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Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
844 FXUS66 KSEW 162210 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 310 PM PDT Tue Jul 16 2024 .SYNOPSIS...Hot and dry weather will continue for the rest of Tuesday. An upper level disturbance will pass through Wednesday with a slight chance of scattered thunderstorms along the Cascades, Olympics and coastal waters, along with some cloud cover in the morning. Due to very light precipitation amounts and the chance of gusty winds with the thunderstorms, there is increased fire danger for the Cascades and Olympics during the day Wednesday. Remainder of the week stays warm and dry, with the potential for cooler temperatures early next week with an upper level low passing by to the northwest of Washington. && .SHORT TERM /TONIGHT THROUGH FRIDAY/...Upper level pattern continues to show upper level ridging, with the axis just to the east now over western Montana, and southwest flow aloft over much of the state. At the surface a thermal trough continues to build up along the coast of the state. Satellite remains relatively clear over the western part of Washington, with a few mid-level cumulus clouds over the north Cascades, and a cloud deck of altocumulus over the coastal waters. Temperatures have already soared into the mid and upper 80s in south Puget Sound this afternoon (with a few 90s in the Southwest Interior). Lows tonight will be noticeably warmer than this morning, with more widespread low 60s and a few upper 50s (warmest temperatures east of Puget Sound over to the Cascades, and the Olympics). Heat risk remains predominately orange (moderate) across the region (although a couple isolated areas may see red/major heat risk). The pattern changes slightly for Wednesday as a small upper level trough moves up into the state from Oregon/Pacific. Cloud cover will increase as the trough approaches early Wednesday morning. An unstable airmass in place (via the thermal trough) will help bring ingredients in place (CAPE of 800-1600 J/kg in the Cascades & Olympics, limited PWAT with 1 inch, and up to 100 m2/s2 of 0-3 km SRH) for a slight chance of elevated scattered thunderstorms. The best chance of seeing a storm will be over the coastal waters early Wednesday morning, and the Cascade/Olympic foothills Wednesday afternoon and evening. No severe weather is expected, but given the drier nature of the storms, there is an increase in fire danger due to the potential for several cloud to ground lightning strikes (see the fire discussion below for further details). The high temperatures for Wednesday will depend on how much marine air/stratus makes its way into the sound during the morning. The model spread in high temperatures remains relatively large. If the cloud cover rolls in Wednesday morning (most likely scenario), highs will range from the low to mid 80s. If there is no cloud cover in the morning, there is a 25% chance (per NBM) that areas south of Puget Sound may reach low 90s. Heat risk for Wednesday will be a mix of minor (yellow) and moderate (orange), but may increase as temperature confidence changes (based on cloud cover Wednesday morning). Thursday and Friday will remain dry as weak ridging builds back into the region Thursday. A few models have a weak trough passing on Friday, but at this time it appears to pass through dry. High temperatures remain in the low to mid 80s for inland areas, with highs in the 60s along the coast. Light southwest winds Thursday should help make the air feel cooler to persons outside. Lows will bottom out in the mid and upper 50s both nights as well. .LONG TERM /SATURDAY THROUGH TUESDAY/...The pattern is expected to remain warm and dry through the weekend. Saturday appears to be the warmest day with the ridge from Friday builds over eastern Washington (widespread upper 80s and low 90s). The forecast cools down slightly going into next week as the ridge moves eastward, and a low passing to the northwest returns stronger onshore flow. There are more ensemble/deterministic models in agreement with this pattern change, so high temperatures as a result will drop into the low 80s Monday, and potentially the upper 70s Tuesday. The threat of precipitation is very low in this period. HPR && .AVIATION...Southerly flow aloft with an upper ridge across the Pacific Northwest. An upper trough will slide northward across Oregon and Washington on Wednesday. Clear skies this afternoon with lingering stratus along the coast. Stratus will spread inland again tonight, mainly staying west of Puget Sound, with lower LIFR/IFR cigs along the coast and perhaps in the Strait of Juan de Fuca. In addition, mid to high clouds will increase early Wednesday morning moving northwards into the afternoon. Thunderstorms are forecast to develop over the Cascades and Olympics Wednesday afternoon as well, but expected to remain over the mountains. W/NW winds into this evening will become light later tonight and a bit more southerly into Wednesday morning. .KSEA...Clear skies through this evening. Increase VFR mid to high clouds later tonight into Wednesday morning. Thunderstorms are expected to develop and remain over the Cascades Wednesday afternoon. N/NW winds through this evening are expected to transition to light S/SW Wednesday morning. JD && .MARINE...Light northerly onshore gradients strengthening Wednesday into Wednesday evening. High pressure will remain over area waters Thursday through the weekend with varying degrees of onshore flow. A slight chance of thunderstorms early Wednesday morning over the coastal waters may result in locally gusty winds. Otherwise, strongest winds continue to be expected over the Central and Eastern Strait of Juan de Fuca Wednesday afternoon into early Thursday morning - with Small craft advisory westerlies. Seas 4 to 6 feet will linger generally over the coastal waters through the forecast period. && .FIRE WEATHER...The overall pattern continues to be driven by very persistent high pressure over the Pacific Northwest with warm and dry conditions. This has resulted in elevated overall fire weather conditions over the region. Low-level onshore flow across the region has maintained the marine layer helping limiting minimum relative humidities and supporting good overnight relative humidity recoveries. The primary issue of concern remains the potential for thunderstorm development Wednesday as an upper-level disturbance moves across the region. This will enhance instability and provide for thunderstorm development over the Cascades and Olympic Mountains - resulting in critical fire weather conditions. High resolution ensembles, like the HREF have shown uncalibrated 6 hour probabilities of lightning as high as 65% along and near the Cascade Crest Wednesday afternoon and again Wednesday evening. Similarly, the HREF has produced 6 hour probabilities of lightning as high as 40% over the core of the Olympic mountains during the afternoon and again over the evening hours. Given the overall unstable conditions, fuel conditions and lightning risk with very low chances of wetting rains a Red Flag Warning has been issued for zones 658, 659, 652 and 661. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Red Flag Warning from 8 AM to 11 PM PDT Wednesday for East Portion of the Olympic Mountains-West Portion of the Olympic Mountains-West Slopes of the Central Cascades Generally above 1500 Feet-West Slopes of the North Cascades Generally above 1500 Feet. PZ...None. && $$