Thursday, April 22, 2010 Severe Weather Forecast

There are three things I analyze (as do most meteorologists) when forecasting the severe weather potential for an area: instability, moisture, and wind shear. Here are a few, basic definitions of each of these:

Instability: the tendency for an air parcel (blob of air) to move, in this case rise. Warm air will rise when it is in air cooler (i.e. less dense) than it is. The colder the air of the environment is compared to the parcel, the more easily the parcel can rise. This is necessary for updrafts to develop in thunderstorms.

Moisture: fairly obvious, but the amount of water in the atmosphere. This is necessary to produce condensation and precipitation within thunderstorms.

Wind shear: change in wind speed and/or direction with height or distance. In the case of severe thunderstorms, we need changing wind direction with height. This allows for rotation to develop. If you want to visualize this, take a pencil and hold it horizontally between your hands. Move one hand toward you and the other away from you and you will notice that the pencil will rotate. This is the basic concept of wind shear (we won't get into any differential equations!).

When examining instability, there are two major parameters to look at: convective available potential energy (CAPE) and convective inhibition (CIN or CINH). CAPE is the amount of energy available in the air to produce updrafts. The vertical velocity of updrafts is determined by this energy (again, we won't get into the physics here, but if you want to, let me know via comments). CINH is the amount of energy required to start the process of lifting parcels and is produced by air aloft that is warmer than the parcels we are trying to raise. This cap can be eliminated by dynamic forcing (forces the parcels above the cap) from cold fronts, dry line, and other things.

Here is the forecast CAPE from the GFS and NAM forecast models for 7:00 pm CDT April 22:

The GFS model places a bulls eye of 2000+ J/kg (energy per mass) over southwester Oklahoma in the vicinity of Altus while the NAM has two areas of 2500+ J/kg over western Texas, one southwest of Childress and another northeast of Del Rio. This is plenty enough instability to sustain updrafts, but can they form.

Below is the model forecast CINH for 7:00 pm CDT April 22:

Both models show CINH of less than 25 J/kg over the same areas the models forecast the strongest CAPE. This means it won't take much energy to get updrafts started. The combination of daylight heating and dynamic forcing should be enough to allow rising air to break the cap. Thunderstorm development should not be a problem.

Looking at moisture, dew points will be well into the 60s over the region of greatest instability. 60s are generally good enough for strong convection to development, though upper 60s and into the 70s are generally better (you can't really have dew point too high, especially in this week's set up). In the graphic below (GFS Forecast Dewpoints for 7:00 pm CDT 4/22), we can clearly see a dry line over the high plains of the Oklahoma and Texas Panhandle. This is the boundary we will be watching for thunderstorm development on Thursday. Note the southeasterly winds over northwestern Texas and down the dry line, this will be important later when we talk about wind shear.

In addition to surface dew point, we must look at what is known as the "lifted condensation level", or LCL for short. In basic terms, the LCL is the height at which a lifted parcel becomes saturated (its relative humidity reaches 100%) due to cooling with height. In general, this is where we find the cloud bases. If this is too high, it will be difficult for tornadoes or supercell structures to develop. Looking at the LCL forecast for Thursday evening, the LCLs look to be low enough to support low-level cloud formation across southwestern Oklahoma to around the Childress area, but the rise in LCLs over western Texas and farther south will make this low-level cloud formation difficult.

Lastly, we look at wind shear. There are two types of wind shear we must look at: deep layer and low-level. Deep layer wind shear is required for rotating supercells to develop. Low-level wind shear is what's required for tornadic development. Another parameters that we can examine is called helicity. Helicity is the tendency for air to rotate. The graphic below is the forecast helicity for Thursday evening.

The NAM is forecasting helicity of over 400 in a small area near Childress on Thursday evening with regional helicity values at or above 200. This means that wind shear will be high enough to support the possibility of supercell development and just maybe, a tornado or two.

Putting this all together, we will now examine the forecast soundings for Childress, Texas, the area that appears to be the target for severe weather at this time.

