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Joaquin Intensifying; Potential Threat for Bahamas and U.S. Coast

By: Bob Henson and Jeff Masters 9:08 PM GMT on September 29, 2015

Tropical Storm Joaquin is gaining strength as it moves slowly west-southwest toward the eastern Bahamas. Joaquin’s top sustained winds were upgraded from 50 to 65 mph in the 5:00 pm EDT Tuesday advisory from the National Hurricane Center (NHC). An Air Force hurricane-hunter aircraft found a central pressure of 990 millibars, a substantial drop from this morning. From aboard the aircraft, NOAA’s Stepped Frequency Microwave Radiometer detected surface winds on the order of 60-65 mph (see Figure 2) at around 2:00 pm EDT. Joaquin has struggled in the face of wind shear of 20-25 knots due to strong upper-level winds from the north-northwest; earlier Tuesday, the 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. However, the thunderstorms maintained their vigor with the help of very warm waters and excellent outflow toward the south side of Joaquin, and on Tuesday afternoon an intense burst of thunderstorms developed atop the low-level center. With this consolidation, Joaquin has a much better chance to intensify further in the next 24 hours. Wind shear will be dropping below 20 knots by Wednesday and may dip below 15 knots for several days, lending more support to intensification. NHC now projects that Joaquin will be a hurricane by midday Wednesday.

Figure 1. Infrared satellite image of Tropical Storm Joaquin, collected at 1945Z (3:45 pm EDT) on Tuesday, September 29, 2015. Image credit: NOAA/NESDIS.

Figure 2. MODIS image of Tropical Storm Joaquin near the Bahamas as seen from NASA's Aqua satellite on Tuesday, September 29, 2015, at approximately 10:30 am EDT. At the time, Joaquin had top winds of 45 mph. Image credit: NASA.

Figure 3. Surface winds (blue line) and precipitation (red line) as measured by the Stepped Frequency Microwave Radiometer (SFMR) instrument on an Air Force hurricane hunter aircraft on Tuesday, September 29, 2015, between 1:35 - 3:34 pm EDT. Surface winds peaked at about 57 knots (66 mph) at 2:13 pm, during a period when heavy rain was observed. Surface winds of 51 knots (59 mph) were observed during a period of low precipitation. Image credit: Levi Cowan, tropicaltidbits.com.

Over the next couple of days, models agree that Joaquin will continue moving slowly toward the west-southwest, which will pose an increasing threat to the eastern Bahamas. The NHC may issue watches or warnings for the Bahamas later Tuesday night. Most of the 12Z Tuesday computer models stop Joaquin in its tracks just short of the Bahamas, but the usually reliable ECMWF model suggests a greater motion toward the southwest, indicating a potential threat to some of the eastern islands. During its slow westward trek, Joaquin will be passing over ocean temperatures near 30°C (86°F)--the warmest seen there since record keeping began in 1880. Oceanic heat content--the amount of energy stored in the upper 50 meters (160 feet) of the ocean--is another index of potential strengthening. Joaquin’s track should take it across the boundary into oceanic heat content values greater than 50 kilojoules per square centimeter by Wednesday. For tropical storms and Category 1 hurricanes, values of heat content greater than 50 kJ/cm2 have been associated with more rapid intensification rates, assuming that wind shear is less than 15 knots, mid-level relative humidity is greater than 50 percent, and SSTs are at least 28.5°C. Joaquin should meet most or all of these criteria over the next couple of days, which raises the possibility of robust intensification.

Figure 4. On Tuesday, September 29, 2015, Tropical Storm Joaquin was located in an area of ocean with record Sea Surface Temperatures (SSTs) of 30°C (86°F). Image credit: NOAA/NCEI.

Figure 5. Oceanic heat content across the Northwest Atlantic as of September 28, 2015. Values of greater than 50 kJ/cm2 are related to more rapid intensification of tropical cyclones up to Category 1 strength, assuming that other conditions are supportive (warm SSTs, moist mid-level air, and light wind shear). Image credit: NOAA/AOML.

The long-range outlook for Joaquin
The 12Z Tuesday model runs did not clarify the unusually muddy waters surrounding the future track of Joaquin. By Friday, the eastern U.S. trough should begin to influence Joaquin and give it a northward component of motion, but the exact configuration of that trough--which will be critical to Joaquin’s track beyond Day 3--is still unclear. The models continue to show huge differences from run-to-run and with each other on just how this trough will develop and interact with Joaquin. The general tendency is for this trough to take on a negative tilt (a NW-to-SE configuration), with upper-level ridging becoming stronger off New England. All else being equal, this would tend to result in a more northwesterly path for Joaquin. Complicating the forecast is the presence of several areas of disturbed weather, including the remnants of Invest 99L (now over northern Florida); Invest 90L (incorporating the remnants of Tropical Storm Ida); and a frontal zone across the Northeast that will produce torrential rain (see below). All of these systems will pump large amounts of heat into the atmosphere through condensation, and this in turn will influence how the situation evolves.

