Ask a Pro Met

[ohleary]

I got a handful of questions.

How does a Mesoscale convective system form a tropical cyclone sometimes?

How do annular hurricanes be able to survive hostile conditions (Epsilon) and what gives them the ability to do that?

How do derechos or bowechoes sometimes form areas of low pressure?

Why did erin intensify over land lol?

Why is it so hard for miami to reach 100 degrees?

Derechos are the straight-line winds usually associated with bow echos which are usually classified as mesoscale convective systems...which in turn are usually associated with mesohighs, not areas of low pressure...though a wake low can form west of the mesohigh but this is fairly unusual.

[am19psu]

Derechos are the straight-line winds usually associated with bow echos which are usually classified as mesoscale convective systems...which in turn are usually associated with mesohighs, not areas of low pressure...though a wake low can form west of the mesohigh but this is fairly unusual.

Yeah, he didn't really clarify at the surface or aloft. Downdrafts cause mesohighs to form at the surface, but the latent heat release generates vorticity and mesovortices aloft.

[hm8]

On some weather models like the RUC and HRRR I see the variable "Skin Temp" being offered as a product...is this similar to heat index or something different?

[metalicwx366]

Yeah, he didn't really clarify at the surface or aloft. Downdrafts cause mesohighs to form at the surface, but the latent heat release generates vorticity and mesovortices aloft.

I meant at the surface. I shouldve said that. Thanks for replies. Miamis all time record high i found was 100. Areas like jacksonville D.C Nyc and sometimes seattle portland savannah and los angeles see above 100 more often than miami which is odd when they are on the coast also.

[patrick05]

i have a question

some nations show the radar with the rain rate (either in in/hr or mm/hr), instead of dBz that NWS shows... is there any difference??

[ohleary]

On some weather models like the RUC and HRRR I see the variable "Skin Temp" being offered as a product...is this similar to heat index or something different?

Skin temp is usually the temperature of the surface, whether it be land, water, ice, etc.

[ohleary]

i have a question

some nations show the radar with the rain rate (either in in/hr or mm/hr), instead of dBz that NWS shows... is there any difference??

The two represent totally different things, but a reflectivity image and its corresponding rainfall rate image would probably look similar....higher reflectivities would generally give higher rainfall rates. There are established mathematics for calculating rainfall rate from reflectivity, which have shortcomings in certain situations, such as when hail is falling...but in these cases there are algorithms that detect and adjust for this.

[patrick05]

how about the radar themselves are they different or both doppler??

i prefer dBz but maybe they are using /hr in order for the public to understand more easily

[isohume]

i have a question

some nations show the radar with the rain rate (either in in/hr or mm/hr), instead of dBz that NWS shows... is there any difference??

No, there is no difference. It's just another way of describing the same thing (returned power). Reflectivity (Z) is a related to the number of hydrometeors per unit volume and dBZ is a way of normalizing the very high Z range in terms of decibels. However, dBZ also has a logarithmic, yet distinct, relationship to rainfall rate...such as mm/hr. One can be explicitly defined in terms of the other.

[radarman]

No, there is no difference. It's just another way of describing the same thing (returned power). Reflectivity (Z) is a related to the number of hydrometeors per unit volume and dBZ is a way of normalizing the very high Z range in terms of decibels. However, dBZ also has a logarithmic, yet distinct, relationship to rainfall rate...such as mm/hr. One can be explicitly defined in terms of the other.

That's a bit simplistic... the relationship isn't so clear, as the polarization of the transmitted wave and the droplet shapes affect the Z-R ratio... For example, tropical systems have different droplet shapes than sub-tropical convective precip. (tropical tends to be more spherical, sub-tropical convective tends to be more oblate)

On the whole though they are highly correlated.

[isohume]

That's a bit simplistic... the relationship isn't so clear, as the polarization of the transmitted wave and the droplet shapes affect the Z-R ratio... For example, tropical systems have different droplet shapes than sub-tropical convective precip. (tropical tends to be more spherical, sub-tropical convective tends to be more oblate)

On the whole though they are highly correlated.

Yeah I wasn't trying to get real specific or go into the various convective/tropical Z/R relationships. But what it boils down to is different scales of measurement, which can be used to infer storm intensity.

[phil882]

Miamis all time record high i found was 100. Areas like jacksonville D.C Nyc and sometimes seattle portland savannah and los angeles see above 100 more often than miami which is odd when they are on the coast also.

It all has to do with the specific heat of water in comparison to the atmosphere. The specific heat of water is much higher than air, meaning that it takes a lot more energy (heat) to raise the temperature of water a degree than it takes to raise the same amount of air the same value. It all breaks down to a simple mathematical equation.

Heat (q) = Mass x Specific Heat x Temperature Change (Delta T)

Specific Heat of Water: 4.18

Specific Heat of Air: 1.01

Ex.

4.18 J = 1g x 4.18 J/g deg C x 1 deg C

1.01 J = 1g x 1.01 J/g deg C x 1 deg C

Thus water takes more heat to warm the same degree Celsius than air... so when the air is saturated with more moisture, it takes more heat to warm the air the same amount. That's why arid locations can often reach and exceed 100 degrees during the warmest months of the year, while tropical regions struggle to get above the low 90's because of all the moisture keeping the temperature down.

[Ed Lizard]

Los Angeles and NYC aren't surrounded on basically three sides by water at the end of a long peninsula surrounded by water. A Southwest wind in NYC has a substantial overland component, ditto an East wind at LAX.

BTW, SW wind and an overland component, check out Islip sometime with a SW wind and compare it to LGA or EWR. EWR apparently has had a sea breeze the last hour, but there is a 6ºF delta in the daily high EWR to ISP.

[SacrydDreamz]

Can one of the more knowledgable mets explain how to tell what kind of tornado we are viewing when watching a U-Tube video.

I guess we can all tell the multi-vortex monsters from a supercell, but tropical funnels, land spouts, water spouts, and how to tell which is which. How high into the parent clouds structure they go. Are gustnadoes and land spouts the same animal?

If some pro-met was really motivated, they could even link U-Tubes of the various types of tornadoes.

Bonus question- if tropical funnels generally form in low shear conditions, than a tornado in a hurricane's rain band wouldn't be a tropical funnel, no?

Tropical funnels, landspouts and water spots are all very similar (if not the same thing in many cases) as they form along localized shear boundaries, such as where outflow boundaries collide. The environment is characterized by relatively weak winds/shear, unlike supercells. Also unlike supercells, the circulation usually builds from the ground (water) upwards... They are usually weak and short-lived... but some have produced signiifcant damage (particularly landspouts). Gustnadoes form along gust front boundaries and in this way are also similar to the previous three... difference is that they are associated with severe storms of which many are indeed supercells.

As far as a hurricane's rain bands are concerned, think waterspout or landspout on steroids.

[Thunder Road]

Hopefully this is a simple question:

What's the difference between BV and SRV on a WSR-88D? I have a basic understanding of how a BV scan works, but hoes does a SRV scan work?

Thanks.

[The Great Zo]

Hopefully this is a simple question:What's the difference between BV and SRV on a WSR-88D? I have a basic understanding of how a BV scan works, but hoes does a SRV scan work? Thanks.

BV stands for Base Velocity (often called just Velocity or V), and is one of the base moments captured by a WSR-88D. It shows the pure radial component of the wind (toward or away from the radar site), with no additional calculations.

SRV stands for Storm Relative Velocity (often called Storm Relative Motion or SRM). SRM is not actually sampled by the radar -- rather, it's calculated from velocity and an assigned storm motion vector. This calculation "corrects" the velocity image from displaying ground-relative flow to storm-relative flow, which makes it much easier to pinpoint circulations on a storm-scale. The storm motion vector can be calculated in a number of different ways, but one way of doing it is to use an automated average of the tracked cell motions on the 88D.

