This is going to be a strongly worded discussion. After one of the wettest Augusts on record and a top 5 wettest summer, we are seeing the biggest signal yet for a widespread heavy rain event to impact Connecticut Wednesday and Thursday.
In fact, as posted earlier on our Facebook page, the Weather Prediction Center (WPC) has issued a Day 2 High Risk of Excessive Rainfall for parts of Connecticut. It is not hyperbole to say that this is extraordinary. High risks for rainfall are confined to high end rainfall events, and are rarely issued this far in advance of an event, suggesting high confidence that a severe flash flooding event is likely to occur somewhere within that zone.
Given the setup, we do not disagree with their assessment.
The Overall Setup
First things first. Forget about Henri. This is a much different setup and one that is less likely to underperform. Even tossing the computer models aside, the atmospheric setup is such that this individual event should be taken seriously.
Not much has changed from the forecast on Sunday with regard to the setup. This is a situation where a combination of factors are likely to make this a high impact event.
First, the remnants of Hurricane Ida continue to move toward the region around a ridge. As that happens, the remnants will merge with a trough diving in from the west. This will create what looks more like a nor'easter synoptically--something that is standard in the winter, and means widespread precipitation.
What is not standard about this setup, however, is the tropical moisture that will be part of it, the very strong jet that will allow for exceptionally efficient forcing, and warm front that will be positioned in a way that will allow for even more frontogenesis, increasing rainfall potential.
Here is the 18z GFS surface depiction of the progression of the event. All guidance depicts heavy rain for CT, on Thursday morning, even as the axis of the heaviest rainfall shifts north and south between model runs.
We are anticipating a high impact event in Connecticut. This will be driven primarily by the amount of rain that falls and the rainfall rate we anticipate Wednesday night into Thursday morning.
Timing: Moderate Confidence
We expect rain to begin tomorrow morning into afternoon, as the storm approaches. It won't be much early on, but expect some scattered showers increasing in coverage and intensity through the day. Plans do not need to be changed on Wednesday. However, heavy rain will move into the state overnight and continue into Thursday morning. The guidance has been moving faster, meaning that Thursday afternoon is looking ok at this point.
The worst period for rainfall will likely be Wednesday night after 9-10pm through Thursday morning between 5-7am. More adjustments are possible to this part of the forecast.
Rainfall: High Confidence
We are forecasting a broad 3-6 inches of rain for Connecticut. Higher amounts are possible depending on where the best "banding" sets up. Currently, we believe that will be in the high risk zone of southern Connecticut, but it is very important to recognize that any part of the state could see very high rainfall amounts. Everyone needs to pay attention to this.
Below is a model we usually use during the winter, the HREF. This is a combination of high resolution guidance and generally gives an excellent projection of what can be expected in some rain or snow events. It is squarely within our forecast range, which I have to emphasize is very significant. We do not see 1-2" rainfall events very often, let alone 3-6" events. This is uncommon to say the least.
This range is further backed up by both GFS and European Model ensemble guidance.
As important as the rainfall total, the rainfall rate gives us a sense of whether flash flooding is possible. An inch of rain over the course of a day is far easier to deal with than an inch of rain in an hour.
Here is where this summer's rainfall comes into play. We've seen a lot of rain, which has saturated water tables and increased local stream flows. In fact, according to the Boston NWS discussion, streamflows across CT, RI, and MA are all much above normal, greater than 90%. This means that there is not nearly as much capacity for normal storm flow zones to handle a lot of rain quickly.
Unfortunately, that's what is coming. Rather than a two day event, the bulk of the rain is likely to fall in a relatively short period of time. This part of the forecast is tricky, because the best rainfall rates are likely to be along a warm front that is forecasted to be nearby during this period. We could easily see 1-2 inch an hour rates near that front.
Even if we don't see the most prolific rainfall rates, we're still talking about weeks worth of rain in less than a day. That's not good given all the rain we've already seen.
Significant basement and flash flooding is likely, and severe flash flooding is possible in areas where the rain just won't slow down.
Wind: Moderate Confidence
Originally, it looked like wind would be an afterthought, but not so much now. With a strong jet and a low of decent intensity, the guidance has all added a signal for some gusty to briefly strong winds early Thursday morning. This is something we'll continue to watch, but for now we think the following will be the peak wind gusts:
Inland areas: max gusts of 20-40 mph
Shoreline areas: max gusts of 40-50 mph
Given the amount of rain we expect, that could cause some issues, but again, rain is the headline here. The highest gusts are more isolated in this forecast. This could lead to some scattered power outages as it stands.
Severe Weather: High Confidence
We think that there is a low chance of severe weather. For most of CT, it will be a cool rain reminiscent of a late fall nor'easter, but for the southern parts of CT, where the warm front is likely to be nearby, the combination of a strong low level jet and the front will create an opportunity for some storms that are capable of producing an isolated tornado. If the front is further south this doesn't happen, but it is something to keep an eye on in CT as we watch where the front goes on Thursday.
We have high confidence that a significant flash flooding event is likely late Wednesday into early Thursday. There could be significant delays or cancellations in parts of the state due to whatever falls overnight. We think that significant to major river and stream flooding is possible.
This is likely to be a high impact event given the amount of rain expected, the rainfall rate early Thursday, and the rain we've received this summer, making it one of the wettest on record. We are less certain about wind, but there could be some impacts if the wind forecast trends stronger.
Stay tuned to the forecast and if you see flooded roads, please turn around. High risk days for rainfall tend to be dangerous and costly in both property and lives. Take this rain event seriously.
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