Two more chilly days to go before we head back above average, but it won’t be quite as windy today. Flurries and snow showers are possible, mainly this afternoon and evening.
The biggest chances will be for my 95-3 MNC listeners in South Bend AND in Q102-land around Evansville! An inch or two of snow is possible in Lake Effect areas today. An inch or two of wet snow is possible in southern Indiana tonight. Including the very areas struck by tornadoes Friday. An Alberta Clipper diving to the southeast late this afternoon and tonight could bring an inch, maybe two, of wet snow to Metro Louisville. (Weather history note: Xenia Ohio had snow following a famously devastating tornado in 1974.)
Three completely quiet days follow before rain – just plain rain – Thursday. As of today, no severe weather is expected across the country this week.
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I’ve had the question a few times this weekend: what happened Friday to cause such an outbreak?
Thunderstorms, including the super-cell, or rotating storms that can spawn tornadoes, have some basic requirements: moisture, instability, and some sort of lift, or trigger mechanism.
Moisture first streamed northward Friday with a warm front. A good way to gauge the amount of moisture in the atmosphere is the the dew point temperature. The dew point in Louisville jumped almost 20% – from 51 to 60 – between 10am and noon Friday.
Instability is what lets the air in the low levels of the atmosphere rise into the upper levels. Without that the atmosphere can’t support big storms. Instability can be increased through daytime heating, and Louisville’s weather station reached 74 before the storms hit. There are many trigger mechanisms, such as what’s called “low-level convergence,” Convergence is air streams flowing into one another or stronger wind moving into weaker wind. When air piles together in the low levels of the atmosphere it has no place to go but up. That air was piling into a deep surface low pressure that moved out of southwest Illinois to the northeast through the day.
The low’s associated warm front, and (eventually) a cold front both went through the affected area Friday. Surface winds turned from an easterly direction at 5am to southeasterly in the morning as the warm front approached. South/southwest winds then prevailed until the late-afternoon passage of the cold front switched the wind direction to west.
An active jet stream is an important component to super-cell enhancement, with speed shear created by super speedy wind flow, and directional shear created by winds roaring in from a different direction compared to the surface. A potent wave, or trough of lower pressure in the upper atmosphere was approaching Missouri at this time, providing an extra “tilt” for the thunderstorms.
This kind of outbreak is unusual for the its northward extent so early in the season, but it’s always worth noting that although tornadoes are most likely “in-season,” they can occur at any time, given the proper conditions. In fact, the last deadliest tornado event in Indiana came in November 2005, when 25 lost their lives in southwest Indiana.
INDY ALMANAC:
Average high today is 47, and the average low is 29. Records are 78 and 2. The sunrise time is 7:13 and the sun sets at 6:41.
DETAILED INDIANAPOLIS FORECAST:
Today: Mostly cloudy with flurries and snow showers. High 38. West wind 10-15 mph.
Tonight: More snow showers possible. Low 25.
Monday: Partly sunny. High 40.
Monday Night: Mostly clear. Low 28.
Tuesday: Sunny. High 56.
Tuesday Night: Mostly clear. Low 42..
Wednesday: Mostly sunny and breezy. High 60.
Wednesday Night: A chance of showers. Low 46.
Thursday: Chance of showers. High 56.
Thursday Night: Chance of showers. Low 37.
Friday: Partly sunny. High 49.
Friday Night: Partly cloudy. Low 31.
Saturday: Partly sunny. High 44.
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