[Thunder Road]

BV stands for Base Velocity (often called just Velocity or V), and is one of the base moments captured by a WSR-88D. It shows the pure radial component of the wind (toward or away from the radar site), with no additional calculations.

SRV stands for Storm Relative Velocity (often called Storm Relative Motion or SRM). SRM is not actually sampled by the radar -- rather, it's calculated from velocity and an assigned storm motion vector. This calculation "corrects" the velocity image from displaying ground-relative flow to storm-relative flow, which makes it much easier to pinpoint circulations on a storm-scale. The storm motion vector can be calculated in a number of different ways, but one way of doing it is to use an automated average of the tracked cell motions on the 88D.

Ok thanks.

[LithiaWx]

Hi! A question I have came up today. How many feet above sea level is the 850mb level? What about 700mb and 500mb? Take Atlanta for instance, we are at or above 1000ft in elevation. The 850mb level is closer to the ground here than a place on the coast but my question is how close? Is it safe to determine the 850mb level by going in terms of feet above sea level?

[Ellinwood]

Hi! A question I have came up today. How many feet above sea level is the 850mb level? What about 700mb and 500mb? Take Atlanta for instance, we are at or above 1000ft in elevation. The 850mb level is closer to the ground here than a place on the coast but my question is how close? Is it safe to determine the 850mb level by going in terms of feet above sea level?

The technical answer is that the height of the pressure level is constantly changing... but in a more "standard atmosphere" sense, sounding plots have an approximation on the right hand side (with mb on the left side):

[Thunder Road]

The technical answer is that the height of the pressure level is constantly changing... but in a more "standard atmosphere" sense, sounding plots have an approximation on the right hand side (with mb on the left side):

So then why does a reporting site like Aurora, CO with an elevation greater than 5500 feet report 1010.3 mb? At such an elevation would they already be approaching the 850mb level? Or are the pressure-to-feet approximations AGL?

Thanks

[LocoAko]

So then why does a reporting site like Aurora, CO with an elevation greater than 5500 feet report 1010.3 mb? At such an elevation would they already be approaching the 850mb level? Or are the pressure-to-feet approximations AGL?

Thanks

Yes - those pressures are corrected to Mean Sea Level so that you can compare across the country at a standard level and make out pressure systems.

[Analog96]

So then why does a reporting site like Aurora, CO with an elevation greater than 5500 feet report 1010.3 mb? At such an elevation would they already be approaching the 850mb level? Or are the pressure-to-feet approximations AGL?

Thanks

Pressure is always adjusted to sea level so it can be fair to compare pressures across the entire country.

[am19psu]

For the mathematically inclined reader: http://en.wikipedia.org/wiki/Hypsometric_equation

[Thunder Road]

Ok but if I had a hand-held non-automated barometer would it read 850mb (or the corresponding inches)? Otherwise that makes sense, so thanks everybody

[OKpowdah]

Ok but if I had a hand-held non-automated barometer would it read 850mb (or the corresponding inches)? Otherwise that makes sense, so thanks everybody

Your barometer will read whatever the actual atmospheric pressure is at that location -- aka the station pressure.

But as already stated that is pretty useless for any sort of analysis (e.g. if you were to map station pressure, you'd pretty much just be getting a topographic map).

The NWS uses a formula that assumes a standard atmosphere lapse rate of 6.5K/km, and the station temperature to imitate a thermal profile "underground", and then uses the hypsometric equation to calculate the pressure reduced to sea level.

[Mencken_Fan]

I meant at the surface. I shouldve said that. Thanks for replies. Miamis all time record high i found was 100. Areas like jacksonville D.C Nyc and sometimes seattle portland savannah and los angeles see above 100 more often than miami which is odd when they are on the coast also.

Outside of Florida, most Eastern cities are relatively near the Appalachian Mountains. Winds with a westerly component have been compressed and heated by "downsloping" off those mountains. This can generate significant heat (a bane in summer but a blessing in winter for those who dislike bitter cold Arctic air.)

Inland southern Florida has a lot of lake and swamp water which prevents "overheating" of the air. Inland air north of Florida sits over dry land; it can more easily heat up.

Northern cities are geared for heat; Miami is not.

[am19psu]

Outside of Florida, most Eastern cities are relatively near the Appalachian Mountains. Winds with a westerly component have been compressed and heated by "downsloping" off those mountains. This can generate significant heat (a bane in summer but a blessing in winter for those who dislike bitter cold Arctic air.)

Inland southern Florida has a lot of lake and swamp water which prevents "overheating" of the air. Inland air north of Florida sits over dry land; it can more easily heat up.

Northern cities are geared for heat; Miami is not.

This answer is mostly wrong. Please let the meteorologists answer the questions in the "Ask a Pro Met" thread.

[Quixotic1]

Any books that you good folks would recommend as far as teleconnections and global pattern prediction? Thanks in advance

[nzucker]

This answer is mostly wrong. Please let the meteorologists answer the questions in the "Ask a Pro Met" thread.

I'm not sure I agree that the answer is incorrect....many of our hottest days come with W/NW winds here in NYC metro, causing compressional warming off the Catskills/Poconos/NNJ hills. During July 2010, which was +4.8F at Central Park, quite a few of the torrid afternoons had flow from this direction. The Park was 94/71 on July 12th with a peak wind at 340 degrees; Central Park had a 4-day heat wave from 7/16-7/19, and those days all had W/NW winds for peak gusts. Often in summer, the hottest days come after cold fronts, which cause offshore flow and downsloping. Also, westerly winds tend to make it much warmer at night, limiting the effectiveness of radiational cooling.

I also think the poster is correct in asserting that the humid/swampy environment in Florida prevents the real heat from entering. NYC had dewpoints in the 40s on July 4th, 2010 when the temperature soared to 96F; these low dewpoints allow air to warm faster since its specific heat is lower than that of water vapor. It would be impossible to have Tds in the 40s during a Florida summer, so it takes more solar energy to warm the air the same amount.

[am19psu]

I'm not sure I agree that the answer is incorrect....many of our hottest days come with W/NW winds here in NYC metro, causing compressional warming off the Catskills/Poconos/NNJ hills. During July 2010, which was +4.8F at Central Park, quite a few of the torrid afternoons had flow from this direction. The Park was 94/71 on July 12th with a peak wind at 340 degrees; Central Park had a 4-day heat wave from 7/16-7/19, and those days all had W/NW winds for peak gusts. Often in summer, the hottest days come after cold fronts, which cause offshore flow and downsloping. Also, westerly winds tend to make it much warmer at night, limiting the effectiveness of radiational cooling.

I also think the poster is correct in asserting that the humid/swampy environment in Florida prevents the real heat from entering. NYC had dewpoints in the 40s on July 4th, 2010 when the temperature soared to 96F; these low dewpoints allow air to warm faster since its specific heat is lower than that of water vapor. It would be impossible to have Tds in the 40s during a Florida summer, so it takes more solar energy to warm the air the same amount.

Compressional warming certainly helps NYC with a west wind, but that doesn't explain why JAX or BHM have average highs much warmer than MIA. And I'd argue the dewpoint is a much larger control on overnight lows in the NE than the direction the wind is blowing. Being surrounded by an ocean on 3 sides is by far the #1 factor in keeping MIA's temperatures controlled in the summer.

Nope, it's not the swamp (well, I mean it might have a small effect, but it's not anywhere close to the dominant one), it's the Gulf of Mexico. SAV/JAX are east of the Okefenokee swamp and can hit 100 no problem.