Last week it was a major snowstorm, but this week we may be looking at an even higher impact event. As it stands there is significant potential in a multi-hazard weather event on Christmas Eve and Christmas Day. There is still a fair amount of uncertainty, but confidence is increasing.
The overall progression that I discussed a few days ago hasn't changed much. A powerful storm is expected to develop to our west in a highly amplified pattern. A powerful cold front is expected to push through the state. As that occurs, a strong wave of low pressure will develop and ride the front, bringing heavy rain, strong winds, and warm temperatures to the state. The timing of the passage has slowed down, meaning that while a good portion of Christmas Eve morning and afternoon look ok, Christmas Eve night and much of Christmas Day now look awful. I use the word awful intentionally.
This period will see strong winds, heavy rain, and with high dew points a rapidly melting snow pack that will increase the chances of localized flooding. If your basement floods frequently, get ready to possibly deal with that on Christmas morning. All of that will be followed with rapidly dropping temperatures and possible icy conditions by Christmas night.
The worst of the rain should exit by early afternoon according to current guidance, but we will be dealing with strong to damaging winds that will make outdoor activities difficult if not impossible. In addition, once the front passes temperatures will begin to drop. While we're not expecting a flash freeze, this is something that we'll be watching closely in case the ending precipitation and dropping temperatures are more closely tied.
Let's look at specific impacts.
High Wind Watches have been issued for SE CT, and I expect this to be expanded to the rest of Connecticut eventually. This is the most impactful part of the forecast and most uncertain item at this time. The two images above show why.
Above and to the left you will find a GFS averaged sounding during Christmas Day in central CT. It's a small image here but what you will notice is just off the ground there are very strong winds inland. Not all of this will "mix" to the ground, but I think enough will right now to cause significant issues. One possible saving grace would be an inversion and weaker lapse rates that prevent mixing.
Above and to the right is a Euro model depiction of 925mb winds on Christmas Day. Again, not all of this will mix to the surface but the image itself is a signal of a robust storm and low level jet that signals a damaging wind threat.
As it stands, I think we're looking at a moderate statewide wind event that but could become high end should some of the signals we see become stronger.
Unlike some of the other events where strongest winds are isolated at the shoreline, I think the potential exists across the state, though not every town will see high end wind gusts.
At this time I think maximum wind gusts will vary between 45 mph--strong but not terribly damaging, and 65 mph--which would be much worse. We're not at the stage where we're able to pinpoint with confidence which areas of the state could be near the higher end of that range.
Those gusts would likely be focused on Christmas morning and early afternoon, diminishing during the latter part of the day but remaining fairly strong.
If these winds verify, we can expect scattered to widespread power outages on Christmas Day due to tree damage.
As I said earlier in the discussion, there is much higher confidence in a heavy rain event than a high end wind event. The image above also comes from today's European model, and it shows a very impressive swath of rainfall across the state. This is a signal that exists across guidance.
Right now we are looking at a statewide 1.5-3" rainfall range.
Maybe some of that precipitation is overdone, but in advance of the cold front we're also going to see higher dew points that will obliterate the snow pack from last week's storm. What this means is that moisture will be released as well in the form of water runoff. Combined with heavy rain Thursday night/Friday morning, localized flooding in the form of basement trouble, small stream flooding, and road ponding is likely.
River flooding looks less likely at this time, especially among the big rivers, but that's something to watch as well in the days after the event.
Crashing Temperatures/Flash Freeze
This is looking like the least likely threat at this time as we see a longer lag time between the end of precipitation Friday afternoon and temperatures falling below freezing. It's something to watch I suppose, but the cold really looks to be in place Saturday.
That said, folks will need to watch out for some black ice Christmas night. If you see ponding or wet conditions on your walkway/driveway later in the day you will likely want to salt. While some of the guidance wants to bring in some snow showers Christmas Day, don't hold your breath. This may very well be the king of Grinch storms.
We're looking at a potential high impact event. Wind is the biggest threat but the details still need to be worked out. Heavy rain is likely to cause some flooding and make outdoor activities much of Christmas Day difficult. After the storm passes, we head back to the cold. At least for the weekend.
We will be fine tuning the forecast as we get closer. The signal for this one looks legitimate. Be prepared.
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