I provide a weather email service for people in MD. I am a storm chaser & Skywarn spotter. For daily weather tidbits follow me on Twitter: @wxmanMD.
By: cws6196 , 10:10 PM GMT on January 18, 2013
Ok, let me start by saying that yes, my forecast for yesterday went bust. I won't make excuses, but I do want to explain what happened and what could have been done to foresee what happened. As I have reviewed the data and tracked the storm yesterday, a few items of interest began to appear. But, keep in mind that in my post prior to the event I did detail my concerns for the event producing as forecast. Unfortunately, those concerns, and then some, came to fruition. This was always meant to be a southern storm. And if you recall in my first post about this storm I stated that I had not mentioned anything about this to you sooner because I didnt see it as an issue, and I felt it was going to stay south of MD. That is what ultimately happened, but I changed my forcast when I saw solid data indicating the storm would move further north. And in actuality, the storm did just that! Here's my take:
1. If you looked at the radar yesterday you would have seen that the precip was all over MD. Actually, the precip field extended into New England. I would say that it drifted further north..... Also, last evening, looking at the radar it was showing that it was snowing throughout S MD, yet that didnt happen. What was happening is called "virga". This is when the precip doesnt make it to the ground. The radar beam looks into the upper atmosphere, and there the precip and snow was, but it was evaporating as it neared the ground. Those in central and N MD didnt see anything because of that. There was a dry layer of air just below the layer of precip. The precip couldn't saturate that lower layer.
2. In S MD there was a lot of rain! The issue here was not virga, but a warm surface layer. This was due partly as a result of a warmer Bay water temp and warmer, wet ground (both of which I listed as my concerns in my previous post).
3. Actually, the forecast was "right" when it came to accumulated snow - had that rain in S MD actually changed to snow. If you recall my previous post I explained how snow accumulation is calculated. In S MD I recorded 0.68 inches of rain yesterday. Converted to snow that would have equaled around 5-6 inches of snow. The NWS forecast was for up to 8" and I surmised around 5". Had the warm layer not been present the forecast for S MD would have been on target.
4. So, why wasnt virga and the warm layer taken into account in my forecast? Not to make excuses, but variables such as those are rarely able to be identified prior to the event. What happened yesterday to allow the dry and warm air layers to exist was not simply one factor out of place. What we saw was that the secondary Low that was to form and move off the Carolina coast early in the day, bringing in colder, moist air, failed to form until later in the day. When it did form it was weaker than expected and further SW than predicted. Also, the high to our north ended up being weaker than expected. Combine those factors and the air flow around the High and Low were not srong enough nor positioned properly to bring in the cold, moist air. Instead we had a layer of dry and warm air. Could that have been accounted for? Possibly, but not likely. Especially not prior to the event. As the day progressed I started to see what was happening and began to realize that central MD wouldn't see anything and I cut down my totals for S MD. But, I still held out that the Low, even though weaker, would eventually position itself to bring snow. It never did, allowing for the warm air to stay in place and so deliver rain.
I hope this summary helps. I do not write it as an excuse, but in the hope you will better understand the complexities of forecasting. I also write this to help me learn from the event.
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The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.