Forecasting Virga

[Nicolas Bennet]

Are any forecast maps available that help predict virga?  I have the impression when there is a lot of mid level moisture (e.g. skew-t below) there is likely to be virga.  But I am not the best at interpreting Skew-Ts, and in any case checking Skew-Ts at multiple points is not as useful as, for example, Skysight map graphics that predict CU, thermal height, etc.

Thanks in advance for any suggestions

[Eric Greenwell]

Have you looked the Rain forecast for days you've seen virga? Since virga is rain that evaporates before reaching the ground, Rain might be a better forecast than cloud cover.

Eric G

[Nicolas Bennet]

Thanks for the reply.  Unfortunately, in my experience, rain forecasts have not been much help forecasting virga here in Nevada, so I am still interested in learning about any forecast maps that might help predict virga.

[Roy Bourgeois]

If you think about what is happening to create virga, starting with initial lower level humidity content, surface heating, lapse rate,  cooling, condensation, precipitation and then evaporation - it's a pretty complex phenomenon. I'm not surprised that it is hard to predict in a specific location.  There are a lot of moving parts to it!   

[krasw]

Since virga is not reaching ground (by definition), weather model rain forecast (=precipitation reaching ground) should not show it in any way, if model is working correctly, and almost always this is the case.

[Nicholas Kennedy]

Eric says Virga is RAIN that evaporates before hitting the ground which I think is technically correct.

OK

Often time out west where the cloudbase is high like 18-20+ ice crystal's fall out of the bottoms of the  clouds, is this also Virga?

In my experience virga with all water droplets in it can cause strong downdraughts, the ice crystal variety not nearly as  much.

Nick

T

[Tom Seim]

I look at the iCAPE (unbounded convective available potential energy) in the GSD sounding. I don't use the CAPE value in Skysight because it is bounded and doesn't provide a good forecast. Bounding means it is limited by an inversion, but if the forecast lapse rate is off a little then the unbounded value is much more accurate.

Tom 2G

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[*Eric Greenwell1*]

"iCAPE" is rare on the web - no help there, and all I found for it on the GSD site is this reference, and references to a Java applet:

29-Jun-2005 (beta)

Changed the calculation of iCAPE (CAPE for an interactive parcel) to only include the area at and above the Level of Free Convection (LFC). (This area is indicated by red hash marks.) Also, the LFC for the parcel (iLFC), if it exists, is shown at the right of the plot. Routine changed: sdg.SkewTPlot.

That doesn't seem to be what you are talking about - no mention of bounding, for example. What's link to a good explanation?

Eric G

[Tom Seim]

This is from the GSD webpage:

• CAPE (Convective Available Potential Energy) in J/Kg. For the RAP, we use the most unstable CAPE (MUCAPE) using the parcel with highest theta-e in lowest 300 mb (variable CAPE_P0_2L108_GLC0(2) in the document above).

Tom 2G

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[Matthew Scutter]

CAPE is poorly defined and most models/services land on a different definition for what they think is best suited to the user's application.

You can read a bit about the differences and which suits which situations here (https://www.weather.gov/media/unr/soo/scm/BKZ02.pdf)

SBCAPE (Surface-Based Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the total amount of potential energy available to a parcel of air originating at the surface and being lifted to its level of free convection (LFC). No parcel entrainment is considered.
MLCAPE (Mixed Layer Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the mean potential energy conditions available to parcels of air located in the lowest 100-mb when lifted to the level of free convection (LFC). No parcel entrainment is considered.
MUCAPE (Most Unstable Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the total amount of potential energy available to the most unstable parcel of air found within the lowest 300-mb of the atmosphere while being lifted to its level of free convection (LFC). No parcel entrainment is considered. 

SkySight uses SBCAPE which I think is generally the best choice for soaring forecasting. If you are particularly interested in the possibility of mid-level thunderstorms not originating from the surface, ML/MUCAPE is a better choice.

There seems to be a general misunderstanding that CAPE = thunderstorms, which is not strictly true.

I teach users that they should look for the thunderstorms actually forming on the thermal charts (big blue holes growing and filling with rain). (SB)CAPE only makes sense prior to the formation of thunderstorms, and gives you an indication of how quickly thunderstorms may form and grow if they do form, and from this you can deduce how soon you need to head home after you see them forming.

As a rough guide, not necessarily transferrable to all soaring regions, if I am on task and I see thunderstorms forming;

If CAPE was >2500, it's time to head home fast.

If CAPE was 1200, maybe I have some time but I'm bringing up the satellite picture and monitoring.

If CAPE was 500, perhaps it will be a non-issue.

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[Tom Seim]

I am not a meteorologist, but I am an experienced glider pilot with decades of mountain flying. Before each flight, I check the weather from half a dozen sources to get a consensus. First, thunderstorms in the mountains are dangerous and bear careful attention. Even widespread overdevelopment is a big problem. You can be a hundred miles or more from the nearest airport in eastern Nevada, and the other outlanding possibilities may be nil. Through years of experience, I have found the muCAPE provided by the GSD sounding to be the most reliable predictor of thunderstorms and overdevelopment. Also, read the National Weather Service hourly prediction and discussion is very useful. I have sent Matthew examples where Skysight predicted no overdevelopment or thunderstorms and we had both in all quadrants. His reply was that Skysight accurately predicted the weather when it clearly didn't! My recommendation is to totally ignore the CAPE and overdevelopment functions in Skysight - they are unreliable and useless. Instead, use Height of Thermals; when the height collapses earlier than they should, it indicates overdevelopment. 

Tom 2G

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[Cliff Hilty]

Hmmm, I'm in AZ, near Prescott, and use XC Skies. If the HRRR predicts precip and cloud base is above 6K AGL and the relative humidity at the surface is below 30% or so (guessing at humidity) we get virga instead of rain. Raise the humidity and it's rain, lower the cloudbase its rain. Makes sense to me. After all virga is rain thats evaporating before reaching the surface.

Cliff

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