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!

EDIT: The Storm Prediction Center has removed Davidson and Williamson counties from their severe risk. Too much cloud cover has prevented the development of an unstable atmosphere. Strong storms are still *possible*, but the overall threat for severe weather is low.

The Storm Prediction Center still believes that we could see some severe thunderstorms this afternoon, as a trough moves through and causes a disturbance that will initiate thunderstorms. At first, it was looking as if this morning would be the main thunderstorm threat, but the line that was heading towards Tennessee weakened considerably and is now just a line of showers with a few isolated thunderstorms. However, when this trough kicks things off, we should see several clusters and bow segments of thunderstorms that could be severe in nature. Timing as of right now is uncertain to me, but storms will begin to die off after sunset. Also, speaking from opinion and limited experience, we will only see this severe weather risk come to pass if the sun can come out. If the sun doesn’t come out, then our chances diminish by a lot. It doesn’t eliminate the risk, but it certainly makes things more difficult for the atmosphere. So, keep in mind that cloud coverage will play a big part in how the day pans out!

Right now, the best window for thunderstorms is in the afternoon, which means somewhere in between 12 PM and 7 PM. I know that a time frame like that is very unhelpful, but model data is still coming in and I have a lot of uncertainties about this event. However, realistically, the window can be narrowed down to something around 3-7 PM. This morning’s upper air sounding is actually quite interesting. The forecasted wind shear I described in the previous forecast has not panned out, and was likely an anomaly or error. However, looking at the green and red lines (green = dewpoint, red = temperature), one can see that they are very close together. When this happens, the atmosphere is said to be saturated (very moist). This saturation goes fairly high into the atmosphere, which poses a threat for wet microbursts, or damaging straight line winds in the downdraft of a thunderstorm. This will be the main cause for our severe weather threat for today, as we will see bowing segments (indicative of downbursts) in several areas. There is also a risk for severe hail in the area, and will have the potential to reach 1″, but will likely be slightly less. I have a feeling that *most* hail will melt before it reaches the ground because of how moist the atmosphere is, but in the stronger cells, it is not out of the question to have some hail. Also, it looks as if the bulk of the unstable air will sit over southern portions of the Middle TN area, so the further south you live, the more likely you will see a severe thunderstorm.

So, if I haven’t made it very clear, today’s threat is a bit uncertain to me. It depends a lot on the cloud cover. However, if the sun comes out, we will see thunderstorms that could reach severe criteria, with wet microbursts and hail possible. Keep abreast of the weather as we get into the afternoon and evening, and also be mindful of flash flooding! Flash flooding is not out of the question, because we have a very moist atmosphere. If we get another stationary storm like yesterday in Cool Springs, it could happen again. However, it is hard to predict such things, so just keep it in mind, and TURN AROUND, DON’T DROWN!

The fine details are actually quite interesting and I was surprised several times during this forecast at some of the elements coming into play. For starters, there is quite a bit of wind shear in the atmosphere for this time of year. The image at right is a Skew-T plot, and if you look to the right, you will see barbs that represent wind speed and direction. If you look, you will notice that they rapidly change direction in the lower atmosphere, and such wind shear is what causes storms to rotate. However, this is just one particular model. The global model is not agreeing with this image, which leads to uncertainty as to what kind of severe weather is expected. However, because the atmosphere will be unstable, hail and damaging winds can be expected. If this forecast model is correct, we may see an isolated tornado. However, if the other model is right, then we won’t see as much of a chance for a tornado. Also, an upper level trough (low pressure) will move over Tennessee while there is a stationary front sitting in Central/Western portions. This will act as a lifting mechanism and force storms to develop. Because these storms will be lifted rather than rising on their own, they will be stronger than the storms we have seen over the last few days. So, extra caution should be exercised tomorrow when dealing with these thunderstorms. Here is the text from the Storm Prediction Center.

…MID-ATLANTIC STATES WSWWD INTO THE TN VALLEY…

A BAND OF 25-35 KT MID LEVEL WINDS AND CYCLONIC FLOW/LARGE SCALE FORCING WILL BE LOCATED ACROSS THE REGION…SOUTH AND EAST OF THE GREAT LAKES/OH VALLEY TROUGH. ALSO…A QUASI-STATIONARY FRONTAL BOUNDARY FROM DELMARAVA SWWD INTO THE TN VALLEY SHOULD PROVIDE FOCUS FOR CONVECTION. DURING THE MORNING THE VERY MOIST AIR MASS IS LIKELY TO CONTAIN POCKETS OF CLOUDS/CONVECTION AND SOME LOCATIONS MAY NOT DESTABILIZE SUFFICIENTLY FOR SEVERE STORMS. HOWEVER…WHERE STRONGER HEATING OCCURS…MODERATE INSTABILITY AND 30-40 KT DEEP LAYER SHEAR WOULD SUPPORT A THREAT FOR MULTICELL STORMS WITH SEVERE HAIL/WINDS DURING THE AFTERNOON AND EARLY EVENING HOURS.

So, in summary, storms will have the potential to produce hail and damaging straight line winds in the afternoon hours of tomorrow, dying off by sunset. I also believe that if the NAM (North American Model) pans out, we could see the potential for an isolated tornado. However, that is uncertain at this time, because the other models are not in agreement. So, we will have to wait and see. I will attempt another forecast in the morning. Stay safe, and “when thunder roars, go indoors!”