![National Oceanic and Atmospheric Administration](/build/images/header/noaa.d87e0251.png)
![National Weather Service](/build/images/header/nws.4e6585d8.png)
![United States Department of Commerce](/build/images/header/doc.b38ba91a.png)
Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
631 FXUS66 KSEW 181655 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 955 AM PDT Sun Aug 18 2024 .SYNOPSIS...A strong upper level low will slowly begin moving northeast off the coast of Washington today. Showers with a few thunderstorms will continue early Sunday morning along and east of the Interstate 5 corridor. More showers and thunderstorms will develop west of Interstate 5 late Sunday morning, and spread across portions of the coverage area Sunday afternoon. While a couple of the storms may be strong, severe weather is not expected. The remainder of the work week remains unsettled with more chances for showers and thunderstorms as upper level troughing continues. && .SHORT TERM /TODAY THROUGH TUESDAY/...No significant changes made to the forecast this morning. The previous discussion can be found below along with updated aviation and marine discussions: Same upper level low from Saturday will slowly begin moving northeast, from off the coast of Oregon to off the coast of Washington during the day. A small embedded shortwave trough will move from Oregon into Washington during the late morning and afternoon. This will help push a jet streak from Oregon into Washington during the day. An associated vorticity max will also aid in the jet streak for another chance of showers and thunderstorms late Sunday morning and afternoon. The expected scenario (based on HREF and CAMs) is for an area of scattered showers and thunderstorms to develop in far Northwest Oregon/Southwest Washington during the late morning hours as the vorticity max moves up the coast into a modestly unstable airmass. The storms will likely increase in coverage with additional individual cells or multicell clusters. SRH/hodograph profiles show quick movement again to the north with activity that develops in the Southwest Interior. Most of the activity will track north into the Olympics into Canada. Some models show convection east of I-5, but the confidence is lower. The best chance for convection in this region is primarily north of I-90 into the North Cascades. Some of the instability will recover for afternoon convection, as temperatures rebound into the mid and upper 70s, and dew points remain in the upper 50s to low 60s. CAPE values increase to 200-300 J/kg for areas east of I-5, and 300-500 J/kg west of I-5. PWATs still remain relatively high with anomalies between an inch to an inch and a quarter. DCAPE takes a hit (dropping to 500 J/kg), as well as lapse rates dropping to 6.2 to 6.5 degrees/km. Taking this all into account, the thunderstorms will generally produce moderate to heavy rain, with a couple stronger storms potentially producing small hail and gusty winds. QPF totals will be highest in the Olympics, with a quarter to a third of an inch of rain possible. Amounts are light in remaining areas, with several hundredths to a tenth of an inch of rain possible. Lightning is not expected to be as widespread as last night, but there will be the danger of fires being started due to lightning strikes, and not enough rain to extinguish the fires (see fire discussion). Monday through Tuesday: the upper level low will move northeast into Canada, with troughing still remaining just off the coast. The majority of Monday will remain dry (some clearing will take place late Sunday evening/Monday morning, so will have to watch for areas of fog). The trough however will begin to slide east over the state on Tuesday. This will return the chance of showers across the entire coverage area late Monday night into Tuesday. There is a slight chance of an isolated thunderstorm over the Olympics, otherwise the thunderstorm threat remains low for Tuesday. High temperatures will cool down from Monday into Tuesday, with temperatures ranging from the mid 70s to the low 70s. Lows will remain mild with widespread mid and upper 50s. .LONG TERM /WEDNESDAY THROUGH SATURDAY/...The likelihood of troughing remaining over the northwest remains high through next weekend in the ensembles/deterministic models. Another large-scale upper level low will track south from Canada into Oregon Wednesday through Saturday. The models show it attempting to cutoff, but then absorbing back into the main meridional pattern over Canada. Wednesday will likely see widespread showers with scattered thunderstorms across the region, with a break Thursday afternoon as the flow temporarily becomes neutral behind the low. Another broad shortwave may bring showers across the region again Friday into the first part of Sunday. The ensembles early next week show the potential for a zonal to ridge flow pattern taking place of the troughing (which may warm up temperatures from the expected low to mid 70s Wednesday through Saturday). HPR && .AVIATION...Southerly winds continue this morning with isolated shower, mainly along the coast. A few pockets of lower ceilings but should trend toward VFR this afternoon. Another round of isolated thunderstorms possible this afternoon over the coast and interior - although far less coverage compared to last night. KSEA...VFR today. Southerly winds may be breezy at times this afternoon. There will again be a low potential (10%) for a thunderstorm to reach the terminal today but confidence is low. 33 && .MARINE...Low pressure remains well offshore with southerly winds developing over the coastal waters. Isolated thunderstorms possible over the coastal waters and interior. Weak west pushes through the Strait the next several afternoons but widespread advisory conditions are not expected. Elsewhere winds become increasingly southeast through Monday with low pressure remaining centered over the outer coastal waters. With stronger winds in the lower atmosphere and some lingering instability near the low center, could see a few occasional gusts to 25 kt over portions of the coastal waters on Sunday, but these don`t appear frequent or widespread enough to warrant an advisory at this time. Seas will build a bit through the early part of the week in response to the persistent southerly winds and remain choppy but only building to around 6 feet. Diurnal west pushes through Strait of Juan de Fuca also likely through the week each afternoon and evening, possibly approaching advisory strength around midweek. 33/Cullen && .FIRE WEATHER...An upper level low will continue to remain in the region Sunday into Monday. Showers continue in Puget Sound and the Cascades this morning with isolated thunderstorms along the crest. Lightning has been frequent with these morning storms, and with the speed of the storms, the rainfall may not be enough to extinguish any fires that start. Another disturbance is expected to trigger scattered thunderstorms this afternoon from the Southwest Interior into the Olympics, and potentially also including the North Cascades. Rainfall amounts with afternoon convection will be significantly lower (best chance of wetting rain in the Olympics), but the threat of lightning will be significantly lower. Still could see gusty winds if a storm or two get strong enough. HPR && .HYDROLOGY...There will be an additional round of showers and thunderstorms late Sunday morning and afternoon. Activity is primarily expected west of I-5, but a couple pop up cells are possible east of I-5, and north of I-90. The highest QPF totals will be in the Olympics, with a quarter to a third of an inch of rain from storms moving through. Remaining areas will only see several hundredths to a tenth of an inch of rain. The threat for heavy rainfall triggering a debris flow/flash flooding will be low with the afternoon activity. The remainder of the week will see several chances of showers and thunderstorms. Precipitation amounts remain low except for possible convection Wednesday in Western Skagit/Whatcom Counties. HPR && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$