textproduct: Seattle/Tacoma

SYNOPSIS

An elongated upper-level low offshore SE AK to Vancouver Island will consolidate and evolve to be offshore the WA coast tonight. Its attendant surface low will be weakening as it evolves east into BC with a broader low developing near the northern Continental Divide. This upper-level low will settle in the western CONUS this weekend with slow eastward movement across the Pacific NW-Northern Rockies with the jet stream south of WA. Meanwhile, strengthening upper-level high pressure will build Saturday-Sunday in the eastern North Pacific with a building surface high in the offshore waters. This amplified pattern is expected to persist with ridging west and troughing east before the next upper-level disturbance drops southward into WA late Wednesday-Thursday.

ET

SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/

Latest Day Cloud Phase RGB shows a few areas of clumping- agitated cu with evidence of glaciating cloud tops across northern interior, more scarce south over King and Pierce county. Latest water vapor imagery shows multiple embedded disturbance with the complex low offshore BC with its primary shortwave rotating eastward over Vancouver Island and another moving in southeast OR. Primary concerns revolve around lightning risk this evening.

Latest WoFS runs continue to support a low non-zero chance that bears watching with respect to lightning. This could be attributable to a northerly developing convergence zone focused western Skagit and Whatcom before drifting south after 00-03Z today into western Snohomish. This forcing and enhanced lift will combine with marginal instability with MUCAPEs per REFS and HREF around 200-300 J/kg in the western Interior. Latest WoFS show a low flash extent density signal based on the 90th %tile and ens max forecasts across Whatcom and Skagit through its run ending at 01Z. WoFS ens max shows a few simulated storms forecasting lightning along the western slopes of the central/northern WA Cascades, albeit lower chance/worst case. Any lightning is expected to be infrequent with all neighborhood probabilities showing scant chances for greater than > 10 flashes. Thus the main threat is infrequent CG strikes, likely sporadic and isolated from an embedded storm, with the larger neighborhood emphasizing Whatcom and Skagit. This scenario is bolstered by low 3-hr NBM pot thunder (10-15%) and the meager REFS lightning density prob across the aforementioned northern interior. NSE-wise, very shallow instability with limited quantity, if any, tapping into -10C based on forecast soundings. As such, increased confidence in an isolated, infrequent lightning outcome. With loss of daytime heating, threat for lightning should decline after 2-4Z over time.

Otherwise, largely light precip is expected for the central/south sound and southern Interior with greatest chances for half inch or more from repeated showers across the Olympics and northern Interior/Everett north, and west slopes. Of which, HREF and REFS LPMM support 24-hr totals exceeding 1 inch in the Olympic park and slopes of the Snohomish-Whatcom. 10-20% chance for more than 1.5 in isolated spots of the higher terrain. A largely drying trend will then take hold outside low mountain precip chances this weekend as the upper-level low evolves the northwest CONUS into MT. This will keep highs cooler than normal, generally reading 4 to 8 below, with largely average highs outside the eastern Olympics.

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LONG TERM /MONDAY THROUGH FRIDAY/

The large scale pattern early Monday will be characterized by a deep upper-level low over the Northern High Plains and amplified trough over the western CONUS with amplified ridge over the eastern North Pacific. This unsettled cool pattern will keep periodic mountain precip chance with drier conditions largely over the lowlands/interior with limited change in highs and lows. Confidence growing with the next upper-level trough diving southeastward into the region around Thursday based on the cluster tool (confidence 80%). This will spread precipitation across the lowlands and mountains, and promote a slight uptick in highs, albeit still reading below normal. Latest NBM probs show modest chances for 24-hr QPF greater than 0.5 inches across the Olympics and central/northern Cascades ending Friday (30% or more).

ET

AVIATION

Scattered showers with a mix of VFR and MVFR cigs this afternoon, with lower visibility in shower activity. Not expecting improvement at the coast this afternoon and MVFR cigs expected to prevail well into Saturday evening. Another round of MVFR cigs is expected tonight/Saturday morning for the interior terminals, with latest probabilities 30-55%. A convergence zone is expected to develop this evening in Skagit County after 00z and drift southwards into Snohomish County. There is good agreement among guidance that the convergence zone will impact PAE and will not impact SEA/BFI. In addition, there is a slight chance for thunderstorms over the Cascades (20-25% chance) and within the convergence zone (15-20% chance). Any thunderstorm that does develop will bring gusty/erratic winds, lightning, and heavy rain. Breezy southerly winds this afternoon along Puget Sound terminals and the coast, with speeds 10- 15 kt and gusts 20-25 kt. Northerly winds expected within convergence zone activity.

KSEA...VFR cigs this afternoon with the chance for scattered showers. No impacts expected from the convergence zone at the terminal, as it will remain near PAE. There is a slight chance (15%) for thunder this evening; however confidence is not high enough to include it in the TAF. Guidance has a 40-55% chance of ceilings deteriorating overnight between 10z-19z. There is a slight chance of IFR cigs between 13z-16z. Improvement to VFR expected in the afternoon. Breezy southerly winds this afternoon through this evening with gusts to 20 kt. Winds ease overnight to 6-8 kt.

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MARINE

A cold front continues to push inland, bringing breezy winds to Puget Sound and the Central and East portions of the Strait. A Small Craft Advisory continues for these locations. Broad high pressure over the northeast Pacific will starting building eastward into Washington's waters on Saturday. The dominant weather feature through the middle of the week will be high pressure over area waters. This will bring increased northerly flow over the coastal waters with increased chances (70-95%) for small craft winds, particularly over the outer coastal waters. With the prolonged northerly winds over the coastal waters, seas will build to 10-15 ft through Tuesday. A Small Craft Advisory is set to go into effect Saturday afternoon and will likely be extended into next week. High pressure will weaken on Wednesday and seas will gradually decrease on to 8-11 ft and look to fall below 10 ft in the afternoon/evening. In addition, weak high pressure will also bring diurnal westerly pushes through the Strait of Juan de Fuca. Latest guidance suggests a strong push will occur Saturday evening and linger into Sunday morning, with the latest guidance at 70-85% chance of small craft winds.

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FIRE WEATHER

Low concerns overall with threat tied to low-end infrequent CG lightning potential. Greatest risk across west slopes of the northern WA Cascades and Whatcom-Skagit counties as a convergence zone sets up and drifts south early this evening into Snohomish. Overall confidence in occurrence is low (10-15%). Threat would be tied to any areas that don't receive wetting rains. Cool and unsettled conditions continue tomorrow. Overall scant concerns going forward with little lightning risk and high afternoon humidity.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory until 5 AM PDT Saturday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 8 PM PDT this evening for Puget Sound and Hood Canal.

Small Craft Advisory from 2 PM Saturday to 5 PM PDT Sunday 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.