Loop of KESX radar from 12PM Sunday – 4AM Monday
GOES17 Visible Satellite Loop:
Snowpack on Ground Monday, February 19th
The Las Vegas Valley Snow Event:
Did we see this coming?
Yes and no. We had been seeing very small chances for snow in the valley for a few days, but it wasn’t until Sunday afternoon that the hi-resolution models were consistently indicating that western parts of the valley could see up to 3 inches and up to an inch elsewhere. That’s when the decision was made to issue the Winter Weather Advisory. This is uncommon for the models but it happens, and for a number of different reasons, models often overdo the snowfall in the valley. This can be because models struggle with the very dry desert climate in the low levels of the atmosphere, they can have a difficult time because snow levels can fluctuate up and down depending on which direction the wind is originating from, or they can just completely miss some things. The models aren’t perfect, and they are one of the most important guides we have for making a forecast.
Why did some parts of the valley get snow but others didn’t?
Surface temperatures, snow levels, and the intensity of the precipitation coming down. All three of these things complicated the forecast and thus, what ended up happening. Surface temperatures in the valley were above freezing much of the night, but the temperatures were much closer to freezing in the west side of the valley where elevations are a bit higher. Also, the temperatures were fluctuating up and down a few degrees resulting in rain, then snow, then back to rain in some cases. This happens often when heavier precipitation falls. The atmosphere is trying to evaporate the precipitation as it falls, and this cools the atmosphere to its wet bulb temperature. This is known as the web-bulb effect and while we can approximate the potential of this effect, it’s notoriously difficult to accurately forecast. Temperatures can fall over 10 degrees in an hour depending on the intensity of the precipitation. So while most areas were above freezing, where the more intense precipitation developed was where accumulations occurred. These areas included Goodsprings, Sandy Valley, I-15 between Primm and Las Vegas, Boulder City. Basically, where the colors are brighter on the radar loop above.
You can see the extent of the snowpack as seen on satellite Monday on the loop above as well.
via Watts Up With That?
February 21, 2019 at 08:35PM