textproduct: Seattle/Tacoma

UPDATE

Showers and thunderstorms have subsided this evening as solar forcing wanes. Now, the anticipated cool down has begun as low- level onshore strengthens. Satellite imagery shows marine stratus marching stretching along the coast and marching inland as areas across W WA wake up to mostly cloudy skies Thursday morning. Low temps are forecast to the lower to upper 50s. The previous discussion is below along with an updated aviation section:

SYNOPSIS

An upper level ridge and thermal trough will begin to move inland today into Thursday. Onshore flow will begin to increase tonight into Thursday, bringing in some clouds in the morning, as well as much cooler temperatures. A frontal system will move across the region late Thursday into Friday, with widespread beneficial rain showers, and also the chance of isolated thunderstorms, especially with a post frontal convergence zone Friday afternoon north of Seattle. Breezy southwest winds Friday afternoon with gusts up to 25 mph are also likely. The pattern remains unsettled this weekend going into next week.

SHORT TERM /TONIGHT THROUGH FRIDAY NIGHT/

Main sensible weather concerns include revolve around lightning chances and heat today followed by unsettled weather tomorrow afternoon onward.

Latest day cloud phase RGB show increasing building with glaciating cloud tops in the northeast Olympics. Latest GLM and ground-based networks showed a few flashes/strikes over the last hour. Regional radar mosaic depicts greater coverage and convective activity along the upper eastern slopes of the WA Cascades. Current water vapor shows a shortwave trough ejecting into western WA at the base of the upper low located in the Gulf of AK.

Marginal instability in place late this morning with MUCAPEs > 500 J/kg per SPC mesoanalysis that aligns with current REFS soundings in the non-mountain areas of the sound and eastern Olympics. Current thinking is there will be potential for a few strikes this afternoon in the northeastern Olympics with main hindrance initially revolving around the dry air in the lower levels that needs to be overcome. Latest SPC mesoanalysis shows low RH within LCL-LFC, and similarly see this drier air in REFS soundings that will contribute to dry air entrainment. This seems most likely given how the initial updrafts/cell weaken. As such, best window for any lightning most likely within the window this afternoon from 2pm-7pm for cells/lightning chances from additional moistening from repeated cells/failed CI. Low confidence in crossing the sound at this time and reaching the I-5 corridor but bears monitoring with confidence 10-20% based on REFS, NBM, and last 4-5 HRRR runs. Current analysis supports the best chances for any lightning in the I-5 corridor being along far western Snohomish (albeit low chances).

Otherwise, today will be the last day of hot temperatures with highs 15-20 F above average and near certain moderate HeatRisk along western Pierce-King-Snohomish and far eastern Olympics. Cooling trend starts tomorrow with highs forecast 15-20 F cooler than today, except around 10 F cooler along the western Olympics. An initial weak marine push anticipated late afternoon/early tonight with a stronger push late tomorrow/tomorrow night with reinforcing onshore flow Friday as the main low drops to Vancouver Island by Friday. This will setup an unsettled pattern late Thursday night to Friday night as the low meanders southward along the offshore the WA Coast. Initial rain chances ramp-up along the Olympics by 5 pm Thursday spreading east to the sound and westward Cascade slopes 5pm-11pm PDT Thursday (chances for 0.01 or more 50-70% based on the REFS). On Friday, marginally instability in place will promote 15-20% chances for lightning across the Sound-western slopes of the WA Cascades with lesser thunder chances over the Olympics and west. 24-hr LPMM ending Friday afternoon (00Z) show 0.5 to 1.25 inches along the foothills and westward slopes of the central and Northern WA cascades and up to 1 inch in the western Olympics.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

The large scale pattern early Saturday will be characterized by a mid-level low over WA that will be slowly evolving east across the Northern Intermountain West to northern Rockies by Sunday. This evolution will continue to keep highs reading cooler than normal, around 4-8 F., this weekend. Well offshore upper-level ridge will be building behind this feature in the eastern North Pacific with mean forecast ECMWF EPS heights exceeding the 90th %tile. Strong signal for moderate winds within the Strait of Juan de Fuca with hourly probabilities showing enhanced chances for 22 kts or higher, 30-60%, Saturday- Saturday night with 50th percentile peak daily gusts, most likely outcome, around 30 kts. While lowlands and non-mountain areas will start to see chances taper off Saturday, chances will remain elevated in the Olympics and westward slopes into early next week under northwest flow. Elsewhere, low chances will prevail in the interior non-mountain areas through Wednesday (generally 5-15%).

