Thursday, July 18, 2013

Two (More) Cases for CoCoRaHS

One of the stated goals for CoCoRaHS is to have one observer per square mile in urban/developed areas, and one every six miles in rural areas. That kind of observer density would go a long way to filling the gaps in precipitation measurements. I know I may be preaching to the choir in this post, but here are two interesting examples where the presence of CoCoRaHS observers (or lack thereof) make a difference in how we interpret a situation. One of these occurred in South Carolina this past weekend, and the other occurred today near me.

Map zoomed in on station SC-PC-13
in southern Pickens County, SC
On Saturday morning the national CoCoRaHS  "dot map" was mostly gray and blue early in the morning, usually a sign that there was a very high value on the map, either a real value or perhaps a decimal error. On closer inspection I found an amount of 8.96 inches in Pickens County, South Carolina, near Clemson.  Most of the amounts surrounding the station in question had amounts from 0.50 to 2.00 inches, with the closest station was about seven miles east.

Wider view of precipitation in Pickens County, SC
A check of the observation detail for that station made it immediately clear that this was a good value.  Here are the comments from the observer:

8.96 inches of rain!
Extreme heavy rainfall. First round late afternoon. Second round began in the middle of the night with a Flash Flood warning waking me up at 4AM. Frequent cloud-to-ground lightning and strong wind. Power fluctuations with lightning. My automatic tipping rain bucket reported at 5:40AM a 1-hour rainfall amount of 3.67". Diminished to light rain at 6:30AM. **EDIT** I'm changing the flooding statement from Unusual to Extreme based on observations in daylight. Creek backing onto our property, as well as Nettles Park on the other side of the creek, has burst its banks and flooded playing fields up to the score board level! Lifetime locals claim they've never seen this in their 40-50 years.

Two lessons here.  First, comments are extremely valuable!  The second lesson is that even with 12 to 15 other observations within a 15 mile radius, not one came even close to this rain amount.  More observers would have really helped define this storm. Here is a comment from the observer (a trained meteorologist) in an email:

It really is too bad there aren't more observers because I would hazard a guess that areas in and near Pendleton, SC which is just 3 miles south of me had really heavy rain before we did.  And that's where the rain seemed to be reforming, right over Pendleton on north.  Then as the storm tried to diminish, the whole thing just shrank right over southeast Clemson.  As it was shrinking, the intensities went way up.  The lightning/thunder was incredible as well.

Case #2 is an example of a storm occurring between observers. I live in Champaign County in east-central Illinois, Despite 45 to 50 active observers in the county, not one was located under this storm.  We need more observers!

Rain shower about 3 miles NW
On my way home this afternoon I noted there were some good building cumulus clouds. About 20 minutes into my 30 minute trip, I saw a well-defined rain shaft to the northwest and stopped to snap a photo.  Once home I checked radar and it turned out this was the only shower or thunderstorm within 90 miles and about 8 miles northwest of my location. As near as I could tell this cell started to rain about 2:45 p.m.

By 3:30 p.m. the storm had intensified and was no doubt producing heavy rain. At it's maximum the storm was about 3 miles in diameter, and during the course of its lifetime (a little less than 2 hours) it moved a distance of only 2 miles. Winds from the surface up to 20,000 feet were only 5 to 10 knots.

Left:  Radar image a 3:37 p.m. CDT when storm was its most intense.
Right: Radar image at 4:19 p.m.
The cell was totally dissipated by 4:30 p.m. Champaign-Urbana is at the left edge of the image.
Interstate 74 is marked by the east to west red line.

This storm moved very little, so unless it was sitting over rain gauges when it developed it's unlikely any nearby gauges would have measured much, if any rainfall.  What about our CoCoRaHS observers?  Here is an image of the radar estimate rainfall with this cell, plotted with the location of the active CoCoRaHS observers.

So how much rain?  We have observers to the east, observers to the west, observers to the north, and observers to the south, but not one where that cell developed and died out! According to radar, someone's corn and soybeans may have received a nice 1.00 to 1.70 inches of rain. But, we'll never know for sure.

We can never have too many observers!


  1. Excellent synopsis! For CoCoRaHS it truly is a case of; 'the more the merrier'!

  2. Similar situations recently here in KY with all the popcorn cells... e.g. the other day, rained all around us, just poured 3- 4 miles away all directions. Watched one shaft must have been 3 miles wide. Not a drop at my location.