textproduct: Portland

SYNOPSIS

Onshore flow persists through much of the next week, while high pressure building offshore will see temperatures trend warmer through Friday. A frontal system Saturday will bring the next chances for light rain and usher in a broad pattern change this weekend into next week.

SHORT TERM...Now through Friday Night

Broad upper-level ridging currently centered over the Rockies will build and retrograde westward over the Great Basin through Friday, increasing heights aloft locally and supporting a warming trend across the region. A weak shortwave is moving over the region this afternoon which is increasing instability over the high Cascades. CAPE values are exceeding 150 J/kg which is not necessarily enough to produce widespread thunderstorms, but may produce some towering cumulus over the Cascades. Temperatures near normal in the mid 60s on the coast and low 80s within inland valleys are expected through Thursday as weak waves within the southwesterly flow aloft will keep heat from rising to hazardous levels. Even on Friday, while still going to be the warmest day of the week, temperatures will most likely peak in the mid 60s to low 70s at the coast and mid to upper 80s inland. These conditions are fairly typical for this time of year. Otherwise, a typical summertime pattern will see dry weather and morning clouds, especially along the coast, clearing to mostly sunny skies each afternoon. -27/36

LONG TERM...Thursday through Wednesday

A more substantial pattern shift will begin Friday night into Saturday as an upper trough moves from the Gulf of Alaska inland over the Pacific Northwest. This system will bring a cold front into the coast on Saturday. Rainfall is expected with around a 50% chance of accumulating rain along the coast and southwest Washington in the afternoon. Accumulation will vary greatly, and will be orographically impacted. Any rainfall will most likely be very light, amounting to no more than 0.05-0.10 inches. The 90th percentile shows some areas of around 0.25 inch along the coast north of Tillamook, but in order for that to occur, the system will have to drop further south than current models are suggesting. Near to slightly below normal high temperatures in the 60s and 70s are expected through the weekend in the wake of the frontal passage.

As the progressive upper trough exits eastward, there is increased uncertainty in the evolution of the pattern by early next week. Long-range ensembles depict a high probability of strong upper-level ridging developing over the Four Corners, but whether that will translate to hot weather locally is more uncertain. 500 mb ensemble heights show around half of the outputs with a trough over the coast early next week, while the other half show substantial ridging. In the case that we see troughing, we cannot rule out another cool start to the week, while if the ECMWF rings true, more heating would be in the forecast and a thermal trough would form at the coast. While warmer temperatures compared to the weekend are all but certain, the spread of potential solutions remains wide. The "best forecast" (mean) at this lead time shows Tuesday with the warmest daytime highs; in the 70s along the coast and upper 80s to low 90s inland. However, there is a 10% chance that we could see highs inland around 95-100 deg F, and in the low 80s along the coast. There are equal chances for highs in the upper 60s along the coast, and mid 80s inland. This wide spread leads to questions regarding whether or not the thermal trough forms on Monday and Tuesday. If it does so on Monday, northerly winds will increase with gusty conditions up to 25 mph possible. -27/36

AVIATION...Through 00z Thursday

Satellite imagery and terminal observations as of early Wednesday afternoon shows VFR conditions with mostly sunny skies. Expect predominately VFR conditions through the TAF period for Valley terminals with FEW/SCT high clouds (>25 kft) at times. There is a 20-30% chance for MVFR stratus re- developing across Valley terminals at any given hour between 10-17z Thu as the low level atmosphere moistens overnight. After 06-07z Thu, there is high confidence for low-end MVFR stratus (1-2 kft) returning along the coast and persisting through Thursday morning. Northwesterly winds 5-10 kt today, strongest in the afternoon with gusts up to 20 kt along the coast. Winds ease overnight.

KPDX AND APPROACHES...VFR conditions through the TAF period with high clouds. 20-30% chance for MVFR CIGs at any given hour between 10-17z Thu. Northwesterly winds 6-8 kt today, weakening under 5 kt tonight. -10

MARINE

Buoy observations from late Wednesday morning show seas continuing at 4-5 ft, which are expected to persist through much of the week with a dominant west to northwest swell at 7-9 seconds and a secondary long-period south-southwesterly swell. A frontal system approaching the waters late this week will result in northerly winds turning more southwesterly ahead of the weak surface front beginning Thursday, mainly for the waters north of Cape Falcon and the waters beyond 30 NM south of Cape Falcon down to Cape Foulweather. Chances for light rain peak late Friday night into Saturday morning during the frontal passage.

Behind the boundary, initial westerly flow will veer out of the north through the weekend as surface high pressure builds offshore. Chances for maximum wind gusts of at least 20-25 kt are greater than 90% across the waters from Sunday through at least Tuesday. Chances that peak afternoon gusts reach gale-force is the highest on Monday afternoon but have gone down since the last forecast package, with some 25-35% within 10 NM south of Cape Falcon and 15-25% beyond 10 NM. Chances for gale-force gusts north of Cape Falcon are lower, only 1-5%. -10/36

PQR WATCHES/WARNINGS/ADVISORIES

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