000 FXUS66 KSEW 282037 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 137 PM PDT Thu Sep 28 2023 .SYNOPSIS...An unsettled Thursday continues with showers, and a couple isolated thunderstorms continuing to move through the region. Heavy rain is the most likely hazard with the strongest storms, but lightning and graupel cannot be ruled out. The upper and surface disturbance move out of the region Friday, with high pressure building in over the weekend. Skies will clear on Saturday and Sunday, but temperatures will remain cool with highs in the 50s and 60s, and lows dropping to the 40s, with a couple 30s in the mountains. The next chance of rain returns early next week with the next disturbance. && .SHORT TERM /TONIGHT THROUGH SUNDAY/...The pattern remains a broad upper level trough centered over the northwestern CONUS, with a 500 mb shortwave trough swinging through WA from Canada today. Strong positive vorticity advection with the shortwave, along with CAPE values in the 200-300 J/kg range, have aided in the development of showers and a couple thunderstorms across Puget Sound late this morning and this afternoon. Heavy rain is expected with the strongest storms/showers, with QPF amounts this afternoon through tomorrow around a quarter of an inch to a half an inch. Lightning and graupel remain possibilities with the strongest storms that are able to elevate (so far have just seen the lightning). For all remaining areas not affected by convection, off and on showers remain possible through the remainder of today into Friday morning. Higher elevations may see some snow with the precipitation activity this afternoon, mainly in the north Cascades and around Rainier. For Friday into Sunday, the upper level trough slides eastward, replaced by an upper level ridge, and surface high pressure. Heights will build in the region, and skies will clear from Friday afternoon into Sunday. Any overnight and morning clouds that do develop should clear by late morning. Winds behind the trough will become northwesterly, but generally remain light at 5 to 10 mph. Temperatures during this period remain steady, with highs in the 50s and 60s expected region-wide. Lows will drop into the 40s, with a couple 30s possible in higher elevations. The next disturbance arrives Sunday night into early next week, bringing the next chance of precipitation into the region. See the long term section for more details. HPR .LONG TERM /MONDAY THROUGH THURSDAY/...The pattern next week remains somewhat uncertain with disagreements in the deterministic, and probabilistic models. Most models are pinpointing an upper level disturbance for Monday into Tuesday, with precipitation chances. The ridge is expected to broaden over the Pacific Monday with a trough swinging through Monday into Tuesday. With some uncertainties again with timing of the trough pattern (in terms of persistence into the week), the confidence for rain is highest Monday into Tuesday, and much lower Wednesday into Thursday. Amounts appear to be highest in the Cascades with a half an inch of QPF possible Monday into Tuesday. Remaining areas appear to receive around a tenth of an inch of QPF. As confidence increases regarding the timing of the disturbance, will include updates regarding the details. Again, temperatures remain steady with highs in the 50s and 60s, and lows in the 40s and 50s. HPR && .AVIATION...Westerly flow aloft with upper level troughing. Surface flow across Western WA is west to southwesterly around 5 kt. Scattered showers are present across the region. An area of convergence and limited but sufficient instability has maintained a line of more intense showers and some occasional lightning strikes in a line north of Bremerton to central Snohomish county. Showers and, possibly, an isolated thunderstorm will continue until the evening. Winds will shift to northerly tonight but speeds will decrease tonight. With lighter winds, clearing skies overnight, and sufficient moisture from the recent rains, fog and low stratus is going to be of concern from around 12-18Z for the Puget Sound area terminals. VFR conditions are expected tomorrow with northerly winds 5-10 kt, potentially gusty at times. KSEA...Scattered showers just north of the terminal will hang around for the next hour. Ceilings are hovering around 2500-3500 ft, but will begin to rise to VFR this evening. Southwesterly winds around 10 kt will switch to northerly around 01Z and begin to decrease overnight. Low stratus and fog look to develop tomorrow morning from around 12-18Z. IFR conditions are likely (30%), with periods of LIFR possible (20%). Ceilings lift to VFR after 18Z with northerly winds 6-10 kt. LH && .MARINE...Light onshore flow is prevailing over the coastal waters. Winds have begun to come up in the Strait of Juan de Fuca. A push of westerlies are expected in the Central and Eastern Strait this afternoon and evening, prompting a Small Craft Advisory through 8 PM tonight. A weak low pressure system will swing south into the Oregon Coast tonight into tomorrow morning. Northwesterly winds look to pick up down the Strait of Georgia Friday evening, which may lead to occasional gusts over 20 kt north of the San Juan Islands. Following, high pressure will build back into the coastal waters into the weekend. The associated increase in northerly flow over the outer coastal waters may reach Small Craft level speeds on Saturday. Winds look to remain west to northwesterly 5-15 kt through the beginning of next week. Seas from 7 to 9 ft slowly easing to 4 to 6 ft beginning Friday. Seas may temporarily rise to 8 to 10 ft in the outer waters Saturday. Long period swell from the northwest will elevate seas again to 8 to 10 ft on Tuesday. LH && .HYDROLOGY...No river flooding is expected over the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 8 PM PDT this evening for Central U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 8 PM PDT this evening for East Entrance U.S. Waters Strait Of Juan De Fuca. && $$
NWS SEW Office Area Forecast Discussion
Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather