April 2011 Tornado Summary

While historically May and June see more tornadoes than any other month, April set a new record for the number of reported tornadoes, estimates so far total more than 600 tornadoes. This is over 300 more tornadoes than any other April on record. The recent two-day tornado outbreak (Apr. 26-28) will also set a new 24-hour record with 211 reported tornadoes. The last record for a single outbreak was 148 tornadoes in 1974. These staggering statistics are estimates right now as they are only preliminary reports. You can read more about April's tornado statistics here: NOAA News.

Its difficult to pinpoint what caused this increase in tornado producing storms; the obvious setup includes cold, dry Arctic air colliding with warm, moist air from the Gulf of Mexico. Occasionally in the spring these two, extremely different, air masses come together triggering powerful storms along their boundary. The satellite imagery from the GOES-East satellite has been released by NOAA. Typically during severe weather, NOAA places the satellite into a higher resolution scan, taking twice as many pictures. Watch the satellite imagery of the outbreak below:

The recent increase in these events are better understood by looking at global climate patterns. The balance of atmospheric energy has been disrupted by changes in ocean currents. The cooler current of the Pacific ocean (remnants of this year's La Niña pattern) and the changes in atmospheric pressure over the Arctic (the Arctic Oscillation) may have played an important role in this month's weather systems.

Sea surface temperatures in the Gulf of Mexico have been running a couple degrees warmer than usual, acting to intensify the available energy being pulled north into the path of the advancing polar air masses. I should mention that I'm not aware of any links between global warming and the warmer sea temperatures in the Gulf.

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Tornado Graph Added

I've added a graph from the Storm Prediction Center to the site. The graph shows the running total of yearly tornadoes, plotted for the past 5 years. The image above is static, but the graph in the left side bar will update as SPC adds data.

There is an apparent trend from year to year that can be observed. However it is important to note that the tornado count is from local storm reports and is only preliminary data. There will always be an increase in tornado reports as the population increases, technology advances, and as an overall interest in severe weather increases. When the linear trend from the 1954-2007 tornado counts was adjusted for this inflation, the upward trend was removed. Read this page for more info on tornado report inflation: http://w1.spc.woc.noaa.gov/wcm/adj.html.

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Heat and Instability Builds Across Mississippi River Valley

Today, much of the Mississippi River Valley and areas as far north as southern Lake Michigan are under heat advisories as temperatures are expected to exceed 90 degrees with heat indexes over 100ºF. Areas in the south, where moisture from heavy rains have saturated the ground, will experience much more intense heat indices. In addition to the heat, severe weather is threat, including flooding, damaging winds, and large hail.

A large swath of moisture is riding on the back of a region of dry air that extends from the Gulf up the East Coast. Along this axis, there is good theta-e advection pumping warm, moist air eastward.

Dry air aloft had greatly limited the vertical growth of any storms in the Illinois/Indiana region this morning. However, current radar data indicates echo tops extending beyond 40kft on developing convection near Lafayette, IN. Unfortunately there is a lack of ground data in this area. Radar image is from IWX at 3.10º.

Surface cape values are near 6000 J/kg in Central Illinois as instability is ramping up. Heavy rainfall could be dangerous for areas that have not seen much rain this past week, as flash flooding will be possible.

Once the front pushes through the Midwest, stable air will finally break the heatwave somewhat, temperatures may not drop significantly but drier air will keep things feeling more comfortable. Areas in the south may have to deal with the front a little longer as high pressure seems to be fighting the trough that is dragging eastward from a low pressure center in Canada.

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Approaching Storm Pictures

A storm just rolled through at the airport where I work. Nothing severe here in South Bend but the approaching cell had a beautiful shelf cloud attached. I took these photos with my cell phone so the quality is sub par.

