Thunderstorms And Heavy Rain As We Head Into The Weekend

Hello everyone!

We have two interesting weather systems coming our way in the next few days. The first will be a cold front dropping south out of Canada tomorrow afternoon. It will bring with it the threat for showers and storms tomorrow afternoon and into the evening. No severe weather is expected. That front will lose steam and stall somewhere just to our south Friday morning at which point a wave of low pressure will try to develop and move NE along the front. Where that low tracks and how strong it is will determine the outcome of our second weather event and how much beneficial rain we receive. Quieter weather is expected to follow that event with mild temps and sunny skies expected into next week.

Thursday Storms

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12Z GFS Showing Some Instability Present Tomorrow Afternoon. Image Credit: Weatherbell

Let’s do a quick rundown of the three ingredients needed for strong/severe storms. Doing this will explain why some storms are likely but also why severe storms are not. Instability is a go as temps soar into the 80’s and low 90’s while dew points climb as well, into the upper 60’s to low 70’s. Nothing incredible, but certainly enough for some booms.

12Z NAM And GFS Both Showing Upper Level Energy And A Surface Cold Front To Trigger Storms.
12Z NAM And GFS Both Showing Upper Level Energy And A Surface Cold Front To Trigger Storms.

How about a trigger? An upper level disturbance will be pinwheeling around the base of an upper low over Eastern Canada tomorrow. Meanwhile, a surface cold front will be sagging south across the region tomorrow. Both of these are shown in the image above with the left two panels representing one model forecast and the left two representing another. While there are some differences, the general idea remains the same. The general idea is that there will be enough of a trigger to get at least a few storms going. The big question then becomes, will they become organized enough to produce severe weather?

12Z NAM Showing Very Little Shear To Organize Storms. Image Credit: Accuweather
12Z NAM Showing Very Little Shear To Organize Storms. Image Credit: Accuweather

The short answer to that question is no. While guidance indicates some marginal shear, perhaps enough to trigger an isolated severe storm, there is not enough shear to support widespread severe weather. This means that you can expect your typical garden variety thunderstorms tomorrow with heavy rain, frequent lightning, and possibly some gusty winds. Remember, storms don’t have to be severe to be dangerous. Lightning can strike up to 15 miles from a storm and is just as dangerous as strong winds or large hail. When thunder roars, go indoors.

12Z 4kNAM Showing Scattered Storms Tomorrow Afternoon. Image Credit: Weatherbell
12Z 4kNAM Showing Scattered Storms Tomorrow Afternoon. Image Credit: Weatherbell

Putting that all together, here’s the big picture for tomorrow afternoon/evening. Scattered storms look to develop in the early afternoon over the mountains and will slowly move SE through the afternoon and into the evening hours before reaching the coast in a weakened state later in the evening. Storms are most likely in the mountains and least likely along the coast and especially along the midcoast.

Friday Rain

12Z GFS Showing The Setup Leading To Possible Rain Friday. Image Credit: Weatherbell
12Z GFS Showing The Setup Leading To Possible Rain Friday. Image Credit: Weatherbell

Tomorrow’s cold front will stall just off the coast Friday morning and a low pressure area will develop along its southern end over the Mid Atlantic states. The GFS shows this situation well with the map shown depicting conditions at 8:00 Friday morning. Scattered showers are possible during this time but steady rain, if it happens, is likely to occur Friday afternoon/evening. There is still quite a bit of uncertainty in the forecast for Friday as guidance offers several different possibilities. There are two scenarios, the dry scenario, and the wet scenario. I break each down below along with which I think is more likely.

A Comparison Of The 12Z NAM (Right Panels, Wet Solution) And The 12Z GFS (Left Panels, Dry Solution).
A Comparison Of The 12Z NAM (Right Panels, Wet Solution) And The 12Z GFS (Left Panels, Dry Solution). Click To Enlarge.

The NAM and GFS models from this afternoon present the two more extreme solutions: almost no rain on the GFS and tons of rain on the NAM. The map above is data-rich and small, so click it to enlarge it so you can see all the details, if you want. The main difference between the models revolves around the amount of upper level energy in the atmosphere and where that energy is located. The NAM has way more energy (lots of red, top right panel) while the GFS has very little (thin stripe of red, top left panel). Also of note is the location of the kicker disturbance. The software I use to get the data for these graphics makes any boundaries very hard to see so I highlighted New England as well as the Quebec/Ontario border to show the differences in location of the kicker disturbance. Notice that the NAM has it farther west, closer to the border while the GFS has it well east of the border. Keep in mind these maps are valid at the same time. By keeping the kicker farther west, the NAM allows for more moisture to stream northward and also gives the low more time to strengthen, bringing more rain. The GFS on the other hand hurries the kicker along, pushing the storm and the moisture offshore quickly.

12Z NAM (Right) vs 12Z GFS (Left) Both Showing Available Moisture (PWAT)
12Z NAM (Right) vs 12Z GFS (Left) Both Showing Available Moisture (PWAT)

The difference in moisture can be seen in the Precipitable Water maps which show how much moisture is available. Notice how the blue (super moisture laden air) is much more abundant and closer to the coast on the NAM (right) compared to the GFS (left). Also notice that on both models, SE MA gets in on the action. Heavy rain is likely there but rain chances become more uncertain as you head NW. Notice also how the mountains are in fairly dry air (brown/yellow). Very little rain is expected there. The battleground so to speak will be those areas in between the SE MA coast and the ME/NH mountains.

Current (12Z Today) Upper Air (500mb) Analysis. Image Credit: Meteocentre
Current (12Z Today) Upper Air (500mb) Analysis. Image Credit: Meteocentre

What do I think will happen? Right now I am leaning towards the GFS’s scenario because the larger scale pattern supports it. This morning’s upper air map shows this well. Winds in the upper atmosphere are overwhelmingly west-east over North America with only shallow ridges and troughs. This “zonal” pattern does not lend itself to troughs digging and amplifying which is what the NAM depicts. Right now, the WNW flow around the heat dome over the SW US looks likely to simply shove the fledgling low off the coast, leaving us with just a few showers or perhaps a brief period of steady rain. SE MA is likely to see the steadiest and heaviest rain with showers making their way all the way up to the base of the mountains. The mountains are likely to stay mostly dry as they are simply too far removed from the deep moisture to our SE. I’ll have more on this tomorrow along with updates on the thunderstorm threat.

Quieter weather is expected through much of next week.

-Jack

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