I’m going to use an ingredient-based approach to this forecast. What is needed for hail, specifically large hail? There needs to be rapid cooling with height, as well as robust updrafts that can sustain the weight of the hail. In essence, we need to know the freezing level, and the amount of atmospheric instability. We’ll start off with the freezing levels. See the two dashed white lines in the picture to the right, the lines that go from bottom left to upper right? The leftmost of those lines represents the -20C line, and the right one represents the 0C line. Where these lines cross the red line represents the place on the graph where these temperatures are located. The 0C line is roughly 3,500 meters above ground. That’s fairly low! Then, the -20C line is roughly 6,000 meters above ground. These relatively low levels of cold air mean that strong thunderstorms have the ability to produce hail. How large that hail gets, however, is represented by the amount of instability in the atmosphere. Mid level lapse rates, or the change in temperature with height, were fairly steep, being well over 7.5C/km. The Convective Available Potential Energy, or CAPE, was also quite high, in the area of 1,800 j/kg (joules per kilogram). You don’t necessarily have to know what the numbers mean, but this tells me that the atmosphere has enough energy to sustain thunderstorm updrafts and hold up the hail for a longer period of time. You should also note that this number was higher than yesterday’s, and yesterday had some fairly large hail.

There is one glaring issue that is preventing the severe potential from being stronger – lack of veering winds. When there’s strong wind shear, it allows the updraft to separate from the downdraft, allowing the thunderstorm to be long lived. In today’s case, there’s not a whole lot of that. This means that eventually the updrafts will collapse. However, due to the reasons listed above, large hail is still quite possible. In summary, thunderstorms have already developed. Visible satellite imagery shows updrafts popping up, while doppler radar confirms that the heavy rain has already begun with several cells. Several of them have even been severe warned! The environment they are in is relatively favorable for hail. Using yesterday as a comparison, today should be about the same, if not more severe.

Our forecast starts with how things are supposed to begin. Right now, there is a tight temperature gradient (specifically THETA, for you other meteorologists) over in the Central Plains. See the area circled in white, and how it goes from blue to yellow within a relatively short distance? This distance will decrease as cold air continues to pour over the Rockies (cold air advection), creating a strong frontal boundary. This will get picked up by an upper level shortwave trough, and in summary, create a surface low pressure with a strong cold front by tomorrow morning. This low pressure is the culprit for all of our severe weather worries! On top of that, a surface low pressure north of the Great Lakes will cause warm temperatures to be brought up (advected) from the Gulf of Mexico, creating our warm and moist temperatures. This will create the setup for our current situation. However, a concern of mine is about how much cloud cover we may, or may not, have. Something called isentropic analysis is indicating that moisture lift will take place, which could cause clouds to cover our area. If this happens, our surface heating is reduced, and the atmosphere cannot mix and destabilize as much. If that happens, instability will rely on whether or not cooling happens in the upper levels. The Storm Prediction Center is also concerned that wind shear will not be as strong, or even present, in some areas. If this is the case, then tornado potential plummets, but still warrants a damaging wind/hail risk. In other words, we have a rough idea as to what will cause the severe weather, roughly when, and what we’re dealing with. However, smaller scale details are eluding us, and weather models are not doing a good job of resolving these issues. We won’t know whether or not enough shear will take place until tomorrow, so this will be something to watch right now.

Lastly, there is concern about the evolution of the shortwave troughs that will dictate the weather tomorrow. Right now, it looks like a shortwave (denoted by the arrow in the image) will progress down the jet streak and into the central/southern plains, where the temperature gradient is sitting. From there, it is believed that it will pick up the front and create a surface low, and then progress back Northeastward. However, the wild card in this situation is the upper level closed/cutoff low off of the coast of Mexico. Depending on how this low moves, it can alter the course of the jet streak carrying the shortwave trough, and therefore alter the dynamics/track of the surface low pressure.  Again, the weather models are not doing a great job of convincing meteorologists that there is an accurate prediction of what will happen. Weather models do not handle features like these very well, so it will likely be an issue to be investigated tomorrow morning. However, this is where good old fashioned hand analysis can make or break a forecast! (read this article: http://tempestchasing.com/17/02/2012/other/met/the-dying-art-of-hand-analysis/ ). This is the other issue that is causing uncertainties in the forecast for how severe the weather will be tomorrow.

