Monday Ice Storm Summary

We just recently released a Premium forecast for clients on upcoming storm threats, including a snow threat on Thursday and early next week, but we also wanted to freely pass along our storm summary for today and highlight threats below.  Should you wish to subscribe to our Premium service and receive daily emails highlighting how weather may impact your life and upcoming schedule, please sign up here.  Otherwise, the post is attached below.

The storm today acted so well at first that it almost seemed eerie.  The scenario I envisioned with a slight last-minute cold trend allowing us to remain frozen all day across almost the entire region and allowing us to avoid the severe icing did occur, with the timing such that we saw the changeover to sleet/freezing rain right around when it was expected, with snowfall accumulations right around in line with what I was expecting with the front-end of the storm.  It was a success of both model guidance and forecast, but more forecasting, as I’ll explain in a little bit.  As has been the case this winter, however, every storm seems to bring its own surprise that is not properly picked up on by weather model guidance ahead of time.  Today, it was the final band of snow that swung through between 2 and 5 PM across SWCT.

This band was not picked up on well at all by weather model guidance.  Before the storm, there were indications that there could be some light snow on the back end.  When crafting accumulation expectations, I worked in the front-end snowfall, then expected that compaction from freezing rain/sleet on the snowpack would cancel out with any back-end snowfall, with back-end snowfall maybe adding at most another inch or so to the snowpack.  If this had happened, snowfall amounts would have been perfectly inside the expected range, as up until 12 PM the forecast was going perfectly.  However, that final band of snow had snowfall rates upwards of 2 inches per hour as it intensified rapidly and swung through the region, resulting in widespread accumulations of up to 3-5 inches.  This ended up boosting most areas to the top or even above the snow/sleet accumulations that I had expected for the storm.  And thus, though something no one could have known was coming, this storm over-performed, trying to at least partially make up for the under-performing blizzard last week.  Only this week, instead of some weather models showing heavy back-end snow and others not, there was no medium or short-range guidance that began showing this banding.  It was not until about 2-3 hours before it formed that the short-range RAP/HRRR guidance indicated that it would form, and by then I highlighted the threat on Twitter and on the blog to Premium members.

The National Weather Service similarly greatly underestimated the extent that this banding would form, expecting at most an additional 1-3 inches from it.  And thus, some snowfall accumulation forecasts busted slightly low.  NWS reports indicate that across the region, accumulations from 6 inches to 14.3 inches fell, quite a large range.  Winds were just gusty enough to cause some weak drifting, which could account for some of the slightly higher amounts, but in general we saw amounts of 8-10 inches at the coast and 10-12 inches inland or so, just slightly above the forecast ranges.  The much better part of the forecast was the impact analysis for Premium clients, which was close to spot on.  Roads remained extremely treacherous through the day, and all schools closed as expected.  Road conditions going downhill early overnight did not improve at all through the day, and early on I was able to identify that residual impacts would last into the day on Tuesday.  Thus, despite the accumulations being off, this was one of the higher-verifying forecasts for sure this winter.  In a winter where there has not been an “easy” storm to forecast, this one performed fairly close to what was expected, providing some relief in what has other been a very stressful last few weeks.

And while the storm is now gone, some of the impacts still linger across the area.  We are seeing numerous school delays tomorrow, as expected.  We are also seeing some isolated school closures, in line with what I was expecting.  The updated forecast calls for all schools to either delay or close.  Premium clients get detailed school forecasts regularly, but I just wanted to highlight here that it is important that you check in with your district tomorrow morning, as many of them may choose to close after announcing delays tonight depending on conditions.  Reports I am getting now indicate inland districts are more likely to do that than coastal ones, but all districts have a chance of that.

Finally, it is worth noting that the GFS was too warm, the ECMWF was too cold, and the proper forecasting came with what I normally do in a scenario like this, which is to blend the model solutions together.  We saw a clear warmer trend at a point in the storm, and that had me worried about accumulations, but the end solution was that the low pressure center actually formed further south than almost all guidance had called for, which is how that extra snow band on the backside got to form.  Thus, all model guidance was off, but obviously not as bad as the blizzard, and it gave us a good indication of the main impacts of the storm through a blend of the available scenarios.  Essentially, a perfect example of why we have meteorologists and not just use computers to forecast the weather, as we had to pick and choose weather models for certain parts of the storm to get the best final scenario; there was no one “right” model.

A free forecast will be available late tomorrow or early on Wednesday for the Thursday snow threat, so please stay tuned for the latest.

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