CoastalWx Posted May 29, 2011 Share Posted May 29, 2011 CAPE is a measure of instability... convective available potential energy. The higher the CAPE the more buoyancy air has. Vertical wind shear is basically the change of wind with height... both speed and directional. Shear plays a huge role in determining the mode of convection... whether it's ordinary, multicell, or supercellular. The stronger the shear you generally see more organized convection for a couple reasons... one is that shear can help keep outflow from undercutting the storms updraft and another is that vertical wind shear helps keeps storms organized in lines or clusters by promoting new updrafts on a specific flank of a storm. Strong shear also produces horizontal vorticity (or spin) than can be tilted into an updraft to produce a mesocyclone. Good summary. An important ingredient to help sustain storms. I think people get confused at times because they hear that shear is bad for hurricanes yet we want shear for our convection here. Updrafts in convection that are not related to those from a tropical system are usually much stronger than those found in a tropical system. Tropical systems are heat engines where convection is forced from strong low level convergence and not necessarily from buoyancy since they are heat engines. It's a finely balanced system. Sure there is some CAPE, but convection that we think of around here and most especially in the Plains contain much, much, more buoyancy. Link to comment Share on other sites More sharing options...
weatherwiz Posted May 29, 2011 Share Posted May 29, 2011 But as much as I posted about shear... strong shear in the absence of high CAPE can cause all sorts of problems by disrupting updrafts. Yeah if you have too much shear and not enough cape updrafts are just going to get torn apart and not be able to really grow...you'd need a helluva lot of mid/upper level support from either s strong s/w or being in RRQ or LFQ of a jet. Link to comment Share on other sites More sharing options...
CoastalWx Posted May 29, 2011 Share Posted May 29, 2011 Good point, Scott. One thing that makes me perk up is a NW flow event because you can still have fairly impressive shear with a SW or SSW sfc wind flow which can bring sig svr to the coastal plain. Yep, true. I also like w and nw flow events because even though they don't always have an EML...considering the source region alone...you're bound to have better lapse rates aloft. So what if we don't have 8C per KM. 6.5Cto 7C is much better than a convectively tainted airmass from the southeast of 5.5C/KM. Link to comment Share on other sites More sharing options...
weatherwiz Posted May 29, 2011 Share Posted May 29, 2011 Still worried that the better shear values are displaced from the higher instability values...better shear to the north and west, better instability off to the south. Link to comment Share on other sites More sharing options...
CT Rain Posted May 29, 2011 Share Posted May 29, 2011 Well shear is important for t'storms and for severe wx b/c it helps to allow for the storms to organize. When you have strong vertical shear values (0-6km shear) in excess of 30-35 mph this helps to allow for thunderstorms to become organized, when you get above 30-45 knots of vertical shear it can lead to the updrafts beginning to rotate some and make some storms take on supercell characteristics. Shear is also important b/c it really helps out with mid level and upper level support...being in the Right Entrance Region or Left front quadrant of a mid-level jet or upper level jet stream can really enhance the upward vertical motion and also help to allow for updrafts to build bigger. Nice summary but you sort of lost me here. Strong shear can be presence in the absence of synoptic scale lift (like you imply from the RRQ or LFQ of the jet streak). Shear is important for storm organization and updraft maintenance/longevity. One thing that is quite helpful for convective initiation is synoptic scale lift (the lift here is orders of magnitude smaller than the speed of an updraft) but that lift (be it from a jet streak or differential cyclonic vorticity advection) helps remove convective inhibition (CIN or a cap). More often than not falling heights and some type of synoptic scale lift is associated with a cold front advancing from the northwest or a lee trough developing on this sides of the Apps. Link to comment Share on other sites More sharing options...
CT Rain Posted May 29, 2011 Share Posted May 29, 2011 Yep, true. I also like w and nw flow events because even though they don't always have an EML...considering the source region alone...you're bound to have better lapse rates aloft. So what if we don't have 8C per KM. 6.5Cto 7C is much better than a convectively tainted airmass from the southeast of 5.5C/KM. lol exactly. Which is what we're dealing with most times. Link to comment Share on other sites More sharing options...
CT Rain Posted May 29, 2011 Share Posted May 29, 2011 Yep, true. I also like w and nw flow events because even though they don't always have an EML...considering the source region alone...you're bound to have better lapse rates aloft. So what if we don't have 8C per KM. 6.5Cto 7C is much better than a convectively tainted airmass from the southeast of 5.5C/KM. It seems recently even when we've had an OK looking NW/W flow event we've been screwed by ongoing convection or even some stratiform precip that has released enough latent heat to screw our solid lapse rates. Link to comment Share on other sites More sharing options...
weatherwiz Posted May 29, 2011 Share Posted May 29, 2011 Nice summary but you sort of lost me here. Strong shear can be presence in the absence of synoptic scale lift (like you imply from the RRQ or LFQ of the jet streak). Shear is important for storm organization and updraft maintenance/longevity. One thing that is quite helpful for convective initiation is synoptic scale lift (the lift here is orders of magnitude smaller than the speed of an updraft) but that lift (be it from a jet streak or differential cyclonic vorticity advection) helps remove convective inhibition (CIN or a cap). More often than not falling heights and some type of synoptic scale lift is associated with a cold front advancing from the northwest or a lee trough developing on this sides of the Apps. Good points there...actually didn't think of that...especially where it can help to remove CIN. Link to comment Share on other sites More sharing options...
CoastalWx Posted May 29, 2011 Share Posted May 29, 2011 It seems recently even when we've had an OK looking NW/W flow event we've been screwed by ongoing convection or even some stratiform precip that has released enough latent heat to screw our solid lapse rates. I think last year was one of them. We had an MCS out in Michigan that killed the mid levels for us, when it was modeled to be quite good. Link to comment Share on other sites More sharing options...
weatherwiz Posted May 29, 2011 Share Posted May 29, 2011 It seems recently even when we've had an OK looking NW/W flow event we've been screwed by ongoing convection or even some stratiform precip that has released enough latent heat to screw our solid lapse rates. Yeah that really screwed us last year on June 6th, we had no cap and we had all this pop up shower crap and the lapse rates I think were a bit less than what everyone thought they would be...they never looked impressive but I remember models having them just up over 6 C/KM but they were more around 5.5-6 C/KM. At least with Wednesday it looks like we might be capped a bit early on, especially if we can get an EML to work in here that should at least help. We really haven't had an EML with the past few NW/W flow setups...except the May 26th event last year. Link to comment Share on other sites More sharing options...
CoastalWx Posted May 29, 2011 Share Posted May 29, 2011 Good points there...actually didn't think of that...especially where it can help to remove CIN. If you think about it..it makes sense. Get that air to rise even a little bit, and you cool it slightly..perhaps just enough to weaken CIN. Link to comment Share on other sites More sharing options...
CT Rain Posted May 29, 2011 Share Posted May 29, 2011 Some of our best severe days tend to have a strong s/w passing near the US/Canada border. It's too bad this upper low is progged to be so far northwest. Link to comment Share on other sites More sharing options...
weatherwiz Posted May 29, 2011 Share Posted May 29, 2011 If you think about it..it makes sense. Get that air to rise even a little bit, and you cool it slightly..perhaps just enough to weaken CIN. Yeah it really makes perfect sense. Great way to look at it too. Link to comment Share on other sites More sharing options...
weatherwiz Posted May 29, 2011 Share Posted May 29, 2011 Some of our best severe days tend to have a strong s/w passing near the US/Canada border. It's too bad this upper low is progged to be so far northwest. Yeah that part blows...I remember an event in the early 2000's...this was when I used to just watch the local news/TWC and read the weather forecasts in the newspaper and there was a good setup but in the paper it said if the system wasn't as far northwest a sit was we would have a major outbreak...I was pissed. Link to comment Share on other sites More sharing options...
