WindRunner wrote:HurricaneHunter914 wrote:http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/wg8sht.GIF
Shear should become favorable for development down the road.
I have been seeing shear maps used rather excessively this year, and people treating them like they were the bible of tropical cyclone development or something, and you have to remember that these maps can be inaccurate sometimes, by up to 20kts or so. These maps, whether computer or human generated, are subject to inaccuracies just like everything else in the weather world, and need to be taken into consideration with these thoughts in mind. From what I hear, a WV image is much more accurate when it comes to showing how badly a system is getting sheared. Of course this is somewhat more difficult to interpret, but it should lead to better conclusions as to the strength of the shear.
More importantly, shear is not an advective property and is often NOT static! Just because a "system" is forecast to move into an area that has little shear NOW does
not mean that shear will remain low in that area LATER. In addition, the magnitude of shear does not necessarily 'advect'. Shear is the difference (dir and magn) of flow between two levels. If flow at one level changes, so does the shear. Oftentimes, winds at both levels change, creating situations where shear can change rapidly. This also means that minor errors in wind speed and direction in model forecasts can create major errors in vertical wind shear forecasts. We all know about the dearth of observations (both surface and aloft) in the tropics, so that adds yet another complication / caveat.
Why not examine 500mb and 850mb model output to get an approximation of shear 24+ hrs in advance? Looking at this evening's GFS output, it looks like a 250mb cut-off low move slowly off the southeast US coast, resulting in 20-35kt westerly flow. Any storm that moves westward in this environment will face strong upper-level storm-relative flow, yielding stronger shear (storm-relative shear, since environmental shear is independent of system motion). The 0z NAM output is stronger with this cut-off upper-level low, indicating 40-75kt 250mb northwesterly flow east of Florida in a couple of days. Again, I can't imagine seeing a tropical depression or storm in such an environment. A slightly better bet for an environment favorable for convective organization may present itself if the 0z NAM has it's way in a few days in the Caribbean, south of a cut-off high progged to develop. FWIW, the GFS shows northwesterly upperlevel flow across much of the Caribbean in 48-72hrs, which just goes to show the variability of upper-level forecasts.
Re: the previous discussion about some folks not "wanting" development... Maybe there are some, but there are also some that always "want" development. The fact of the matter is that, as has been mentioned previously, only a fraction of INVESTs become tropical cyclones. Many pro mets value their FAR (False alarm ratio), which means that many don't want to forecast a cyclone when one doesn't develop. So, they call them as they see them. It seems that some folks don't like this -- they want us all to be like JB and always mention the "possibility" of development. Sure, he may call some storms well, but how many times does he mention development only to not see any development? I'm not slamming JB, but let's all remember that tropical cyclone forecasting is extremely difficult, particularly cyclone genesis.
How many INVESTs have we had this year? How many posts contain nothing but "Wow, I bet this one will be Beryl!"? I think it's good that Storm2k mods are trying to increase the signal-to-noise ratio, but it's never bad to post a reminder of the golden rule of maintaining high quality -- before you post, ask yourself if others will benefit from your post. If you think they will, then post away. If you think they won't, use the Private Message system. It's just now July 1st, and we've had how many threads about non-cyclones? There've been thousands of posts about convective areas that aren't even tropical depressions... I'm not saying we shouldn't discuss these, but it's tough (and time consuming) to weed through all the one-liner and emoticon-only posts to get to the substance.