In summary, this looks like it will be more of a severe squall line event. Damaging winds and large hail will be possible with thunderstorms, despite the limited instability. Tornado probability is uncertain due to some of the dynamics, including low level wind shear, but a few isolated tornadoes are not out of the question. Further south, things will get a little more intense than here in Tennessee, but again, nothing apocalyptic. Areas further south and east will get the squall line later in the evening, probably after 8:00 PM CST, and will probably continue into overnight hours. I do expect we will see a handful of damaging wind reports and a few hail reports. More will be known about the risk by tomorrow morning.

As far as my explanation, I am at the American Meteorological Society conference in New Orleans, LA. The image above (this morning’s sounding by LIX) is the fruit of this conference; Patrick Marsh released his new sounding analysis program called SHARPPy. I’ve been here since last Friday, attending talks, meetings, presentations, and the whole lot. I got to meet Reed Timmer, the now retiring Director of the National Hurricane Center, and Dr. Josh Wurman of the Doppler on Wheels (DOW). In other words, I’m in weather nerd heaven and having a great time. I return home Saturday and will be able to resume normal life, if life permits.

No severe weather is expected for the Middle Tennessee area over the next few days.

Generally speaking, in the last 2 years we have had at least 1 significant tornado risk during fall severe weather season. If this trend continues, it is likely that we will see at least one significant risk this year. However, the atmosphere does not like to be put in a box. Therefore, at this point we can only assume what will happen. Based on long range modeling, however, it looks like Tennessee will experience above average temperatures over the next month. We cannot guarantee a lot of things from this alone, but one can guess a few…

Warmer temperatures do not guarantee more moisture, but it certainly increases our chances of getting that moisture. With the jet stream in better position, warmer temperatures in place, and the potential for good moisture could indicate an active season for the Southeast. Will this actually come to pass? We won’t know until it happens.

Until that happens, however, enjoy the lovely weather! Things should stay cool and dry through the next week and rain free. Though not my favorite kind of weather, I know the rest of the world enjoys it, so enjoy it while it lasts! I’ll be happy when fall severe weather comes around .

Also, keep checking back for updates, as I am planning on making a trip to Norman, OK, for the National Weather Festival. I’ll post about it if it comes to pass!

As said above, a strong mid-level jet is currently situated around a cutoff low, perfectly positioned to provide copious amounts of wind shear through the atmosphere. These strong winds extend through the lower atmosphere, and there is even a 40-50kt low level jet forecast to take place after dusk, bringing plenty of moisture out of the Gulf. By my limited understanding, if this takes place, surface instability will continue into the evening and allow for a better chance of severe thunderstorms, mainly across northern portions of Mid-TN. If this is the case, the strong, shearing winds in the upper-air profile to the right will provide an environment favorable for severe weather. However, most of this is speculation, and the models are inherently imperfect. The challenge is knowing how to sift through the data. Unfortunately I do not have much more to add, but through the afternoon and early evening, I am going to continue to monitor the weather and try to further analyze the situation. Even though severe weather isn’t staring us in the face (yet), be prepared for some storms to be severe tonight. The National Weather Service is saying that places West of I-65 have the best shot; I’m going to say West of I-65 and North of I-40.

I’ll keep updating the website as things change. Thank you for reading, and don’t forget to check for updates on Facebook and Twitter!

Irene is forecast to become a category 4 hurricane at it’s peak, which would be approximately 2 AM friday. A category four hurricane has *sustained* winds between 131-155 MPH. Irene is currently a category 2 hurricane (96-110 MPH), forecast to be a category 3 (111-130 MPH) hurricane by tomorrow. Infra-red satellite imagery shows that Irene has begun to intensify quite a bit over the last 12 or or so hours, and is really about to kick it into high gear! Irene is moving over very warm waters with a relatively unobstructed intake of that heat, which will result in further intensification. Because of this, it is completely within the realm of possibility for Irene to become even stronger more quickly. Irene needs to be watched very closely, because she could become quite strong and may even make landfall as a category 4 hurricane instead of the forecast category 3.

Irene has not yet developed a well defined eye wall (see left image), which is interesting. I am not well versed in hurricane forecasting, this is a first for me, so I am not quite sure what this indicates. However, I would guess that this means that Irene is not a “mature” hurricane yet. The fact that it is not fully organized might have to do with the fact that portions of Irene are over the Dominican Republic. Once Irene moves out into the bahamas, I expect we will see an eye wall develop on satellite and it will probably correspond with the category 3+ conditions. If you look at this visible satellite image from this morning, you can see the lack of a well defined eye wall. However, you can also see a very intense updraft and the further strengthening of Irene.

I’ll close this off with the forecast text from the National Hurricane Center. If you are in the path of Irene, you need to start getting prepared! If you are chasing Irene, I wish you the best of luck. Stay safe!

HURRICANE IRENE DISCUSSION NUMBER 12

NWS NATIONAL HURRICANE CENTER MIAMI FL AL092011

500 AM EDT TUE AUG 23 2011

SMOOTHING THROUGH ALL OF THE AIRCRAFT FIXES SINCE IRENE MOVED OFF THE COAST OF PUERTO RICO MORE THAN 18 HOURS AGO YIELDS A LONG TERM MOTION OF 295/10 KT…WHICH IS USED FOR THIS ADVISORY. THERE IS NO SIGNIFICANT CHANGE TO THE PREVIOUS FORECAST TRACK REASONING. THE 23/00Z G-IV JET AIRCRAFT AND AIR FORCE C-130 DROPSONDE DATA APPEAR TO HAVE SETTLED DOWN THE MODELS…AND THERE IS CONSIDERABLY LESS DIFFERENCE AMONG THE VARIOUS MODEL SOLUTIONS NOW. THE OVERWHELMING CONSENSUS IS THAT IRENE WILL GRADUALLY TURN NORTHWESTWARD OVER THE NEXT 2-3 DAYS AND THEN MOVE NORTHWARD THROUGH A DEVELOPING BREAK IN THE SUBTROPICAL RIDGE OVER THE SOUTHEASTERN UNITED STATES. THE OFFICIAL FORECAST TRACK IS SIMILAR TO THE PREVIOUS ADVISORY AND LIES VERY CLOSE TO CONSENSUS MODELS TVCN AND TVCA.

AN EARLIER RECON FLIGHT INDICATED THE PRESSURE HAD ONLY FALLEN BY 2 MB DOWN TO 978 MB…AND THE OVERALL APPEARANCE IN SATELLITE IMAGERY HAS CHANGED LITTLE SINCE THE PREVIOUS ADVISORY. AS A RESULT…THE INITIAL INTENSITY IS BEING HELD AT 85 KT…WHICH COULD BE GENEROUS. IRENE IS FORECAST TO REMAIN IN A RELATIVELY LOW VERTICAL WIND SHEAR ENVIRONMENT AND OVER SSTS NEAR 30C. THAT COMBINATION…ALONG WITH EXPANDING OUTFLOW IN ALL QUADRANTS…SHOULD ALLOW FOR IRENE TO BECOME A MAJOR HURRICANE WITHIN THE NEXT 24 HOURS ONCE THE CYCLONE CLEARS THE EFFECTS OF HISPANIOLA…AND PROBABLY MAINTAIN MAJOR HURRICANE STATUS THROUGHOUT THE REMAINDER OF THE 5-DAY FORECAST PERIOD. THE INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS ADVISORY AND CLOSELY FOLLOWS A BLEND OF THE HWRF AND GFDL MODEL INTENSITY FORECASTS…AND CONVERSION OF THE SURFACE PRESSURES DEPICTED IN THE GFS AND ECMWF MODELS.

WIND RADII WERE EXPANDED BASED ON A BLEND OF RECON FLIGHT-LEVEL WINDS…SFMR WINDS…AND SURFACE OBSERVATIONS.

