textproduct: Seattle/Tacoma

SYNOPSIS

Unsettled weather remains on track to move into western Washington this weekend with a trough moving through. This includes a slight chance of thunderstorms Saturday afternoon, along with a steadier rain Saturday evening into Sunday. Many locations will see at least half an inch of rain in total, with a higher potential for over an inch of rain in the Cascades south of Interstate 90. Drier weather will move in next week, with temperatures warming back up from Wednesday onwards.

SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/

A zonal flow continues overhead the region this afternoon with onshore flow continuing. Cloud coverage will remain partly sunny through the afternoon with cloud coverage increasing tonight into Saturday. An upper level low will dive southeastward this weekend from B.C. and deepen as it comes ashore over the weekend. A surface trough and low pressure will also progress inland this weekend as well. The ensembles are indicating an abundance of moisture available with this system compared to normal for this time in September (as the ECMWF shows anomalies for precipitable water and integrated vapor transport at 4.0 and 4.9 standard deviations above normal). With the abundant moisture available for this time of year, the region will see some beneficial rain this weekend.

The timing of the precipitation will start Saturday, with the coast seeing some wrap around precipitation to start the morning. As the low progresses inland, more wrap around showers will begin to form across the interior and the Cascades. The initial shower activity has a 15-20% chance of thunder. While SPC has all of western WA in a general thunderstorm risk for Saturday, the actual thunder may be more concentrated into portions of the south interior and Olympics with upslope/orographic lifting as the precipitation moves northwest. Otherwise, the precipitation coverage will become more stratiform going into Saturday night/Sunday, and cover primarily the Cascades (south of I-90), and adjacent portions of the south interior.

Most locations will see at least 0.50" of rain with this system from Saturday through Sunday evening. The bulk of the precipitation is expected again in the south Cascades, where there is a 60-80% chance of rainfall exceeding 1" and 1.50". Rain rates are not expected to exceed 0.25" to 0.35" an hour with this system (except potentially in thunderstorms), which should keep hydrologic impacts minimal (except for some ponding). Winds this weekend will remain generally light, but could get a little breezy Sunday across portions of the interior, and along the Cascade Crest with gusts up to 20 mph. High temperatures will remain in the upper 60s to around 70, with lows in the mid 50s.

LONG TERM /MONDAY THROUGH FRIDAY/

Ensembles have the upper level low/trough moving out of the region late Sunday, with northwest flow aloft increasing. There could be a few remaining showers in the Cascades going into Monday morning, otherwise conditions will drying out through the day for most, but with more clouds than sun keeping temperatures the same. Another trough will pass through early Tuesday, however the models cap any moisture with this trough moving through. Wednesday is where the models have an upper level high building over the Pacific, with a split difference as to how long the high remains offshore going into late next week. A couple models even show an omega block over the Pacific. Regardless, most will see a warm up with the surface pressure centered over southern B.C., and lower pressure to the southwest off the coast. Highs in the lowlands will likely climb back into the mid/upper 70s (with perhaps a few 80s).

HPR

AVIATION

VFR at the terminals today and Saturday. A weather system is expected to impact the terminals Saturday afternoon/evening. Latest guidance suggests precipitation will start impacting the coast around 20Z and the interior terminals after 00Z Saturday. Additionally, there is a threat for isolated thunderstorms Saturday afternoon/evening. Highest chances (20%) will be over the Olympics. As for the interior terminals, there is a 10-15% from PAE southwards. Chances for thunderstorms will subside Saturday night. Southwesterly winds this afternoon will transition northerly this evening around 4-6 kt. Light NE to calm winds overnight.

KSEA...VFR today and Saturday. Precipitation chances will increase into the afternoon, with precipitation likely impacting the terminal after 00Z. There is a slight chance (10%) for isolated thunderstorms Saturday afternoon/evening. Southwesterly winds this afternoon will shift northerly after 02Z-03Z at 3-5 kt. Northerly winds will increase to 5-7 kt Saturday afternoon. Southwesterly winds return Saturday evening after 00Z.

MARINE

High pressure will continue to weaken today as a low pressure system moves across area waters towards Oregon today into Saturday. Behind the system, high pressure will begin to rebuild over area waters and strengthen through the middle of the week. Westerly winds will increase through the Strait of Juan de Fuca with the potential to reach small craft thresholds Saturday evening and Sunday evening. Latest probabilities are around 40-55% on Saturday and 70-85% on Sunday. Seas generally around 4-6 feet this weekend, decreasing slightly early next week to 3-5 feet.

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FIRE WEATHER

Onshore moisture will increase this weekend as a trough moves across western Washington. The initial precipitation Saturday afternoon will be more showery (with a 15-20% chance of thunder), but coverage will become more stratiform late Saturday night into Sunday, increasing the chances of a wetting rain for most (especially the south Cascades). As a result, the fire weather concerns will remain minimal going through the weekend and early next week. High pressure will build in south B.C. by midweek, with lower pressure offshore at the surface. The pattern will become drier with warmer temperatures, and additionally some models are showing brief breezy east to southeast winds Wednesday evening/Thursday morning. A few Cascade locations may see RH values dip into the 20s, though there remains some uncertainty in the ensembles at this time as to how strong and long the high pressure will remain late next week.

HPR

SEW WATCHES/WARNINGS/ADVISORIES

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