Archive for September, 2009

Wind Shear Explanation
September 30, 2009

If you’ve read our recent posts on the severe weather for Thursday, you’ve probably noticed the term “wind shear”.   Wind shear is the changing of wind direction with height.

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Here’s a diagram of a typical wind shear situation.  Winds at the surface are out of the south, while winds aloft are flowing out of the west-southwest.   Of course as you go higher in the atmosphere the wind speeds increase due to the lack of friction.

Storms that develop in a highly sheared environment can develop an internal circulation known as a mesocyclone.  Rotating thunderstorms are usually known as supercell thunderstorms; they are the most dangerous of the convective storm types.

Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (F2-F5) tornadoes, as well as downburst damage and large hail.

Drew Michaels

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Severe Weather Update for Thursday
September 30, 2009

 Not a lot of changes in the severe weather threat for Thursday.  The computer models have been consistent moving a cold front into our area from 3 until 9 p.m.  The cold front will be fast moving; by 10 p.m. the rain and storms should be out of our area.

The atmosphere ahead of the front is what we’re concerned about.  Moisture will continue to spread north late tonight and throughout the day on Thursday.  Dewpoints in the 60’s will act as sufficient fuel for the showers and storms. 

Wind shear will also increase ahead of the upper level trough.  Strong wind shear will help to rotate thunderstorms; however, the big question will be the strength of the instability.

Clouds will likely race across the area on Thursday.  CAPE values are forecast to remain below 1000 J/KG.  The models are trying to hold the low level clouds throughout the afternoon; however, the clouds will likely break and the surface temperatures should warm quickly.  Increased sunshine will help to boost the CAPE values. 

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There will be a CAP in place during the afternoon.  That CAP should break when the trough approaches from the west.  Notice the cold air aloft in the picture above.  If the sun can come out on Thursday afternoon expect the severe weather threat to increase.

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The area remains under a slight risk for severe weather.  The isolated tornado threat will be confined to the late afternoon with peak heating.  If individual cells develop they will eventually congeal into a squall line of storms by the evening.  The largest threat with a squall line will be damaging winds of 60 mph.  Check back on the blog on Thursday for updates throughout the day.

Drew Michaels

STRONG MOISTURE SURGE THURSDAY…
September 30, 2009

Our winds will be ramping up out of the south and southwest by Thursday morning and that will lead to a big increase in moisture.  The best way to see the movement of moisture is to examine the dewpoint temperatures.  The definition of dewpoint temperature is the temperature the air has to cool in order to reach saturation (100% humidity) at a constant pressure.  The basics with dewpoint temperatures, the higher the dewpoint…the higher the moisture content of the atmosphere.   Here is a look at the dewpoint forecast for the next 2 days…notice the high increase in the dewpoint temperatures ahead of the cold front!

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Monday we had a cold front move through, but the moisture was not available to produce any storms, but as you can see our next cold front will have plenty of moisture!  Therefore, we will be under a slight risk of severe weather Thursday for the late afternoon and evening hours.  Here is the latest on our severe weather risk.

THURSDAY SEVERE WEATHER OUTLOOK

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RAINFALL ESTIMATE (THURSDAY AM – FRIDAY AM)

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This is not looking like a major rain event with only 0.5 inch to just over 1 inch of rainfall possible.  It’s the winds that we will have to watch very closely…we could be getting gusts well over 50 mph within these storms and there could be some rotation, so an isolated tornado can’t be ruled out.  Make sure you read the detailed blog by Drew Michaels titled “SEVERE WEATHER THURSDAY”…there is a great explanation on why we are looking out for a possible tornado threat.

BLOG ENTRY BY: PATRICK CRAWFORD (9-30  5:56 AM)

Severe Weather Thursday
September 29, 2009

We’ve enjoyed a rather tranquil weather pattern over the last several weeks; however, that trend is about to change as a strong upper level storm system heads our way for Thursday!

The forecast models are in agreement that strong to severe thunderstorms will develop across our area by late Thursday afternoon and evening.  Here’s the set-up:

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A strong trough at 500 mb, or 18,000 ft, will move into our region for Thursday afternoon.  This trough is negatively tilted, and is reminiscent of a spring like upper level pattern. 

A negatively tilted trough increases the wind shear within the atmosphere.  The winds aloft will flow out of the west southwest, while the surface winds blow from the southeast.  The turning of the winds with height is known as wind shear.

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Wind shear will be enhanced by Thursday evening.  SRH values are forecast above 300 which would enhance the threat for rotating storms.

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Right now a slight risk for severe weather has been issued across the central plains.  I wouldn’t be surprised if this possibly becomes elevated to a moderate risk by Thursday. 

The strength of the upper level winds really makes this type of scenario conducive for the development of severe weather.  We’ll have more updates..so check back on the blog.