IRENE IS FORECAST TO BECOME A LARGER THAN AVERAGE HURRICANE. IT IS IMPORTANT TO REMIND USERS NOT TO FOCUS ON THE EXACT FORECAST TRACK…ESPECIALLY AT DAYS 4 AND 5…SINCE THE MOST RECENT 5-YEAR AVERAGE ERRORS AT THOSE FORECAST TIMES ARE 200 AND 250 MILES… RESPECTIVELY.

These thunderstorms will be coming from the north, heading south (due to the upper level high pressure’s circulation altering the jet stream) and the storms will be lined up from west to east. Convective Available Potential Energy (CAPE) will be well over 5,000 even into 6,000 j/kg due to our very warm environment, with low level lapse rates already over 8.5c/km! For a quick crash course, CAPE is the amount of potential energy available to a storm, and a lapse rate is how fast the atmosphere cools with height. 8.5 degrees celsius for ever kilometer is considered nearly extreme! In other words, this damaging wind setup is beginning to really take shape, and it will be interesting to see what these storms do. The atmosphere is already primed, all we are waiting on is for the front to pull the trigger. The image to the right is the experimental High Resolution Rapid Refresh (HRRR) model showing simulated radar reflectivity, and is confirming the chance for thunderstorms this evening. Even though there is a margin for error, it is looking increasingly likely that we will see storms tonight.

What could prevent us from seeing this? Well, this whole forecast goes to pieces if the frontal boundary doesn’t show up, or shows up too late. We’ll have to keep an eye on whether or not that happens. Enjoy tonight’s storms, they should be quite fun!

..TN AND MID MS VALLEYS…

MORNING CONVECTION OVER PARTS OF KY/WV/VA HAS ERODED THE AREA WHERE STRONG INSTABILITY REMAINS…WITH GREATEST THREAT OF SEVERE STORMS REMAINING OVER PARTS OF KY/TN INTO SOUTHEAST MO. HOT /100F+/ TEMPERATURES AND MUCAPE VALUES OVER 5000 J/KG WILL PROMOTE STRONG UP/DOWNDRAFTS…ALTHOUGH VERTICAL SHEAR IS RATHER WEAK. LATEST STORMSCALE MODEL SOLUTIONS INDICATE POTENTIAL FOR UPSCALE GENERATION OF MCS SAGGING INTO NORTHERN MS/AL THIS EVENING. DAMAGING WINDS ARE THE MAIN THREAT WITH THESE STORMS.

Today, the atmosphere will be pretty unstable, with CAPE values well over 2,000 j/kg. Our initiation mechanism today will be an upper level shortwave trough making it’s way through Kentucky. This shortwave trough will continue to circulate around the upper level high pressure over the central US, eventually making it to Tennessee. Forecast models have this shortwave trough kicking off storms over the next few hours, and then storms should die down after the sun sets. This event isn’t expected to be anything major or anything complicated. Again, we have a lifting mechanism (shortwave trough), unstable atmosphere, and clear skies. The ingredients are in place, now we just have to see what happens.

Don’t be surprised to see a few severe thunderstorm warnings today if things do kick off. Also, heed them! Downburst winds can be just as strong as dangerous as tornadoes in some situations, do don’t take warnings likely.

Even though the National Weather Service and the weather community have been expecting this heat wave for the last week, it never really caught my attention until last night when I was drawing my own weather analysis map. When I saw how large this high pressure was, I knew we were in for it. This was confirmed with this morning’s weather map analysis, where the high pressure actually GREW overnight! Take a look at the image to the right. I’ll explain things a little. This is a 500 millibar map (millibars is a unit of pressure). Now, 500mb is just a pressure reading, and can be at varying heights above ground. So, we contour them (the black lines), or lines of a constant height. The higher the height (center of the map right now) the higher the pressure. The lower the height (Pacific Northwest) the lower the pressure. (Confused as to why this is the case? check out this link here.) Now, areas in blue are areas that dropped in height/pressure over the last 12 hours, while areas in orange are height/pressure rises. Now you can kind of get a picture of what the atmosphere has done. A low pressure off of the West Coast as brought the heights down a little bit, but not by much. Also, a Canadian low pressure dropped the heights a bit, but it is also leaving the area and will not affect us much. The high pressure, however, enlarged and grew in intensity, expanding to the Northwest. This and similar things will likely be the pattern for the next week. This pattern is called a Ring of Fire, because areas on the outer edges of the high pressure are unstable enough to support some summertime thunderstorms. The general storm motion tends to rotate clockwise around the high pressure.

