Saturday, May 4, 2013

Snow in Plain Sight

Friday, May 3, 2013
8:24 A.M.

The last couple days have been pretty darn nice around the Pacific Northwest, but if you are a fan of the weather now, just wait until the weekend. Highs are expected to top 80 Sunday and Monday here in Seattle, and it will be even hotter near the Cascades. The average high for this time of year is 62.

Photo taken from Barbara Robertson (my aunt) from Belton, Missouri on May 2, 2013
But while we’ve been simmering in summerland, my mom’s side of the family two time zones to the east has been seeing some extremely rare early-May snowfall. The last time Kansas City saw this much snowfall this late in May was back in 1907, when Teddy Roosevelt was president and people were driving the Ford Model N, which made the Model T look like a Bugatti Veyron in comparison. As you can see from the picture above, it didn’t take a whole hell of a lot of snow to break a 100 year old record… I believe 1.7 inches fell on Kansas City in 1907. By now, 2-3 inches have fallen in most of the city and suburbs, with another possible inch in spots before the snow is over and done with.

What’s causing all this snow? Well, we’ve got an extraordinarily cold pool of air for this time of year that is plunging southward across the plains, and when you sprinkle some precipitation over that air mass due to an upper-level trough over the area, you get snow. There have been some pretty hefty storm totals from this trough, with some portions of the Upper Midwest receiving over 18 inches of snow from this storm. Had this occurred in the winter, it would likely be regarded as a fairly run-of-the-mill storm. For this time of year, however, a storm like this is unprecedented.

Ford Model N - 1906: Photo retrieved from Wikipedia Commons. Uploader - Harz4

The snow is starting to wind down over much of the area now, but it is being replaced with a likely greater threat: the risk of flash flooding. The same frontal boundary responsible for all this snow will continue to crawl eastward, but a surface low will develop due to the aforementioned upper low closing off over the Lower Mississippi Valley. This low will not be particularly strong pressure-wise, but it will be fed by ample subtropical moisture from the Gulf and Atlantic, and, most importantly, will be slow to make eastern progress through the states. The combination of ample, subtropical moisture and the lack of upper-level winds to carry the storm eastward is very conducive to flooding events because certain regions have the potential to get soaked with tons of rain for long periods of time.

As you can see, there are a bunch of flood warnings currently up along the Mississippi River. Just by looking at this radar, you can tell that the Mississippi and points east are getting absolutely drenched. As of 12:44 A.M. PDT Saturday, Alabama and central Tennessee look to be taking the brunt of the rainfall.

The good thing is that we will not see much in the way of severe thunderstorm activity from this system. Powerful, long-lasting supercells need wind shear to keep the updraft and downdraft separated and the storm structure intact, and this wind shear can lead to a rotating updraft if the conditions are right. Because the winds aloft are weak and there is very little shear going on, tornadoes/hail/high winds are not a major threat from this system.

Enjoy the weekend here! I'm definitely planning on going swimming in Lake Washington. Water temperatures are still in the mid 50s, but with our first heat wave of the year upon us, you shouldn't have any problem staying warm.


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