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!

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.

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!”

Here are some of the details. This severe risk depends on how well the atmosphere recovers after the morning convection. If it remains too cloudy, the chances will go down with each hour the sky is not clear. However, if the sky does clear, these parameters are what we could see: CAPE over 3,000 J/kg, Helicity over 100, sufficient capping, some slightly curved hodographs, 65-70F Dewpoints, and low LCLs. Each of those ingredients above are indicators of severe weather. The capping in place through the afternoon would help prevent weaker thunderstorms from forming while the atmosphere recovers. However, it has to remain weak enough to be broken. Hopefully the cap will break, which is indicated by the 18z NAM. Based on the current models, it looks like there might even be an isolated tornado threat, and the SPC also agrees with this. The image to the right shows the forecast EHI levels, which look rather nice. EHI is a composite of energy and wind shear, and the higher the value, the more likely the environment is sheared enough to produce a tornado.

Here is the text from the SPC…

…LWR OHIO AND TN VALLEYS INTO ERN AR…

SURFACE FEATURES ARE RATHER WEAK ASSOCIATED WITH THE ERN TROUGH…HOWEVER A COOL FRONT SHOULD BRING DRIER AIR SEWD ACROSS MIDDLE MS RIVER VALLEY INTO WRN TN/OH VALLEY BY EVENING. IN ADVANCE OF THE FRONT…A MOIST AND MDTLY UNSTABLE AIR MASS SPREADS EWD ACROSS THE TN/LWR OH VALLEY WHICH WILL SUPPORT THE EXPECTED ACTIVE MCS ACTIVITY TONIGHT AND EARLY WEDNESDAY.

WITH REGARDS TO THE SEVERE THREAT THE CONVECTIVE CLOUDINESS REMAINING FROM OVERNIGHT MCS/S COULD DELAY THE DAYTIME HEATING SUFFICIENTLY TO LIMIT THE OVERALL COVERAGE OF SEVERE THUNDERSTORMS IN THE SLIGHT RISK AREA. SINCE THE PRIMARY ASCENT WITH THE S/WV WILL BE SHIFTING EWD TOWARD SRN APPALACHIANS E OF BETTER THERMODYNAMICS BY LATE AFTERNOON…INITIATION WILL BE DEPENDENT MORE ON SURFACE BOUNDARIES.

HEATING SHOULD BE SUFFICIENT JUST AHEAD OF THE COLD FRONT GIVEN THE MOIST WARM SECTOR TO DEVELOP A MDTLY UNSTABLE AIR MASS BY MID AFTERNOON WITH MLCAPES AOA 2500 J/KG TN VALLEY SWWD INTO NRN MS/AL. OVERALL WESTERLY SHEAR SUPPORTS CLUSTERS…LINE SEGMENTS WITH WIND DAMAGE AND HAIL THE RESULT. SUPERCELLS COULD DEVELOP ALONG LOCALLY BACKED FLOW NEAR OUTFLOW BOUNDARIES WHICH WOULD INCREASE POTENTIAL FOR ISOLATED TORNADOS.

LOW LEVEL WAA AND HEIGHT FALLS EXPECTED ACROSS THE CENTRAL/SRN APPALACHIANS WEDNESDAY NIGHT WILL LIKELY MAINTAIN CONVECTION AND EMBEDDED TSTMS WITH EWD EXTENT POSSIBLY REACHING THE CENTRAL/SRN APPALACHIANS BY 12Z THURSDAY. HOWEVER…OVERALL SEVERE WEATHER THREAT SHOULD DIMINISH AFTER DARK AS INSTABILITY DECREASES WITH EWD EXTENT.

I will make another post if I can tomorrow morning with the details. I might try to chase tomorrow, and if I do, coverage and updates will be limited. Check back for more updates!