And then it rained

Update:  Albany rain total now up to 3.73″.  That may well be an all-time summer season record for them.  Still only 1.60″ here.

Summer weather here can be pretty boring.  Either the wind blows off the land, in which case we have hot days, cool nights, lots of sun, and no rain; or the wind blows off the sea, in which case we have cooler days, milder nights, partly cloudy skies, and still no rain.  That is all a function of the ocean being cooler than the land and the air above it.  (In the winter, when the ocean is warmer than the land and the air, we see lots of evaporation and convection, with rain becoming the dominant pattern.)

There are two ways we can get rain in the summer, and they are about equally common.  First, we can get an unseasonably cool air mass dropping down over the ocean from the north, which essentially revives the winter pattern in a half-assed way.  This brings cool, gentle rains with little to no thunder, and we last saw this happen in the summer in July of 2011.

Second, we can have a low pressure system set up to our southwest over the ocean, which is pretty much the only way we get summer thunderstorms.  In the Midwest, there is a fairly frequent northward flow of moist air from the Gulf of Mexico.  In the Southwest, a similar pattern happens in the summer creating the monsoon season.  Here, because we are so close to the cold, non-storm-inspiring ocean, we need a very specific pattern to bring moist air up from the south or southeast.  With a low offshore to the southwest, moister air from over the ocean off California is pulled onshore and northward over the southern Oregon mountains.  There it is heated by the warm land, triggering convection and thunderstorms.  If the flow is just right, these storms then continue to move northwestward over the Willamette Valley.  The pattern itself occurs several times each summer, but usually the moisture is too limited and the storms peter out soon after drifting away from the mountains.

This time the flow over California tapped the remains of a tropical system, bringing moisture in the atmosphere to 2-3 standard deviations above normal.  A east-west band of storms developed, moving northward into our area and being stretched westward out over the ocean by the circular flow.  It arrived with a bang just after 2 pm, bringing more lightning than I’ve seen since a similar event on June 4, 2009.  That storm, which also came from the southeast, was stronger but passed over quickly, dropping a quick inch of rain.  In this storm, the lightning kept up for two hours, striking five times within a mile of the library where I was working and once just down the block (or possibly hitting the building itself).  The center of the low-pressure circulation moved overhead shortly after the rain arrived, essentially trapping the band in place.  Instead of moving, it began to rotate around us, keeping us under moderate to heavy rain for 4.5 hours.  It has finally stopped now, leaving us with 1.59″ for the day.  That is admittedly not high by Minnesota standards, but it is above our monthly average of 1.43″, more than double the daily record of 0.70″, and the third-highest one-day total in September since 1889.  Albany got hit by a stronger cell in addition to the stalled band, bringing totals there to around 2.5″ which is probably an all-time September record.

With the soil parched from three months of little to no rain, there was almost no runoff.  Just two hours of thunder to warm a Minnesotan’s heart and almost two inches of rain to moisten the earth and spark off fall mushroom season.

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