Ask a Pro Met

[AtticaFanatica]

Is it possible to have 60 dbz snow bands in a Nor'easter? I will say yes given the right conditions, but I have never ever seen it before.

Without any sleet or partially-melted flakes (as seen in the bright band), that seems awfully high. You'd certainly need very wet, large-diameter snow flakes, and a lot of them within the radar's resolution volume. Even then, the weakly reflective properties of snow compared to rain makes 60 dBZ pure snow bands virtually impossible.

[Nic]

What are Li's? They are mentioned when there is a chance of severe weather

[Analog96]

What are Li's? They are mentioned when there is a chance of severe weather

Lifted Index. More negative= more unstable.

[LocoAko]

Lifted Index. More negative= more unstable.

Just to add to this, the actual number is the degrees (in C) of the difference between the environmental temperature at 500hPa and a parcel lifted adiabatically (usually from the surface) to 500hPa. A negative value implies that the lifted parcel has a temperature warmer than the environment, so that it will continue rising. The more negative this is, the stronger the difference in temperature is and the stronger the lift can be. It does not take into account what happens to the environmental temperature on the way from the surface to 500hPa (ie: caps, etc.)

[isohume]

Just to add to the discussion a bit more...LI numbers are dependent on the micro climate, meaning for example an LI of -3 in Iowa is most likely more deeply unstable than an LI of -3 in South Carolina on any given day due to higher sfc heating in the lower latitudes. In other words...the LI in IA generally accounts moreso for the true sfc-h5 lapse rate, while the LI in SC is generally weighted moreso with a shallower lapse rate.

Also...the Best 4-layer LI is more commonly used operationally, at least it is here at GSP. In this variant, the LI is calculated for a few levels between the sfc and 1600 m. Then the best, or most unstable, LI value is considered. This is useful to use when the sfc LI alone may be misrepresenting the true instability, such as a 12z raob etc.

[hm8]

I know that CAPE is measured in J/kg....but what exactly do those units mean? Joules per kilogram of what?

[isohume]

I know that CAPE is measured in J/kg....but what exactly do those units mean? Joules per kilogram of what?

It's a kg of air or ~0.8 m3, which is interesting because CAPE doesn't really account for a specific volume or mass. It is defined by the integration of temperature differences btw two specific height levels and the work performed counter to gravity.

Joules are defined as kg*m2/s2...or work over time. With joules having kg in the numerator and CAPE being defined as joules/kg...the kgs cancel out.

I imagine kg is left in the denominator to indicate one SI unit of measurement. Perhaps someone else can shed some more light on this.

[hm8]

It's a kg of air or ~0.8 m3, which is interesting because CAPE doesn't really account for a specific volume or mass. It is defined by the integration of temperature differences btw two specific height levels and the work performed counter to gravity.

Joules are defined as kg*m2/s2...or work over time. With joules having kg in the numerator and CAPE being defined as joules/kg...the kgs cancel out.

I imagine kg is left in the denominator to indicate one SI unit of measurement. Perhaps someone else can shed some more light on this.

Makes sense.

I just looked up CAPE elsewhere, and it defined CAPE as "the integration of the positive area on a Skew-T sounding". I understand integration and all of that, but where is the positive area of a Skew-T sounding?

[isohume]

Makes sense.

I just looked up CAPE elsewhere, and it defined CAPE as "the integration of the positive area on a Skew-T sounding". I understand integration and all of that, but where is the positive area of a Skew-T sounding?

Positive CAPE is the area under the curve btw the actual parcel (left) and the moist adiabatic lapse rate (right) temp traces...the area in red below.

[hm8]

Positive CAPE is the area under the curve btw the actual parcel (left) and the moist adiabatic lapse rate (right) temp traces...the area in red below.

Thanks. I'm in Calc 2 now in high school...and while I understand its nothing compared to what I'll have to eventually go through if I pursue a career in meteorology, its pretty interesting seeing what I'm learning being applied in real life.

[isohume]

Thanks. I'm in Calc 2 now in high school...and while I understand its nothing compared to what I'll have to eventually go through if I pursue a career in meteorology, its pretty interesting seeing what I'm learning being applied in real life.

That's pretty good being in Calc II already. You'll have a good head start if you go into meteorology. You'll find all levels of calc and diffyQ apply directly to meteorology in theory and in practicality.

[hm8]

Weird radar glitch from last night on the KMKX radar...check out the returns showing up down through IL

Ever seen that before?

[wx.1028]

Is it possible for the NYC metro area too experience a dryline. or is it mostly a southern plains condition ?

[Analog96]

Makes sense.

I just looked up CAPE elsewhere, and it defined CAPE as "the integration of the positive area on a Skew-T sounding". I understand integration and all of that, but where is the positive area of a Skew-T sounding?

It also stands for Convective Available Potential Energy, so that's why it's important for the formation of large thunderstorms, and that's why it is measured in joules per kilogram.

[am19psu]

Weird radar glitch from last night on the KMKX radar...check out the returns showing up down through IL

Ever seen that before?

It looks like anomalous propagation to me.

Is it possible for the NYC metro area too experience a dryline. or is it mostly a southern plains condition ?

Not in the true synoptic sense, with a large cT airmass. I've seen "cold" fronts pass with a dewpoint gradient and little temperature gradient, but it's usually modified cP air, not cT air.

