Heavy Rains Friday

Wednesday, August 12, 2015

10:18 pm
Hello everybody! I have a day off from my counselor duties at Hidden Valley Camp so I thought I’d go ahead and write a blog about the upcoming rains later this week. We could get dumped on Friday, but there’s still a lot of uncertainty. 
The weather down here sure was interesting though. I heard some thunder up at camp, and although I missed it, I heard through the grapevine that an intense thunderstorm passed right over Seattle. Today, Sea-Tac got more rain in 24 minutes than the previous 71 days combined. Also, Iwakuma tossed a no-hitter. I gotta say… today was a good day. Too bad it’s cloudy for the Perseids meteor shower tonight, but hey, you can’t get too greedy. 
Anyway, let’s take a look at what we may be dealing with later this week. I get back to camp Thursday afternoon, and everything we do out there is in the great outdoors (including sleeping and eating), so even though I absolutely love the rain, it definitely puts a damper on activities. Me and my tent group of six 12-year-old boys are hiking Mt. Pilchuck on Friday, so I’m definitely interested to see what hiking in the rain with these kids will be like. By the way, the hike is only 5.4 miles roundtrip, but features 2,300 feet of elevation gain. I’m super excited, rain or shine.
Since I’m talking about Friday, I might as well give a brief talk about the weather we can expect Thursday. The newest models from the UW don’t show much precipitation at all, but there will be a significant amount of CAPE (Convective Available Potential Energy) over the area in the afternoon, so some thunderstorms may develop. CAPE is defined as the amount of energy a parcel of air would have if it rose until it could no longer rise (i.e. it becomes colder and thus denser than the surrounding air). The more CAPE there is, the more unstable the atmosphere is, and the faster parcels rise. Because of this, high CAPE is an important ingredient for convection, but just because there is high CAPE does not mean there will be convection. There are many other factors that can inhibit convection… strong low-level inversions, high pressure systems, low vertical velocities of air parcels in the lower atmosphere to initialize convection… I could go on. There are plenty others. 
Valid 05:00 pm PDT, Thu 13 Aug 2015 – 24hr Fcst: Retrieved from UW Pacific Northwest Environmental Forecasts and Observations Modeling Page
Anyway, let’s move onto Friday. I’m only going to give a brief overview, because the details are pretty murky for reasons I will explain shortly.
The rains are predicted to start heading into the area Friday morning.
Valid 08:00 am PDT, Fri 14 Aug 2015 – 39hr Fcst: Retrieved from UW Pacific Northwest Environmental Forecasts and Observations Modeling Page
A blob of rain will form south of the Olympics (at least in this model run)…
Valid 11:00 am PDT, Fri 14 Aug 2015 – 42hr Fcst: Retrieved from UW Pacific Northwest Environmental Forecasts and Observations Modeling Page
… and this blob will intensify and move eastward as the day progresses.
Valid 05:00 pm PDT, Fri 14 Aug 2015 – 48hr Fcst: Retrieved from UW Pacific Northwest Environmental Forecasts and Observations Modeling Page
This model run gives around an inch of rain to Mt. Pilchuck on Friday. This morning’s run gave two. We’ll see what tomorrow morning’s run gives. All this variance in precipitation is due to a cut-off-low off the Northern California Coast. Since cut-off lows are not connected to the jet stream, they are rather erratic and hard to forecast. This cut-off low is responsible for Friday’s uncertain forecast. There is an old weather saying: “cut-off low, weatherman’s woe.” That saying definitely holds true right now!
Right now, rain seems like a pretty good bet for Friday, but where and how much is up in the air. If the models are correct however, some places could see pretty heavy amounts, especially places north of Everett. But regardless of how much it rains, I’m prepared to have an awesome time climbing Mt. Pilchuck on Friday. 
Thanks for reading!
Charlie
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