textproduct: Seattle/Tacoma

SYNOPSIS

An upper level trough will move east of the area tonight. An upper level ridge will build offshore tonight and Monday then move inland Tuesday and Wednesday. The ridge will shift eastward Thursday and Friday with increasing low level onshore flow at the end of the week.

SHORT TERM /TONIGHT THROUGH WEDNESDAY/

Latest radar imagery shows scattered showers from this afternoon have largely fizzled out, although a stray shower over the Cascades remains a possibility this evening. Switching to satellite, starting to see some high level smoke already work into the area, mostly from Canadian wildfires. This will remain aloft and as such have little to no impact on air quality at this time and will mainly manifest as some haziness. Inherited forecast remains on track with no evening update needed. For additional forecast details, please refer to the Previous Discussion section below. 18

From Previous Discussion...Isolated showers continue to linger around the area early this afternoon, but will diminish with skies clearing this evening as an upper trough axis shifts east of the Cascades. An upper ridge offshore will gradually build into the region Monday into Tuesday. Thermally induced low pressure near the surface will expand into the area Monday night and Tuesday. This will lead to a pretty good jump in high temperatures areawide on Tuesday...with upper 80s to lower 90s from Seattle metro southward. This will begin to put a large portion of the interior from Seattle southward into likely Moderate HeatRisk. During this same period, north to northeasterly flow aloft, especially around 600 millibars and below, will begin to import regional wildfire smoke into the region. It will initially begin aloft on Monday, but smoke models are showing near surface smoke increasing by Tuesday. Moderate HeatRisk expands to most of the lowland forecast area on Wednesday with temperatures slated to warm a few degrees further as a 588-ish decameter ridge axis slides overhead and the surface thermal trough remains oriented just inland from the coast. However, with that said, the smoke factor is a wildcard and may knock 2 to 4 degrees off of high temperatures - models rarely handle this scenario well.

LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/

Low level onshore flow is expected to increase by later in the day on Thursday as thermal low pressure makes the jump toward the Cascades, but not before much of the interior sees another very warm day with Moderate HeatRisk continuing away from the immediate coast. Onshore flow increases further on Friday and the flow aloft turns more zonal. This is expected to lead to modest cooling, but temperatures, especially across interior areas, are expected to remain a little above average. Ensembles maintain the idea of some weak upper troughing arriving by next weekend. But the negative height anomalies are small which, as previous shifts have alluded to, simply point to a return to "normal" temperatures for early August along with Minor HeatRisk and dry conditions

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AVIATION

Northwesterly flow aloft into the region tonight as an upper trough continues eastward. Low level flow becomes northerly, brining some elevated smoke from regional fires. A shallow IFR/LIFR stratus layer is may from along the immediate coast early Monday morning, but is not expected to impact interior terminals. Northerly surface winds increase again during the day Monday.

KSEA...VFR conditions under generally cloud-free skies this evening. Some elevated smoke from regional fires remains nearby during the period. Otherwise, surface winds easing overnight, increasing again tomorrow from the north at 9 to 12 kt by afternoon.

MARINE

A strong surface ridge will remain centered well offshore into midweek as thermally induced low pressure expands northward across the interior of Western Washington. The thermal trough will shift eastward toward the second half of the week for increasing onshore flow. Winds will generally remain under small craft advisory levels into Wednesday. However, the presence of the thermal trough over Western Washington will allow winds over the outer coastal waters to approach low end advisory levels each afternoon/evening Monday and Tuesday. Increasing onshore flow late in the week may necessitate headlines for westerlies in the central/east strait.

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FIRE WEATHER

A strong warming trend will start on Monday and extend into at least mid-week with Wednesday expected to be the hottest day of the stretch. While this will result in low daytime RH values throughout the CWA, the Cascades, the Cascade valleys, the adjacent lowlands of the Chehalis river valley and the south Puget Sound area will be of particular concern. Min RH values each of those days are expected to slip into the teens. Wednesday will be of particular concern as Min RH values could get as low as the low teens to upper single digits. No significant winds or instability is expected during this time frame, so Red Flag concerns are minimal. However, with RHs this low, any wind will contribute to fire spread and as such, maximum caution is emphasized at least into mid-week. Thursday sees some slight cooling and thus a slight nudge upward in RHs, but not enough to end concerns. Friday and beyond will see a much more significant rebound to daytime RH values as onshore flow returns.

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 2 AM PDT Monday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 3 AM PDT Monday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm- Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm.