Mesoscale Discussion #91

MESOSCALE DISCUSSION 0091
   NWS STORM PREDICTION CENTER NORMAN OK
   0344 PM CST THU FEB 02 2012

   AREAS AFFECTED...NRN PORTIONS OF WRN TX THROUGH TX PANHANDLE AND WRN
   OK

   CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 

   VALID 022144Z - 022345Z

   THUNDERSTORMS ARE EXPECTED TO DEVELOP FARTHER SWD OVER PORTIONS OF
   THE TX PANHANDLE BY 00-01Z AND EVENTUALLY INTO WRN OK THIS EVENING.
   INITIAL PRIMARY THREAT WILL BE SUPERCELLS WITH ISOLATED LARGE HAIL.
   HOWEVER...A WINDOW WILL EXIST DURING THE EARLY EVENING FOR ISOLATED
   TORNADOES FROM THE SRN TX PANHANDLE INTO NWRN TX AND SWRN OK. AREA
   WILL CONTINUE TO BE MONITORED FOR A POSSIBLE WW BY 23-00Z. 

   WV AND OBJECTIVE ANALYSIS DATA SHOW AN UPPER JET STREAK FROM SRN AZ
   THROUGH SRN AND ERN NM WITHIN BASE OF TROUGH SITUATED OVER THE CNTRL
   AND SRN ROCKIES. MID-UPPER ASCENT WITHIN THE EXIT REGION OF THIS
   FEATURE AND WITHIN STEEP LAPSE RATE ENVIRONMENT IS PROMOTING
   DEVELOPMENT OF ELEVATED CONVECTION FROM NERN MN INTO THE TX AND OK
   PANHANDLES. THIS AREA REMAINS WELL NORTH OF GREATER MOISTURE AND
   INSTABILITY ACROSS W-CNTRL AND WRN TX...AND AS A RESULT IS EXPECTED
   TO REMAIN BELOW SEVERE LEVELS NEXT COUPLE HOURS. 

   LLJ IS STRENGTHENING OVER WRN TX WITHIN UPPER JET EXIT REGION AND IS
   CONTRIBUTING TO MOISTURE ADVECTION AND NWD DESTABILIZATION. STORMS
   SHOULD INCREASE IN COVERAGE AND INTENSIFY THIS EVENING FROM PORTIONS
   OF THE TX PANHANDLE...NWRN TX AND WRN OK AS THE ATMOSPHERE
   DESTABILIZES NWD WITH TIME. STRONG VERTICAL SHEAR PROFILES WITH
   LARGE 0-2 KM HODOGRAPHS WILL SUPPORT A FEW SUPERCELLS...BUT SOME OF
   THE ACTIVITY MAY EVOLVE INTO LINE/BOWING SEGMENTS. A WINDOW MAY
   EXIST FOR ISOLATED TORNADOES AS STORMS DEVELOP ON NRN FRINGE OF
   GREATER MOISTURE RETURN AND BEFORE ONSET OF NOCTURNAL COOLING.
   OTHERWISE...ISOLATED LARGE HAIL AND DAMAGING WIND WILL BE THE MAIN
   THREATS.

   ..DIAL.. 02/02/2012

   ATTN...WFO...OUN...SJT...LUB...AMA...MAF...

Texas Drought Monitor - January 31, 2012

After the rainfall event we experienced last week, we knew that we would have some good news once the new drought monitor was released. Indeed, we do have good news. For the first time in a long time, parts of Texas are officially out of drought conditions. That’s good news for them, but much of the state still remains in drought conditions. However, lets look on the positive side. Compare the amount of Texas that is in the highest drought category now verses back in September. Most of us do have a reason to be thankful today. More rain is coming!

InsuranceBusters.net

InsuranceBusters.net was established on July 30, 2009.  The personal goal of each and every individual who is employed by InsuranceBusters.net will always be focused on educating the public first. When we do our job correctly, and inform all who inquire, the subsequent result will be them hiring a public adjuster.  Whether that is InsuranceBusters.net, or another licensed public adjuster. I started InsuranceBusters.net to become the voice of the insured.  A voice, that until recently, had no option to be heard, as well as no knowledge of exactly how much money the insurance companies have saved, by merely saying “NO”.

Visit InsuranceBusters at www.InsuranceBusters.net

Storm Prediction Center - Day 2 Convective Outlook

Lets start out with the latest Storm Prediction Center outlook for Thursday. With their latest convective outlook issued earlier Wednesday Afternoon, they have a standard slight risk in effect for portions of West Texas and Southwest Oklahoma. While this outlook will likely change based on mesoscale features, at the time of this post, the risk extended from San Angelo, northwest to Childress, eastward to Ringgold and Eastland County.

0Z NAM: 850 millibar winds at 6 PM Thursday

We like to start off by analyzing the mid to upper levels of the atmosphere to get a general diagnosis of the amount of wind shear/vorticity that will come into play during an event. At 6 PM tomorrow, we have a positive tilt shortwave in place across the four corners region with the right-entrance region of the jet beginning to influence West Texas. In terms of wind shear, we like to see winds at or above 35 knots to support organized convection in the mid to upper levels of the atmosphere. By 6 PM tomorrow, it looks like we’ll have 50 to 70 knots of wind overspreading West Texas at 6 PM.

0Z NAM: 850 millibar winds at 9 PM Thursday

Taking a look at the low levels, specifically at 850 millibars, the 0Z NAM already has a sustained low level jet in place by 6 PM on Thursday. When determining the potential for specific severe weather hazards, we like to check the direction of the wind at 850 millibars. If the wind is coming from the southeast with 500 millibar winds from the southwest, that creates turning with height. If the winds at 850 millibars and 500 millibars are out of the southwest, that creates a linear type setup, which favors squall lines verses discrete supercells. It looks like we’ll have southeasterly 850 millibar winds tomorrow evening. At 6 PM, the NAM has the low level jet at 30 knots.

Fast forward three hours and you still have southeasterly winds at 850 millibars, but notice that instead of the 25 to 30 knots we were seeing earlier that we’re cranking up to 40 to 50 knots! That’s actually normal for a low level jet. After sunset and once the initial daytime heating subsides, the low level jet will increase. The process on why that occurs will make an excellent education post one day So, wind shear looks favorable for severe weather on Thursday across West Texas, but that’s only one of three necessary ingredients for severe weather. Lets take a look at instability…

The first thing to note from this graphic is that these instability values are for 9 PM on Thursday, nearly three hours after sunset. Even with the loss of daytime heating, instability remain high (for a cool season event) along the dryline and warm front. The warm front can be identified by the sudden drop off in instability across Oklahoma, while the dryline is your north-south feature located roughly along Interstate 27. During cool season events, I like to see instability values in excess of 1,000 joules per kilogram (J/kg). That doesn’t look like its going to be a problem tomorrow afternoon and evening. In fact, instability values appear to be in access of 1,500 joules per kilogram which is pretty impressive for early February. Now we have confirmed that wind shear and instability won’t be an issue.

0Z NAM: Surface dewpoints at 9 PM Thursday

Now for our final ingredient… Moisture! While wind shear and instability look supportive of severe weather, I’m a little concerned about instability. No doubt it will be sufficient for severe weather as any value above 50 degrees should be enough for severe thunderstorms, I’m looking in terms of the hazard of more significant severe weather, like tornadoes. For surface based thunderstorms capable of producing tornadoes, I generally like to see dewpoint values in excess of 60 degrees. During the spring time, that number increases to 65 degrees. Yet, there are a few factors that may help thunderstorms tomorrow develop low level rotation even with the lower dewpoint values. The first factor is the terrain and elevation. If thunderstorms form on the caprock, the higher elevation may allow thunderstorms to produce organized low level rotation with dewpoint values in excess of 55° verses the 60° dewpoint values I look for. The second factor is that surface temperatures won’t get too warm, likely upper 60s to lower 70s. That means we won’t have a large temperature/dewpoint spread which will keep cloud bases lower. The third factor is that storms may remain surface based well into the evening, when they will be able to use the abundant wind shear in place. We’ll find that out tomorrow…

Either way, it does appear we will see a complex of thunderstorms form across the eastern Texas Panhandle and Northwest Texas between 5 and 9 PM Thursday, before moving northeast into Oklahoma. The primary hazards with a squall line would be damaging winds and the possibility of large hail. In discrete thunderstorms, large hail and the possibility of a tornado would be possible. If we end up with higher moisture values then currently anticipated, the tornado threat would increase substantially. See the video for complete details…

Storm Prediction Center - Day 3 Convective Outlook

0Z NAM: Instability at 3 PM Friday

The severe weather threat will spread east to the Interstate 35 corridor on Friday as the upper level storm system begins to push northeastward. Compared to Thursday, instability values appear to be much lower. In fact, the NAM doesn’t have any instability values over 500 J/Kg across North Texas on Friday. While instability levels will be higher across South Texas, the upper level storm system will be too far north to cause any issues that far south. There may be a few severe wind reports as the squall line passes across North Texas on Friday, but at this point the severe weather threat looks fairly marginal. We’ll keep an eye on it though and provide an update with our February 2nd Daily Weather Video.

I am planning to chase in West Texas on Thursday. My laptop is still down for repairs so I’ll have to put my desktop in the car as the backup computer. I’m not sure if it has the processor power to stream live video, but if it does I’ll certainly have that available on Thursday! Stay tuned for the latest updates on our Facebook and Twitter pages. I hope to have our next weather video posted by 9:30 AM on Thursday.

Dr. Ted Fujita

The Fujita scale, or “F-Scale” is one of the few pieces of weather jargon that has made it into modern vocabulary and is familiar to everyone from Grandma to Hollywood actors.   Just recall how many times Bill Paxton and Helen Hunt, stars in the movie “Twister”, shouted various F-Scale ratings while racing towards those CGI crafted tornadoes!

The original damage rating scale, the “F-Scale”, was introduced as part of a research paper published in February 1971.

Original F-Scale

Dr. Fujita’s goal was to provide a way to rate each tornado by its intensity and size, and to estimate wind speed in correlation to the damage it caused.  However, it wasn’t until the Super Outbreak of 1974 that the new F-Scale could really be put to the test.  Following the outbreak, Dr. Fujita lead a team of meteorologists that surveyed the path of each tornado, putting together a map of each of the 148 tornado paths and assigning each one a corresponding F-Scale rating.

Even though the original F-Scale was used for well over 20 years, it had a few shortfalls.  Its damage assessment protocols did not take into account how well certain types of buildings were constructed, it did not provide a way to estimate wind speed in cases where there was no damage (i.e. tornado in an open field), and in some cases it allowed for gross over-estimation of wind speeds (damage to weaker structures).

In February 1997, the scale we use today, the Enhanced Fujita Scale “EF-Scale”, was officially introduced to the meteorological world with enhanced protocols for better assessment of storm damage not just to structures of different types, but to vegetation as well. Tornadoes have been rated using the EF-Scale since February 1, 2007.

Current EF-Scale

And so you’ll understand the process…immediately after a tornado hits, storm damage survey teams are deployed by the local National Weather Service office to survey and document the types of damage found.  Only after careful analysis and application of the EF Scale assessment process is a rating given to each tornado.  It’s the amount of damage that determines the rating, not just the wind speed.

Most of us only know Dr. Fujita as the “EF Scale” man, but he has also been credited with being the first to discover “Wall Clouds” (the low and rotating base of a thunderstorm), Downburst winds such as the one that brought down Delta flight 191 at DFW Airport on August 2, 1985, and the “Suction Spot”, a mini-vortex which rotates around the primary tornado funnel.

For more information on the current EF-Scale rating system:

http://www.crh.noaa.gov/arx/efscale.php

For additional information on Dr. Ted Fujita:

http://www.notablebiographies.com/supp/Supplement-Fl-Ka/Fujita-Tetsuya.html

Severe Thunderstorm Watch #200

 URGENT - IMMEDIATE BROADCAST REQUESTED
   SEVERE THUNDERSTORM WATCH NUMBER 20
   NWS STORM PREDICTION CENTER NORMAN OK
   1235 PM CST WED FEB 1 2012

   THE NWS STORM PREDICTION CENTER HAS ISSUED A
   SEVERE THUNDERSTORM WATCH FOR PORTIONS OF 

          CENTRAL AND SOUTHERN ARKANSAS
          NORTHERN LOUISIANA
          EXTREME SOUTHEAST OKLAHOMA
          NORTHEAST TEXAS

   EFFECTIVE THIS WEDNESDAY AFTERNOON AND EVENING FROM 1235 PM UNTIL
   800 PM CST.

   HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70
   MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.

   THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 95
   STATUTE MILES NORTH AND SOUTH OF A LINE FROM 60 MILES SOUTHWEST
   OF DE QUEEN ARKANSAS TO 50 MILES EAST NORTHEAST OF PINE BLUFF
   ARKANSAS.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE
   ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU0).

   REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE
   FAVORABLE FOR SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
   AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
   THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
   AND POSSIBLE WARNINGS. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY
   DO PRODUCE TORNADOES.

   DISCUSSION...A CLUSTER OF THUNDERSTORMS OVER SOUTHEAST OK WILL
   SPREAD EASTWARD INTO AR...WHILE NEW STORMS POTENTIALLY DEVELOPING
   SOUTHWARD INTO NORTHEAST TX.  STEEP MID LEVEL LAPSE RATES AND
   MODERATE CAPE VALUES WILL PROMOTE THE RISK OF A FEW INTENSE CELLS
   CAPABLE OF LARGE HAIL AND GUSTY WINDS.

   AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT
   TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60
   KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 400. MEAN STORM
   MOTION VECTOR 27035.

   ...HART

Before we can use hodographs for our forecasting and analysis, we first must have at least a basic understanding of vectors. A vector is a quantity that, unlike a scalar which has just a magnitude, consists of both a magnitude and a direction. Let’s relate these terms to meteorology! When you check your local forecast, the first thing you may see that your forecast high is 52°F. This quantity is a scalar, because it has only a magnitude. What do you look for next, most likely, the chance of rain! A 70% chance of rain has no direction, just a magnitude, so this value too is a scalar. But what are you likely to look for next? The wind, of course. For the sake of this example, let’s say that the wind today will be 20 miles per hour out of the north-northwest. This value, unlike the other two examples, is a vector. With a magnitude of 20 mph and a direction of south-southeast, or 158°. Important: When people refer to a “north wind,” they usually are talking about wind that is blowing from north to south. When talking direction, north is 0°/360°, east is 90°, south is 180°, and west is 270°. The NNW wind in this is example is blowing to the SSE, and because the direction of a vector is given as the direction it points, we assigned it a direction of 158°. However, to stay consistent with the way things are done in meteorology, from here on out all winds will refer to the direction they are coming from, so a NNW wind will be about 338°.

What does this have to do with hodographs? We’ll get to that! First, let’s show you a blank hodograph just to get that image in your head.

The above image is the most common of several ways a hodograph may be presented. It is the same concept as a polar coordinate chart. The lines directed outward from the center indicate direction, and the different sized rings encircling the center represent wind speed. This is the fundamental part of hodographs that must be understood. Two things: speed, direction. Does that sound familiar? Speed and direction? It should! Remember that wind is a vector, so it has both a magnitude (speed) and a direction.

Let’s start by plotting the wind speed at the surface on this hodograph. Let’s say the wind outside is blowing at 20 knots out of the east, toward the west.

The red dot on the hodograph indicates where the surface wind in this situation would be plotted! This represents the wind vector. In case this is difficult to visualize, here’s what the hodograph would look like with the vector drawn in as an arrow.

Technically, this first image we posted could be used as a real hodograph! It has the chart, with the wind at at least one level plotted. But we will almost never encounter a hodograph with only one level plotted, as it defeats the purpose of such a useful graphical display. Let’s plot the wind speeds in a hypothetical atmosphere all the way up to six kilometers above the ground! We will use the same easterly surface wind, and add in a few more points to show other levels of the atmosphere at the same time.

We can now see the wind direction at five different heights above one location on a single plot! The wind at the surface is blowing at 20 knots from the east. One kilometer above the ground, the wind is blowing 30 knots from the ESE. At 3 km, the wind is 35 knots from the SE. At 4.5 kilometers above the ground, the wind is blowing at 40 knots from the SSW, and at 6 km the wind is blowing 60 knots from the WSW! As before, to help us visualize all of these directions, let’s take a look at the same diagram, but with the vectors plotted.

Like before, the longer the arrow and the farther the plotted point from the center, the higher the wind speed! Without knowing what the atmosphere looks like before hand, we would have no idea which point was which, so points on a hodograph will usually be labeled with either a height or a pressure for reference.

Now that we’ve plotted several points from 0-6 kilometers, there is one more step before we are done. While digitally generated hodographs will usually have more than five data points, this illustrate the same point just as effectively. When a hodograph is created, it is helpful to “connect the dots” of all of the plotted points. This helps visualize how the atmospheric wind profile actually looks more effectively than to just look at several dots. To do this, we will draw a line from the lowest point (the surface), to the second-lowest point (1 km), and continue this all the way to the highest point (6 km). Let’s take a look!

There we have it! This is what a hodograph would look like in the environment we used. The line used to connect the dots shows perfectly that the wind speed increases with height, and the wind direction veers with height. A veering wind profile is one that rotates clockwise with height, like this one. When the wind turns counter-clockwise with height, it is said to be backing. A wind profile that veers and increases with height like this one is extremely favorable for supercells and tornadoes! Here are a couple more examples of hodographs that can be useful for forecasting.

Straight-line hodograph:

This is often called a straight-line hodograph. These do not have to be, and almost never will be, perfectly straight, but hodographs that generally exhibit a straight line fit into this category. Even though significant speed shear can be present, the lack of directional wind shear tends to favor splitting supercells that are more likely to produce large hail than tornadoes.

Weak wind shear environments

In environments like this, winds are weak and sporadic throughout all levels. Coming from several different directions, this hodograph has no winds that exceed 10 knots. Environments with weak wind shear can still have severe weather if instability is high, but it will likely be in the form of multicellular storms with hail and wind as the main threads. Supercells and tornadoes are rare in these environments, but they can happen, especially with extreme instability and local boundaries. For example, the environment near Jarrell, TX, on 5/27/97 looked much like this, but the presence of incredible instability along with a gravity wave moving through the region helped a southward-moving supercell produce a violent F5 tornado.

Values that can be drawn from hodographs

In addition to the assumptions that can be made simply by glancing at a hodograph, a slightly more in-depth look at an environment’s hodograph can reveal a bit extra at times. Here we’ll discuss a couple of these!

Bulk shear and bulk wind difference

Bulk wind difference is the difference between the wind vectors at two levels of the atmosphere. We usually see 0-6 km bulk wind difference, which means the difference between the winds at 6 km and at the surface. We can see this easily on a hodograph by drawing a vector from the surface wind to the 6 kilometer wind! Once we’ve drawn this vector, we can redraw an identical vector that originates at the center of the hodograph.

From the vector we’ve added at the origin of the plot, we can see that this hodograph has an 80 knot 0-6 km bulk wind difference in the ENE direction. When taken into consideration with other factors, this is very favorable for severe thunderstorms! Bulk shear is very similar to bulk wind difference, except that “shear” is normalized over the depth over which it is taken. A wind difference of 100 m/s over 6 km, or 6000 meters, results in a bulk shear value of .0167 s^-1. ((100 m/s)/(6000 m) = .0167 s^-1)

Storm Motion and Storm Relative Helicity

When forecasting for severe weather and possibly supercells, storm motion and storm relative helicity (SRH) are two very important factors that must be considered. The two are related, and both can be estimated using hodographs, although exact values are difficult to ascertain with out help from a computer! Storm motion tends to be near the “mean wind” of the environment, so without going into too much detail, we can estimate that the storm motion in this environment will be somewhere near this area:

The actual mean wind in an environment like this would likely be a bit more northerly and possibly a bit faster, but because tornadic supercells often move right of the mean wind, we have placed our estimated storm motion a bit farther to the east. Once we’ve plotted our storm motion, we can begin finding our storm relative helicity. SRH is typically measured either from 0-1 km or from 0-3 km, and represents the amount of “spin” in the atmosphere between those levels. For supercells in general, many meteorologists use 0-3 km SRH, while 0-1 km SRH can be very helpful when forecasting tornado potential. To calculate 0-3 SRH using this hodograph, we will draw two lines from the storm motion data point to the 0 km (surface) and 3 km data points. The area between these lines and the plotted hodograph represents the SRH in meters-squared per second-squared (m²/s²). It would be difficult to calculate an exact SRH by hand for a hodograph like this, but this would be an environment with a significantly high value! Over time, after observing many hodographs, it becomes easier to estimate SRH by looking at the hodograph. 0-3 km SRH values over 250 m²/s² and 0-1 km SRH values over 100 m²/s² are considered by many to be guidelines for the minimum needed for tornado formation with supercells, but there is no exact threshold. It all depends on the environment!

We hope this has been educational and you have learned something about hodographs. We plan to add more educational postings here with time. If you have any questions or special requests, let us know through Facebook, Twitter, or our contact page. Thanks!

Most people already have the “where” figured out…the basement or a small interior room or closet away from windows…but have you also thought about what you should have with you when you go into your shelter?  It may sound silly, but you may want to consider stocking your safe area with not only a flashlight, but also a pair of old shoes, a jacket…perhaps a blanket…and a bicycle or motorcycle HELMET!

Me (Jenny Brown) Sporting Sean Casey's helmet

A study conducted by the University of Alabama at Birmingham suggests a number of fatalities from the April 27, 2011 Super Outbreak were caused by trauma to the head and neck.  They suggested that motorcycle helmets, football helmets and bicycle helmets offer a practical, inexpensive solution to reducing the risk of head injuries during a tornado.  Even if it’s only minimal protection from being hit by debris tossed about at 100+ mph, a helmet is certainly better than nothing and can help prevent a more serious head injury such as a concussion or laceration.  For children, just purchase an inexpensive bike helmet dedicated to your tornado safe spot.

This advice could also benefit my fellow chasers, especially the ones who think they are fast enough to run between the hailstones a mach speed to grab the biggest one.  Put a helmet on your head and making that dash could be a lot less painful.

To view an article by the Universality of Alabama summarizing their research, click here.

For a complete list of severe weather safety tips, visit the Storm Prediction Center.

This is only the first in a many of discussions meant to help increase your knowledge about severe weather. We’ll have more on what you can do to protect your family during severe weather in subsequent discussions.