textproduct: Seattle/Tacoma

SYNOPSIS

An upper level trough over the area today will bring chances of thunderstorms across the mountains and critical fire weather conditions. Thunderstorm potential will continue along the Cascades on Saturday, but will wane throughout the weekend as an upper level ridge starts to builds into the area from the southeast. Another upper level trough will move into the region early next week. Uncertainty then increases in the forecast heading into midweek as guidance remains split between a warmer, drier pattern and a cooler, wetter one.

SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/

Latest satellite imagery shows stratus draped inland along the coast, while it scatters out across the interior this afternoon. Radar also shows some development of some isolated showers and thunderstorms along the southern Cascades this afternoon. With a weak upper level trough remaining over the area today, expect another round of thunderstorms along the Olympics and Cascades this afternoon and evening as conditions over the higher terrain become increasingly unstable. Critical fire weather conditions are expected again across the mountains as any thunderstorms that do develop could produce abundant lightning and gusty, erratic winds and not much in the way of rain as conditions remain very dry across the region. The exception remains any storms that remain stationary over an area, which will need to be monitored for enhanced rainfall, especially in the vicinity of any burn scars. Afternoon highs will be in low 80s across the interior and 60s along the coast in stratus.

The upper level trough will weaken across the area tonight, allowing for a ridge to build into the area from the southeast over the weekend. Southwesterly flow will develop aloft and will help to push smoke out of the area this weekend. Low level flow will remain onshore, allowing for marine stratus to push inland during the overnight periods both Saturday and Sunday. While overall thunderstorm chances will primarily wane throughout the weekend, another round of thunderstorms will be possible across the Cascades on Saturday afternoon and evening as the air mass becomes unstable during the afternoon hours and some monsoonal moisture works its way up into the region. By Sunday, any thunderstorms that do develop will mostly be confined to the Cascade crest and areas east of the crest. Afternoon highs over the weekend will be in the upper 70s to low 80s across the interior and in the mid to upper 60s along the coast, while overnight lows will be in the 50s.

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LONG TERM /MONDAY THROUGH FRIDAY/

The first part of next week remains to follow a rather typical pattern of onshore low-level flow maintaining morning clouds, with some clearing in the afternoon. A passing trough around Tuesday could provide a reinforcing push of clouds and knock temperatures a few degrees cooler than normal. Otherwise, ensemble guidance begins to spread a bit larger toward the middle of next week with a fairly even split between a cooler trough pattern and a warmer (near normal) pattern under zonal flow or a weak ridge. In any event, there's not a significant signal for any precipitation of note.

AVIATION

Stratus has cleared across the interior with generally VFR conditions. However, some thunderstorms will continue to develop off the mountains, mostly over the Cascades, this afternoon and evening. Otherwise, stratus will slowly retreat to the coast this afternoon. Another surge of stratus is likely later tonight through Saturday morning for most of the area, with IFR ceilings and visibility expected for most terminals.

KSEA...VFR through the remainder of the day. Some thunderstorms will develop well east of the terminal area and continue into the evening, but is not expected at KSEA itself. Winds predominantly from the southwest, becoming west/northwest (but 5 kt or less) this evening around 02z. A return to light southwest winds then follows overnight, along with the return of low stratus around 12z tonight with IFR/LIFR conditions into the morning tomorrow.

MARINE

High pressure remaining off the coast of California will keep winds over the coastal waters light through Saturday. High pressure will then rebuild offshore British Columbia and move southward on Sunday and remain situated off the Pacific Northwest into next week. This will bring the return of northwesterly winds to the coastal waters. Diurnally driven westerly winds down the Strait of Juan de Fuca will occur each afternoon and evening over the next several days, with winds approaching Small Craft Advisory strength at times.

Seas will primarily remain between 3 to 6 ft through the weekend and through much week.

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FIRE WEATHER

A Red Flag Warning remains in effect for the Olympics and Cascades through this evening for another round of convection. Similarly to Thursday, most of the storms are not expected to produce much in the way of wetting rains. Given the dry conditions, any storms that do develop could have gusty and erratic outflow winds, which combined with the lightning and the dry conditions present, could produce new fire starts. While most storms will not produce much rain, there remains a small chance of flash flooding in any storms that remain stationary over an area and produce enhanced rainfall, especially over burn scars.

Elevated fire weather conditions will continue across the Cascades into Saturday with chances of thunderstorms possible again along the Cascade crest on Saturday afternoon and evening. Overall chances for convection will decrease through the weekend, however, as an upper level ridge starts to build into the area from the southeast. By Sunday, expect convection to mainly be confined to the Cascade crest and areas east of the crest. Another upper level trough will move into the area next week and could bring some additional elevated fire weather concerns to western Washington at times.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...Red Flag Warning until 9 PM PDT this evening for East Portion of the Olympic Mountains-West Portion of the Olympic Mountains-West Slopes of the Central Cascades Generally above 1500 Feet-West Slopes of the North Cascades Generally above 1500 Feet.

PZ...Small Craft Advisory until 5 AM PDT Saturday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.