Good evening from Southern Connecticut Weather.
You know us. We do not hoist the high end warning flags unless necessary. Despite a higher degree of uncertainty than we'd like, we're hoisting.
Our level of concern is increasing as we've watched a complex evolution between a coastal low, inverted trough, and anomalously high precipitable water and easterly fetch combine tomorrow to bring a widespread significant rain event, and potentially major rainfall event in some part of the region.
Adding to the concern is the very high rainfall we've seen this summer and September, which has caused a lot of basement flooding issues and higher water tables.
It goes without saying that tomorrow and Saturday are now a SCW Period of Interest.
You know us. We do not hoist the high end warning flags unless necessary. Despite a higher degree of uncertainty than we'd like, we're hoisting.
Our level of concern is increasing as we've watched a complex evolution between a coastal low, inverted trough, and anomalously high precipitable water and easterly fetch combine tomorrow to bring a widespread significant rain event, and potentially major rainfall event in some part of the region.
Adding to the concern is the very high rainfall we've seen this summer and September, which has caused a lot of basement flooding issues and higher water tables.
It goes without saying that tomorrow and Saturday are now a SCW Period of Interest.
Above: the latest Weather Prediction Center (WPC) excessive rainfall outlook for tomorrow. While a moderate risk of excessive rainfall is different than having a moderate risk of severe thunderstorms, the synoptic setup tomorrow into Saturday suggests that a lot more rain is coming to Connecticut. Image courtesy of Pivotal Weather.
Overall Setup
You may be wondering "where did this come from?" but we have been talking about the potential for Friday rain the last few days. We were intentionally silent on the setup because of the uncertainty but now inside 24 hours there is greater certainty in heavy rain.
A coastal low develops well offshore, but because of the upper flow extends an inverted trough over some part of the region. If you're wondering why you're hearing about an inverted trough in fall and not winter, you are onto something. Usually we only talk about inverted troughs (IVTs) during winter storms, but this time we have a setup that is ripe for a heavy rain event.
IVTs are notoriously difficult to predict. This time is no different.
What we think is that as the coastal low develops tomorrow, the IVT sets up over eastern NJ, NYC, the lower Hudson Valley, and some part of western CT. SW CT in particular back to New Haven is a particular risk zone that the WPC has highlighted, but it should be noted that anywhere from around Hartford and points west could see heavy rain.
The inverted trough will be enhanced by anomalously high moisture aloft, a persistent easterly flow pushing moisture into the trough, and a slow moving coastal low. That's tomorrow. The low is slow to depart, which puts Saturday at risk. Rain would be gone by early Sunday, but like last weekend, we have to watch for a lower probability delayed but not denied event with the track of the low.
Let's look at the global camp and high resolution camp.
Globals (Euro/GFS)
As the low meanders to our south, Saturday could bring moderate to heavy rain to eastern areas, as depicted in the 12z Euro depiction below along with its projected rainfall total. Rather than the intense high end rainfall totals of the high resolution models, the global models have a more widespread and spread out (over time) rain event. It's 2-4 inches of rain which would cause some issues, but nothing like what the high resolution guidance attempts to do.
It should be noted that just a few inches would make September more wet than our outrageously wet July in a number of places. A 4" rainfall equates roughly to a month's worth of rain in September.
Overall Setup
You may be wondering "where did this come from?" but we have been talking about the potential for Friday rain the last few days. We were intentionally silent on the setup because of the uncertainty but now inside 24 hours there is greater certainty in heavy rain.
A coastal low develops well offshore, but because of the upper flow extends an inverted trough over some part of the region. If you're wondering why you're hearing about an inverted trough in fall and not winter, you are onto something. Usually we only talk about inverted troughs (IVTs) during winter storms, but this time we have a setup that is ripe for a heavy rain event.
IVTs are notoriously difficult to predict. This time is no different.
What we think is that as the coastal low develops tomorrow, the IVT sets up over eastern NJ, NYC, the lower Hudson Valley, and some part of western CT. SW CT in particular back to New Haven is a particular risk zone that the WPC has highlighted, but it should be noted that anywhere from around Hartford and points west could see heavy rain.
The inverted trough will be enhanced by anomalously high moisture aloft, a persistent easterly flow pushing moisture into the trough, and a slow moving coastal low. That's tomorrow. The low is slow to depart, which puts Saturday at risk. Rain would be gone by early Sunday, but like last weekend, we have to watch for a lower probability delayed but not denied event with the track of the low.
Let's look at the global camp and high resolution camp.
Globals (Euro/GFS)
As the low meanders to our south, Saturday could bring moderate to heavy rain to eastern areas, as depicted in the 12z Euro depiction below along with its projected rainfall total. Rather than the intense high end rainfall totals of the high resolution models, the global models have a more widespread and spread out (over time) rain event. It's 2-4 inches of rain which would cause some issues, but nothing like what the high resolution guidance attempts to do.
