textproduct: Seattle/Tacoma

SYNOPSIS

Onshore flow will continue to keep western Washington cool through the end of the week, with clouds gradually clearing by the weekend. A warm pattern via an upper level ridge and thermal trough will build over the region this weekend into early next week. Heat is the main concern, as highs in the 80s and 90s result in widespread moderate HeatRisk for the region. Additionally, dry, offshore and unstable air will produce elevated fire weather conditions during this period. The ridge will move inland Tuesday as onshore flow returns, slowly cooling temperatures down.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

Upper level analysis shows an upper level low and trough axis in southeast Washington/eastern Oregon this afternoon with a mini surface trough also in this area. Descent onshore flow continues in wake of this low this afternoon. Quite a bit of low level moisture has kept cloud coverage in place (as seen on satellite). There are a couple pockets of showers in the foothills of the Cascades this afternoon, but the steady rain has wrapped up. Temperatures remain cool this afternoon, with highs only reaching the low to mid 60s with the cloud coverage in place (which will begin to erode going into the evening as the trough axis is replaced with a shallow ridge that will pass overhead Thursday. Winds will also switch from the south to the north in most spots, remaining light at around 5 to 10 mph. Lows tonight cool into the 40s. More sunshine is on tap for Thursday, with the flow remaining mostly zonal with a weak trough passing to the north of the region. This will keep the temperatures in the upper 60s to low 70s Thursday and Friday, with plenty of sunshine. Lows also remain in the 40s with low to the north shifting to the east on Friday. A ridge is expected to build late Friday to the west, and this along with a thermal trough underneath the ridge will be the main culprit for some warm and dry air in the long term forecast.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

An upper level ridge remains on track to build over the region this weekend into next week, with a thermal trough underneath it along the coast. Ensembles have the ridge building up to Yukon/eastern Alaska. This will result in an offshore pattern, with temperatures warming through the weekend, peaking Monday, before cooling down midweek next week. The hottest days will be on Sunday and Monday, with the warmest temperatures from the Seattle metro into the south interior, where widespread low to mid 90s are likely. Remaining areas away from this warm region will see highs in the mid 80s. Moderate HeatRisk remains widespread for all but the mountain peaks on Sunday and Monday, with portions of the south sound/interior having a 30-50% chance of seeing major HeatRisk. This latest forecast has temperatures slightly warmer overall, and may cause issues for those sensitive to heat (especially given the overnight temperatures Monday and Tuesday morning will only cool into the low 60s for urban areas in the region.

In addition to the heat, the dry and unstable air caused by the thermal trough will create elevated fire weather concerns during this period (see the fire weather discussion below). Otherwise, ensembles finally bring an onshore return on Tuesday into Wednesday as the ridge departs to the east. The initial onset of onshore flow will only cool off the immediate coast, with the interior still seeing highs into the 80s on Tuesday with moderate HeatRisk in the Seattle metro. Wednesday will see a more widespread cooldown, with highs only reaching the low to mid 70s in the interior. There is a 20-30% chance of a few showers in the interior Wednesday, but overall moisture is nonexistent in the long term.

HPR

AVIATION

Low scattered to broken clouds continue to hang around the terminals this afternoon, with generally VFR conditions. The flow aloft remains northwest, with said winds mixing down to the surface this afternoon. Winds at 4-8 kt this afternoon will decrease tonight, and become light and variable in spots. Most of the clouds will clear tonight. Despite the light winds, the flow aloft/onshore gradients should keep clouds forming aloft (rather than fog/mist). IFR probabilities remain greatest in the south Cascades, Olympics and the coast with a 30-50% chance of development. The interior terminals have a low chance (20-30%) of MVFR ceilings in the morning, but this remains conditional with overnight winds/cloud coverage. The period to watch for low clouds is from 12-18Z Thursday morning. Winds increase again Thursday 5-10 kt.

KSEA...VFR conditions through the evening. There is a 30% chance of MVFR ceilings developing in the morning, but this is conditional based off of overnight winds/cloud coverage into tonight. Otherwise, VFR is likely to continue into Thursday. Winds turning to the northwest/northeast this afternoon into Thursday. 4-8 kt will decrease to less than 5 kt overnight (may become light and variable), then increase 5-10 kt Thursday.

HPR

MARINE

High pressure will rebuild offshore, with lower pressure inland today through the end of the week. Various degrees of onshore pushes will continue through the Strait of Juan de Fuca. The current advisory out for the east Strait of Juan de Fuca will be allowed to expire at 3 PM PDT this afternoon. While a couple higher resolution ensembles hint at a couple wind gusts greater than 20 kt this evening, confidence was not high enough to extend the advisory (based on how winds this morning decreased below advisory threshold with similar probabilities). Another push Thursday evening has a higher likelihood of gusts greater than 20 kt in the same areas.

Later this weekend into early next week, a ridge will build over the waters, which will turn the flow north/easterly at times. The far outer coastal waters may see winds approach 20 kt Friday afternoon, as well as in Puget Sound/inner coastal waters briefly on Saturday. The return of onshore flow Tuesday with the high moving inland may result in areas of stronger winds, particularly in the Strait of Juan de Fuca where there is an elevated risk of gales next Tuesday (as well as small craft winds for the coastal waters).

Seas hold around 4-6 ft for a majority of the next 7 days. A brief uptick to 8 ft Friday may occur, as well as an increase to 8-10 ft next week Wednesday.

HPR

FIRE WEATHER

A warm, dry and unstable pattern will setup across western WA Saturday through Monday, with offshore flow at times. While wetting rains will briefly help the fuels from reaching critical thresholds this weekend for red flag conditions, minimum relative humidity (RH) values may drop to as low as 15 to 20 percent for portions of the south sound/interior Saturday, Sunday and Monday. These dry humidity values, combined with an unstable airmass (via the thermal trough), and offshore winds (although light, aiding in drying the air), will likely lead to elevated fire weather concerns across the region this weekend/early next week. Fuels that will be most susceptible to the elevated fire weather concerns are dry/shrubby grassy areas (especially those near roads), as well as piles of dry fuels. The onshore flow will return on Tuesday, with a gradual cool down and improvement of RH values.

HPR

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 3 PM PDT this afternoon for East Entrance U.S. Waters Strait Of Juan De Fuca.