weatherwiz Posted May 29, 2011 Share Posted May 29, 2011 Kind of far out but the 9z SPC SREF has some pretty decent parameters for svr potential on Wednesday. Link to comment Share on other sites More sharing options...
snowNH Posted May 29, 2011 Share Posted May 29, 2011 Good summary. An important ingredient to help sustain storms. I think people get confused at times because they hear that shear is bad for hurricanes yet we want shear for our convection here. Updrafts in convection that are not related to those from a tropical system are usually much stronger than those found in a tropical system. Tropical systems are heat engines where convection is forced from strong low level convergence and not necessarily from buoyancy since they are heat engines. It's a finely balanced system. Sure there is some CAPE, but convection that we think of around here and most especially in the Plains contain much, much, more buoyancy. Thanks Guys... What do we need to get the shear and cape to be in order to get a substantial severe weather threat? I know you guys usually type CAPE in joules or something and I'm not sure what shear's units are. Can you give me a threshold for CAPE and shear where we'd likely see severe weather in NE? Oh and you guys always mention the "CAP" breaking... what it that too? Link to comment Share on other sites More sharing options...
free_man Posted May 29, 2011 Author Share Posted May 29, 2011 Thanks Guys... What do we need to get the shear and cape to be in order to get a substantial severe weather threat? I know you guys usually type CAPE in joules or something and I'm not sure what shear's units are. Can you give me a threshold for CAPE and shear where we'd likely see severe weather in NE? Oh and you guys always mention the "CAP" breaking... what it that too? It depends on the individual setup. You can get organized severe with 1000 to 2000 mucape and 35kt mostly unidirectional shear during the nighttime hours. 8/10/00 was like that. Shear can be minimal but with 90/70 and 3000 j/kg sbcape little cap and a weak trigger you can get some serious cores and storms. Remember that severe storm that shredded Topsfield, MA or whatever north shore town several years ago...there was little shear at all that day and had golfballs or close to tennis balls for 10 minutes! It all depends on what type of storm mode you prefer Link to comment Share on other sites More sharing options...
TalcottWx Posted May 29, 2011 Share Posted May 29, 2011 Just looked at the bufkit stuff that was most recent for nam and gfs... I think it looks pretty good right now still.. still have an EML.. thought gfs looked better than NAM though on the hodo's Link to comment Share on other sites More sharing options...
weatherwiz Posted May 29, 2011 Share Posted May 29, 2011 Thanks Guys... What do we need to get the shear and cape to be in order to get a substantial severe weather threat? I know you guys usually type CAPE in joules or something and I'm not sure what shear's units are. Can you give me a threshold for CAPE and shear where we'd likely see severe weather in NE? Oh and you guys always mention the "CAP" breaking... what it that too? Well in order to get a good amount of cape you want a warm low-level airmass and plenty of sun to pump up those sfc temps, you want high dewpoints, and you want steep mid-level lapse rates (or a good deal of change in temperature with height). Cape is measured in J/KG. For example, Friday was pretty damn warm out every where, temperatures into the 80's, dewpoints were in the mid 60's, and mid level lapse rates were up around 7-7.5 C/KM...this combination really made for an extremely unstable airmass and Cape values were in excess of 2500-3500 J/KG. Shear is just basically winds aloft...it can be measured in a variety of ways, m/2, knots, or mph...usually though it's done in knots, especially here in the US...some other countries do m/2. Being on the crest of a strong summer heat ridge is usually where the stronger mid level and upper level winds are located during the summer. However, when you have strong low pressure systems with cold fronts or strong pieces of s/w energy these can be associated with stronger jet energy as well (with winds in the low, mid, and upper levels). As far as the capped is concerned a Cap can be a very good thing, however, it can also be bad b/c if it doesn't break then you don't get convection to form (More on this in a bit). Anyways, with a Cap in place this prevents cumulus and clouds from forming too early in the day which allows for better potential for substantial heating and hotter sfc temps which then in turn means potential for greater instability. Under the cap since things are highly unstable once the cap is able to break all hell breaks lose and storms can not only develop but become extremely strong/severe very quickly. Think of it like a soda bottle with a cap on...if you violently shake the bottle it builds up alot of CO2 and potential energy...once you open the lid...boom everything rushes up ans soda spills everywhere. As far as for thresholds there really are no thresholds...different combinations of cape/shear can lead to different outcomes. Usually though you want to see at least 1000 J/KG of Cape. As for shear values there really is no threshold either...usually though for low level winds (925 and 850mb winds) you want to see at least 20-25 knots of wind, for mid-level winds (700 and 500mb) you want to see at least 40-50 knots, for upper-level winds you want to be either in the Right entrance region or left exit region of about a 70-90 mph upper-level jet. As said earlier being in these regions help enhance vertical motion and upward momentum. Link to comment Share on other sites More sharing options...