It should be noted that just a few inches would make September more wet than our outrageously wet July in a number of places. A 4" rainfall equates roughly to a month's worth of rain in September.
Here's the GFS, which has been slow to make this a widespread rain event but the latest model run is now on board. It is disfavored for now given how much correcting it has had to do. Note that even on the GFS, heavy rain happens in western CT, even though it's later Friday into early Saturday. Unlike the Euro, there is less rain east on Saturday, but it could still be a blah kind of day verbatim.
Images courtesy of Tropical Tidbits and Weathermodels.com.
Images courtesy of Tropical Tidbits and Weathermodels.com.
The globals having a widespread rainfall Friday and Saturday gives us confidence in a floor of a significant rain event. Discounting the GFS, I think the general overall rain totals in the state Friday-Saturday will be 2-4" with higher totals the further west you are.
It'd be great if it remained that cut and dry, for lack of a better term, but it's not. Let's look at the high resolution guidance and why this could be a major rainfall event somewhere in CT.
It'd be great if it remained that cut and dry, for lack of a better term, but it's not. Let's look at the high resolution guidance and why this could be a major rainfall event somewhere in CT.
High Resolution Guidance (HREF/NAM/HRRR)
Let's start with a recap of the rainfall this month. Thanks to a series of coastal storms, including the remnants of Tropical Storm Ophelia, much of Connecticut has seen 1.5x to double normal rainfall this month alone, and this could be the biggest rainfall event of the month for some.
Let's start with a recap of the rainfall this month. Thanks to a series of coastal storms, including the remnants of Tropical Storm Ophelia, much of Connecticut has seen 1.5x to double normal rainfall this month alone, and this could be the biggest rainfall event of the month for some.
The high resolution guidance has gone big with a narrow area of extremely high rainfall totals tomorrow. Synoptically, a narrow area where a "firehose" of moisture makes sense. You can see it in the global models and certainly in the high resolution guidance. The questions really are 1) where does this firehose set up and 2) how high end can it be.
I think some of these are overdone, hence the general 2-4" rainfall forecast. However, there is likely to be an area, most likely in western and SW CT, where even more rainfall occurs.
In consulting with GP and SA, we don't think that area is going to be very wide, but anyone from Hartford (in case the IVT is further east) to western CT needs to be weather aware tomorrow. This is something where we need to watch radar trends to see exactly how thing set up.
Here's the latest HRRR. Rather than post the (likely) overdone high res totals of the HRRR, it's just worth saying that it's not out of the question to see 6+ inches of rain in the areas where the heavy rain trains. If that were to occur, expect flash flooding as early as tomorrow morning.
Once again, stay weather aware as this forecast evolves. Do not drive through flooded roads.
I think some of these are overdone, hence the general 2-4" rainfall forecast. However, there is likely to be an area, most likely in western and SW CT, where even more rainfall occurs.
In consulting with GP and SA, we don't think that area is going to be very wide, but anyone from Hartford (in case the IVT is further east) to western CT needs to be weather aware tomorrow. This is something where we need to watch radar trends to see exactly how thing set up.
Here's the latest HRRR. Rather than post the (likely) overdone high res totals of the HRRR, it's just worth saying that it's not out of the question to see 6+ inches of rain in the areas where the heavy rain trains. If that were to occur, expect flash flooding as early as tomorrow morning.
Once again, stay weather aware as this forecast evolves. Do not drive through flooded roads.
The HREF mean is concerning as this is an ensemble of high resolution guidance. Do not take this verbatim, as any shift east could put more of CT in the more high end rainfall zone and a shift west could take much of CT out of it.
Overall
Given the rainfall this month, the synoptic setup for a firehose of moisture somewhere between NJ, NYC, and western CT, this event requires close attention by everyone.
Stay tuned as frequent updates will likely be required tomorrow. Take this one seriously.
-DB/GP/SA
- Flood watches have been issued for Fairfield, New Haven, and Litchfield counties. It is possible those get expanded further east.
- A significant rainfall event is likely Friday and Saturday, with general rainfall amounts of 2-4". Higher rainfall totals are more likely in western CT, and flash flooding will be possible.
- A major rainfall event is possible tomorrow, particularly in western and SW CT as an inverted trough becomes a focal point for heavy rainfall. In this area, some spots could see rainfall totals in excess of 6", with flash flooding likely. Central CT should watch out in case the inverted trough sets up further east.
- Saturday is more uncertain, but some rainfall seems likely, making at least the early part of the day messy.
- Rain should clear by early Sunday, and Sunday looks like a decent day.
Given the rainfall this month, the synoptic setup for a firehose of moisture somewhere between NJ, NYC, and western CT, this event requires close attention by everyone.
Stay tuned as frequent updates will likely be required tomorrow. Take this one seriously.
-DB/GP/SA
