textproduct: Seattle/Tacoma

SYNOPSIS

Widespread showers and the threat for thunderstorms returns on Thursday as a low pressure system passes through, exiting the region this weekend. High pressure builds over the area next week, bringing a return to warm and dry weather.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

Broad upper-ridging exists over much of CONUS at this time. It's influence has been been felt across W WA due in part to the recently observed well above-normal temperatures the last couple of days. Upstream, an upper-low is positioned just offshore of the PNW. Its gentle progression landward will be responsible to the upcoming pattern change as thunderstorms along the OR and WA Cascade crest this afternoon is a sign of what's to come.

Tonight, the aforementioned upper-low will spread moist, diffluent southerly flow aloft across W WA into the early morning hours on Thursday. With orographic enhancement - the first wave of rain and showers will track along the Cascades after midnight. Hi-res guidance (REFS) is indicating sufficient mid-level instability with 7-8 C/km during the onset of precipitation along with a couple hundred J/kg of CAPE. The next impulse looks to arrive by mid-morning along the coast and Olympic Peninsula. NBM 12-hour probabilities of thunder ending a 11 PM Thursday range between 20-30% across all of W WA including the lowlands with the highest probabilities along the Olympic Peninsula. Thunderstorms may produce small hail, gusty winds, and additional fire starts as a Red Flag Warning has been issued - more details are provided in the Fire Weather section below.

The upper-low will lift into BC on Friday as conditions trend drier. While some low-end chance to slight chance PoPs will linger over the northern third of the CWA, generally dry conditions are expected. Cooler temperatures on Thursday with highs mostly in the lower to mid 70s followed by a warm up into Friday with highs in the mid 70s to near 80 F. Overnight lows are forecast into the 50s.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Saggy troughing over the region this weekend but mid-level heights are slated to rise. As a result, conditions will remain dry and trend warmer but Minor HeatRisk is expected. However, upper-ridging will build into the area on Monday with widespread Moderate/Orange conditions with some isolated splotches of Major/Red. The heat now looks to potentially stick around into midweek with nominal cooling across the interior.

AVIATION

Southwesterly flow aloft as an cut off upper level low remains offshore. All terminals VFR this afternoon will continue throughout the evening. Diurnally driven winds generally from the N/NW 5 to 10 knots will continue this evening, and will turn more southerly to southwesterly overnight and into Thursday morning. Terminals along the coast such as KHQM may see winds slightly stronger, in the 8 to 12 knots range. Conditions will lower into Thursday morning along the coast with a return of MVFR/IFR marine stratus starting 04-06z and continuing throughout majority of Thursday morning.

Conditions over the interior will likely remain VFR, although there will be introduction for rain showers and a chance of isolated thunderstorms across the area through Thursday, which could lower conditions down to MVFR/IFR briefly if a shower or thunderstorm drifted over a terminal.

KSEA...VFR conditions at the terminal this afternoon continuing through the evening. Northwesterly winds 5 to 10 knots will shift more S/SW late tonight and into Thursday morning, maintaining the same speeds. Have introduced VCSH throughout Thursday afternoon as chances for showers continue to remain. There is a slight chance for thunderstorms tonight throughout Thursday, but not enough confidence to include it in the TAF as of now.

Mazurkiewicz

MARINE

A weak surface low will meander over the coastal waters through Friday with variable changes in winds over the coastal waters. Diurnal westerly flow will continue through the Strait of Juan de Fuca each afternoon but continues to remain under Small Craft Advisory criteria at this time. A more typical summertime pattern returns this weekend with northerly winds over the waters and gusts up to 20 kts at times.

Coastal seas 4 to 6 feet building slightly to 5 to 8 feet tonight and into Thursday with a period of 7 to 8 seconds. Seas will build even further beyond 30-40 NM to 8 to 10 feet over the weekend, but again will remain in the outer coastal water zone.

FIRE WEATHER

A Red Flag Warning is in effect for the mountains on Thursday. Showers and scattered thunderstorms return to the forecast tonight through Thursday evening. The main concerns with these storms will be dry lightning (storms resulting in less than 0.25 of an inch of rainfall) and the potential for strong outflows between 35-50 mph. Storms that develop on the west slopes of the Olympics are more likely to produce a wetting rain (0.25 inches or more), while storms along the Cascades will produce sub- wetting rain amounts of 0.10-0.20 inches.

Fire weather concerns linger into early next week as a hotter and drier pattern sets up over Western Washington, potentially increasing fire activity from any new lightning starts from Thursday's convection.

SEW WATCHES/WARNINGS/ADVISORIES

WA...Red Flag Warning from 1 AM to 11 PM PDT Thursday 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...None.