Drew Michaels

Why The Colors Change?
September 28, 2009

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I love this time of the year.  The cooler air from the north signals the possibility of a beautiful fall color change across the Ozarks and Ouachitas, but what exactly causes the change?  Our friends at www.arkansas.com have a great explanation I have included below. 

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Frost rarely triggers fall foliage. The first heavy frost seldom occurs until the colorful period is nearly over. Two factors are primarily responsible for starting the fall color process. The first is the changing of the season. Plants react to less light intensity as the earth tilts on its axis and the days grow shorter. To the human eye, the sun appears to move further south on the horizon. The second major contributor is weather during the critical period of mid-September to mid-October.

Many warm days and cool nights at the beginning of fall are very good for production of red colors. According to Dr. Don Culwell, a University of Central Arkansas botanist: “As the temperature drops, the cell’s machinery begins to shut down. Cool nights change sugars to red pigment.” Leaves produce some sugars for food during the day, but cooler temperatures at night keep these sugars from moving out of the leaves and into the tree.

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Weather patterns during September and October (in Arkansas) are far more important. For instance, if it’s too rainy during this period, we will not have the warm, sunny days or the cool, crisp nights described earlier. If it turns too cold, we will have cool nights, but the lack of warm days will discourage leaves from manufacturing sugars while the sun is shining. If it stays too hot at night, sugars will continue to pass into the tree instead of collecting in the leaf until late in the critical period, when frost or heavy wind may strip the foliage from the trees before much color is apparent. When the right things happen at just the right time, we enjoy a statewide outbreak of the picturebook color that is always present in one area or another each year. Most years, great color appears in different regions at various times in October and November, which is why the weekly updates are valuable.

Speaking of updates, we’ll have a weekly update on the color change so you can plan your perfect hike or drive.  Check back on the blog.

Drew Michaels

LATE WEEK STORMS…
September 28, 2009

Another cold front will be arriving later this week and unfortunately this cold front will have enough moisture to produce some showers and storms.  Some of these storms could be severe with hail and damaging winds.  The Storm Prediction Center is already keeping a close eye on the set up for Thursday.

THURSDAY

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THURSDAY SURFACE MAP

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We are not anticipating a huge rain event like we had early last week, but over an inch of rain will be possible in parts of the viewing area.

PRECIPITATION FORECAST (FRIDAY 1AM)

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BLOG ENTRY BY: PATRICK CRAWFORD (9-28  8:10AM)

Big Changes Tonight….
September 27, 2009

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This afternoon temperatures warmed well into the 80’s and 90’s thanks to a southerly wind and a mostly sunny sky.  Average highs for late September are in the upper 70’s and lower 80’s.  Fall will quickly return for Monday as a cold front arrives from the northwest.  

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The cold front will usher in cooler air aloft for Monday and Tuesday.  850 mb temperatures will go from 20-22 degrees Celsius on Sunday to around 8 degrees for Monday.

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Expect the winds to shift after midnight as the front moves through.  Northwest winds will drop highs in the 70’s by Monday afternoon; that’s 10 to 15 degrees cooler than Sunday.  The front will move through dry with the lack of low level moisture.

Drew Michaels

The Coolest Air Of The Season Arrives Tuesday Morning
September 26, 2009

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Highs will reach the mid 80s to low 90s Sunday afternoon. It may be the last good opportunity to hit the lake or pool considering there will be plenty of sunshine as well. A cold front will slice temperatures 10 to 15 degrees by Monday afternoon. The bulk of the cold air will arrive on Tuesday morning as lows will dip to near 40 degrees in northwest Arkansas and the upper 40s for the river valley. The next best chance for storms will be on Thursday night and Friday.

Ross Ellet

1 Day Of Summer On The Way
September 25, 2009

The sunshine has returned and now the temperatures are about to increase. Below is the 850 mb temperatures for Sunday evening.

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Temperatures are expected to top out in the mid 80s in northwest Arkansas and near 90 in the river valley. The summer like temperatures will only last one day before fall returns. A cold front will move through on early Monday morning and as a result the cooler weather will return. The 850 mb temperatures for Monday evening are below.

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Highs on Monday should be a solid 10 degrees cooler from Sunday’s highs. Temperatures will bottom out on Tuesday morning with lows in the mid 40s in northwest Arkansas and the low to mid 50s in the river valley.

Ross Ellet

DENSE FOG ADVISORY…
September 25, 2009

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A large portion of the area is under a DENSE FOG ADVISORY this morning.  Be prepared to take it slow this morning and maybe leave a few minutes early to your morning denstination.  The advisory will last until 9am and then the sunshine we are expecting for Friday will help to move the fog out of the area.

BLOG ENTRY BY: PATRICK CRAWFORD (9-25  3:25 AM)