Downslopes and thunderstorms

[Steve Dedman]

Living in Knoxville, we have an interesting geological/geographical situation regarding thunderstorms. Situated in the Tennessee River valley between the Cumberland Plateau and the Great Smoky Mountains of the Appalachians, we are in a bowl between two sources of orographic lift.

In watching the way storms behave over the last decade that I have lived here, I have a question: how does a rapid drop in terrain effect thunderstorms?

Since a rapid rise in altitude is generally conducive to thunderstorm development, is the reverse then true? IOW, will a drop in terrain (and the accompanying loss of isentropic lift) help to weaken thunderstorms?

Here are some observations that have led to this question:

When storms approach Knox from the west or NW, they tend to fall apart or weaken significantly just west of the metro. When they approach from the south they tend to strengthen just north of the metro. The Cumberland Plateau runs from NE to SW about 30 miles west and NW of Knoxville. This would seem to indicate that the loss of orographic lift that makes storms strong as they approach the Plateau is a big inhibitor of continued strong storm capability. Often we see a re-strengthening between Knoxville and the Smokies as the terrain starts moving upward again.

The storms that really seem to be the strongest and most long-lived approach from the SW - straight up the Tennessee River valley; with no source of orographic forcing.

I know that there are many days here that, were I to feel the same humidity and temps in Oklahoma, I'd be buying canned goods for the storm shelter. Then we get little to nothing in the way of strong storms.

What I am thinking is that as the warm moist air crests the plateau, it runs into a situation where there is air of similar properties (hot and sticky) at lower altitudes, negating the lift. The elevation drop from Crossville (atop the plateau) to Knoxville is not enormous - only about 1000 feet over about 40-50 miles. But is that enough for the bottom to drop out from underneath the storms?

For reference, here is a google terrain map of the area:

MAP

Perhaps someone can shed a little more light on this phenomenon?

 

[Scott Weberpal]

Living in Knoxville, we have an interesting geological/geographical situation regarding thunderstorms. Situated in the Tennessee River valley between the Cumberland Plateau and the Great Smoky Mountains of the Appalachians, we are in a bowl between two sources of isentropic lift.

The word you're looking for in regard to the lifting mechanism is orographic lift, not isentropic lift. Isentropic life describes what happens when warm rises up over cold air because it prefers to move toward air with a similar density. Isentropic lift is what often happens along and north of warm fronts that creates rain/elevated t-storms.

See:
http://www.theweatherprediction.com/advanced/isentropic/
and
http://www.theweatherprediction.com/habyhints2/447/

 

[Steve Dedman]

Whoops. Should have typed slower...

 

[Andrew Khan]

Well, as I live only within 50 miles of you, I can indeed agree with you. Most of the time, Orographic lift here will give me some really nice lenticularis clouds. However, with the mountainous area and constant changes is terrain and altitude in makes a very tricky and challenging environment for the storms which come through. They are intrinsically put through a test of strenghth against the constant negative orography which is inherent in TN. I have it probably worse than you. I live in extreme east TN, in the tri cities, and the mountains are really quite copious. A few times, storms can become acclimated to the mountains over time during their life, and as they reach me, they utilize the orography as a fueling mechanism for their mesocyclonic rotation.

The ones that do make it through, don't last to long before they gust right on out.

 

[Randy Bowers]

It's unlikely that downsloping has a detrimental affect to thunderstorms in your area. Of course, orographic lifting is important because, as you said, it acts as a means of lift. Often in steep terrain, orographic lift can be quite a bit more significant than a simple convergence boundary, outflow boundary, or frontal boundary that we may be looking for to initiate convection at the same time further west in the state.

Using the Rockies as an example, it's quite common to see either orographic lifting or elevated heating, or a combination of both, result in thunderstorm development over the mountains. This activity then propagates eastward into the high plains and many times holds together throughout the afternoon. In some cases convective complexes can evolve from this activity and may continue a substantial distance eastward (down the slope). The dying off of convection immediately east of the mountains would more likely be associated with the loss of the impetus for its development (i.e., loss of orographic lift or absence of elevated heating further east). It's not necessarily a caused because the terrain slopes downward as the storms propagate eastward.

The terrain over eastern Tennessee isn't near as dramatic as Colorado and Wyoming, obviously. Nevertheless, the process for convective development is similar with elevated heating likely being considerably less of a factor.

You mentioned that storms tend to be a little more robust and long-lived when they move southwest to northeast. Could this be more of a function of the type of weather system they are associated with and thus the general storm environment? Storms associated with large intense synoptic scale weather systems in the spring and fall months in our area tend to bring the most intense activity. This activity most commonly moves southwest to northeast. In the late spring and summer months, when low shear but a considerable amount of instability is present, storms typically are slow moving and not well organized, and typically don't follow the southwest-to-northeast rule that their more intense counterparts often follow. They tend to last only a short time due to their vertical or near vertical updrafts. My thought is that perhaps this observation you made is less of a function of the direction the storms are moving relative to the terrain and more of a function of the direction well-organized/intense storms typically move. Obviously, certain trajectories of the flow are going to be associated with better orographic lifting though.

I would be curious to see any papers/research that NWS MRX has done on these issues. I'm sure they've done some extensive local studies that are published somewhere. The ideas that I've posted here are nothing more than "ideas." My over all knowledge/experience in forecasting in the higher terrain in the eastern part of the state is pretty limited. Perhaps someone else could contribute a better explanation.

 

[Angie Norris]

That was something I noticed when I lived close to Knoxville. It always seemed like the systems would fall apart when they hit the Cumberlands, so I attributed the effect to upslope friction possibly disrupting storm organization in some way. Very rarely did storms make it intact over the Cumberlands, but when they did, then we got major, severe storms, and in a couple of cases tornadoes (Feb 21, 1993 and Nov 10, 2004 most notably).

I took Randy's advice, and I'm checking out the research page for the MRX WFO (www.srh.noaa.gov/mrx/research/research.php), and I'm finding some light summer reading (thanks, Randy !!). The extremely simplified explanation seems to be the Cumberlands inhibit moisture return to the valley, so you need the southwest flow to in effect bypass the mountains and supply the needed moisture for storm development. Cold air damming in the valley also inhibits instabilty in the region, so the southwest flow would aid in warm air advection as well. The upslope flow does appear to aid in the development of pulse severe storms in the late spring and summer, and would also be a major component to the flood events in the Smokey Mountains.

Cool stuff!!!!

 

[Steve Dedman]

The extremely simplified explanation seems to be the Cumberlands inhibit moisture return to the valley, so you need the southwest flow to in effect bypass the mountains and supply the needed moisture for storm development. Cold air damming in the valley also inhibits instabilty in the region, so the southwest flow would aid in warm air advection as well. The upslope flow does appear to aid in the development of pulse severe storms in the late spring and summer, and would also be a major component to the flood events in the Smokey Mountains.

Cool stuff!!!!

That would go a long ways toward explaining why middle Tennessee can be getting hammered, and the same system will just dump huge amounts of rain on Knoxville. When the tornado hit Murfreesboro right before Easter, the same storm complex did make it to Knoxville and was TOR warned for Knox County. However, it was radar-indicated rotation aloft and no funnels were sighted. My own observations were that the storm had too high of a base to be a big tornado threat. The rotation was located (on radar) about 2 miles north of my house, so I had a really good vantage point from my bedroom window. I didn't see anything rotating in the base of the storm, there wasn't even a wall cloud.

That was the event that started me down this thought path - that perhaps when the storms come off of the plateau, the base is (within a short time) 1000 feet higher than it was in the Crossville area.

In looking at the MRX tornado database, Knox county has more than twice as many documented tornado reports over time than any of the surrounding counties. But I wonder if that might be because the rest of the area is very rural, and there were simply more people in Knoxville to report tornado damage to the NWS in the days before doppler and home video.

 

[Wes Carter]

I live in Middle TN but spent much of my life in East TN in the Tri-Cities. I travel frequently between the here (near Manchester, TN) and Kingsport to visit family. I have noticed much of what you all are discussing and have even made it a point to watch radar as things here progress east.

I have noticed that storms here in Middle TN seem to have lower cloud bases than the ones in East TN. Randy Bowers mentioned that the elevation changes are not as dramatic here as they are in the Rockies. In the Rockies the cloud bases are usually much higher both relative to sea level and the base terrain. In the supercell that spawned the Murfreesboro tornado I dare say the cloud base was not much more than 1000ft above the terrain.

Could the terrain play as large a role in storm propagation even if it it less dramatic than the Rockies *IF* the change in elevation was a large percentage of the cloud base height?

 

[Steve Dedman]

Could the terrain play as large a role in storm propagation even if it it less dramatic than the Rockies *IF* the change in elevation was a large percentage of the cloud base height?

That's exactly what I'm thinking. That since the storms over middle Tennessee can so very low-based, that when the upslope falls away near Crossville, it's kind of like the storm doing a Wile E. Coyote off of the cliff. Also, the difference in elevation from Nashville to Crossville is almost identical the the difference from Crossville to Knoxville - Crossville is about 900 feet higher than either of the larger cities.