Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 191252 CCA AFDSEW Area Forecast Discussion...CORRECTED National Weather Service Seattle WA 452 AM PST Sun Nov 19 2017 Corrected to include the aviation and marine segments. .SYNOPSIS...A cold front will continue to head towards the area from the northwest during the day today. This front will sweep across the region tonight. Expect locally heavy precipitation and gusty winds ahead of and with the front. A weak stationary front will result in some light precipitation over parts of the area on Monday. A strong Pacific Storm will bring locally heavy rain, gusty winds, and a warmer air mass to the region on Tuesday. && .SHORT TERM...
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Light precip was falling across the far northwest part of the CWA at this time. Expect the precip to gradually overspread the rest of the CWA during the day today. Meanwhile, the models have slowed down the timing of the cold front slightly, with the ECMWF solution being the slowest. The forecast was adjusted accordingly. There is a possibility that strong, gusty winds could persist over some areas through early this evening. Therefore, the end time of the Wind Advisory was extended until 6 PM PST, this evening. Will need to keep an eye on the wind speeds over the Admiralty Inlet area as there is a possibility that winds could reach low end warning criterion. Expect locally heavy precipitation on the coast and in the mountains ahead of and with the front. It still looks like about 2 to 4 inches of rain will fall on the Skokomish River Basin from this system. This should be enough to cause the Skokomish River to rise above flood stage (17.0 feet) later this afternoon or early this evening. The north Cascades could pick up about 7 to 12 inches of snow during the day today, mainly above the 4000 foot level. Look for the cold front to sweep across the area from the northwest tonight before becoming stationary over eastern Lewis County early Monday. The presence of the stationary front will result in a continued threat of light precip over the southeast part of the CWA during the day Monday. Otherwise, expect zonal flow across the region. A warm front will approach the area from the south Monday night for an increasing threat of precip. The front is anticipated to lift north across the CWA on Tuesday. Expect strong warm air advection to develop Monday night. This will cause the snow levels to dramatically rise. By Tuesday morning, the snow levels are forecast to be above 8000 feet across much of the CWA. The air mass will also be quite wet. This combined with the presence of a strong low level southerly jet will result in heavy rain on the southwest slopes of the Olympics and Cascades on Tuesday. There is a chance that the rainfall amounts could be a little more than currently forecast (generally 4-7 inches in the mountains, highest on the Olympic range). This amount of rain could lead to flooding on a few rivers besides the Skokomish. In addition to the possibility of significant rainfall on Tuesday, it will also become locally windy across the area. Needles to say, temperatures will be above normal. .LONG TERM... The medium range models were somewhat in agreement during this period. The main differences were in the details. Anticipate above normal temps to persist Wednesday and Thursday, with a few places possibly getting close to or breaking daily records on Wednesday. Temps are forecast to be closer to normal for the latter part of the period. Otherwise, expect wet and unsettled conditions to persist during this period. There will also be windy periods and the possibility of flooding on some area rivers.
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&& .AVIATION...
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The early morning 12Z terminal forecasts will show a slower evolution of the impacts of the incoming frontal system by 2-4 hours from the previous forecast based on the slower movement of the front seen on satellite imagery and in the latest model solutions. A strong cold front extending southwestward from the northern tip of Vancouver Island will spread down the British Columbia coastline this morning and move southward through Western Washington later this afternoon and this evening. The front will be preceded by heavy rain and strong southerly surface wind. Moderate westerly flow aloft will become strong today while strong south to southwest flow develops between 1500 and 6000 ft MSL midday today and continues until the front passes this evening. The air mass is stable and will become increasingly moist from the northwest this morning through this afternoon. There will be some drying at times to the northeast of the Olympics as strong flow aloft interacts with the terrain. Expect MVFR to VFR conditions this morning to rapidly deteriorate to low MVFR or IFR in precipitation and low ceilings from the northwest during the late morning to afternoon hours. Albrecht KSEA...Mid and high clouds early this morning will rapidly lower to BKN-OVC040 20Z-22Z. Rain is expected to develop around 00Z with ceilings lowering to around BKN015 02Z-06Z with conditions occasionally lowering to 4SM RA BR OVC008. Some improvement is expected after 06Z as the front pushes south of the area. South winds 8-10 kt will increase to 15G25kt about 22Z then will diminish after 06Z behind the front. Winds aloft will be quite strong this afternoon and this evening ahead of the front with winds at 2500 feet increasing to 19055-60KT and winds at 050 22070KT. Winds aloft will diminish rapidly behind the front. Albrecht
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&& .MARINE...
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A strong cold front extending WSW from the northern tip of Vancouver Island into the Pacific will slowly drag southeast down the British Columbia coastline this morning then push southeastward through the coastal and inland waters late this afternoon and this evening. In advance of the front, winds will rise. The wind increase will progress from north to south today with gales expected over most waters ahead of the front. Winds will rapidly ease and shift to the northwest behind the front this evening. High pressure will bring light winds to the waters on Monday. A strong warm front will push northward through the waters on Tuesday. A strong low pressure center developing well offshore will push northeast into Haida Gwaii on Wednesday. Albrecht
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&& .HYDROLOGY... There is still the possibility of minor flooding on the Skokomish River later this afternoon or early this evening due to about a six hour period of heavy rain on the southwest slopes of the Olympic range. Therefore, the Flood Watch remains posted. Strong warm air advection will develop Monday night, causing the snow levels to dramatically rise. By Tuesday morning, the snow levels are forecast to be above 8000 feet across much of the CWA. The air mass will also be quite wet. This combined with the presence of a strong low level southerly jet will result in heavy rain on the southwest slopes of the Olympics and Cascades on Tuesday. There is a chance that the rainfall amounts could be a little more than currently forecast (generally 4-7 inches in the mountains, highest on the Olympic range). This amount of rain could lead to flooding on a few rivers besides the Skokomish. Beyond Tuesday, it still looks like much of this period will be wet. The snow levels will remain atypically high (above 8K feet) through at least Wednesday night before lowering during the day Thursday. This means that the precip will fall primarily as rain in the mountains. As stated in a previous discussion, it appeared that this firehose will be a bit different in that instead of staying on for X amount of time, it will be more of an off and on type of deal. Thus, expect occasional heavy rain on the mountains and coast. Although rainfall amounts may not be significant for any given time period, the cumulative rainfall for the Wednesday through Thursday period might be enough to drive a few rivers above flood stage. Beyond Thursday, there might be residual flooding on a few rivers on Friday. Otherwise, flooding is not anticipated. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Weather Advisory above 4000 feet until 10 PM PST this evening for Cascades of Whatcom and Skagit Counties. Flood Watch in effect from 10 AM PST this morning through Monday morning for Mason County. Wind Advisory in effect until 6 PM PST this evening for the northwest interior. PZ...Small Craft Advisory for rough bar until noon PST Monday for Grays Harbor Bar. Gale Warning until 7 PM PST this evening for Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm- Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Gale Warning until 3 PM PST this afternoon for Admiralty Inlet- Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-East Entrance U.S. Waters Strait Of Juan De Fuca- Northern Inland Waters Including The San Juan Islands-West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 6 PM PST this evening for Central U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 6 AM PST early this morning for Puget Sound and Hood Canal. Gale Warning until 3 PM PST this afternoon for Puget Sound and Hood Canal. && $$ www.weather.gov/seattle An illustrated version of this forecast discussion can be seen at www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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