While Arizona typically brushes off their heat as "at least it's a dry heat", Seattleites can usually brush off the raindrops that fall in our frequently rainy days without getting too wet -- "a dry rain," as we might say.
But that is certainly not the case on Thursday, as strong thunderstorms were expected to bring drenching rains that could tally around 1 inch per hour -- similar to the heavy rains that fell on Aug. 29. Contrast that with usual Seattle rains that maybe bring 0.01"-0.05" per hour.
(The rains are expected to be so intense that staunch Seattleites should be given a 24 hour exemption to the rule that you can not carry an umbrella without being subject to ridicule!)
So, why are Thursday's rains going to so effective in instantly drenching someone than a typical rainy Seattle day? It all comes down to vertical winds.
Large raindrops mean that there are strong updrafts inside the clouds above your head. These upward blowing winds can hold raindrops inside the clouds for a longer period, allowing them to continue to grow in size until they finally become heavy enough for gravity to finally win the battle over the updraft, allowing the raindrop to fall to the ground. The stronger the updraft, the larger the drop can grow. (Plus the warmer the air, the more moisture it can hold, and here in summer, we have plenty of that around.)
That's why thunderstorms tend to have large raindrops, as they tend to also have strong updrafts. And that will certainly be the case Thursday as an area of low pressure is bringing a lot of moisture and instability, allowing for towering thunderstorms with strong updrafts that will allow raindrops to keep growing for some time before their date with Mother Earth...or your windshield... or what could be your umbrella (but just this once!)
Now on Friday, it'll still remain wet, but as the low moves inland and the winds shift to a cooler, more northerly direction on the backside of the low, the airmass will stabilize, the thunderstorm threat will cease, and instead will be left with more widespread, steady rains with weaker updrafts and -- you guessed it -- smaller raindrops. Just it'll be more along the lines of an October/November rainy day than a morning drizzle, but less intense than the 5-seconds-can-ruin-a-hairdo showers of Thursday. (Maybe it'll take a full minute.)
The updrafts will continue to weaken as this low pressure center moves away on Friday night and early Saturday and not only will the showers decrease in coverage, but raindrop size. So by late Friday -- umbrellas will be off limits, once again.
Video by Mike McLaughlin shows the big raindrop showers of Thursday.
Hail depends on updrafts too...
In addition to the heavy rains, some of the stronger thunderstorms Thursday could also include some hail.
Hail works in a similar fashion to what causes large raindrops, only here we are talking about the upper reaches of the thunderstorm where it's cold enough for rain to be ice pellets. The updrafts push the raindrop so high it freezes, then the ice pellet gets high enough away from the wind and begins to falls to Earth -- think of water shooting up out of, say, Seattle Center's International Fountain. It reaches skyward until it gets far enough away from the jet, then falls back.
Only in our ice pellet's case, the hail stone is picking up a coating of rain on its way down until it hits the updraft again and gets pushed back skyward, where that coating of water freezes and adds another layer to the ice. The stronger the updraft, the more this process can repeat and the larger the stone will grow. An updraft of 56 mph will support a golf-ball-sized hailstone.
(P.S. Umbrellas are OK during hailstorms too.)