BULLETIN - IMMEDIATE BROADCAST REQUESTED
SEVERE THUNDERSTORM WARNING
NATIONAL WEATHER SERVICE NORTHERN INDIANA
133 PM EDT WED SEP 10 2014

THE NATIONAL WEATHER SERVICE IN NORTHERN INDIANA HAS ISSUED A

* SEVERE THUNDERSTORM WARNING FOR...
  NORTHERN GRANT COUNTY IN CENTRAL INDIANA...
  WESTERN HUNTINGTON COUNTY IN NORTHEAST INDIANA...
  SOUTHERN WABASH COUNTY IN NORTH CENTRAL INDIANA...

* UNTIL 215 PM EDT

* AT 131 PM EDT...A SEVERE THUNDERSTORM WAS LOCATED NEAR LA
  FONTAINE...AND MOVING NORTHEAST AT 55 MPH.

  HAZARD...60 MPH WIND GUSTS.

  SOURCE...RADAR INDICATED.

  IMPACT...EXPECT DAMAGE TO ROOFS...SIDING AND TREES.

* LOCATIONS IMPACTED INCLUDE...
  ANDREWS...HANFIELD...SERVIA...LANDESS...LINCOLNVILLE...BANQUO...
  PLEASANT PLAIN...MOUNT ETNA AND LANCASTER.

THIS INCLUDES INTERSTATE 69 IN INDIANA BETWEEN MILE MARKERS 269 AND
276.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

FOR YOUR PROTECTION MOVE TO AN INTERIOR ROOM ON THE LOWEST FLOOR OF A
BUILDING.

Geography and severe weather probabilities remained pretty much the same when the National Weather Service Storm Prediction Center issued its updated “Day 1 Convective Outlook” at 12:28 p.m. EDT.

SPC meteorologists expect Thunderstorms, some severe, to develop near a cold front from the southern Great Lakes southwestward through the lower Ohio and mid-Mississippi valleys into the Ozarks. Damaging winds will be the primary threat this afternoon through early tonight.

Some risk will exist for isolated tornadoes in addition to locally damaging wind through late afternoon from eastern Illinois east-northeastward into parts of Ohio and Lower Michigan.

The next update is due out by 4 p.m. EDT.

The National Weather Service Storm Prediction Center is monitoring trends in the area indicated on the map above for any increase in thunderstorm activity late this morning through early afternoon and for a conditional tornado threat, according to a “Mesoscale Discussion” it issued at 11:34 a.m. EDT.

A leading shortwave trough is currently moving across northern Illinois and will continue rapidly northeastward across Indiana, northwestern Ohio and Lower Michigan.

Around 10:30 a.m. EDT, a weak thunderstorm showed brief supercell characteristics over central Illinois. This suggests the environment is close to being favorable for at least brief tornadoes.

A limiting factor is extensive cloud cover ahead of the leading band of precipitation, but partial heating is occurring across southwestern Indiana.

It is unclear whether atmospheric destabilization will be sufficient for further supercell development today before the aforementioned shortwave trough moves through. Once that wave passes, early potential for severe storms should be largely over, with any additional and primarily wind threat focusing southwestward along the trailing cold front.

Nearly the entire state of Indiana has a slight risk of severe weather between 9 a.m. EDT today and 8 a.m. EDT tomorrow, according to the “Day 1 Convective Outlook” that the National Weather Service (NWS) Storm Prediction Center issued at 8:56 a.m. EDT today. The slight risk area also includes about half of the 37-county warning area of the northern Indiana NWS office and all of IMO SKYWARN quadrant two.

The primary threat is damaging straight-line thunderstorm winds or gusts of 58 mph or stronger, although an isolated tornado cannot be ruled out. As you can see on the maps at right, the probability of damaging straight-line winds in northeastern Indiana is 15 percent, which is about 7.5 times the average probability for this time of year (2 percent). The probability of a tornado is 2 percent, which is about 20 times the average probability for this time of year (0.1 percent).