North end of the cloud (and storm):

There has been several reports of trees and power lines down, due to 65-70 mph winds in the southern counties of Michiana. The storm was tornado warned but nothing confirmed yet. Here's the radar image of what your looking at (actually this is an earlier image, radar data for the time of those photos is missing):

Plotted along side the radar is downdraft CAPE. The larger values indicate stronger downdraft potential with convective storm cells. When you have convection in a storm, air is lifted and condenses into precipitation. This precip has a cooling effect and thus you get cold air that sinks ahead of the convection. This cold, sinking air is what shapes the shelf cloud. They form ahead of storms that have strong downdrafts. If you get enough downdraft from a cell it can form a gust front at the outflow boundary, which is basically a storm induced cold front. In the radar image you can see the gust front as it formed some smaller thunderstorms ahead of the larger cell.

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Current Forecast: Complicated

After a week of unseasonably warm temperatures and no rain, Northern Indiana finally got a good watering on Memorial Day. Scattered thundershowers left some areas with over 3 inches of rainfall while others saw slightly more than a trace.

The current mesoscale analysis is a bit messy. A well organized MCS fell apart as it moved east into northern IN last night. As it fell apart several convective boundaries formed. These boundaries are visible in the satellite imagery:

An impressive line of convection that pushed south from the MCS has fired up some storms along the western Ohio River Valley a few hours ago. This is seen beautifully in the visible satellite imagery from this afternoon:

Upstream there is clearing and temperatures are reaching near 80 degrees. In addition, instability seems to be expanding with CAPE (the energy available for vertical storm growth) values breaching 1000 J/KG. A decent line of convection has formed in western Indiana. These storms probably won't become severe as wind shear values remain somewhat stable at the moment. The latest RUC KSBN sounding forecast indicates precipitable water values near 1.5" but doesn't bring the upstream instability into the picture.

Overall it seems the next 72 hours will be a waiting game. Forecasters are leaning towards lowering the probability of precip based on the current situation and this morning's weaker than expected rainfall. Keep checking the latest forecast products from the NWS in North Webster at http://www.crh.noaa.gov/iwx/ as forecasters will be watching the complicated mesoscale developments.

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Resuming...

It seems I've manage to lose a few posts as I've moved to a new web server. Anyway, things seem to be settled in and hopefully I can get back to writing soon. Thanks for reading!

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Unwelcomed Cold Snap to End Midweek: El Niño Returns

The expected mild-winter effects of the current El Niño event have been thwarted by a negative North Atlantic Oscillation (NAO-) index, ushering in extremely cold air from Canada. This pattern has persisted for all of January for the Northern Plains, the Midwest, and the Eastern US. Fortunately strong zonal flow will dominate beginning midweek, preventing arctic air from intruding on us.

The NAO describes the relationship between the semi-permanent Icelandic Low and an area of subtropical high pressure situated off the Northwestern Coast of Africa. The relationship between the two systems is important to weather patterns in the Eastern US and Western Europe because the effects can be prolonged. When the difference between the two systems is low, the NAO Index is said to be negative. Cold air from the arctic can easily dip south during a negative NAO. By midweek however, flow will be more zonal (west-east), this is familiar to an El Niño type pattern and temperatures should allow for thawing.

Forecasters are keeping an eye on an area of low pressure in Northern Mexico that could end up being a nasty MLK Day storm for the East Coast. This amplifying trough  is expected to get a boost from some quick moving areas of upper level energy diving south from California on Friday (according to the GFS). This system is positioned in a way that will pump a lot of moisture from the Gulf and eventually from the Atlantic into the Southern and Southeastern US. Precipitation could get interesting as this warm moist air combines with the colder air mass exiting the Eastern US this weekend.

Forecasts suggest that once this flow is in place, mild temperatures should dominate for a while. Initially, for the Midwest, temperatures won't get as mild as they normally would with this type of pattern given the amount of very cold snow on the ground. Depending on the duration of mild weather, temperatures could  reach into the low 40s later next week across the Great Lake States and into New England. Welcome back El Niño.

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