textproduct: Portland

SYNOPSIS

Drier and warmer conditions persist through Thursday with temperatures gradually warming and peaking on Wednesday with the development of low-level offshore flow. Cooler temperatures Friday into Saturday with increasing chances for rain. The forecast becomes more uncertain Sunday into Monday, with the potential for weather conditions to range anywhere from cool and showery to warm and dry depending on the evolution of the large scale weather pattern.

DISCUSSION...Tuesday morning through Monday

Satellite imagery early Tuesday morning shows mostly clear skies across NW OR and SW WA with a few pockets of developing stratus and fog in sheltered valleys due to light winds and saturated soils from recent rains. This stratus or fog should dissipate by late morning with sunny skies and warming temperatures expected across the area today. Ensemble guidance is in good agreement that upper heights over the PacNW will raise today into Wednesday as the upper low from previous days continues moving east into the Northern Plains and high pressure over the desert SW slowly spreads into the Western US. At the surface, a thermal trough will push north from CA, bringing weak offshore flow (east winds over terrain and the coast and north winds for the interior lowlands). These features will bring continued dry weather as well as a brief warming trend into Wednesday. Daytime temperatures are expected to rise into the upper 70s to around 80 degrees for the interior lowlands today and low to mid 80s on Wednesday. Coastal communities will also be warmer due to the offshore flow, in the upper 60s to low 70s today and low to mid 70s on Wednesday. There's around a 10-15% chance that coastal cities off the immediate coast could reach 80 degrees on Wednesday if increased east winds continue into Wednesday afternoon.

Ensemble members have come into much better agreement on the weather pattern for Thursday into Saturday than 24 hours ago. On Thursday, a weak trough begins pushing south into the PacNW, lowering heights and pushing the surface thermal trough south. This will allow temperatures to lower back to near to slightly below normal for early September (upper 70s for the interior lowlands), though dry conditions will continue.

On Friday, ensemble members indicate another trough develops over the western Canadian border and drops south into the PacNW Friday into Saturday. Uncertainty still exists on the exact track and timing of this low with 80% of members suggesting the trough will deepen along the coast while the other 20% suggest the trough will deepen a little more inland over WA and OR. Then, 60-70% of the members indicate the trough will move east of the Cascades by Sunday morning where the other 30-40% suggest the trough will linger longer into the day on Sunday with around 10-15% keeping the trough over the PacNW into Monday. Even with the uncertainty, nearly all members indicate rain will return to the area sometime Friday night into Sunday, though the exact track will determine how much rain and where it will fall. Friday night into Saturday has the highest confidence with the latest NBM deterministic forecast now indicating a 40-50% chance of rain late Friday into Saturday evening across the entire forecast area. Part of the reason for the lower probabilities is likely more to do with the timing of when the trough will push through the area as opposed to whether it will actually rain or not. There's a 65-75% chance that any location will receive at least 0.01 inch of rain in the 48 hours ending at 5 AM Monday. When looking at probabilities of reaching or exceeding 0.25 inch in the same 48 hour period, there's a 25-40% chance for locations west of the Cascades with a 40-55% chance over the Cascades, including the foothills. Temperatures will also fall fairly dramatically. Daytime temperatures will be anywhere from 6-10 degrees cooler on Friday than Thursday with another 2-6 degrees of cooling on Saturday. However, the forecast temperature spread increases Saturday into Sunday as the uncertainty in the weather pattern increases. The 10th to 90th percentile spread for the interior lowlands on Saturday is from the low 60s to low 70s increasing to low 60s to upper 70s on Sunday. If you have outdoor plans this weekend, keep an eye on the forecast and plan accordingly for the rain potential.

Even with the 10-15% of ensemble members keeping the upper trough over the region on Monday, members indicate it would weaken and limit precipitation chances. The rest of the members indicate dry and warmer conditions for Monday, which is reflected in the deterministic NBM with precipitation chances less than 10% with temperatures in the 70s for the interior lowlands. -03

AVIATION...Through 12Z Wednesday

High pressure continues to build across the area with northeast to easterly flow aloft. Predominately VFR conditions through the TAF period at all terminals. However, through 16-18z Tue, there's a 15-25% chance of shallow low stratus/fog producing MVFR/IFR/LIFR conditions at any terminal which would develop due to calm winds and moist surface conditions due to recent rain. Winds along the coast have shifted east to northeasterly early Tue morning and will shift northerly and increase to 10-15 kt with gusts up to 25 kts after 16-18z Tue. Inland winds are mostly northerly less than 5 mph and will increase after 16-19z up to 8-12 kts with gusts around 20 kts in the southern Willamette Valley.

KPDX AND APPROACHES...Expect VFR conditions through the TAF period, with an exception of a 15-25% chance of shallow low stratus/fog causing IFR/LIFR conditions through 16Z Tue. Light and variable winds will become northwesterly around 7-9 kt by 19-21Z Tue. -03

MARINE

High pressure offshore will maintain predominately northerly winds across the coastal waters through the week. A strengthening pressure gradient on Tuesday will bring stronger northerlies with gusts up to 25 kt as there is an 80% chance of wind gusts exceeding 21 kt by Tuesday evening. As a result, Small Craft Advisories remain in effect from 11 AM Tuesday to 6 AM Wednesday for the outer coastal waters south of Cape Falcon, and from 2 PM to 12 PM Tuesday for all other coastal waters. Breezy north winds continue through Thursday, but should remain 20 kt or less. Seas of around 4 ft with a dominant northwest swell at 10-12 seconds will gradually build to around 6 ft by Tuesday evening as wind chop increases with the northerly winds. A weak low pressure system is expected to approach the coastal waters late Friday into Saturday. /DH/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory from 2 PM this afternoon to midnight PDT tonight for PZZ210-251>253-271.

Small Craft Advisory from 11 AM this morning to 6 AM PDT Wednesday for PZZ272-273.