The sounding on the right (NAM) paints a much more ominous picture than the one on the left does. The green line on the left indicates dew point and the red line indicates temperature. The flags on the right side of each chart indicate the direction that the wind is blowing FROM and the speed (long lines mean 10 knots, short lines mean 5 knots, and triangles mean 50 knots). The winds on the NAM graphic show intense low-level shear while the GFS seems to be showing much weaker shear. This shows that both models still have fairly strong disagreements. If the NAM were to play out, the probability of severe weather, and tornadoes, would likely be much higher than if the GFS were to plan out. So here is the bottom line:

Probability of thunderstorms: high (70%)
Probability of severe thunderstorms: moderate (50%)
Probability of one or more tornadoes: moderate (40%)
Probability of one or more strong tornadoes: low (10%)

Area to most likely experience severe weather: southwest Oklahoma/northwest Texas
Timing: late Thursday afternoon and early evening

I will probably put out a "follow up" blog should things significantly change regarding this event. Remember that these are the thoughts of an amateur. Listen to products from the National Weather Service for decisions regarding the protection of life property.

A Needle in a Haystack

This morning I woke up and noticed the Storm Prediction Center's Day One Convective Outlook. A simple slight risk area over northwestern Oklahoma with a 2% risk for tornadoes. I already knew I couldn't storm chase (though I don't consider myself a storm chaser by any stretch of the imagination), but being in Norman, Oklahoma, I know many friends who are devoted storm chasers. Dynamics weren't bad (30 knot low-level jet), decent wind shear, and a little CAPE. Dew points were severely lacking though, and this lack of moisture was one of the biggest factors keeping most recreational storm chasers from going out today.

Fast forward to 11:30 am. Over lunch, me and a friend looked at the setup (but neither of us are available to chase). CAPE was really starting to ramp up, but in Texas, but both of us came to the conclusion that while today was a solid slight risk day, not something we would want to chase (linear mode with little tornado threat). The SPC had shifted the slight risk area farther south and east.

I finished up classes today at around 2:30 pm central time, and examined the weather conditions yet again. I held my previous conclusion that the event had potential to produce severe weather (in Texas), but was not chaseable. At around 5:00 pm, I left my apartment to pick up a friend from the Oklahoma City airport. I was approaching the I-235/I-35 interchange when I got a phone call that changed the day. "Jason! There is a huge tornado on the ground near Elk City!" My immediate reaction was more astonishment than anything else. How could a tornado, especially a strong one, form in western Oklahoma? Looking back at the radar, it's kinda hard to pinpoint exactly where the tornado was located (it's somewhere in the red "blob").

In conclusion, any storm chasers who "missed" this tornado thinking there wouldn't be anything out there today, should not feel bad for missing this storm. It was virtually impossible to predict anything like this would have developed. With the SPC only putting out a 2% risk for tornadoes (which was still quite accurate considering there was one isolated tornado), it's no surprise many people didn't chase. The only chasers that saw this tornado were extremely devoted (and lucky) storm chasers. The severe weather season has a long way to go!

Spring is in the Air

Now that spring is finally here (meteorologically though not astronomically), WHEN IS IT GOING TO WARM UP!!! If you're like me, you're probably quite sick of this cold, miserable winter we have had, thanks to a largely negative Arctic Oscillation, but forecasts from the Climate Prediction Center (CPC), are beginning to show we could be moving away from this negative trend. This trend will produce warmer conditions (or is that just spring?).

The GFS is developing what I consider to be the first "spring-like" weather system over the high plains on Monday. You can see from the image below, that a 996 mb low causes decent warm air advection inland across Texas and into southwestern Oklahoma. There is a slight possibility of a few severe thunderstorms, but due to limited moisture, I don't expect anything significant, maybe a few severe reports at best.

So how long until severe weather season really cranks up? Well if you look at the 00z March 7 run of the GFS (http://hoot.metr.ou.edu/models/gfs/surf/dewp), it looks like it could be a while. Dry air spills into the plains following the storm system forecast to pass through this week with the 60 F dew point line moving very far south. Also, if you look at the sea surface temperatures in the Gulf of Mexico (10 C is about 50 F, 15 C is about 59 F, 20 C is about 68 F, and 25 C is about 77 F), the Gulf waters are quite cool (though interestingly, you can make out the loop current in the southeast Gulf of Mexico), this means there is not very much heat content in the Gulf, which will limit warm air and moisture advection, a key ingredient for severe weather development. Looking at this, we are probably in for a quiet start of severe weather season.

