It has been a while since the meteorological community has had the chance to analyze the potential for higher latitude blocking. It has also been a while since we’ve had the opportunity to analyze a synoptic heavy rain event. Both of those look to come to fruition, in multiple facets, over the next five to seven days. A dramatic pattern change will unfold across North America this week, with anomalously strong ridging and surface high pressure building into Canada and the Northwestern Atlantic Ocean. Precariously timed with the formation of a tropical cyclone, this blocking high pressure will lead to a forecasting headache — and the potential for heavy rains and impacts from a Tropical Storm along the East Coast.
For those without a technical background, high latitude blocking is a broad term for higher then normal pressures/heights in the higher latitudes. These “blocking” ridges of high pressure to our north, sometimes over Canada and the Atlantic and sometimes as far north as parts of Greenland, slow down the weather pattern closer to our area. The slower weather pattern can allow disturbances to interact and phase — forming much larger, more powerful storms that otherwise would have continued on their own way if the pattern was moving at a normal progressive speed.
This week, forecast models are in agreement that higher latitude blocking will develop over much of Canada into the Northern Atlantic Ocean. Ridging builds into these areas in the mid levels of the atmosphere, and a very strong surface high pressure builds east and southeast into Canada and even parts of New England. This is one important piece to the forecast headache, and one reason why meteorologists are slightly more concerned than normal at this range: The tropical system, or storm system that forms, cannot simply escape north or northeast. The blocking will slow down the pattern considerably.
Obviously, a main piece of the forecast puzzle is Tropical Depression 11 itself. Tonight, the system continues to hold at its relatively weak state — with maximum sustained winds of 35 miles per hour. The storm is expected to continue encountering wind shear, making the environment unsupportive for strengthening, for the next 24 hours or so. From that point forward, forecast models (and the National Hurricane Center) suggest the storm will begin a westward movement, and wind shear will weaken, allowing for further organization and a higher likelihood that the storm will obtain Tropical Storm characteristics.
This leaves us with two important pieces — a Tropical Storm or tropical system in the northern Caribbean, and a blocking high pressure system to the north over Canada and the Northern Atlantic. The final piece in the “puzzle of concern” as we’ve been calling it today in the office, is a broad but energetic disturbance which forecast models suggest will slide from the Ohio and Mississippi Valleys into the Southeast States. This mid and upper level trough is forecast to drive east/southeastward toward the Tropical Depression because of, you guessed it, the higher latitude blocking.
And that’s the proverbial forecast trifecta. Three pieces in a puzzle that could lead to a major storm system on the East Coast later this week into this weekend.
Here’s the problem: Exactly how these individual pieces behave will have incredible impacts on the outcome of the storm system. The very little nuances in the tropical system, the exact positioning of the blocking, and the timing of the incoming shortwave trough are just the tip of the iceberg in terms of uncertainty.
Forecast models, today, showed us a few distinct scenarios which are possibilities. The European model, the most extreme of the group, showed a Hurricane developing off the Southeast US Coast, phasing with the incoming upper level trough and being tucked into the Mid-Atlantic States. Such a solution would bring widespread heavy rain, strong winds, and coastal flooding to the Mid Atlantic, mostly keeping our area drier — with high surf and winds remaining a potential threat.
The GFS and other models take Tropical Depression 11, strengthen it slightly, and move it northwestward, eventually merging along a frontal boundary and becoming more of a hybrid system. The system then phases with the upper level trough over the Mid Atlantic, and gets tucked inland near New Jersey and New York, bringing heavy rain and strong winds to much of the coast.
Other model guidance simply takes the storm system and meanders it seaward, with very little impacts. These solutions have been appearing less and less frequently over the past few days.
Okay, that’s enough information. What can I expect, and what are you guys looking for?
You don’t need to run out and get the generator just yet. What you should do, however, is understand the potential that exists. There is enough uncertainty, still, that the probability of a dangerous storm occurring in our area is very low. The potential envelope of solutions is very large. That being said, high latitude blocking almost always catches meteorologists attention. It has historically been present for some of the stronger, more memorable storms on the East Coast. So it will be prudent to pay close attention to the forecast over the next few days. Even if a tropical system doesn’t directly impact the area, heavy rain and winds will be possible this weekend — with localized flooding possible.
The potential for heavy rains will begin as early as Tuesday Night and Wednesday, unrelated to Tropical Depression 11. But the frontal boundary associated with those rains will remain a significant part of the forecast.
As always, stay tuned over the next few days. We’ll have a slew of updates and information available on the website.