Thursday, February 15, 2018

Current Rain Amounts/Radar Data

Graphics shown here are from about 6:45 am MST to 7:00 am. I checked the ALERT network last evening at 5:00 pm, and amounts were small but coverage of measurable precipitation was already 100%. Light sprinkles began here around 10:45 am, but it was mid-afternoon before light rain began. The plot above shows amounts for past 24-hours across the network. Generally, amounts currently range from about half an inch to well over an inch - very large amounts indicated on the Catalinas. Have to admit that I haven't yet ventured outside to read our gauge, since the rain continues.

I thought that I'd take a look at some products from our KEMX radar data, but as processed at other places than the NWS webpage. Above and below are the reflective fields for tilts 1 through 4 from the College of Dupage weather page (link to the right). The base scan at top graphically shows the severe terrain blockage that affects our radar. The radar beam is blocked to the southwest all the way through the 4th tilt (second below). The blockage, especially for low-top non-thunderstorm events, significantly impacts the KEMX estimations of rainfall.

The higher tilts show a very distinct bright band circle for our current setting. The morning sounding indicates the top of the bright band (the ice to water melting zone) has its top just at or below 700 mb.
The bright band impacts composite radar displays, spreading out the apparent extent of reflectivites reaching 40 dBZ or so. The estimated rainfall will be biased toward high amounts when the data bins used to estimate rainfall rate happen to also be in the melting zone. A complicated situation that will result in overestimates when just the radar data are considered.

At the bottom is the current echo tops product - this indicates the precipitation is low-topped, reaching only to around 500 mb. A check of the CG flash data reveals absolutely no flashes in Arizona and northern Mexico during the event to current..

Final note: the CoD radar products indicate elevation tilts of 0.5, 1.5, 2.5, and 3.5 degrees. However, commercial radar analysis software (GR2Analyst) indicates the radar is operating in the VCP 212 mode - this scan strategy has 7 tilts between 0.5 and 4.0 degrees. The CoD products are either interpolated or using the nearest data to their fixed tilt elevations.

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