earthlight Posted May 29, 2011 Share Posted May 29, 2011 The EML is so important..it's probably at least a little bit coincidental...but in recent years, every time we have a decent EML present we get a good a severe weather event. It's slightly concerning to see the models worsening the lapse rates between 00 and 18z Wednesday...but the setup is still good. We have a great thermodynamic environment and seemingly just the right amount of shear. Link to comment Share on other sites More sharing options...
TalcottWx Posted May 29, 2011 Share Posted May 29, 2011 Wednesday may be a chase day... SPC mentioned supercells in its afd.. even if I head west to worcester I might see something Link to comment Share on other sites More sharing options...
CT Rain Posted May 29, 2011 Share Posted May 29, 2011 We could see a nice light show tonight... models are very unstable through the overnight. Link to comment Share on other sites More sharing options...
CT Rain Posted May 29, 2011 Share Posted May 29, 2011 Anyway the GFS looks great for Wednesday. Too bad it's not a 24 hour forecast lol Link to comment Share on other sites More sharing options...
TalcottWx Posted May 29, 2011 Share Posted May 29, 2011 We could see a nice light show tonight... models are very unstable through the overnight. What indices or signs are you looking at for nighttime stuff like that? Link to comment Share on other sites More sharing options...
Damage In Tolland Posted May 29, 2011 Share Posted May 29, 2011 We could see a nice light show tonight... models are very unstable through the overnight. Nice..is it from the MCS rolling thru the midwest? What timeframe are you contemplating? Link to comment Share on other sites More sharing options...
earthlight Posted May 29, 2011 Share Posted May 29, 2011 We could see a nice light show tonight... models are very unstable through the overnight. agreed. I am interested in tomorrow & Tuesday too...the nam has been flagging potential for a north to south moving MCS in the east end of the ridge falling off the ridge axis and diving south into unstable air. Link to comment Share on other sites More sharing options...
TalcottWx Posted May 29, 2011 Share Posted May 29, 2011 That MCS is some form or another should come right at us.. but who knows how strong it will be Link to comment Share on other sites More sharing options...
TalcottWx Posted May 29, 2011 Share Posted May 29, 2011 NAM doesn't even have the MCS existing on the hour 0 reflectivity thats impressive Link to comment Share on other sites More sharing options...
CT Rain Posted May 29, 2011 Share Posted May 29, 2011 agreed. I am interested in tomorrow & Tuesday too...the nam has been flagging potential for a north to south moving MCS in the east end of the ridge falling off the ridge axis and diving south into unstable air. The forecast is a total mess to be honest I'm not too excited about Monday or Tuesday... though I'm focusing on tonight's forecast right now. Monday could have some potential as long as the cap breaks. That seems more likely up here though. Link to comment Share on other sites More sharing options...
TalcottWx Posted May 29, 2011 Share Posted May 29, 2011 The forecast is a total mess to be honest I'm not too excited about Monday or Tuesday... though I'm focusing on tonight's forecast right now. Monday could have some potential as long as the cap breaks. That seems more likely up here though. Really quick again what do you look at for signs or indices for night time events? Link to comment Share on other sites More sharing options...
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