textproduct: Seattle/Tacoma

SYNOPSIS

Cool and unsettled conditions will continue across Western Washington for the first half of the week as a pair of systems move through the region. A pattern shift toward dry and considerably warmer conditions is expected late in week as strong high pressure aloft builds into the region.

SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/

We've gotten a brief reprieve from precipitation this afternoon across the area as weak shortwave ridging slides onshore ahead of the next approaching system. As mentioned in the update this morning, model solutions are trending further northward with the surface low which implies that much of the forecast area will see a soggy stretch of weather Monday afternoon into much of Tuesday. Unlike this past Saturday, we're looking for stratiform precip to spread onshore during the day Monday. Between Monday afternoon and Tuesday morning, lowland areas from around Seattle southward could see 0.50" to 0.75" of rain with higher amounts in the Cascades. Snow levels will be higher this round keeping any accumulating snow only in the highest of elevations. Onshore flow ramps up on Tuesday as the trailing upper trough moves onshore. This is expected to generate a fairly potent convergence zone. As a whole, convective indices are pretty minuscule across the CWA on Tuesday, but it wouldn't be surprising to get the well-known one clap wonder in the PSCZ. Temperatures throughout the short term forecast will remain on the cool side. As for the cool temperatures: to quote a song title by The Sundays, "Here's Where the Story Ends".

LONG TERM /WEDNESDAY THROUGH SUNDAY/

An upper level trough exits the area on Wednesday making it something of a transitional day. Clouds and a few, mainly mountain, showers will be present early in the day. The trend toward clearing and modest warming will be underway by Wednesday afternoon. An upper level ridge centered offshore begins to build into the region on Thursday. Some residual upper troughing over the western Canadian provinces will temper it's strength initially, but only looks to temporarily slow what now looks like an inevitable climb in temperatures to well above seasonal normals.

By the weekend, a high amplitude ridge is expected to be centered just offshore with 500 millibar heights over Western Washington in the mid to upper 580s decameters. Near the surface, thermally induced low pressure will expand into the region and the low level flow is expected to turn offshore Saturday night into Sunday as the thermal trough shifts toward the coast. Broadly speaking, a large portion of the forecast area will be looking at potential high temperatures in the mid 80s to mid 90s by the early portion of next week. Probabilistic guidance is pointing towards likely Moderate HeatRisk for a substantial portion of the lowlands (coast included) by Sunday. Record highs for Sea- Tac for Sunday and Monday are well within reach at 86 and 88, respectively. The record high on Monday has stood for 63 years.

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AVIATION

Northwest flow aloft over Western Washington will back to southwesterly again tonight as the next in a series of upper troughs and associated frontal systems approaches the area. High and mid level moisture ahead of the next system will spread onshore this afternoon and tonight. VFR ceilings are expected to prevail areawide into this evening. Ceilings will deteriorate back to MVFR in increasing rain along the coast mid to late Monday morning with said conditions gradually spreading to most interior terminals 21Z Monday onward.

KSEA...VFR ceilings expected into this evening with ceilings gradually coming down overnight. MVFR ceilings in rain are expected to develop around 21Z Monday. Surface winds S/SW 7 to 10 knots gradually ease this afternoon and veer light W/NW for a period this evening. Winds are expected to become light E/SE late tonight before backing northerly 9 to 14 knots late Monday afternoon.

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MARINE

A slow moving front and associated sub-1000 millibar surface low will move into the coastal waters late tonight and early Monday then gradually drag onshore and weaken late Monday afternoon and evening. Onshore flow will increase behind this system on Tuesday and continue into Wednesday as a surface ridge rebuilds into offshore and coastal waters. Onshore flow weakens and turns more northerly late in the week as thermally induced low pressure expands northward into Western Oregon.

Small craft advisory winds are expected over the coastal waters late tonight and on Monday with the incoming front and surface low. The onshore flow on Tuesday looks sufficient to generate small craft advisory winds for nearly all of the interior waters and possible gales in the central and east strait. Onshore flow relaxes enough on Wednesday to keep small craft advisory westerlies isolated to the strait before quieter conditions take hold across all waters at the end of the week. Coastal seas are expected to remain below 10 feet.

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FIRE WEATHER

As upper level ridging builds into the area starting Thursday, conditions, including fuels, will need to be monitored as temperatures are expected to climb to above normal values by the end of the extended forecast period. Given the steep climb in temperatures, extra caution would be advised in any planning during this time. Current NBM minimum relatively humidity values are running in the 25-35 percent range by day 7 across portions of the area. These are likely a tad higher than what will transpire. If the cross Cascade gradient goes negative by Sunday as advertised in a number of models, humidity values dropping into the teens in the Cascades valleys and portions of the Southwest Interior become a distinct possibility.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory from 5 AM to 1 PM PDT Monday 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 11 PM this evening to 1 PM PDT Monday 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.