NWS talk at BDR/Gama 20 Aug 2012
- If the edges of a cumulus cloud are well defined then it will continue building. If the edges are torn or indistinct then it will not build.
- Updrafts in large storms may exceed 50 kts. If a strong updraft is close enough to the ground it may evolve into a tornado.
- There is no lightning without ice.
- Ice will form in cumulus clouds 3,000 to 5,000 feet above the freezing level.
- Lightning is possible from the anvil well after the storm has passed.
- Internet lightning reports are only for cloud-to-ground. Most lightning is inter-cloud.
- Thunderstorms can form well in advance of a cold front because of compression induced instability.
- Thunderstorms rarely last more than 1 hr but can pop up in minutes (literally, between Nexrad scans).
- The accuracy (observed vs forecast) of TS forecasts in the first 3 hours of a TAF are about 20% and drop rapidly after that.
- TS do not have intensity, +TSRA refers to heavy rain.
- IFR forecasts are about 30% accurate, MVFR about 60% and VFR 90%, i.e., the better the forecast, the more likely it is to be true.
- PROB30 will not appear in the first 9 hours of a TAF.
- TEMP refers to an event with a probability of at least 50% and lasting at least 30 minutes during the forecast period.
- RA is forecast only if the POP is at least 50%.
- Radar return intensity is, in part, based on the size of water droplets. Cumulus clouds tend to form larger droplets than do stratus so the returns may show a higher intensity for the same total amount of water.
- Radar shows only precipitation so there is no way to know if a storm is convective or not.
- Radar is line-of-sight so 100 miles from the installation Nexrad may be reporting no lower than 5,000 feet.
- Composite Nexrad uses the highest intensities found in any scan.
- Nexrad dishes rotate at 2 rpm increasing azimuth by 1/2 degree after each rotation. It takes 4 1/2 minutes for a complete scan.
- Some TAFs are issued every three hours, although they may not have changed.
- For TAFs, the "terminal area" has a radius of 5sm, "vicinity" has a radius of 10sm.
- ATC does not understand that TAFs are only for the terminal area and may fly you into bad weather on approach.
- Birds perching on ASOS equipment may alter the readings significantly.
- ASOS wind is a 2 minute average and gusts are the highest in 10 minutes.
- Only 15% to 20% of weather balloon instrumentation is ever returned.
- Balloons are inflated to 5 to 6 feet in diameter on the ground and rise to 100,000 feet where they may expand to as much as 20 feet in diameter before they fail.
- Balloons rise about 1,000 fpm.
- There are no known aircraft collisions with weather balloons.
- The reason that POU and SWF TAFs are so different is that they are authored by different people.
- Read the discussions to determine how accurate the meteorologist thinks the TAF is.
- NWS has run out of cloud posters because of budget cuts.