Figure 6. 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. The latest model run (8 am EDT Sept 29) reverted back to a solution close to the one shown above on the left. Image credit: wundermap with the "Model Data" layer turned on.

Figure 7. Ensemble members from the GFS model run at 12Z Tuesday, September 29, 2015, show a huge spread in where Joaquin might end up. The ensemble is produced by running the model multiple times, each time varying the starting-point conditions slightly to mimic observational uncertainty.

The model guidance from 12Z Tuesday shows two main possibilities by Days 5 and 6. The HWRF, GFDL, and UKMET models all depict a hurricane heading toward the mid-Atlantic by this weekend, tucking into the north side of the negatively tilted trough. In contrast, the ECMWF and GFS models show Joaquin heading out to the open Atlantic well before approaching the U.S. East Coast, as the trough pushes Joaquin eastward rather than pulling it into its northeast side. Various members of the ECMWF and GFS model ensembles have a wide range of solutions (see Figure 7 at right). Given the complexity of the factors involved, it is far too soon to put too much stock in any particular model solution. This is a good time to keep in mind that the width of the National Hurricane Center’s “cone of uncertainty” is based on average historical forecast errors from the last five years. In the case of Joaquin, the long-range uncertainty indicated by the models right now appears even larger than the cone’s historical database would suggest, which calls for even more caution than usual in focusing on any possible outcome. Data from the first dropsonde mission into Joaquin this afternoon by the NOAA Gulfstream-IV jet should make Tuesday night's 00Z suite of computer model forecasts more reliable than the Tuesday morning runs.

Regardless: heavy rain event coming to northern Appalachians and New England
Independent of Joaquin, an unusually intense heavy-rain event will be striking the northern Appalachians and New England over the next 2-3 days. Deep tropical moisture streaming northeastward from the Gulf of Mexico will lead to near-record 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 widespread 3-5” rainfall from 00Z Wednesday to 00Z Friday from eastern New York across most of New England to southeastern 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. Dry conditions have prevailed over the mid-Atlantic and New England over the last few weeks, moderate drought near the coast, so most locations could handle several inches of rain before general flooding became an issue. However, if intense rains fall in a short period over mountainous areas, some smaller creeks and streams could respond quickly and produce a localized flash flood threat. Urban areas from New York to Portland may also experience street flooding if the heaviest rains materialize there, as the drainage systems struggle to keep up. If Joaquin happens to move into New England early next week, there would be a much greater risk of widespread flooding.

Which track model should you trust?
According to the 2014 National Hurricane Center Forecast Verification Report, issued in March 2015, in 2014 we had two track models that at times out-performed the official NHC forecast, a feat that is tough to do. NOAA's HWRF model did slightly better than the NHC official forecast for 2-day and 3-day forecasts, while the UK Met Office's forecast did slightly better than NHC's 4-day and 5-day forecasts. Once again, the European Center (ECMWF) and GFS models excellent performers, but the GFDL model, a excellent performer in recent years, had substantially poorer forecasts than the other four models. The Canadian CMC model and simple BAMM model had accuracies comparable to the GFDL model. The European Center model and GFS models were virtually tied as the best performing models when averaged over the three-year period 2012 - 2014.

Wunderground has a web page with computer model forecasts for Joaquin for many of the best-performing track models used to predict hurricane tracks. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on this page (you can use our wundermap with the "Model Data" layer to look at ECMWF forecasts, though.) Here are some of the better models NHC regularly looks at:

ECMWF: The European Center's global forecast model
GFS: NOAA's global forecast model
UKMET: The United Kingdom Met Office's global forecast model (not evaluated by NHC in 2013)
GFDL: The Geophysical Fluid Dynamics Laboratory's hurricane model, initialized using GFS data
HWRF: The intended successor for the the GFDL hurricane model, also initialized using GFS data
NAVGEM: The Navy's global forecast model (which replaced the defunct NOGAPS model in 2013)
CMC: The Canadian GEM model
BAMM: The very old Beta and Advection Model (Medium layer), which is still useful at longer ranges

If one averages together the track forecasts from the first five of these models, the NHC official forecast will rarely depart much from it. These are the five models used to formulate the TVCA consensus model seen in Figure 8; the TVCA model was very close to the official NHC forecast in 2014.

Figure 8. Skill of computer model forecasts of Atlantic named storms in 2014, compared to a "no skill" model called "CLIPER5" that uses just climatology and persistence to make a hurricane track forecast (persistence means that a storm will tend to keep going in the direction it's current going.) OFCL=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; ECMWF=European Center for Medium Range Weather Forecasting model; UKMET=United Kingdom Met Office model; TVCA=one of the consensus models that blends together up to five of the above models; CMC=Canadian Meteorological Center (GEM) model; BAMM=Beta Advection Model (Medium depth). