No two forecasts are the same. As we usually do, we are grading our latest big forecast, covering the final call forecast for the remnants of Ida. This was one of the most important forecasts we've ever provided, given the hazards associated with the system. Although we think the forecast went well, we deal in data. Every forecast is a learning opportunity.
Before we begin, some context. We began talking about the potential of Ida's remnants to bring rainfall to CT on Sunday in the weekly forecast discussion. In fact, I declared Wednesday/Thursday a SCW Period of Interest. This is a distinction we use when we think there is the increased potential of a significant weather event. Here's what I said on Sunday:
"As the remnants of Ida move northeast around a large ridge, it interacts or merges with a trough coming through the midwest. What this essentially means is that the remnants of Ida get a bit of a "charge". This is aided by the presence of a frontal boundary across our region as the system approaches on Wednesday/Thursday.
This is the kind of setup that can bring a lot of rainfall. The position of the boundary and track will be critical to the rainfall potential and what eventually falls. We will be watching closely as flash flooding will be possible.
For now, with the uncertainty on the heaviest rain axis, we are going to go with a general 1-3" of rain on Wednesday/Thursday, with the heaviest rain falling late Wednesday into early Thursday, but some signals and the official WPC forecast is higher."
We don't get points here officially, but providing a heads up is important as it provides you with information that you can begin to plan ahead with.
Hopefully you did.
Now let's get to the grading. What will the final grade say? Let's take a look.
Also above: A flooded Labor Field in East Hartford on Thursday morning. On a normal day, you would see none of this water. The Hockanum River recorded its fourth top 5 river flood event on record due to the heavy rains brought by Ida's remnants.
We had moderate confidence that rain would begin Wednesday morning into the afternoon as the storm approached. That's what happened. We called for intensity to increase over the course of the day, and that happened as well. That said, I must admit I didn't see the moderate rain coming in as soon as it did, but we also didn't predict the nonexistent lull that some of the guidance tried to highlight. That was a good call.
The worst time period was covered very well, with the extreme heavy rain coming in right as we said, but the I think we can deduct some points as the heaviest rain left the area faster than anticipated. The worst was really ending in terms of heavy rainfall around 3-4am rather than 5-7am. That's really pretty minor in the whole scheme of things. The timing aspect of our forecast was right were it needed to be.
This was obviously a key part of the forecast and here we did an excellent job. We forecasted a broad 3-6 inches of rainfall for Connecticut, and it looks like that worked very well, although many locations were on the higher end of this range and some places in far SW CT exceeded it by quite a bit.
We said that higher amounts were possible depending on where the best banding set up, and accurately forecasted that location to be in southern Connecticut. The high resolution guidance did a great job highlighting the potential zones, and we accurately forecasted the setup would keep the warm front which was a significant source of focusing extreme rainfall further south, despite some models like the GFS and Euro wanting to push it further north.
Our forecast was very good considering the convective nature of the rainfall. Although we're using graphics to illustrate the rainfall totals, we also looked at direct reports to come up with our grade. We could have gone with 4-8 inches, and in hindsight I would have liked to add an "isolated up to 9 inches" line of text, but we expressed the threat of higher localized rainfall in our 3-6 forecast and nobody should have been surprised in they ended up higher.
Once again, this forecast was very good, but not as good as the rest of the forecast. We said that the rain would come in a short period of time, that the heaviest rates would be along the warm front, and that significant basement and flash flooding was likely and would be severe in areas where the rain just won't slow down.
We do deduct some points for not seeing that 1 in 500 year type hourly rainfall was possible. We could have argued that 2-3 inch an hour rates were possible, but not many saw that even in the most aggressive forecasts.
Unlike our forecast for Henri, our wind forecast here was spot on. Both inland and shoreline locations gusted within our forecast ranges. Based on what we saw, no shoreline location gusted above 50 mph. This is good, considering that some guidance tried to be more aggressive with gusts.
I am very happy with how this forecast turned out. We thought that there was a low chance of severe weather, and that's exactly what happened. We recognized that the setup for severe weather in CT was different from the Mid-Atlantic, where a rare tornado outbreak took place.
The best chances for severe storms were in southern CT as we expected and although there was a tornado watch, we remained confident that any severe weather was going to be isolated. There were no tornado warnings issued in CT.
We had high confidence that a significant flash flooding event would take place. We thought significant to major river and stream flooding was possible, and that took place. We used strong language to call this an extraordinary setup, and to say that severe flash flooding was likely to occur in the high risk zone. Unfortunately, we ended up being right.
Let's recap the grades that make up our final grade. Each are weighed equally.
Rainfall Rate: B+
Severe Weather: A
Overall Impact: A
This leads to our final grade, an excellent A-.
As I said at the start, no two forecasts are the same. Every storm is different. You cannot assume that because the last storm underwhelmed that the next one would as well. The setup we had here suggested a high impact event, and the models only backed that assessment up.
Considering that life threatening flash flooding was on the table, it feels good to get the forecast right.
This forecast, and the updates during the storm, kept people safe. That's all we can ever ask for.
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Thank you for reading and trusting SCW.