A modest uptick in temperatures by mid-week, albeit with highs still reading 2-6 F below average, with overnight lows reading near average for this time of year.

ET

AVIATION

Onshore flow has started to infiltrate the coastal terminals with MVFR conditions. Onshore flow is expected to continue though the night and into Thursday. MVFR probabilities for the interior 15-60% from 11Z-18Z Thursday while the coast has IFR probabilities of 10- 30% through 18Z Thursday. Winds will remain out of the southwest 5- 12 kt, except for near the Strait of Juan de Fuca where winds will be 10-15 kts with gusts up to 25-30 kts through 20z Thursday. A cold front will move through late Thursday, introducing shower chances as early as 18Z for the coast and widespread shower chances by 03Z Friday.

KSEA...VFR through tonight. There is a 15-35% chance of MVFR stratus reaching the terminal between 12-18Z this morning, which will lift up to VFR later in the day (but remain broken or overcast). Shower chances will increase beginning at 00Z Friday onwards, bringing MVFR conditions. Winds expected to remain out of the southwest at 5-10kt through most of the period. West/southwest winds will pick up after 00Z Friday with gusts up to 20 kts.

HPR

MARINE

Higher pressure offshore will continue with lower pressure inland through tonight as a surface thermal trough continues to move inland. A strong onshore push tonight remains on track to produce gales in the central/east Strait of Juan de Fuca, as well as gusty winds over 20 kt in the Admiralty Inlet (with a small craft advisory there adjacent to the gale warnings). Seas and winds this afternoon have decreased in the coastal waters, which resulted in the small craft advisory being cancelled early. A cold/occluded front will push through the waters tonight into Friday, with widespread rain showers and also a 20% chance of thunder Friday afternoon for the interior waters. Ensembles maintain a high likelihood of widespread wind gusts over 20 kt going into Friday with the front. The unsettled onshore pattern likely continues into the weekend into next week, with winds at times remaining breezy for outer portions of the coastal waters, as well as the Strait of Juan de Fuca.

Seas decreasing to 4-6 ft today, and will hold around 4-6 ft through Saturday before increasing to 6-8 ft and 8-10 ft this weekend into next week.

HPR

FIRE WEATHER

Elevated fire weather concerns focused along WAZ654 and WAZ655, with non-mountain areas seeing min RHs in the teens yesterday, and in areas of the west slopes. While a slight uptick in RH is anticipated today, min RH forecast in mid-upper 20 percent across lower elevations. Meantime, this will occur in tandem with sustained winds 10 mph and higher. Latest NBM shows joint probabilities 60-80% that further increases confidence for this RH-wind combination. Locations of the west slopes of the central Cascades are also seeing gusts approach 20 mph with RH under 30%.

Looking ahead, cooler and wetter conditions will prevail tomorrow night-Saturday night that will limit fire weather concerns. However, the unsettled pattern will yield isolated, infrequent lightning threat on Friday with the chance of thunder around 15-20% across the sound to the west slopes of the WA Cascades.

ET

SEW WATCHES/WARNINGS/ADVISORIES

WA...Heat Advisory until 11 PM PDT this evening for City of Seattle- Downtown Everett / Marysville Area-Eastern Kitsap County- Eastside-Foothills and Valleys of Central King County- Foothills and Valleys of Pierce and Southern King Counties-Foothills and Valleys of Snohomish and Northern King Counties-Lowlands of Pierce and Southern King Counties-Shoreline / Lynnwood / South Everett Area.

PZ...Gale Warning until 6 AM PDT Thursday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 6 AM PDT Thursday for Admiralty Inlet.