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...CUTOFF LOW CONUNDRUM IN THE FORECAST...

9/27/2016

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Disc: One of the terms a forecaster hates to hear is "cutoff low".  A cutoff low is a low pressure system that is "cut off" from the main flow, and as such, doesn't move very much.  We'll have to deal with one of those this week- a little more on that later.

On the current weather map, a cold front has cleared the area, and now stretches from VA to near Nantucket.  We can already see the next players on the field, as the front is going stationary south of the Mason-Dixon line, and an upper level low is centered over Lake Superior.  These players will all play a role in the weather this week across the state.  Also noted on the upper air charts was a strong block over Southern Canada, which will also play a big role in our weather.

First of all, for tonight, fair weather.  Although clouds will be on the increase, not expecting any precipitation.  Temperatures will range from 50 to 60.

Tomorrow: As weak low pressure begins to organize along the stationary front, a stiff Northeasterly wind will develop.  This, combined with cloudy skies, will keep temperatures cooler than today.  Precipitation chances will be low, but not nil, as there could be a sprinkle at any time, especially from noon on.  I don't like the idea of coastal drizzle, since winds will be NE, but sprinkles will be a possibility.  High temperatures should be in the mid to upper 60s.

Tomorrow night and Thursday: Much of the forecast beyond tomorrow afternoon depends on how the cutoff low evolves.  Right now, most of the models keep the majority of the precipitation associated with this low further south and west than before, which would mean SW CT could see a decent soaking, while NE CT would not see a whole lot of rain, but would still be very cloudy, cool, windy, with periods of light rain.  However, I need to point this out, because it's important for the public to know, that forecasting cutoff type lows is never a certainty.  We have seen many times where models come to a consensus forecasting the axis of heaviest precipitation to be in a certain spot and then it turns out to be in a totally different area, even as close in as 12 hours out.  Recent examples of this include Hermine and "Juno" the Jan 25, 2015 fiasco.  A more distant example for some veteran forecasters is March, 2001.  So, as always, stay tuned for updates, and I will try to pinpoint the best possible forecast that I can.  It's just important to know that these are probably the single most difficult systems to forecast and many times, changes occur at the last minute.  Anyway, for tomorrow night, for now, not expecting much rain for most of the state, but there could be a rogue shower in the SW.  For Thursday, rain should arrive during the afternoon and propagate from SW to NE.  As for temperatures, expect chilly weather, with lows in the mid and upper 50s, and highs 60 to 65.

In the longer term, for Friday, expect periods of rain, with wind gusts of around 40 MPH and an all-around raw day.  It's hard to pinpoint when and where the heaviest rain will set up.  It may not rain in the entire state at every minute, but most of the day will be rainy.  High temperatures should be pretty uniform and chilly- 60 to 65 throughout the state.

For the weekend, questions only get larger, as one would expect, heading into the longer term.  The two main questions are how long this cutoff low hangs around and then whether this same system can spawn another coastal low, as well.  Chances of that happening are relatively low, but nothing can be ruled out.  For now, the best weekend outlook that I can come up with would be for Saturday would be relatively ugly and Sunday would be the best of the two days.  For Saturday, expect rain on and off much of the time.  Once again, as I said for Friday, there will be difficulty pinpointing when and where exactly the heaviest rains will fall.  But don't expect nice weather Saturday.  Temperatures should be in the upper 60s.  Sunday could see some sun, but there is still the chance of a shower, especially later in the day or evening, since the upper level low will still be hanging around.  Highs should again be in the upper 60s.

Looking further out, next week should see the chance for another more solid area of rain on Monday, as the upper level low finally swings through on its way out.  Then for Tuesday, the weather should finally clear out, at least for a while.  Expect highs near 70 Monday and in the upper 60s on Tuesday.

Thereafter, all eyes turn to the tropics.  A tropical wave, currently east of Barbados will soon be a named tropical cyclone and be moving west through the low latitudes.  Given the position of the Western Atlantic Ridge and the block over Canada (which will still be in place), there is the *POTENTIAL* for this storm to come up the coast.  Whether it moves up parallel to the coast, moves at  a latitude too far east for many effects, swings west like Sandy did, or even makes landfall in the Gulf of Mexico and tracks northward after that, west of the area, are all options right now, but looking at the positioning of various features in that time frame does signal the potential for an east coast tropical system.

Let's take a graphical look at some of the systems slated to affect the area, starting with this cutoff low.  Here's a map valid Friday morning.  (It's just a sample time I took that seems to be during the "core" of the storm- at least for now).  You can see at that moment there is a good slug of rain across SW CT and adjacent LI and upstate NY.
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Now I want to do something a little different, and post an upper level long range map, but I want to show the blocking that will be in place for any potential tropical system at that time frame.  This map is the CPC average map, which is an average of all long range guidance.

​
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You can see the very strong block centered over the Canadian Maritimes.  When a block like that esets up, usually some type of stormy weather results.  That's all for now! Enjoy the rest of your week (perhaps spent inside reading books or watching movies)!

-GP!
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    Southern Connecticut Weather is brought to you by Timothy Wrightington Jr. (TW), Spencer Aronstein(SA), Greg Petridis (GP), and Don Bell (DB).

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