Archive for April, 2009

Eastern Sebastian County Wall Cloud
April 30, 2009

Here are a sequence of pictures from the lone severe thunderstorm of the day.  They were taken from near Washburn in Eastern Sebastian County.  As I approached the storm a bit of a hook developed and I began to see a well defined wall cloud through the trees.  When I finally got to a good viewing location it had lost some of its definition but still was fairly organized.   These pictures were taken between 6:04 and 6:12PM as the storm rapidly fell apart.  Actually, by 7pm there was nothing at all left of this storm on radar.

Wall Cloud

Wall Cloud

Wall Cloud

Wall Cloud

Wall Cloud

While the wall cloud looked pretty well defined when I first began to see it…it never had any significant rotation.

EDIT:  Jason Hurt caught this image of the same wall cloud as it passed near Dayton in Sebastian County at around 5:40PM.  Jason says “it was very well organized and rotating for a period of about 10 minutes, then fell apart after 5:45”.

Dayton Wall Cloud

Dayton Wall Cloud

Post by Brian Emfinger

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Ozark Tornado Confirmed
April 30, 2009

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Last night at 5:07 p.m. a tornado touched down 4 miles northeast of Ozark.  It was an extremely short lived tornado; however, it damaged 6 homes, 1 barn, and several trees.  No injuries were reported!

You can read about how the Ozark tornado formed in my earlier blog post.

Drew Michaels

Tricky Temperature Forecast For NWA Friday
April 30, 2009

Below is the surface temperature forecast for 7pm tomorrow from the GFS computer model. A stationary front will separate the chilly air to the north from the warm temperatures to the south.

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The front will be positioned right on top of Benton, Delaware, and McDonald counties tomorrow afternoon. This along with a few showers could keep the surface temperature much cooler across the northern tier of the viewing area  compared to the river valley. Areas that do see a mix of sun and clouds and temperatures warming into the upper 70s and low 80s will be very unstable tomorrow afternoon. This combined with dewpoints jumping into the upper 60 to near 70 degrees, and some dry/cold air aloft will the set the stage for some very strong wind gusts and large hail. Hail is the number one severe weather threat tomorrow late afternoon through the early overnight. The tornado threat will exist but it will be low due to a lack of wind shear in the low levels.

Ross Ellet

12:45 PM Update
April 30, 2009

Some showers and thunderstorms are developing near Tulsa, OK. These storms will drift into our area later this afternoon with more rainfall and some hail a possibility. Here is the 12:50 pm Super Doppler Radar image.

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The sunshine will help to destabilize us later this afternoon. This along with cold air aloft and yesterday’s outflow boundaries, it is likely additional storms will develop later this afternoon and evening. Here is the RUC model showing precipitation developing through 4 pm.

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RUC model showing showers and storms between 4pm and 7pm.

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RUC model showing another line moving into the area between 7pm and 10pm tonight.

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Ross Ellet

10:30 a.m. Update
April 30, 2009

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 This morning we’ve seen scattered showers and a few isolated thunderstorms in response to another disturbance aloft.  This trend will continue into the afternoon.

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In order to detect some of these waves you need to look at the atmospheric flow around 18,000 ft or 500 mb.  Look closely in the red box to see the wind barbs change direction. 

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The cloud deck is starting to erode across eastern Oklahoma this morning.  Surface temperatures will begin to warm and the atmosphere may start to destabilize throughout the afternoon. 

More scattered showers and storms will be possible this afternoon.  Some of those storms may contain hail and strong gusty winds.

Drew Michaels

THURSDAY STORMS STORM CHANCES…
April 30, 2009

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We are under a slight risk of severe weather today.  We have a 500 mb (approximately 19,000 feet) shortwave stalling out right over us today and this will be the focus for our shower and thunderstorm chances.  The moisture is readily available and it won’t take much energy to give us some storms.  So an upper level shortwave will do the trick.  If storms develop the main threats will be heavy rain and gusty winds.  Here is the latest short range forecast:

PRECIPITATION FORECAST (THURSDAY 11AM)

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PRECIPITATION FORECAST (THURSDAY 1 PM)

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PRECIPITATION FORECAST (THURSDAY 5 PM)

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BLOG ENTRY BY: PATRICK CRAWFORD (4/29 9:33 AM)

10:00 P.M. Rainfall Update
April 29, 2009

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Drew Michaels

Possible Ozark Tornado Tonight
April 29, 2009

We’re still gathering information regarding a possible tornado that touched down northeast of Ozark tonight.  A bow echo moving northeast shortly after 5 p.m. was likely responsible for the tornado.

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The diagram above is of a bow echo.  The winds in the middle of the bow are known as a rear inflow jet.  These winds are responsible for the straight line winds associated with these lines of storms; however, it’s important to also note the circulation that develops on the northern end of the line.  This circulation is known as a book-end vorticy.

An enhanced area of wind shear occurs around this northern end, and is sometimes responsible for the formation of tornadoes.  It’s likely that the Ozark tornado was formed from this type of feature.  They are very hard to detect on radar and are generally short lived!

Drew Michaels

6:00 P.M. Rainfall Updates
April 29, 2009

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Drew Michaels

Flash Flood Watch Through Thursday
April 29, 2009

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Rainfall continues to fall across the area this late afternoon.  Many locations have received between a .25″ to 1.00″ of rain; however, some locations across eastern Oklahoma are approaching 1.50″ of rain.

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Here are some notable totals as of 4:30 p.m.  The National Weather Service in Tulsa has issued a Flash Flood Watch for the entire area until Thursday evening.  A seriesof fast moving shortwaves will move across our area providing more scattered storms starting late tonight and lasting through Thursday. 

The River Valley and Northwest Arkansas will likely get a break this evening; however, more storms are likely overnight.  Total rainfall is expected to range between 1.00″ to 3.00″ by Thursday evening.

Drew Michaels