Updated: Jan 16
It's easy to assume that when you are looking at a reflectivity mosaic like the one below that somehow the cities are the cause of this precipitation event. You can see in this snapshot from 0035Z on January 14th that there is reflectivity reported in and around the major cities of Pittsburgh, Cleveland, Charleston and Dayton. Could this be some sort of "heat island" effect during the winter? Hardly.
Despite what the radar looks like, cities don't somehow attract snow like you might see with respect to the formation of radiation-fog. In metropolitan areas, the heat island effect can greatly diminish or delay the onset of fog in urban areas. But that's not true for snowfall or any large-scale precipitation event for that matter. Instead, snowfall is somehow attracted to areas around NEXRAD sites! Notice the precipitation areas are nearby the NEXRAD sites for Pittsburgh, Cleveland, Wilmington and Charleston.
KPIT 140051Z 32011KT 5SM -SN BKN015 OVC020 M02/M05 A2994 RMK AO2
KCRW 140054Z 34008G14KT 4SM -SN BR BKN009 OVC021 M01/M02 A3000
KCLE 140051Z 35019G26KT 6SM -SN OVC016 M01/M05 A3004
KDAY 140046Z 34009KT 9SM -SN BKN023 BKN029 OVC034 M01/M04 A3012
But what about in the middle near Zanesville, Ohio where the area is void of any precipitation?
The simple explanation to what you see in the radar mosaic is that this light snowfall event across the region is largely due to the fact that the tops of the clouds producing the light snow are rather low and the radar is overshooting the precipitation that is falling at a greater distance from the WSR-88D NEXRAD Doppler radar sites. This is easy to determine using the radiosonde observation (RAOB) from Pittsburgh as an example. This RAOB was launched at 2300Z on January 13th and is referred to as the 00Z RAOB on January 14th.
Notice that in this Skew-T log (p) diagram the cloud top of this rather shallow low-level saturated layer near Pittsburgh is roughly 10,000 feet MSL. Given a fairly cold cloud top temperature of -15°C and sufficient depth to those clouds, the precipitation type that is being produced is likely snow...mostly light snow. Close to the radar site this is easily detected by the radar, but as you move away from the site the lowest elevation angle of the WSR-88D NEXRAD Doppler radar is overshooting the precipitation event in locations like Zanesville. In fact, all pilots should never rely on a single source to explain what is occurring. For example, here are the reports from the Cambridge Municipal Airport (KCDI) near Zanesville. Notice at 0035Z, the airport is reporting light snow reaching the surface.
KCDI 140115Z AUTO 33006KT 10SM -SN OVC020 M01/M03 A3004 RMK AO2
KCDI 140055Z AUTO 36004KT 10SM -SN OVC020 M01/M03 A3004 RMK AO2
KCDI 140035Z AUTO 36007KT 10SM -SN OVC020 M01/M03 A3003 RMK AO2
KCDI 140015Z AUTO 35004KT 10SM -SN OVC022 M01/M04 A3003 RMK AO2
Similarly at 0053Z at the Zanesville Municipal Airport (KZZV), light snow is being reported.
KZZV 140153Z AUTO 33009G15KT 9SM -SN OVC023 M01/M05 A3004 RMK AO2
KZZV 140053Z AUTO 33009KT 9SM -SN OVC025 M01/M05 A3004 RMK AO2 SNB45
The sad truth is that most pilots are not taught to consider multiple sources such as surface observations, RAOBs and even satellite imagery. Moreover, they don't understand the details behind some of the many weather products they rely on with every flight. This is, in part, due to poor training from their CFIs who also know very little about weather in the first place. However, if you found yourself wondering the same thing as you are reading this, then you may want to consider doing some 1-on-1 training with an expert. You can do a single one hour or 30 minute sessions by booking a time here or for more extensive training you can book a ten session subscription plan here which comes with a savings. In this case you can choose from ten 60-minute sessions or ten 30-minute sessions. You have up to one year to use these to your advantage to learn more about topics like this or perhaps to review the weather with an expert prior to making a cross country flight in your plane. In this new year, you owe yourself to make this commitment - CFIs are always welcome.
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So the next time you are looking at a radar mosaic like the one below from the EZImagery MRMS Composite Reflectivity mosaic collection, don't forget to also look at other observational data to validate what you are seeing or not seeing.
If you want to learn more about the Skew-T diagram, you can order your copy of The Skew-T log (p) and Me book that is available in soft cover or eBook format.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist