FastCast--Monday, Oct. 16 to Thursday, Oct. 19:
A potent and fast-moving storm system will move through Western Washington on Monday. The storm will begin with a strong cold front (with heavy rain at times) on Monday morning, during the commute. Then, from late morning to late evening, strong post-frontal showers will move through Western Washington, with a chance of thunderstorms across the region. A strong convergence zone will develop over Island, Skagit, and Whatcom Counties, and heavy showers will move through most of the lowlands in the afternoon. These showers will have heavy rain and gusty winds, with the potential for lightning, thunder, and small hail. Winds will pick up as well, gusting 25-35 mph in the lowlands and 35-45 mph in the North Sound, strongest on Whidbey Island. Rain totals will likely be in the 0.4-0.6" range in the lowlands, with some areas getting less, and areas under heavy showers getting more. Some showers will linger into Tuesday. Expect highs on Monday and Tuesday in the upper 50s to low 60s, with lows in the upper 40s to low 50s. On Wednesday, clouds will decrease, followed by sunny conditions on Thursday. With an atmospheric river to the north in BC, southerly flow will allow the lowlands to warm up significantly, with highs reaching the upper 60s to low 70s, with lows in the upper 40s to low 50s.
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Continue reading the full blog below!
A potent weather system is ahead for Western Washington on Monday, with the main impacts being rain (heavy at times), a chance of thunderstorms, and localized gusty winds. Let's take a look at the forecast!
Rain will come in 2 waves, with the initial cold front from 7-10 AM Monday, bringing heavy rain at times and locally windy conditions. Then, from 11 AM-7 PM, there is a region-wide chance of thunderstorms in heavy post-frontal showers moving onshore. Winds will be breezy (25-35 mph) in the lowlands and windy (35-45 mph) in the North Sound.
First, let's take a look at the European model forecast for rain through Monday night.
The European model shows most of the lowlands (except around Everett) getting 0.6-1" of rain, with up to 1.5" in Skagit County. Areas from Olympia south get 0.5-0.8", and the coast gets 1-1.5". The Olympics get up to 3", with 1-2" in the Cascades.
Let's compare this to the high-resolution NAM model, also showing rain through Monday night.
The NAM forecast shows less rain for the entirety of the lowlands, with 0.2-0.5" expected in this forecast. Areas from Mount Vernon northward get 0.5-0.75", with the coast picking up 0.6-1.2", and the Olympics & Cascades getting 2-4".
Let's compare one more time to the HRRR high-resolution forecast, also through Monday night.
The HRRR sides more with the European than the NAM, showing 0.4-0.6" for the lowlands, but mainly south of Seattle. The HRRR shows a typical rain shadow NE of the Olympic Peninsula, but this will likely not happen on Monday due to Convergence Zone activity in that area. This forecast gives the coast 0.5-1", and gives the mountains 1-3".
Now, let's take a look at the thunderstorm potential on Monday. Thunderstorms are most likely in heavy post-frontal showers from late morning to late evening. Below is the NAM high-resolution forecast for CAPE (measure of instability in the atmosphere). More instability (CAPE) = higher chance of thunderstorms.
The NAM forecast below is for Monday afternoon.
The NAM shows high CAPE values of 250-1000 across the lowlands on Monday afternoon, highest from Seattle north to Vancouver BC.
Let's compare this to the HRRR high-resolution forecast, seen below (also for Monday afternoon).
The HRRR forecast shows CAPE values of 200-750 across the region, highest west of Olympia and from Port Townsend to Bellingham.
Finally, let's take a look at the UW WRF forecast, also showing CAPE on Monday afternoon.
The UW WRF forecast agrees, showing CAPE values of 200-600 across Western Washington on Monday afternoon.
So...the bottom line is that lightning & thunder are possible in any shower from late Monday morning to Monday evening. Heavy rain, sudden drops in visibility, gusty winds, and small hail are also possible in any showers. Remember to go indoors if you hear thunder, and stay inside for at least 30 minutes.
We will end the blog with a look at the extended forecast, which has some interesting things to note. This is the Seattle forecast from the European EPS model.
Notice near-average highs on Monday & Tuesday, followed by significant warmth (low 70s possible) on Wednesday and Thursday. Then, from Friday to Monday, temperatures drop steadily. By next week, highs will likely struggle to get out of the mid 50s. Also, notice low temperatures drop to the low to mid 40s next week after remaining steady in the low 50s this week. We have a lot to watch in the extended forecast, so stay tuned!
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