FastCast—Wednesday, Aug. 17 to Friday, Aug. 19:
An interesting couple days is ahead for the Pacific Northwest. On Wednesday and Thursday, expect hot weather around the area. Wednesday will have lowland highs in the mid to upper 80s, with some areas (likely east of I-5, the foothills, and the Olympia area) reaching the low 90s. Overnight lows will only reach the low to mid 60s. Thursday will be the hottest day, with temperatures in the lowlands reaching the low to mid 90s, hottest in the foothills and Olympia area. The Portland area will reach the upper 90s to low 100s, and Eastern Washington will reach the low to mid 100s, with isolated spots reaching the upper 100s. Lows in these areas will be in the mid 60s to mid 70s. Monsoon moisture will move north into the Pacific Northwest from Wednesday through early Friday. This will bring muggy and unstable conditions, with dewpoints in the low to mid 60s west of the Cascades and a chance of thunderstorms. The highest chance of thunderstorms will be in the Cascades on Wednesday afternoon and from Thursday afternoon through Friday morning. Very little (if any) rain is expected in these storms, elevating the fire risk, hence the Red Flag Warning in effect for the Cascades. There is a chance that a stray thunderstorm may move into the lowlands between Thursday afternoon and Friday evening. Stay tuned for more information and continue reading below!
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Continue reading the full blog below!
Interesting weather is ahead for the Pacific Northwest, with a brief heat wave and monsoon moisture impacting the region on Wednesday and Thursday.
Let’s start with the heat. The HRRR forecast for Wednesday is below.
Expect lowland highs in the mid to upper 80s (cooler near the water). Portland and SW WA will reach the mid 90s to low 100s, and Eastern Washington will reach the upper 90s to mid 100s, hottest in the Columbia Basin. The coast will even reach the low 70s.
Expect more of the same on Thursday, just a few degrees hotter. The NAM forecast is below.
Expect Thursday’s highs to be in the low 90s around the lowlands, with the mid 90s in the foothills and near Olympia. Portland will hit the mid 90s as well. Eastern Washington will be scorching, in the upper 90s to mid 100s, with the Columbia Basin (Tri Cities area) reaching the upper 100s.
As we advance through Wednesday and into Thursday afternoon, monsoon moisture moving north will make its presence known. Muggy conditions are likely west of the Cascades, especially on Thursday. The NAM dewpoint forecast for Thursday afternoon is below.
Dewpoints across the lowlands will be in the low to mid 60s, with areas near the Cascades, SW WA, and the Willamette Valley pushing 70 degree dewpoints.
The combination of hot weather and high dewpoints will fuel a chance of thunderstorms both on Wednesday and Thursday. Wednesday’s chance will be more limited to the mountains, and there is a slightly better chance area-wide on Thursday. The forecast for CAPE (instability) values for Thursday from the UW model is below.
This forecast calls for CAPE values of 200-1000 across the Northwest, particularly in the Cascades, Willamette Valley, and SW WA.
The NAM forecast below is more aggressive with CAPE values.
The NAM shows high CAPE values of 2000-3500 over the Cascades, Olympics, and Oregon’s Coast Range on Thursday evening. Values of 500-2000 are noted over parts of the lowlands. This forecast has consistently shown high CAPE values, but it bears further watching as uncertainty always exists when forecasting thunderstorms in the Pacific Northwest.
There is one major downside to the potential thunderstorms on Wednesday and Thursday, as seen below in the NAM precipitation forecast through early Friday.
Barely any rain is expected to fall over the Cascades and most of Western Washington, except the coast. This means that any thunderstorms have a high possibility of being dry thunderstorms (when no rain falls), bringing a higher than normal fire danger. Due to this, NWS Seattle has issued a Red Flag Warning for the western slopes of the Cascades from Wednesday morning to Thursday night.
Due to the uncertainty of the upcoming situation, there will be an update Wednesday night. Stay tuned on Twitter & the blog!
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