Friday (highest chances at KAST). Otherwise, expect light winds through the TAF period, generally less than 5-10 knots.

KPDX AND APPROACHES...Currently VFR with light and variable winds. Expect VFR conditions to continue through most of the TAF period, then a potential for MVFR conditions just after 12Z Friday (15-25% chance for MVFR conditions). With deterministic guidance not biting on the potential for MVFR conditions after 12Z Friday, confidence is low. Otherwise, light and variable winds continue through the TAF period. ~12

MARINE

Fairly benign conditions persist across the waters today, with seas around 4 to 7 ft. As the southerly swell weakens into Friday, a fresh northwesterly swell of 8 to 9 ft at 15 seconds will arrive on Friday and persist through early Saturday, pushing seas to around 8 to 10 ft. At the same time, north- northeasterly winds will increase and bring gusts around 25-30 kt across the outer waters. As a result, Small Craft conditions are likely through all of Saturday for the outer waters of Cape Falcon southward. Seas and winds will ease back down to 5 to 7 ft by Sunday. Overall seas and winds look to remain fairly benign through early next week, then with another potential for Small Craft conditions beginning on Wednesday. ~12

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.