So what do I predict? I am predicting a slow warming of temperatures across the United States, but slower than normal because of the lack of heat content over the Gulf of Mexico and a negative, but becoming more positive, Arctic Oscillation.

This is my first "serious" discussion about weather and climate and I'm running on only one and a half semesters of actual meteorology courses (though I have been studying the weather for quite some time), so if you have questions or comments, feel free to leave your comments, but please don't be to harsh on me! Just point out any errors (kindly) if you feel something is incorrect.

Tropical Storm Erika has formed east of the Leeward Islands

An Air Force Reconnaissance mission has determined that the vigorous tropical wave that was being monitored during the last few days has developed a closed-circulation and winds in excess of 40 mph, it is therefore classified as Tropical Storm Erika. The initial forecast from the National Hurricane Center projects Erika to pass north of the Antilles and the Bahamas as it moves on a generally, northwestward track during the next five days. Tropical Storm Watches have been posted for parts of the Leeward Islands. A complete listing of the watches can be found on the NHC website at http://www.nhc.noaa.gov.

For the intensity forecast, there is a decent amount of uncertainty. The NHC points out that while global models forecast an increase in wind shear (thus weakening Erika), two very reliable models, the GFDL and HWRF, bring Erika to hurricane status despite the wind shear. In my opinion, it is likely that by day five, Erika will be either a strong tropical storm or weak hurricane (winds in the 70 to 80 mph range).

Related to the uncertain intensity forecast, the track forecast is also somewhat uncertain. Should Erika stay relatively weak or weaken during the next few days, she would take a more southern track, posing a stronger threat to the Bahamas and Florida. Should Erika intensify, she would feel the effects of a weakness in a ridge of high pressure to the north and bend more northwesterly this weekend, possibly posing a threat to northern Florida or the southeastern coast of the United States.

Bottom line:

• Tropical Storm Watches in effect for parts of the Leeward Islands.
• Interests in the Virgin Islands, Puerto Rico, Dominican Republic, Haiti, and the Bahamas should monitor the progress of Erika.
• Erika is less likely to impact the United States the stronger she becomes. In other words, a weak storm will stay on a more westerly course and a strong storm would turn more north-northwesterly.
• Large uncertainty in intensity forecast means significant uncertainty in the track forecast.

Powerful Supercell over Barber County, Kansas

Here are a few radar images of a strong, likely tornadic, supercell thunderstorm over Barber County, Kansas taken at 5:16 pm CDT on April 26. The top image is the Base Reflectivity image, the middle is the Base Velocity, and the bottom is the Storm Relative Motion. All of these images came from the Vance Air Force Base Radar in northern Oklahoma.

March 26, 2009 Severe Weather Pictures

These pictures were taken on the Louisiana State University campus in Baton Rouge, Louisiana after a night of severe weather. According to the National Weather Service, there was a wind gust to 67 mph at Baton Rouge Metropolitan Airport. Also, Entergy reported about 15,000 customers without power during the worst of the storm. Numerous reports of trees and limbs down across Baton Rouge. Damage reported in the Southdowns Neighborhood near the LSU campus. A more complete report on the Baton Rouge area severe weather will be available during the next few days.

March 25, 2009 Storm Reports

Here are some severe weather reports that have come in as of 8:53 pm March 25.

• Hail reports: 27; the largest hail reported was three inches in diameter, or teacup-sized, near Jollyville, Texas in Travis County at 5:45 pm.
• Damaging wind reports: 14; the highest reported wind gusts were 60 mph near The Woodlands, Texas and Cut and Shoot, Texas, both in Montgomery County, between 8:00 pm and 8:15 pm.
• Tornadoes: 1
  

1. A tornado was reported near Huntsville State Park near Huntsville, Texas at 7:45 pm.

Severe weather is forecast to continue over eastern Texas and Louisiana tonight. More reports will be posted as they become available.