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Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
807 FXUS66 KSEW 152157 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 257 PM PDT Mon Jul 15 2024 .SYNOPSIS...High pressure is expected to remain in the region for the majority of the week. A thermal trough will build along the coast, and will cause temperatures to increase Tuesday and Wednesday into the upper 80s and low 90s. With instability in the Cascades via a small upper level trough Tuesday into Wednesday, there will be a slight chance of an isolated thunderstorm for any elevated convection that takes place. && .SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery this afternoon shows the majority of the marine push retreating to along the coast (with some stratus remaining west of the Washington coastline). A small upper level ridge remains over Washington this afternoon. High temperatures today took a penalty due to this marine push, with most areas just now getting into the 70s this afternoon. Most places will see highs top out into the mid and upper 70s (could see an 80 in the Southwest Interior but the chances have decreased with the early morning marine push). By Tuesday, a thermal trough will begin to build along the coast. This will help bring warm air down to the surface from aloft with southwest flow behind the ridge sliding into Idaho/Montana Tuesday. With relatively light winds, and no marine push for Tuesday morning, this will cause high temperatures to sear into the upper 80s and low 90s across the region. The highest temperatures will be in the Southwest Interior. Heat risk will remain moderate (orange) for the majority of lowland areas, with a couple isolated areas in the Seattle metro seeing major (red) level heatrisk. Given the short duration of this heatwave (compared to the one a week ago) and low temperatures expected to drop into the 50s (with Tuesday night only being the best chance of lows in the low 60s) - will not be issuing any heat headlines at this time (although all persons outdoors should continue to practice heat safety, such as taking breaks in air conditioning, and staying hydrated). There`s an additional shortwave trough/mid- level low tracking into Oregon and Washington Tuesday night into Wednesday. This will create some instability over the Cascades late Tuesday night into Wednesday. There`s a conditional 15% chance of an isolated thunderstorm for any convection that is able to initiate along this corridor. Coverage again is expected to remain isolated, and precipitation is expected to be light with any pulse storms that do develop. .LONG TERM /THURSDAY THROUGH SUNDAY/...The pattern largely remains warm and dry with high pressure through the weekend (although some ensembles show the potential for troughing northwest of the state over B.C. Canada). Nevertheless the region will continue to see high temperatures in the 70s and 80s, with another shot of 90 by Saturday with heights building overhead via upper level ridging. Low temperatures will still drop into the 50s during this dry period. HPR && .AVIATION...High pressure over the NW brings dry and stable weather through Tuesday. The flow aloft is light and variable. The low level air mass remains moist and stratus clouds will impact the coast overnight for IFR to MVFR conditions. The strait and south sound may see patchy low clouds by 12z for brief IFR to MVFR conditions as well. Low clouds will burn back to the coast by the afternoon for VFR conditions. 33 KSEA...Northerly winds around 5 kt with VFR conditions. 33 && .MARINE...High pressure over the NE Pacific will maintain onshore flow across western WA this week. A stronger onshore push may bring Small Craft Advisory winds to the strait on Wednesday and again on Thursday. 33 && .FIRE WEATHER...High pressure will persist over the Pacific Northwest for continued warm and dry conditions, with low level onshore flow providing good relative humidity recoveries through majority of the week. There is a main period of concern, Tuesday night into Wednesday, for some southerly flow to provide ample instability for thunderstorm development across the Cascades. Given the dry nature of the fuels and the threat for lightning being present, went ahead and issued a Fire Weather Watch for zones 658 and 659. Relative humidity values will be in the mid 30s through the day, but cannot rule out extremely localized pockets of 25-30. It is worth noting that especially with a pattern like this, with our fuels already being so receptive, all it takes is a rogue lightning strike from a thunderstorm to light these fuels. Mazurkiewicz && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$