Saturday, April 20, 2013

Spring Pulls A Vanishing Act; Returns for May

Spring continues to do its magic and will be pulling a vanishing act until the beginning of May.

Pictured above is the 500 millibar height anomalies from forecast Day 6 to Day 10. Cold colors show low pressure anomalies, while warm colors portray high pressure. In the forecast image above, we see strong low pressure building over the Arctic to allow for a strengthening polar vortex. As a result of this  strengthening, warm weather is typically favored. However, high pressure building south of Greenland and in the Northeast Pacific will coerce the low pressure further south and allow cold air to make itself available in all the areas shown in blue. Again, usually, warm air would be favored as the cold air would be kept up north in the presence of such strong low pressure over the Arctic. However, the angling of those two previously mentioned high pressure systems will let the polar vortex squeeze some of that unseasonably colder air further south.

The long range American ensembles show 500 millibar height anomalies over the next sixteen days from left to right, top to bottom. We can see the polar vortex pushing south towards the US/Canada border, but by the time May rolls around in the bottom row of forecast images, we see abundant high pressure across the nation. This abundant high pressure formation is what is supposed to happen with strong low pressure in the Arctic. It looks like the nation-wide high pressure formation in early May would be tempered in the East US just because of persistent high pressure stirring around Greenland, but cold air should not prevail like it will in the next couple of days.


Snow Rounds Out Atypical April

It looks like snow will be rounding out the last days of April.

The NAM model forecast has two swaths of accumulating snow in the Midwest and Plains. The stripe of snow in the Midwest looks to put down upwards of 6 inches in many spots, especially across Wisconsin and southeast Minnesota. Lighter totals will be observed into northern Nebraska and southern South Dakota. The second body of snow will be more significant, but will cover less ground. Kansas will get hammered with totals flirting with (and in some cases, exceeding) one foot of snow. The heaviest amounts appear to be centered in the northern part of the state, with central Kansas being the cutoff for the heaviest snow. Normally, I would address this as Old Man Winter's last hurrah, but with the way this April has been going, I wouldn't be surprised to see sporadic, light snowfall chances into May.