textproduct: Seattle/Tacoma

SYNOPSIS

A return to an active weather pattern begins today with a trough developing offshore. A few fronts associated with this trough will keep precipitation chances in the forecast through the week, including snow in the mountains. Temperatures this week will also be a couple degrees cooler, with highs in the 40s and lows around freezing for much of the week.

SHORT TERM /TODAY THROUGH TUESDAY NIGHT/

It is mostly dry this morning across western WA this morning, but that is quickly changing. A trough is expected to develop offshore in the Pacific today, and dig its way southward along the coast, with much of the jet energy focused in California. This trough is expected to be the primary trigger for some of the weather expected in western WA through much of the week.

Already on radar this morning, the first showers are approaching the coastline from a cold front offshore that is expected to stall once it gets to the coast. These showers have been light in nature as of midnight, but will increase in intensity/coverage going through the day. The expected mode of precipitation today is bands/areas of showers. There is enough cool air aloft (from 850 mb up to 500 mb) for some instability in the air today (particularly from the coast into the ocean today). This has resulted in a 20% chance of thunder for today's shower activity along the coastline, and offshore. In addition to the chance of thunder, there is a chance of some snow mixing in (especially as snow levels drop below 1,000 ft late tonight, and below 500 ft Tuesday morning). As far as impacts go for today/Tuesday, the main area of concern is the US-101 corridor from Forks to the Lake Crescent area, where HREF/REFS keep probabilities for 1" of snow at 70-80% (but drops to 50% for 2" of snow). This area of US-101 will continue to be monitored for a possible winter weather advisory as the morning evolves (there remains some uncertainty as to if warmer air from the onshore flow may disrupt some of the cool air down at the surface causing flakes to melt).

For the remainder of the region, shower coverage is not expected to be as widespread as the coast today, but a few showers will work their way inland into the interior today into Tuesday. While a few snowflakes may fall in some of the more urban areas, the threat of accumulating snow (an inch or greater) in the interior remains near 0. The snow will be primarily focused in the Olympics/Cascades, with some of the snow in the Olympics potentially reaching down to the Lake Crescent and Hood Canal areas tonight/Tuesday morning. The snow amounts in the mountains Monday-Tuesday are on the lighter side, with just around 1-2 inches in the passes, and 3-6 inches at the peaks of the mountains.

High temperatures Monday/Tuesday will range in the low to mid 40s for the lowlands, and 20s/30s in the mountains. Winds will be out of the south/southwest, with a few gusts to 20 mph possible in the interior Monday afternoon, otherwise decreasing in magnitude into Tuesday to around 5 mph.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

The same trough will remain in place over the region through Friday, before moving further inland. This will keep temperatures cool through the week, with precipitation chances continuing. With the precipitation mode expected to continue to remain showery, some uncertainty remains in exact amounts of the precipitation going into the second half of the week. The snow impacts will shift more towards the mountains (though a few flakes will continue to remain possible in the lowlands next few overnights). The Cascades and Olympics will see snow through the week, with snow rates expected to remain light enough to keep traffic impacts to a minimum. From early Wednesday through Friday the heaviest of the snow looks to be in the central/south Cascades (with potential for 6+ inches of snow in a few of the passes/peaks Wednesday through Friday).

There are split ensemble solutions as to what replaces the previously mentioned trough moving inland. Most members show another trough developing next weekend, while a couple members show a drier/warmer pattern to the east (but just clipping western WA). If a trough does develop, there will be potential for more precipitation, as temperatures increase slightly next weekend. The precipitation will be monitored for possible impacts to travel (as there are signals that some of this precipitation may be heavier in nature).

HPR

AVIATION

A trough will dig along the coast today, with a surface cold front associated with the trough expected to slow as it approaches the coast. This will increase southwest flow aloft. Ceilings this morning are mostly VFR (couple pockets of MVFR are present around KPAE and KPWT, with mist down at KOLM). Scattered showers are expected across the terminals in the TAF period. Most of the showers will be concentrated from the Olympics westward, but a few will track in the interior. Additionally, the air along the coast/Pacific ocean is unstable enough for a 20% chance of thunder today. Some of the heavier showers may drop visibilities down to IFR. Additionally, the cool air overnight may lead to a few snowflakes mixing in overnight/early Tuesday morning. Probability for widespread MVFR ceilings increases tonight/Tuesday morning, but some terminals may stay VFR. Surface winds today are out of the SW 5-10 kt (couple gusts to 20 kt possible in the interior this afternoon). Will drop to 4-6 kt tonight/Tuesday.

KSEA...VFR high clouds in place over the terminal, with a 60% chance of showers/vicinity showers from 18Z through 00Z, then a 20% chance of vicinity showers through the remainder of the TAF period. A few snowflakes cannot be ruled out mixing down Tuesday morning (if a shower passes over the terminal). Probabilities for MVFR becomes likely after 00Z this evening. Winds: SW 5-10 kt with a couple gusts to 20 kt from 18Z-03Z, decreasing to 4-6 kt tonight/Tuesday.

HPR

MARINE

A trough is expected to deepen offshore in the coastal waters today, and remain over the region through the end of the week before moving inland. A couple of surface fronts will pass through with this trough pattern (the first one expected to slow coming up to the shore today/Tuesday). Precipitation will be showery in nature, with there also being a 20% chance of thunder in the forecast (for Monday only). Winds with this front are only expected to exceed 20 kt in the far outer coastal waters this morning through the afternoon. The winds will be breezy elsewhere out of the south (up to 15-20 kt gusts in the interior), but below small craft headline criteria. The hazard concern transitions to high seas, with an increase to 10-13 ft expected late today into Wednesday, decreasing to 7-8 ft Thursday, and 4-6 ft Friday as the low moves inland. Some uncertainty remains if a system will develop next weekend, but there will be a potential for gusty small craft winds and seas building above 10 ft next weekend (depending on how the system tracks).

HPR

HYDROLOGY

No river flooding is expected over the next 7 days.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory from 4 PM Monday to 4 AM PST Tuesday for Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

Small Craft Advisory from 4 AM Monday to 4 AM PST Tuesday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm.