textproduct: Portland

SYNOPSIS

Drier and warmer conditions persist through Thursday with temperatures gradually warming and peaking tomorrow with the development of low-level offshore flow. Cooler temperatures return on Friday and will continue through the weekend, along with increasing chances for rain. The forecast becomes more uncertain by the start of next week, 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 afternoon through Monday

Satellite imagery early this afternoon shows mostly clear skies across NW OR and SW WA. Ensemble guidance remains in good agreement that upper heights over the PacNW will raise into Wednesday as the upper low from previous days continues moving eastward into the Northern Plains and high pressure over the desert SW slowly spreads into the Western US. In addition to these two synoptic features over CONUS, a tropical/sub-tropical low off the coast of California along with an upper level high over the NE Pacific will create a col over the Pac NW. This col will help to maintain rather benign conditions through the middle of the week. 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). This overall setup 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 5-10% chance that coastal cities off the immediate coast could reach 80 degrees on Wednesday if offshore flow continues into Wednesday afternoon/early evening.

Ensemble members remain in good to excellent agreement on the weather pattern for Thursday into Saturday. 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, more robust low will develop over far western Canadian and drop south into the PacNW for the end of the week and the start of the weekend. Uncertainty still exists on the exact track, strength 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 eastern WA and eastern OR. As Sunday approaches, the majority of ensemble models, around 60-70%, 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 between Friday night and 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 night 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 70-80% chance that any location, within our CWA, 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 30-50% chance for locations west of the Cascades with a 45-65% 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 mid 70s increasing to the mid 60s to low 80s 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. /42-03

AVIATION...Through 18Z Wednesday

High pressure will gradually build over the region today through Wednesday. Predominately VFR conditions are expected through this evening under mostly clear skies. There is around a 20-30% chance for low stratus and/or fog to develop at coastal terminals after 12z Wednesday. Northwest winds are expected to increase at the coast this afternoon with gusts up to 20-25 kt. Inland, northerly winds increase to around 8-12 kt with gusts up to 20 kt in the southerly Willamette Valley.

KPDX AND APPROACHES...VFR conditions expected through the TAF period. Light winds become northwest around 7-9 kt after 20z this afternoon. /DH

MARINE

High pressure offshore will maintain predominately northerly winds across the coastal waters through the week. A strengthening pressure gradient today will bring stronger northerlies with gusts up to 25 kt as there is an 80% chance of wind gusts exceeding 21 kt by this 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 to 5 ft with a dominant northwest swell at 10-11 seconds will gradually build to around 6 ft by this evening as wind chop increases with the northerly winds. A weak low pressure system is expected to approach the coastal waters from the north late Friday into Saturday. /DH

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 until 6 AM PDT Wednesday for PZZ272-273.