Ask a Pro Met

[am19psu]

Question: When I have a forecast of 70% chance of rain, what does that mean? I have always thought that it meant that at any given time during the forecast period that 70% of the area covered by the forecast would be seeing rain. Is this correct? If not, someone please explain.

I know what a 70% chance of snow means. It means I'd better plan on going to school on a regular schedule.

It means that 7 out of 10 times you will see measurable precipitation with the expected atmospheric conditions. If you had a sample size of 100 70% POP days, for example, and it rained 68 times, your local office is doing a good job.

[isohume]

Question: When I have a forecast of 70% chance of rain, what does that mean? I have always thought that it meant that at any given time during the forecast period that 70% of the area covered by the forecast would be seeing rain. Is this correct? If not, someone please explain.

I know what a 70% chance of snow means. It means I'd better plan on going to school on a regular schedule.

It means what you think it means. You can look at it as any time during the forecast period there is a 70 percent chance of rain. Or since 70% is in the likely range, it can also mean numerous rain showers will affect the fcst area during that time.

One thing about POPs is, they are subjective. Meaning, a 40% chance of rain doesn't mean the same thing to everyone. Due to personal experience, someone might take their umbrella to work after seeing a 40% forecasted, while another will believe the chance is too low to worry about getting wet.

[am19psu]

It means what you think it means. You can look at it as any time during the forecast period there is a 70 percent chance of rain. Or since 70% is in the likely range, it can also mean numerous rain showers will affect the fcst area during that time.

One thing about POPs is, they are subjective. Meaning, a 40% chance of rain doesn't mean the same thing to everyone. Due to personal experience, someone might take their umbrella to work after seeing a 40% forecasted, while another will believe the chance is too low to worry about getting wet.

I don't think it means what he thinks it means. There is nothing in a POP that has anything to do with areal coverage of precip.

[isohume]

I don't think it means what he thinks it means. There is nothing in a POP that has anything to do with areal coverage of precip.

I meant POPs are subjective.

POPs can be expressed spatially or temporally. (scattered, numerous or chance, likely)

[am19psu]

I meant POPs are subjective.

POPs can be expressed spatially or temporally. (scattered, numerous or chance, likely)

Yeah, I actually like the categorical expression over the probability value, since so few people understand probability. The categorical POPs do a better job of conveying it to the public.

[isohume]

Yeah, I actually like the categorical expression over the probability value, since so few people understand probability. The categorical POPs do a better job of conveying it to the public.

I normally use coverage terms in the short range and probability expressions in the med/ext range. It just sounds better and I think it provides a better visualization of the forecast for most folks.

[BadgerWXman]

I normally use coverage terms in the short range and probability expressions in the med/ext range. It just sounds better and I think it provides a better visualization of the forecast for most folks.

I second this

[Shades]

What is Digital Hybrid Reflectivity in refrence to NEXRAD, and the difference between standard Base Reflectivity (0.5)? Thank you.

[isohume]

What is Digital Hybrid Reflectivity in refrence to NEXRAD, and the difference between standard Base Reflectivity (0.5)? Thank you.

First off, DHR is a volume product (all elevation angles used) whereas as 0.5 base reflectivity is a one elevation angle product.

DHR is used in precipitation processing and it attempts to account for beam blockage and gaps in elevation angles by including any available rain gauge data. Hence the term "hybrid". Basically it calculates a radar bias (high or low compared to the actual rain gauge network) and adjusts the accumulated precipitation output of the radar accordingly.

DHR is used in other software programs like Flash Flood Monitoring and Prediction (FFMP) to help aide in identifying the flooding potential or at risk stream/river basins for flash flooding.

[Master of Disaster]

Looking for a met or two for a quick interview on the severe weather for an article I am working on for the Examiner. Of course all credit will be given. PM me if interested.

[k***]

Looking for a met or two for a quick interview on the severe weather for an article I am working on for the Examiner. Of course all credit will be given. PM me if interested.

[Master of Disaster]

Do you have to be a total idiot on every single thread?

Anywho, thanks guys, its published and was picked up by USA Today. You guys rock.

[Dark Energy]

For anyone that wonders how a wx balloon is launched. See the video below. It is cut down of course, it takes a bit longer than shown, but you get the idea.

[isohume]

For anyone that wonders how a wx balloon is launched. See the video below. It is cut down of course, it takes a bit longer than shown, but you get the idea.

Haha....I remember her! Great met/person. Cool vid.

[NortheastPAWx]

Looking at doppler velocity images -- how can you detect a tornado with them?

[bkviking]

Ok, non physicist here so if someone can explain my issue I'd appreciate it. When looking at things like, say, SSTs, why do areas of warm water promote high pressure whereas areas of cooler water promote low pressure. I would think that rising air would occur where there is higher heat/moisture content, than the opposite. Is not low pressure an area where air rises, and high pressure where air is compressed downwards towards the surface? Thus most years lately we see warm SST anomalies around Greenland, and this seems to be responsible for blocking high pressure in that area. My obviously wring understanding would be that there's more lift in that area, and thus low pressure storms.

