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Tropical Storm Joaquin a Potential Threat to the Bahamas and U.S. East Coast

By: Jeff Masters and Bob Henson 3:02 PM GMT on September 29, 2015

Tropical Storm Joaquin formed on Monday evening in the waters between Bermuda and the Bahamas, and could be a threat to both the Bahamas and the U.S. East Coast late this week. Joaquin was struggling some on Tuesday morning, due to high wind shear of 20 - 25 knots from strong upper-level winds out of the north-northwest. Water vapor satellite loops show that a large area of dry air lay to the northwest of Joaquin, and the strong wind shear was driving this dry air into Joaquin's core, keeping the low-level center of circulation exposed to view with all of the storm's heavy thunderstorms limited to the southeast side of the center, as seen on visible satellite loops. However, the thunderstorms maintained their vigor with the help of record warm waters and excellent outflow toward the south side of Joaquin, allowing the storm to intensify to 50 mph winds by 11 am EDT Tuesday. Ocean temperatures in the region are near 30°C (86°F)--the warmest seen there since record keeping began in 1880. The Hurricane Hunters will fly into Joaquin on Tuesday afternoon, and the first dropsonde mission from the NOAA jet is scheduled for Tuesday afternoon, as well.

Figure 1. Latest satellite image of Joaquin.

Forecast for Joaquin
The forecast for Joaquin is very complex, and the confidence in both the intensity and track forecast for the storm is very low. Joaquin is trapped to the south of a high pressure system whose clockwise flow will push the cyclone very slowly to the west or west-southwest at about 4 - 5 mph. As the storm gets farther from the high, the strong upper-level winds out of the north currently bringing high wind shear of 20 - 25 knots will gradually decrease, allowing Joaquin to strengthen. The 8 am EDT Tuesday run of the SHIPS model predicted that wind shear over Joaquin would fall to the moderate range, 10 - 20 knots, by Tuesday evening, then to the low range, 5 - 10 knots, Wednesday through Thursday. It would not be a surprise if Joaquin was a Category 1 hurricane by Thursday. As Joaquin progresses to the west, the storm will also increasingly "feel" the steering influence of a strong trough of low pressure situated over the Eastern United States. This trough features a flow of winds from southwest to northeast, which will tend to push Joaquin more to the north or northeast, roughly parallel to the U.S. coast or out to sea towards Bermuda. However, the models are showing huge differences from run-to-run and with each other on just how this trough will develop and interact with Joaquin. The Tuesday morning (00Z and 06Z) runs of the GFS model showed the trough absorbing Joaquin and destroying the storm by this weekend. The 00Z Tuesday run of the European model showed Joaquin getting slung northeastwards out to sea, well away from the U.S. coast. The 00Z Tuesday run of the UKMET model had Joaquin interacting with the trough in such a way that the trough would tilt westwards, leading to the unusual situation that we saw with Hurricane Sandy of 2012, where the storm would head towards the coast on a northwesterly track (this was also the solution the European model had in its run 12 hours previously.) The 06Z Tuesday runs of the GFDL and HWRF models had Joaquin heading northeast out to sea, then rotating back to the northwest this weekend to potentially threaten the Northeast U.S. as a strong tropical storm. The models are so different, in part, due to the uncertainty with how long Joaquin's current slow motion will last. Another wildcard how the moisture from tropical disturbance 99L, which pushed ashore into the Florida Panhandle Tuesday morning, will affect the trough of low pressure that will be steering Joaquin later this week. The long-range forecast is complicated by the possible interaction with the remains of Tropical Storm Ida (now designated as Invest 90L), centered at 11 am EDT Tuesday about 1000 miles east of Joaquin. their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 90L 2-day and 5-day odds of development of 10% and 40%, respectively. Data from the NOAA jet from today should make Tuesday night's 00Z suite of computer model forecasts more reliable than the Tuesday morning runs, though.

Figure 2. Two model runs, just 12 hours apart, from one of our top models for predicting hurricane tracks--the European model--showed radically different solutions over 900 miles apart for where Joaquin might be in 5 - 5.5 days. Image credit: wundermap with the "Model Data" layer turned on.

Regardless: heavy rain event coming to northern Appalachians and New England
Adding to the complexity and hazard of this situation, an unusually intense heavy-rain event will be striking the northern Appalachians and New England over the next 2-3 days. These rains would greatly enhance the potential for Joaquin to cause flooding afterward if it happened to move nearby. Over the next couple of days, deep tropical moisture streaming northeastward from the Gulf of Mexico will lead to extremely high amounts of water vapor for the location and time of year. This moisture will intercept a preexisting frontal boundary, as rain-producing impulses move along the east side of the sprawling upper-level trough over the eastern U.S. The NOAA Storm Prediction Center is calling for a band of widespread 3-5” rainfall from central Pennsylvania to southern Maine. Models are in fairly strong agreement that this heavy rain will develop, but there is some uncertainty on where the rains will be heaviest--in particular, the placement of the southwest-northeast stripe where training echoes could lead to particularly large amounts. The 0600 GMT (2:00 am EDT) Tuesday runs of the NAM and GFS models indicate a large swath of 4-5” rainfall, with many embedded areas of 5-10” potential, through 2:00 am EDT Friday, extending from Pennsylvania and New York across northern and central New England. Dry conditions have prevailed over the mid-Atlantic and New England over the last few weeks, with moderate drought near the coast, so most locations could handle several inches of rain before flash flooding became an issue. Any direct impacts from Joaquin would only add to the eventual flood risk.

Figure 3. Projected 3-day rainfall totals from 1200 GMT (8:00 am EDT) Tuesday, September 29, 2015, through 1200 GMT Friday, October 2. Isolated local amounts could be considerably higher. 

Figure 4. Short-Range Ensemble Forecast (SREF) plumes, produced at 0300 GMT (11:00 pm EDT) Tuesday, September 29, 2015, showing potential rainfall in Boston through 1800 GMT (2:00 pm EDT) Friday, October 2. Each trace corresponds to a particular model run. The ensemble average indicates that around 3" - 4" could be expected, though larger amounts are possible. The ensemble consists of three model "families" (WRF-ARW, WRF-NMM, and NAM), each run with varying initial conditions to represent uncertainty in the atmosphere's starting point. Image credit: NOAA Storm Prediction Center.

We'll have a full update on the Joaquin this afternoon by 4 pm EDT, when the latest 12Z (8 am EDT) suite of model runs will be available. We'll also have an update on the action in the remainder of the tropics.

Jeff Masters and Bob Henson


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.