A complicated weather pattern will likely dump tons of rain on the East Coast later this week and this weekend. A wide range of possibilities could unfold—stretching from scattered showers to the unlikely event of a hurricane threatening land—so just about everyone who lives east of the Appalachian Mountains needs to watch the forecast closely.

The Actors

Meteorologists seem to have settled into the “lots of moving parts” cliché to explain this complicated weather pattern, as one little change here or there could result in a large shift in what happens later this week. It’s the butterfly effect in action.

Here are the major features we’re dealing with at the moment:

1) Tropical Storm Joaquin strengthening near the Bahamas

2) Plumes of tropical moisture streaming up the East Coast

3) A low pressure system that’s currently developing over the Mid-Atlantic

4) A sharpening upper-level trough approaching the East Coast

5) A strengthening jet stream

The way that these features come together will dictate what kind of weather we see on the East Coast between Thursday and Sunday, and who gets the worst of whatever happens.



1) Tropical Storm Joaquin

The most serious part of the equation at the moment is Tropical Storm Joaquin, a strengthening cyclone that’s spinning a few hundred miles east of the Bahamas at this hour.

Since (up until now) it’s been a weak storm in a weird environment, the models have had a hard time figuring out what it will do. We’ve seen this play out several times this year, most notably with Tropical Storm Erika back in August.

The model spread is large right now—some have it hitting various points on the East Coast, while others show it moving harmlessly out to sea—and the National Hurricane Center keeps reiterating that confidence in the storm’s future track is “very low.”

The above map shows the NHC’s thinking as of 5:00 PM Eastern Time; Joaquin strengthened quite a bit between this morning and this evening, up to 65 MPH from 45 MPH six hours earlier. This strengthening trend will continue over the next few days as wind shear lets up, and Joaquin will likely be a hurricane by this time tomorrow.

2/3) Tropical Moisture and a Developing Low

The system playing out over land today and tomorrow will be the first significant thump of rain in the forecast.

For the past couple of days, a tropical disturbance hanging out on the northern Gulf Coast has produced extremely heavy rainfall in southern Alabama and the Florida Panhandle. The disturbance is now over land and has a zero percent chance of developing into something more, but it continues feeding tropical moisture far to the north.

Meanwhile, we have a developing low pressure system in the Mid-Atlantic that’s creating a large area of heavy showers and thunderstorms. The rain will probably get heavier as the low gets its act together and moves toward the Northeast.

Given the tropical influence of the air mass in place over the region (air you can wear!), the showers and thunderstorms can tap into the deep reserve of moisture in the atmosphere and pour like crazy. As such, flash flood watches are in effect across many of the areas seeing the heaviest rain. In areas covered by the watch, the ground is already saturated from recent rains, so it’ll only take one sustained downpour to overwhelm natural and man-made water systems and trigger rapid flooding.

4/5) Cold Fronts, Jet Streams, and Joaquin

This is where it starts to get really tricky.

Above is a look at this morning’s run of the GFS model, showing a sharp trough in the jet stream over the East Coast. In fact, this model run kicks Joaquin out to sea, one of the many solutions models are showing today.

This morning’s run of the European model shows a similar situation, strengthening Joaquin into an intense hurricane near the Bahamas before the trough quite literally kicks it out to sea, sending it perilously close to Bermuda before heading into open waters.

However, even though both models send Joaquin out into the ocean, they also both show unbelievable amounts of precipitation across the Carolinas, Mid-Atlantic, and Northeast thanks to the combination of a stalled cold front, the development of a nor’easter-type storm, and a persistent ribbon of tropical moisture feeding into the region.

If Joaquin comes closer to the East Coast and merges with the cold front to turn into a sort of hybrid cyclone, it could exacerbate rainfall totals, not to mention the effects of wind and coastal issues (flooding, waves, rip currents, erosion).

Timing and Impacts

Rain. Rain rain rain.

If one of the stormy scenarios unfolds, the greatest impact will be extremely heavy rain that leads to widespread flooding starting on Thursday and lasting through the weekend. Above is the latest forecast from the Weather Prediction Center showing how much rain they expect to fall over the next seven days. Keep in mind that this is a generalization—some areas will see more or less depending on the exact track of storms.

This forecast itself will likely change (probably dramatically) as models and meteorologists get a better sense of exactly what will happen later this week. As it stands, the WPC seems to expect the storm(s) to hug the shore, producing a large area of five or more inches of rain—areas immediately along the coast from Virginia Beach to Bangor could see seven or more inches of rain, which would cause extensive problems if it comes to fruition.

Meteorologists are working feverishly to get more data to ingest into these models. The National Hurricane Center has scheduled several hurricane hunter missions to fly into Joaquin, and there are a couple of NOAA Gulfstream jets ready to fly into the upper atmosphere to give us a better read on what could happen. The National Weather Service will also launch special weather balloon soundings from all launch locations east of the Mississippi River to aid in our understanding of the pattern and to give models more data to work with.

If you live in any of the states along the East Coast, it would be a good idea to keep up with Joaquin’s whereabouts and stay on top of rainfall forecasts and flood alerts for your area. If you’re expecting heavy rain, make sure no roads along your route to work/school/church/the store flood during major rainfall events. It only takes one foot of moving water to sweep a vehicle off a road. The majority of people who die in floods die in their vehicles after trying to drive across a flooded roadway. Don’t risk your life and the lives of your rescuers because you thought you could make it.

[Images: author, NOAA , GR Earth, Tropical Tidbits ]

Email: dennis.mersereau@gawker.com | Twitter: @wxdam