textproduct: Seattle/Tacoma

SYNOPSIS

A lingering ridge of high pressure will bring one more day of warm temperatures to western Washington today. A transition to onshore flow and cooler conditions will take place on Wednesday along with a chance of some precipitation. Cooler and unsettled conditions will continue into the weekend as a trough of low pressure over the Northeast Pacific sends a series of weak frontal systems across the area.

SHORT TERM /TODAY THROUGH THURSDAY/

Generally clear skies over W WA this early morning although seeing some high clouds starting to move in from the far SW corner of the CWA. Some low-level clouds are possible inland from the coastline, although their eastward extent is pretty limited.

One last day under the ridge will see daytime highs a few degrees warmer than yesterday with widespread upper 70s to lower 80 expected throughout the interior lowlands. Locations along the coast and in the islands will benefit from their proximity to water with highs in these spots much milder in the upper 60s to around 70. The ridge pushes eastward late this afternoon/early this evening allowing for an onshore push. While the most obvious change will be the cooling of temperatures, the incoming upper level trough tonight and associated moisture could give rise to evening and overnight showers. Best chances of this seem to be near the Cascades and associated foothills where orographics will give a boost to the incoming instability, but the lower end PoPs for these showers extends to the eastern half of the Olympic Peninsula. A slight chance for thunderstorms is present over the far southern portion of the Cascades, which lines up with latest SPC thoughts of general thunder...although this is more of a concern for down in PQRs CWA.

Wednesday sees the upper low traverse the area, keeping showers in the forecast and significantly cooler temperatures...mainly in the lower 60s. Best chance for rain is largely confined to east of the Sound, however PoPs are present area-wide for much of the day. Thursday sees a shortwave ridge quickly enter and exit the area. While this will limit PoPs over the area, it does not completely eliminate the chance for showers in some locations...again with a preference over the northern two-thirds of the Cascades. Temps warm slightly...mainly a couple of degrees at best...as lowland highs remain in the lower 60s...although some spots may nudge into the mid 60s.

18

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/

An alternating series of troughs and shortwave ridges for the remainder of this week will keep temperatures cool...upper 50s to lower 60s for daytime highs...and the chances for rain elevated. A broad trough begins to impact the area starting Friday and at the time of this writing models seem to be favoring some level of instablilty over W WA...resulting in a widespread slight chance for thunderstorms in the afternoon and evening hours. While this trough ejects Saturday morning, embedded shortwaves in the upper flow will keep showers in place for much of the remainder of the day. How quickly this moisture is pushed out of the area appears to be a point of contention in the long range model data. While there is consensus on an upper level ridge setting up for the remainder of the forecast period, the amplitude of said ridge will play a role in how quickly dry conditions return, thus introducing some uncertainty. Latest NBM output suggests moisture...and thus showers...lingering Sunday before tapering off on Monday. This transition will also help temperatures find their footing as daytime highs Monday get into the mid to upper 60s.

18

AVIATION

An upper level ridge will transit east over the terminals today, with southwest flow increasing ahead of a shortwave trough passing through tonight into Wednesday. VFR is expected through the day with a few high clouds. Winds will shift from the northeast this morning, to the southeast this afternoon at 5-10 kt ahead of the trough. Couple gusts to 20 kt can't be ruled out in the south sound/interior this afternoon, and the Strait of Juan de Fuca later in the evening. There is a 60-80% chance of showers passing through the terminals east of Puget Sound/Cascades late this evening/Wednesday morning, with another line of showers along the coast later Wednesday morning. There is also a 20% chance of thunder in the far south Cascades (Rainier southward). Ceilings likely lower tonight/Wednesday morning down to MVFR (with a couple pockets of IFR). Winds to remain out of the southwest Wednesday, with a couple spots becoming breezy later in the day.

KSEA...VFR during the day, with NW winds switching to the southwest this afternoon 8-12 kt (couple gusts to 20 kt possible). Tonight/Wednesday: showers likely to pass over the terminal from 06Z onwards, with the first round wrapping up around 15Z, and another round just after 18Z. MVFR conditions likely to develop as a result early Wednesday morning. Winds will remain southwesterly tonight/Wednesday.

HPR

MARINE

A ridge will transit east over the waters today over a thermal trough at the surface. Winds will return to onshore later this afternoon as a trough/front passes over the waters this evening into Wednesday. The interior waters have the highest chance of seeing showers Wednesday morning. Pushes through the Strait of Juan de Fuca will resume with this system passing through this evening, with the strongest one expected Wednesday evening with a marginal chance of wind gusts approaching gale force in the central/east sections. The southern portion of the Puget Sound waters and the north interior waters have a medium chance of seeing gusts over 20 kts for rougher small craft conditions. Seas will hold around 4-6 ft through the week, with a brief bump to 6-8 ft with the midweek system.

HPR

HYDROLOGY

The daily hydrology discussion has ended until the start of the next rainy season; it will only be updated during this time as needed.

SEW WATCHES/WARNINGS/ADVISORIES

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