[AtticaFanatica]

Looking at doppler velocity images -- how can you detect a tornado with them?

Radial velocity measures scatterers moving toward or away from the radar (so 0 velocity could mean no wind or wind blowing tangent to the beam). Rotation shows up as areas of inbound velocity next to outbound velocity. If there are strong inbounds and in the next azimuth over, strong outbounds, that may be indicative of a tornado. There's all sorts of caveats, the radar isn't looking at the surface, the velocity values are estimates over a volume, etc., but that's the gist. Similarly, a mesocyclone will show up as the same signature, just on as larger scale.

[usedtobe]

Ok, non physicist here so if someone can explain my issue I'd appreciate it. When looking at things like, say, SSTs, why do areas of warm water promote high pressure whereas areas of cooler water promote low pressure. I would think that rising air would occur where there is higher heat/moisture content, than the opposite. Is not low pressure an area where air rises, and high pressure where air is compressed downwards towards the surface? Thus most years lately we see warm SST anomalies around Greenland, and this seems to be responsible for blocking high pressure in that area. My obviously wring understanding would be that there's more lift in that area, and thus low pressure storms.

The earth's circulation is driven by the differences in the suns radiation between the equator where it's maximized and the polar regions where it is at a min. The tropics and subtropics are driven by the Hadley circulation. The explanation of the Hadley cell in the discussion below partly answers your question.

http://en.wikipedia....iki/Hadley_cell

Another part of the circulation is a compensating secondary circulation, the Ferrel Cell which is explained in the within this discussion of the Atmospheric Circulation.

http://en.wikipedia.org/wiki/Atmospheric_circulation

[metalicwx366]

Why do some hurricanes produce way more tornadoes than the others? Why did katrina and andrew produce no tornadoes over south florida but did over louisiana? Why do the tornadoes in hurricanes generally are weaker than others? Why do the tornadoes form mostly in the northeast quadrant, and not the eyewall? Why do hurricane produce tornadoes? Lastly why did Ivan Frances and Beaulah produce so many tornadoes?

[isohume]

Why do some hurricanes produce way more tornadoes than the others? Why did katrina and andrew produce no tornadoes over south florida but did over louisiana? Why do the tornadoes in hurricanes generally are weaker than others? Why do the tornadoes form mostly in the northeast quadrant, and not the eyewall? Why do hurricane produce tornadoes? Lastly why did Ivan Frances and Beaulah produce so many tornadoes?

A general answer to your many questions is...the llvl shear and helicity increase dramatically when a hurricane makes landfall.

This is due to increased surface friction, which backs sfc winds and thus leads to high levels of helicity and cyclonically curved hodographs. The surface friction also increases the llvl bulk shear. Since sfc winds have to slow while they are backing...this leads to a significant increase of windspeed with height in the bl....or the overall shear magnitude. Both helicity and shear are important in producing tors in a low CAPE tropical environment.

The reason srn FL doesn't receive many tors from hurricances is because the land is mostly marshy or swamp land and the sfc based friction response is low. Thus the produced helicity and lowering of sfc windspeeds is not that great either. It's almost like the hurricane is still over water in that regard.

[am19psu]

Why do some hurricanes produce way more tornadoes than the others? Why did katrina and andrew produce no tornadoes over south florida but did over louisiana? Why do the tornadoes in hurricanes generally are weaker than others? Why do the tornadoes form mostly in the northeast quadrant, and not the eyewall? Why do hurricane produce tornadoes? Lastly why did Ivan Frances and Beaulah produce so many tornadoes?

So far, we don't really understand the physics that well, but statistical studies have shown the number of tornadoes a tropical cyclone produces is mostly a function of the upper level divergence at landfall and time of day of landfall. It turns out that there seems to be some barrier of divergence aloft around 50 x 10^-7 s^-1. When upper level divergence is greater than the threshold, tornadoes become much more likely. Also, tornadoes are more likely to form in the mid-afternoon (3pm local time is the max). It also appears that once you control for upper level divergence, the intensity of the landfalling hurricane is not a significant predictor of tornado activity.

I'm not a severe expert, but my guess is that tornadoes are generally weaker in hurricanes because the airmass is not particularly unstable. Tropical airmasses are very close to moist adiabatic, which doesn't allow for steep lapse rates and large values of CAPE. I'll defer to the severe experts for more details.

It actually turns out that the downshear direction is a better predictor of tornado location than the NE quadrant. It just so happens for many US landfalls, the shear vector is oriented SW/NE, so it gives the appearance of the NE quadrant being favored.

Here is the conference presentation where I got most of this info: http://ams.confex.com/ams/29Hurricanes/techprogram/paper_168674.htm

[weatherfreak96]

I was watching a system of storms by my place and I noticed a peculiar feature. It looked like a storm cell that was cycling, but they seemed awfuly far away. How far away does a cycling storm form different cells?

[Dino]

So, as you might know, I have been working on my senior thesis for industrial design and I haven't gotten much information from mets who actually use DOW's so any feedback would be appreciated. (hoping someone on the board might have experience with them)

Basically I am designing a winter weather research vehicle (like a DOW, but for snowstorms). DOW's are huge and mainly for researching tornadoes, so when mets started to use them in the snow they realized these things are terribly large to maneuver. That's only one market segment which is fairly small -- that's why my second market segment is local news stations. They could buy their own "DOW" and try to "one-up" other stations with their own mobile Doppler that could intercept heavy precip. Ideally these would be targeted towards the most populated areas on the east coast.

anyways, here's a link: http://www.kdtdesign.com/lynx.html

-Dino

[AtticaFanatica]

It looks pretty cool, but I'm not sure where you would put a radar dish on that vehicle.

Also, advances in technology aren't really going to allow use of smaller dishes, the size (area) of the dish is mathematically related to several other important radar parameters (gain, returned power, etc.) based on the radar equation. For example, the small radar dish shown in the slide in question can be small because it's a very high frequency system with a very specific purpose, it would be useless in winter storms because the signal would completely attenuate after only a few km. Your dish is gonna have to be at least a few feet in diameter.

[Dino]

It looks pretty cool, but I'm not sure where you would put a radar dish on that vehicle.

Also, advances in technology aren't really going to allow use of smaller dishes, the size (area) of the dish is mathematically related to several other important radar parameters (gain, returned power, etc.) based on the radar equation. For example, the small radar dish shown in the slide in question can be small because it's a very high frequency system with a very specific purpose, it would be useless in winter storms because the signal would completely attenuate after only a few km. Your dish is gonna have to be at least a few feet in diameter.

Thanks for the feedback!!!

The dish I have on the vehicle is a little over 3 feet in diameter. It's located where the rear windshield would be (bumper cam for rear view displayed on dashboard) -- I could push the radar dish diameter to 4 feet...maybe 5 ft.

Definitely taking note of the signal attenuation.

[isohume]

I was watching a system of storms by my place and I noticed a peculiar feature. It looked like a storm cell that was cycling, but they seemed awfuly far away. How far away does a cycling storm form different cells?

Truly isolated airmass tstms are actually rare as most tstms have some sense of organization to them and between adjacent tstms.

One of the main mechanisms for "cycling" is tstm outflow and cold pool interactions btw tstms. In a general sense...a tstm will go through 3 stages...developing, mature and dissipating. In the developing stage, the tstm is strengthening it's updraft based on the available instability, moisture and environmental flow. Once a tstm matures, it mostly consists of strong updrafts and the precipitation core is held high in the storm. Once the storm begins dissipating, it is all about downdrafts and the precip/hail core are no longer supported against gravity and fall to the ground.

As this precip falls to the ground, it drags colder air aloft to the surface and helps develop a sfc cold pool underneath the storm. These cold pools are supported dynamically as a meso high froms atop them due to the great downward force of mass. The meso high forms a meso cold front which then propagates outward. Depending the the overall storm motion, the meso cold front helps form new thunderstorms along the flank of the dissipating storm...normally to the SW. It does this by providing lift in an already unstable or "untapped" atmosphere upshear of the dissipating storm.

I hope this makes sense and this is not covering organized convection where tilted updrafts allow tstms to last for very long periods.

[Metsfan]

Where could I find information for a model runs temperature for each layer from surface to the 100 milibar level? Also how would I figure out surface and mixed layer cape from that information? Thanks.

[isohume]

Where could I find information for a model runs temperature for each layer from surface to the 100 milibar level? Also how would I figure out surface and mixed layer cape from that information? Thanks.

You can gather temp data from Bufkit profiles etc. It's hard to calculate CAPE directly because it is the integration of instability or buoyancy through the column and ideally you'd want to use virtual temperature. However, once you've found the mixed layer T and Td you could plot the parcel rising up the dry adiabat until you reach the LFC, then continue up the moist adiabat until you reach the EL. This will give you a visual representation of CAPE and could infer weak, mod, strong CAPE. The best way is to let a computer program do it, like RAOB. This program allows you to mix whatever layer you want and it will then re-compute the thermodynamic parameters.

[RJisLiving]

Hello, I was wondering...in the past year I've really taken an interest in meteorology, whether I wish to pursue that with any formal schooling or not, I don't know. If I wanted to learn outside of that in an amateur capacity, do you have any recommendations? Websites, stuff like that? I would just like to be able to read the maps and have a better understanding of what I'm seeing, be able to predict what's coming and/or going on, and be able to read when others (especially on the boards here) discuss weather in depth and be able to understand what I'm reading. Any help would be greatly appreciated!

[isohume]

Hello, I was wondering...in the past year I've really taken an interest in meteorology, whether I wish to pursue that with any formal schooling or not, I don't know. If I wanted to learn outside of that in an amateur capacity, do you have any recommendations? Websites, stuff like that? I would just like to be able to read the maps and have a better understanding of what I'm seeing, be able to predict what's coming and/or going on, and be able to read when others (especially on the boards here) discuss weather in depth and be able to understand what I'm reading. Any help would be greatly appreciated!

I'm assuming you're looking for non-mathematical intro type courses? There are many good intro books. I'd recommend Meteorology Today - Ahrens

Also...there some good intro type guides online that are free. Here's a couple.

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