It would not be much of a stretch to ask your "Where is winter?" Locally temperatures been anywhere from 4-8 degrees above average. The whole continental United States have been above average over the last couple of weeks in fact. During this period the teleconnection signals showed a (+)NAO and (-)PNA along with phases 5-6 showed a warm pattern. Teleconnections are connections between atmospheric conditions at one point and another. The NAO and PNA are the 500mb height patterns over Greenland and the western United Sates respectively. The MJO or Madden-Julien Oscillation is the movement of strong tropical convection along the ITCZ (intertropical convergence zone) in a 30-60 day cycle. The direct culprit? A strong ridge of high pressure set up shop across the central US allowed for calmer, more unseasonable temperatures to happen. At the same time with a low pressure system to the west, warm, moist air from the Gulf of Mexico was advected or moved horizontally northward. This helped to allow the two severe weather events to occur. Having a quiet end to hurricane season allowed for the gulf to still have amorously warm surface water in mid-December. Analysis12/10/2021 At 12z on Dec 10th a trough was located in the western United States which would deepen as it traveled southeastward. As this was occurring advection of warm moist air from the gulf into an increasingly unstable airmass in the Mississippi river valley. CAPE was in excess of 1000 j/kg, which is quite a bit for mid-December. Helicity was around 400 m^2/s^2. Helicity is just oner measure for wind shear, while CAPE is the most commonly used measure of instability. At 21z a surface low was posited over NE Kansas with a triple point of a cold front, warm front, and dry line. Around 9:00 CST in Valpo as I was walking home from working a volleyball game that the weird feeling air seemed familiar. I actually got 11/17/2013 vibes. Sometimes you can just tell the air isn't right. Earlier in the day during the 16:30z update, the Storm prediction Center in Norman, OK had issued a moderate risk for the target area. Soon enough the event started with storms initiating in Arkansas. Who would have thought that a single supercell would form in the warm sector and remain alive and strong for 400 miles! As of writing this post this quad state tornado was actually two separate tornados both rated EF4 with maximum winds of 190mph. There were quite a few wind reports and a couple EF0 tornados reported in Indiana. 12/15/2021 This event was a record breaker. There were 54 significant wind reports of 65 kts (75 mph) winds within a serial derecho that traveled around 700 miles at 63 mph. Here is a comparison between the latest derecho and the August Iowa derecho. Here is some information on derechos if you need a refresher. This most recent derecho was a serial derecho that traveled ahead of a cold front, like many squall lines that affect the midwest in the summer. A progressive derecho like the Iowa derecho last year or the June 30th, 2012 derecho ride on top of stationary front that we can sometimes see in northwest flow type patterns in the summer. Now for a little analysis time. At the jet stream level (300mb) a trough dug into the four corners region heading northeast. At 850mb, there was a clear low level jet which brought in the tropical air northward. The fact the central plains were in between a trough and a ridge certainly helped advect that unstable air. The surface air was very warm and moist for this time of year. This isnt a great graphic, but it shows the non-thunderstorm winds. This was a strong low pressure system that underwent cyclonegenesis in the lee of the Rocky mountains. Do you see the brown lines and how close they are.? These are isobars or lines representing areas of equal pressure. Generally speaking, the closer the isobars are to each other, the windier it is due to the stronger pressure gradient. When the dryline and cold front passed the pressure gradient increased and what is called a sting jet developed with the stronger winds aloft being mixed down to the surface in the cold conveyer belt of the belt (the idea that there is cold air coming down behind the cold front.) For us locally the highest gust of 63mph were recorded in Whitestown. All that to say the weather has been quite warm recently. It's felt more like spring. So you are probably asking what the chances for a white Christmas are. I will say, the chances are not great...
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