So, just how bad is it going to get for us in the Southeast? Well, as early as Monday we can expect temperatures back in the mid and even upper 90s, but the overall likelihood of *excessive* heat isn’t quite like what’s going on elsewhere. The National Weather Service in Nashville doesn’t expect heat index values to head into heat advisory levels over the next week. So, although it will be hot, it shouldn’t get too dangerous. Most of the dangerous heat will be confined to the Central US for the time being, and maybe even the East Coast as the high begins to move out. Forecast models show this high pressure system beginning to move out sometime next weekend. One model says Friday, the other says Sunday, so it is unclear as to exactly when.

A friend of mine described to me the real danger of heat waves. He said, “The danger of heatwaves is not because of the heat during the day, but it’s the fact that the temperature does not easily drop during the night.  The body needs to have the opportunity to recover from the heat during the day.” Unfortunately, high dewpoints wont allow the temperatures to drop too low. So, stay cool if you are out in the Central US! Those of us in the Southeast are going to dodge this bullet… for now.

…HEAT ADVISORY IN EFFECT FROM NOON MONDAY TO 7 PM CDT TUESDAY…

THE NATIONAL WEATHER SERVICE IN NASHVILLE HAS ISSUED A HEAT ADVISORY…WHICH IS IN EFFECT FROM NOON MONDAY TO 7 PM CDT TUESDAY.

* EVENT…HEAT INDEX READINGS ARE EXPECTED TO REACH 105 TO 110 DEGREES DURING THE AFTERNOON ON MONDAY AND TUESDAY. LITTLE OR NO RELIEF FROM THE HEAT AT NIGHT WITH LOW TEMPERATURES ONLY DROPPING TO THE MID 70S.

* TIMING…HEAT INDEX READINGS WILL PEAK IN THE AFTERNOON HOURS FROM 105 TO 110 DEGREES EACH DAY. LITTLE RELIEF AT NIGHT WITH OVERNIGHT LOWS IN THE MID 70S.

* IMPACT…THE FIRST TO SUFFER FROM THE HEAT WILL BE CHILDREN…THE ELDERLY AND PEOPLE WITH CHRONIC AILMENTS. HEAT EXHAUSTION…HEAT CRAMPS AND HEAT STROKE ARE POSSIBLE DUE TO PROLONGED EXPOSURE TO THE HEAT.

This morning’s upper air analysis provides some insight into why this is happening. An upper level high pressure system has steadily been moving out of the Oklahoma area going eastward. By tomorrow, it should be centered over Tennessee and will not be out of the area until Wednesday. This is a limited understanding explanation, but the reason this high pressure causes heat is because areas of high pressure are places where air is sinking from the upper atmosphere. This sinking motion creates extremely calm and sometimes nonexistent winds, no clouds, and crystal clear skies. This allows for the air to heat up a lot more easily than with low pressure systems, which usually have cold fronts associated with them. That is why this high pressure is causing such excessive heat, but if I got anything wrong in there, feel free to correct me! Also, this high pressure system is fairly slow moving, so it will be just as slow to leave as it was slow to arrive.

Because of the heat, I encourage everyone to take the necessary precautions to prevent dehydration. Drink plenty of water, avoid working or playing in the middle of the day, and recognize the signs of heat exhaustion. Don’t ask me what those are; but if you start vomiting, you probably should seek help! Also, do not leave pets or children in the car. This should be a general rule of thumb, but ESPECIALLY do not do it now! Stay cool over the next few days!