Tropical Storm Joaquin is over 400 miles northeast of the Bahamas on Tuesday, but it’s forecast to turn north toward the East Coast later this week. (NOAA)

Tropical Storm Joaquin, currently in the southwest Atlantic 400 miles northeast of the Bahamas, is expected to turn north over the next few days and potentially converge with a stalled front that will be hovering over the East Coast. Though the forecast remains very uncertain, heavy rain, coastal flooding and strong winds are possible for the Mid-Atlantic and Northeast starting Friday.

Though it’s too soon to tell exactly where the storm will head and how strong it will get by the weekend, the possibility remains that Tropical Storm Joaquin could become a weak hurricane just off the Eastern Seaboard.

[Short-term rainfall forecast — localized flooding, thunder possible tonight]

Over the past 24 hours, the National Hurricane Center has been forecasting a wide margin of possible storm movement, ranging from a landfall along the coast in North Carolina and the Delmarva to a storm that passes well out to sea in the Atlantic, with no U.S. landfall at all. The Hurricane Center has been stressing in their discussions that confidence in the forecast track is very low.


Track and intensity forecast for Tropical Storm Joaquin. (National Hurricane Center)

There’s a very wide range in the model forecasts for the storm’s future track, which is illustrated below. This makes identifying a “most likely” track all the more challenging. Note the tremendous divergence of possibilities, including several models that hook this storm toward the left (west), with potential landfall anywhere from the Outer Banks to Boston.


Each line represents an individual model forecast for Tropical Storm Joaquin, ranging from East Coast landfall to a turn out into the open Atlantic. Uncertainty is very high for the storm’s future track. (tropicaltidbits.com)

The intensity forecast remains equally problematic. The guidance spread is shown below, and the official Hurricane Center intensity remains conservative at 70 mph — nearly a Category 1 hurricane — as the storm comes within striking range of the Mid-Atlantic coastline.

But a sizable number of models do intensify this storm into a Category 1 or 2 hurricane, which the National Hurricane Center notes in their discussion. The rationale for strengthening is the potential for wind shear over the storm to decrease. Wind shear is detrimental to tropical cyclones, and if it weakens it could allow the storm to strengthen to its full potential.


Strength forecast for Tropical Storm Joaquin from a number of different models. Each line represents an individual model forecast over time. (tropicaltidbits.com)

Yesterday afternoon, the European model was predicting an ominous scenario in which Tropical Storm Joaquin strengthened and tracked west into central Virginia. In the very next run, the model changed its tune entirely — a good reason to not place too much confidence in one model run. The latest run keeps the storm well offshore but still maintains a small and intense vortex.


Mean sea level pressure forecast from the European model for Sunday morning. Joaquin is the area of low pressure (green) out in the Atlantic. (weatherbell.com)

A recent run of the GFS model, on the other hand, brings a much weaker and more diffuse area of low pressure into the coastal Mid-Atlantic, then moving northward, albeit very slowly — a “left hook” scenario. One reason for this radically different track, compared to the European, is a significant blocking ridge of high pressure that the GFS builds across New England. This effectively impedes the northward progress of the storm.


Mean sea level pressure forecast from the GFS model for Sunday morning. Joaquin (or what’s left of it) is the large, broad area of low pressure (green) over the Mid-Atlantic. (weatherbell.com)

This scenario from the GFS model would bring Joaquin into the Washington region with impacts beginning early Saturday. One might anticipate a very wet forecast, and indeed the GFS portrays a truly insane amount of rainfall — 7 to 10 inches — for the D.C. region.

Through Tuesday of next week, the National Weather Service is forecasting anywhere from 3 to 5 inches of rainfall for the D.C. area, with higher totals in the higher elevations of northern Virginia, and up to 7 inches in coastal Maryland and Delaware.


Rainfall forecast through Tuesday morning of next week. (National Weather Service)

A hybrid transformation is likely

Tropical Storm Joaquin is unlikely to affect parts of the East Coast as a purely tropical system. It seems likely the storm will undergo what we call an extratropical transition — or a transition from tropical to non-tropical — as it interacts with a stalled front along the coast and the trough in the jet stream. It’s the same front that’s about to bring cooler weather to Washington, D.C., starting tomorrow.

The National Weather Service foresees these weather elements combining, as shown in the forecast surface charts below, for Friday, Saturday and Sunday mornings, respectively.


(National Weather Service)

The process of this transition tends to add an additional source of energy to the storm and has been known to rejuvenate weakening systems. There is potential for the frontal and jet stream interaction to focus additional heavy precipitation along the coast, and potentially some distance inland.

Additional impacts in our region include higher-than-normal tides, with NOAA predictions suggesting at least 2 foot abnormally high tides at Ocean City, Md., beginning on Friday (below).

Very strong wind is potentially another concern, especially along the coastline. Tropical storm-force winds (40-74 mph) remain a possibility, but are presently less than 5 percent per the latest Hurricane Center guidance — although a stronger-than-forecast storm, and/or a more inland track, will up these odds.

There is significant uncertainty in the forecast for this storm, and we will probably continue to see the models toss and turn over the next 24 to 48 hours. Big swings in track and intensity are possible. Social media is abuzz with discussion of a big East Coast hit, but any serious discussion should be tempered at this point concerning specific model solutions. This is something to watch and monitor closely.


Total tide forecast (in black) for Ocean City, Md., due to a combination of daily tides plus additional storm surge from onshore winds (gold). Tides will be running more than two feet above normal later this week. (National Weather Service)