[AtticaFanatica]

Is it possible for the NYC metro area too experience a dryline. or is it mostly a southern plains condition ?

There's very few places that are setup geographically to experience drylines. Latitudinally, they can extend northward into Canada, but longitudinally, they rarely extend too much east of the OK/MO border.

[AtticaFanatica]

Ever seen that before?

I've seen similar markings resulting from radar interference (one radar receiving pulses transmitted by another system), hence the regular spacing. I'm not sure if that's what's going on here though.

[radarman]

It looks like anomalous propagation to me.

Definitely not AP

I've seen similar markings resulting from radar interference (one radar receiving pulses transmitted by another system), hence the regular spacing. I'm not sure if that's what's going on here though.

That is possible. Little surprised about the somewhat substantial azimuthal extent though... Either that or a flaky oscillator

[ArtRosen]

Is this true?

http://www.home-weather-stations-guide.com/waterspouts.html

Unlike tornadoes, fair weather waterspouts grow from the bottom up. The first sign of a waterspout is a dark spot on the water surface marking the beginning of a vortex. As rotation increases, spiral wind lanes extend outwards, and once wind speeds increase enough, water is whipped up to form a small cloud at the base.

As the spout grows a funnel develops and extends upwards towards the cloud base. The waterspout is now at its peak, and has commenced moving across the surface, generally slowly but speeds up to 80mph (130kph) have been recorded. Wind speeds within the spout are typically between 60 and 120 mph (95-195kph), strong enough to be destructive but not devastating. The height of the column is generally less than 300ft (100m), but can reach heights of 2000ft or 600m. Fairweather waterspouts rarely exceed 100ft (30m) in diameter.

Apart from the spray at its base, the funnel is not composed of water, but is outlined by water vapour formed as pressure drops and evaporative cooling occurs.

The final stages of a waterspout occur as the column loses contact with the warm water and rising air that formed it. The column becomes ropy in appearance, and may bend. Rain, which is often falling behind the spout, will kill a waterspout by cooling the air around it, stopping convection and rotation.

[isohume]

Is this true?

Yeah that's correct. We used to get fair-wx waterspouts up in Lake Michigan and Lake Huron when I worked at Gaylord. They generally form later in the summer or early fall when the water is at it's warmest. The formation is generally a reaction to a cold wave passing overhead...the warm airmass adjacent to the water is in an unstable upward flux when colder air passes aloft and significantly increases the llvl lapse rates. Air converges at the surface (where relative low pressure forms due to the upward mass flux) and turns cyclonically...then upward to the cloud base if the the lapse rates are steep enough.

[PrinceFrederickWx]

I have a question about the NAO that hopefully someone can answer. I've always been told that a negative NAO generally increases the chances of the mid-atlantic/northeast being colder/snowier than average in the winter. But... doesn't a -NAO usually have the opposite effect (i.e. makes it warmer than average) in spring and summer? I thought one of the reasons the mid-atlantic had one of the hottest springs ever and the hottest summer ever in 2010 was due to the persistently negative NAO?

I've seen conflicting information from people on the Internet with this, so I figured I'd take the question to the pros.

[ArtRosen]

Yeah that's correct. We used to get fair-wx waterspouts up in Lake Michigan and Lake Huron when I worked at Gaylord. They generally form later in the summer or early fall when the water is at it's warmest. The formation is generally a reaction to a cold wave passing overhead...the warm airmass adjacent to the water is in an unstable upward flux when colder air passes aloft and significantly increases the llvl lapse rates. Air converges at the surface (where relative low pressure forms due to the upward mass flux) and turns cyclonically...then upward to the cloud base if the the lapse rates are steep enough.

So all water/land spouts are caused this way? Even the ones that come from severe storms (not supercells).

[isohume]

So all water/land spouts are caused this way? Even the ones that come from severe storms (not supercells).

No, only fair-wx waterspouts. Waterspouts developing from tstms are formed from descending mesocyclones and processes normal to tornadogenesis.

[ArtRosen]

No, only fair-wx waterspouts. Waterspouts developing from tstms are formed from descending mesocyclones and processes normal to tornadogenesis.

So those are tornadic waterspouts then? I thought a tornadic waterspout looked like a normal tornado just over the water and not those translusent tubes?

[ArtRosen]

Waterspout

Tornadic waterspout

[isohume]

So those are tornadic waterspouts then? I thought a tornadic waterspout looked like a normal tornado just over the water and not those translusent tubes?

I think you misunderstood my answer. You're right, the two types normally do look different...with tornadic waterspouts usually more opaque and wider. You can find examples for the opposite case tho.

[Mencken_Fan]

This is a major tornado (The La Plata, Maryland tornado) heading eastwards into the Chesapeake Bay:

This link shows a few exceptionally good photos of the F4 La Plata tornado moving out over the Chesapeake:

http://www.chesapeak...ie/chessie.html

This is a waterspout on the Atlantic Ocean near Bethany Beach, Delaware:

[hm8]

What is the largest area ever covered in one outlook issued by the SPC?

[Ellinwood]

This link shows a few exceptionally good photos of the F4 La Plata tornado moving out over the Chesapeake:

http://www.chesapeak...ie/chessie.html

That would be a nice link if it didn't have the following two sentences:

"The largest was classified by the National Weather Service as an F5." (Needs to be updated?)

"This is a picture of the smaller tornado with a lightning strike caused by the violent winds." (lol no)

[WinterWxLuvr]

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.