The severe weather risk is conditional, because it depends largely on sunshine. If cloud cover remains heavy all day, the threat of severe weather will be much lower than if the area sees sunshine. This is because the sun’s heating of the earth’s surface would increase atmospheric instability.

SKYWARN storm spotter activation might be needed this afternoon and early this evening, according to a “Hazardous Weather Outlook” issued by the northern Indiana NWS office at 5:27 a.m. EDT.

Residents of the slight risk area should be sure they have a reliable way to be aware of any watches or warnings the NWS might issue today, such as a weather alert radio. You can also keep up to date on weather developments by subscribing to this blog, following the northern Indiana NWS office on Twitter (@NWSIWX) and/or liking the northern Indiana NWS office on Facebook.

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The northern Indiana National Weather Service office has postponed until Thursday the weekly weather radio test that it normally conducts on Wednesdays between 11 a.m. and noon EDT time, due to the threat of severe weather as well as transmitter problems at South Bend.

The National Weather Service has issued a flash flood watch for most of northern Indiana and northwestern Ohio from early this afternoon through this evening.

A flash flood watch means that conditions may develop that lead to flash flooding. Flash flooding is a very dangerous situation.

View the full statement on the NWS website.

No part of the country has a severe weather probability of greater than 30 percent, according to an updated “Day 2 Convective Outlook” that the National Weather Service Storm Prediction Center issued at 1:18 p.m. EDT. The entire state of Indiana and all 37 counties in the county warning area of the northern Indiana NWS office continue to have a slight risk of servere weather between 8 a.m. EDT tomorrow and 8 a.m. EDT Thursdsay, with probabilities ranging form 15 to 30 times higher than a normal day for this time of year.

At midday, forecast models indicated a weaker low pressure system than previously expected. The latest thinking is that an organized squall line or multiple line segments will be the primary storm mode. Extensive clouds and precipitation early in the day should limit atmospheric instability and hamper the development of severe storms. Even so, damaging straight-line thunderstorm winds remain a possibility.

The SPC’s next outlook for tomorrow comes early tomorrow morning, when it issues it’s first “Day 1 Convective Outlook” of the day.

Update: NWS has published a multimedia briefing on this situation.

Much of northern Indiana and surrounding parts of Illinois, Michigan and Ohio have a 45 percent probability of some form of severe weather within 25 miles of a point tomorrow, according to the “Day 2 Convective Outlook” that the National Weather Service (NWS) Storm Prediction Center (SPC) issued at 1:36 a.m. EDT.

To put that in perspective, the historical average probability of severe weather in that area at this time of year is only one percent, according to the SPC website. That means that tomorrow’s probability of severe weather is 45 times the normal probability. Here’s a link to a Web page that will provide a better understanding of outlook probabilities,

There is a chance that morning cloud cover tomorrow will limit atmospheric instability and therefore limit severe weather potential. But if that doesn’t happen, widespread swaths of severe wind gusts with peak gusts approaching 75 mph are possible. A couple of tornadoes are also possible as are heavy rainfall and localized flooding.

The area of 45 percent probability includes essentially all of the 37-county warning area of the northern Indiana NWS office and therefore all four quadrants of IMO SKYWARN.

SKYWARN storm spotters should spend some time today reviewing reporting criteria and methods, charging device batteries and generally preparing for possible activation tomorrow.

All residents of the area depicted in the map above who plan outdoor activities tomorrow should make sure they have a way of receiving notification of any severe thunderstorm or tornado warnings that the NWS issues (don’t rely on being able to hear an outdoor warning siren!). They should also identify in advance how they’ll notify others of a warning and where and how everyone will take shelter.

The northern Indiana NWS office plans to issue a multimedia briefing regarding tomorrow’s severe weather possibilities later this morning. I’ll post it on the “W9LW’s Ramblings blog” when it’s available.

The SPC will issue an updated “Day 2 Convective Outlook” by 1:30 p.m. today. I plan to update the blog after it comes out.

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