What are mb surface winds/low and mid-level lapse rates

[STurner]

I have heard of winds being discussed in nowcasts, forecasts, and other threads on numerous occassions. I am still learning about CAPE, LLJ, LLC's, SRH, and wind shear and still struggle to understand exactly what they mean. What do you all mean by 300mb surface winds at 60kts etc. and so forth. I was also wondering if low/mid-level lapse rates are as well. Basically what are good surface winds and low/mid-level lapse rates in terms of numbers. It seems like a lot of these things are hard to pick on a graph or model, for it looks like a bunch of scrunched up numbers, lines, and circles. Also if this thread needs to be moved go ahead and do so.

 

[Skip Talbot]

Shane,

As for the winds: Mb stands for millibars which is a unit of atmospheric pressure. The lower the number the higher the altitude because pressure decreases with height. 300 mb is where we typically look for the jetstream. 500 mb is what we call the midlevel and we look at the speeds here to see if we have the needed shear for severe weather. Kts stands for knots and is a unit of speed. We'd like to see at least 20 kts at 500 mb for minimum shear. More would be better. Between 500 mb and 850 mb we want to see a change in the wind direction. This gives us directional shear, which is needed for a good supercell setup. Surface winds are the winds at the ground. Strong clockwise changes in direction from the surface to 850 mb are an indicator of a good tornado setup. Strong surface winds also create good low level shear for tornado setups. 20 knots at the surface is very favorable. Tornadoes form in a very diverse environment in terms of shear though (even calm surface winds).

 

[STurner]

Shane,

As for the winds: Mb stands for millibars which is a unit of atmospheric pressure. The lower the number the higher the altitude because pressure decreases with height. 300 mb is where we typically look for the jetstream. 500 mb is what we call the midlevel and we look at the speeds here to see if we have the needed shear for severe weather. Kts stands for knots and is a unit of speed. We'd like to see at least 20 kts at 500 mb for minimum shear. More would be better. Between 500 mb and 850 mb we want to see a change in the wind direction. This gives us directional shear, which is needed for a good supercell setup. Surface winds are the winds at the ground. Strong clockwise changes in direction from the surface to 850 mb are an indicator of a good tornado setup. Strong surface winds also create good low level shear for tornado setups. 20 knots at the surface is very favorable. Tornadoes form in a very diverse environment in terms of shear though (even calm surface winds).

When you mean change in direction do you mean like from west to east or northwest to southeast. Dr. Greg Forbes seems to talk about east or southeast winds being good surface winds or something of that sort. Does these winds come into play with other things such as CAPE, LLC's, SRH, EHI, and LLJ etc? Even though I have not chased much, I do like to learn more about how tornadoes form. I really get excited about tornado outbreaks unfolding even though I am not out on the field.

 

[Paul Austin]

Here are a couple resources (of the literally thousands) online to help with weather station symbols and interpretation as a start:

http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/sfcobs/home.rxml

http://www.state.nj.us/dep/seeds/wssym.htm

As for your questions, yes in the beginning it can be overwhelming looking at so many numbers, symbols, and abbreviations (there are lots of those to get used to), but you'll get a handle on it. 300mb = 300 millibars, a measure of barometric pressure. 300mb compared to typical surface pressure near 1000mb would indicate a much higher altitude than the surface, in this case, a layer closely associated with the high winds of the jet stream. So 300mb surface winds would be a mix-up in terminology. For comparison, consider a strong hurricane with a typical surface pressure not much if at all lower than 800mb. Typical levels used in forecasting are 300mb, 500mb, 700mb, 850mb, and surface winds (including both speed and direction) with a few others heights often observed as well. The lower numbers are higher average altitude and the higher numbers conversely associated with lower altitude.

Lapse rates are a measure of the rate at which temperature falls with altitude. A drop of 20 degrees per 100 millibars would be a steeper lapse rate than a drop of 10 degrees per 100 millibars. Steeper lapse rates provide a stronger potential for higher CAPE (convective available potential energy) development with daytime heating and moisture transport. It also is usually associated with higher hail potential (cooler temperatures aloft)

I hope this is a decent start. I recommend you search Stormtrack for the "educational links" threads. As I mentioned before, there is a lifetime's worth of info freely available on the web, if you know where to look. And this question has been asked a lot on ST. I'm sure several members will post some useful links. But I would search for those already posted.

 

[Paul Austin]

Well, I'm a little late on some points. Actually, I think Skip's explanation is a bit clearer than mine, but hopefully it all helps.

 

[Adam Lucio]

SE winds generally indicate backed flow at the surface. Backed winds are favored because they can enhance shear profiles locally. Since in allot of setups the 850-500mb winds will [generally] be between the S and W....SE winds at the surface adds that much more turning with height.

Usually some sort of boundary is looked for along with backed surface winds such as a warm front.

The above doesnt hold true for every setup though, but it is generally favored. Looking at storm relative winds is a better indicater of the shear profile...there may be times where a SE surface wind sits under SE winds at 850 and 500 and then you have a unidirectional shear profile [more favorable for squall lines and linear messes] so in a situation where winds at 850 and/or 500 are from the SE you would probably want a surface wind from the S....to increase the directional shear and improve chances for turning

How does that come into play with CAPE and everything else...well...each ingredient has a prefered level to produce severe weather and tornados...and its when the best combination of all the facorts is present that the magic happens.

 

[Skip Talbot]

When you mean change in direction do you mean like from west to east or northwest to southeast. Dr. Greg Forbes seems to talk about east or southeast winds being good surface winds or something of that sort. Does these winds come into play with other things such as CAPE, LLC's, SRH, EHI, and LLJ etc? Even though I have not chased much, I do like to learn more about how tornadoes form. I really get excited about tornado outbreaks unfolding even though I am not out on the field.

You'll hear two terms on here quite often: veering and backing. As Adam pointed out, we like backed surface winds. When we talk about backing surface winds we mean the winds at the ground are turning from west to east over time. This enhances directional shear as Adam said, and can also aid in our moisture advection. When you hear the term veering, we typically are talking about how the winds are turning from east to west with height (not time). We want a clockwise change in direction with increasing height to produce the best directional shear. The opposite changes (veering at the surface, backing with height) typically work against supercell formation.

As for the indices you mentioned:
CAPE: Convective Available Potential Energy measures instability and is a product of temperature and dewpoint with height. Winds do not directly affect this value (> 1000 for most tornadoes depending on the kind of CAPE).
LLC: Is how high the bottom of your storms are going to be. This is also dependent on temperature and dewpoint, so once again winds do not have direct influence (< 1600 for most tornadoes).
SRH: Storm relative helicity is a measure of directional shear. This index is the direct result of the winds at different levels. (> 200 for most tornadoes depending on the kind of SRH).
EHI: Is combination of CAPE and SRH, so the winds do contribute to this index, but you won't have any EHI without any CAPE even if you have the best shear. (> 1 for most tornadoes).
LLJ: Is not an index, but stands for low level jet. This is a flow of wind that we usually look for at 850 mb. It enhances our wind shear, and often increases at night because of the way the sun heats up the atmosphere. Look on your map for a place where this jet intersects the main jetstream at 300 mb and you usually have the best location of wind shear.

 

[Mike Krzywonski]

The following links provide good info. as well:
http://extremeinstability.com/wx101.htm
http://weather.adamsphotogallery.com/class/page2.htm