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!

Again, CAPE placement is somewhat uncertain, but it does appear we will have over 1,000 j/kg of cape in the area. There will be a capping layer to the north-east of Nashville today, which may further limit thunderstorm severity. The cap does not appear to “break” as it were, so the presence of this capping layer may hinder us from mainly severe thunderstorms. That is the biggest “if” that I can see with today’s storms for counties north and east of Nashville. There might be enough wind shear to pose a damaging hail threat today, so if you love your car, put it in the garage! As for storm timing, storms are possible on and off all day, so it is hard to say. We are not dealing with a cold front squall line this time, so it will be less large scale and more on a storm by storm basis. Here is the SPC text.

…MID MS/LOWER OH VALLEYS INTO THE LOWER MS VALLEY TODAY INTO THIS EVENING…

SIMILAR TO MONDAY LARGE-SCALE PATTERN WILL SUPPORT THE SWD DEVELOPMENT/MOVEMENT OF ONGOING STORMS OVER THE OZARKS AND MID MS VALLEY INTO THE LOWER MS VALLEY. EARLY VISIBLE SATELLITE DATA SUGGEST THAT STRONG DIABATIC HEATING SHOULD ONCE AGAIN COMBINE WITH A MOIST BOUNDARY LAYER TO CREATE A MODERATELY UNSTABLE AIR MASS BY AFTERNOON WITH MLCAPE INCREASING TO 1500-2500 J/KG. THIS INSTABILITY WILL COINCIDE WITH A BELT OF 30-35 KT NNWLY FLOW IN THE MIDLEVELS WHICH WILL SERVE TO ENHANCE THE SWD /AND EVENTUALLY SWWD/ MOVEMENT OF DEVELOPING TSTM CLUSTERS. DAMAGING WIND GUSTS WILL BE THE PRIMARY HAZARD WITH THIS ACTIVITY THROUGH THIS EVENING.

There is also a risk for severe thunderstorms tomorrow, but I will make an individual post about that later this afternoon. Stay safe and heed all warnings!

The Storm Prediction Center in Norman, Oklahoma, has highlighted Tennessee under 2 slight risks for severe thunderstorms this week. This includes tomorrow (tuesday; upper left) and Wednesday (upper right). Tomorrow, a warm front will travel across Middle Tennessee, providing a lot of heat and moisture for thunderstorms. Along the warm front will be the most wind shear, and with the warm front draped across Middle Tennessee, we will likely see some severe thunderstorms. The only concern I have for Tuesday’s severe weather is the presence of a capping inversion. It does weaken some, but it may not be enough (EDIT: The cap will break between 1 and 4 PM based on current model data). On Wednesday we will have a similar situation, but according to the SPC we may have a more serious risk for supercell here in Tennessee. We also have some capping going on, but much weaker than on Tuesday. This means Wednesday is more likely to see severe thunderstorms. On both days, damaging winds and large hail are the main threat, but I would not rule out an isolated tornado. Even on Tuesday, I believe there to be a chance for a tornado based on wind shear and hodograph profiles; it all just depends on if the cap lifts and if the LCL heights stay low enough.

However, there are a few large “if”s when it comes to this capping layer on Tuesday. On top of that, current model data places the bulk of the instability to South-West Middle Tennessee. If this ends up being the case, the bulk of severe weather will be down there and NOT Nashville, though isolated severe storms would not be ruled out. In the end, we will have to wait and see what happens. It looks as if storms could develop as early as 1:00 PM, but storms will have to either last or develop later in the afternoon for full severe potential to take place. Again, there are a lot of things that are still “iffy”, and this is not expected to be a major severe weather day. Just keep in mind that severe thunderstorms are possible. Here is the text from the SPC.

…ERN KS/NERN OK LOWER MO VALLEY TO SRN APPALACHIANS…

PROGRESSIVE WAVE TRAIN WITHIN A WNWLY FLOW FROM PAC NW TO MID ATLANTIC STATES AS PERSISTENT RIDGING SRN STATES RELAXES. THE TROUGH OF INTEREST FOR SEVERE STORMS TUE WILL BE THE ONE THAT WILL BE CROSSING CENTRAL ROCKIES THIS AFTERNOON AND THEN ACROSS THE CENTRAL PLAINS BY TUE AFTERNOON AND LWR MO VALLEY TUE NIGHT.

QUASI-STATIONARY FRONTAL BOUNDARY WILL EXTEND WNWWD FROM SC/NRN GA ACROSS MO INTO A SURFACE LOW OVER ERN KS MID DAY TUE. WITH THE APPROACHING S/WV TROUGH LOW WILL SHIFT EWD INTO WRN MO BY AFTERNOON WITH COLD FRONT THEN MOVING SWD THRU SRN HI PLAINS LATER TUE/TUE NIGHT.

A 40-50KT SWLY 850MB JET AHEAD OF SFC LOW ACROSS LWR MO VALLEY RESULTS IN A VERY MOIST AND POTENTIALLY STRONGLY UNSTABLE WARM SECTOR DEVELOPING BY TUE AFTERNOON WITH MLCAPES AOA 3500 J/KG. WITH A PRONOUNCED EML SPREADING EWD ACROSS WARM SECTOR AHEAD OF THE S/WV TROUGH…THERE WILL BE CONSIDERABLE CINH MUCH OF THE DAY. HOWEVER ASCENT AND MID LEVEL COOLING SPREADING ACROSS CENTRAL PLAINS IN ADVANCE OF THE S/WV TROUGH SHOULD INITIATE SEVERE THUNDERSTORMS IN VICINITY OF FRONTAL ZONE AHEAD OF SURFACE LOW FROM NEAR MKC ESEWD TO WRN TN VALLEY BY LATE AFTERNOON.

MODEL SOUNDINGS LOWER MO VALLEY SUPPORT SUPERCELLS ACCOMPANIED BY VERY LARGE HAIL AND POTENTIALLY TORNADOS BY LATER TUE AFTERNOON. WHILE THE VEERING SHEAR PROFILES ARE IMPRESSIVE THE ACTUAL DEVELOPMENT WILL DEPEND NOT ONLY ON THE CAP ERODING SUFFICIENTLY TO TAP INTO THE VERY UNSTABLE BOUNDARY LAYER…BUT ALSO THE EFFECTS OF THE EXPECTED EARLY DAY MCS ACTIVITY MO/IL.

HAVE INCREASED THE SEVERE PROBS ACROSS MO INTO WRN KY/NWRN TN ACCOUNT ON THIS POTENTIAL…WHICH MY LEAD TO A HIGHER RISK IN LATER FORECASTS.

As far as Wednesday goes, a more detailed forecast will have to come tomorrow. Models are still not in agreement with what will happen on Wednesday, but it looks like we could see some severe thunderstorms, and according to the SPC, some supercells as well. However, I feel like the wind shear profiles and hodographs are a little bit too messy right now to say anything about tornadoes, so that will come tomorrow. Wednesday looks like a high CAPE event, with the cap completely lifting by 4 PM. Storms will be free to develop, and if there is sufficient wind shear, all types of severe weather could be possible. Here is the text from the SPC.

…TN/LOWER OH VALLEYS WSWWD TO AR/ERN OK…

ECMWF/GFS AGREE THAT NEW TSTM DEVELOPMENT WILL BE LIKELY INTO WEDNESDAY AFTERNOON ALONG THE WARM AND COLD FRONTS WITH SOME POTENTIAL WITHIN THE WARM SECTOR ITSELF ACROSS THE LOWER OH/TN VALLEYS WSWWD THROUGH OZARKS/AR TO ERN OK. A MOISTENING WARM SECTOR AND DIABATIC HEATING BENEATH ERN EXTENT OF STEEP MIDLEVEL LAPSE RATE PLUME WILL SUPPORT A MODERATELY-STRONG UNSTABLE AIR MASS WITH HIGHEST VALUES OVER THE WRN EXTENT OF THE SLIGHT.

DEEP WLY TROPOSPHERIC WINDS SUGGEST LINE SEGMENTS WILL BE THE PRIMARY STORM MODE WITH DAMAGING WINDS AND HAIL POSSIBLE. HOWEVER…STRONGER SHEAR /35-40 KT/ ACROSS KY/TN WEDNESDAY AFTERNOON/EVENING MAY RESULT IN A FEW SUPERCELLS. FARTHER W OVER AR…DEEP LAYER SHEAR WILL TEND TO BE WEAKER. HOWEVER…STRONGER INSTABILITY SHOULD COMPENSATE AND SUPPORT A SEVERE WEATHER THREAT AS WELL.

The National Weather Service is anticipating spotter activation for both days, so don’t forget to call in your severe weather reports if you are a spotter. Please remember to stay safe and heed all watches and warnings appropriately!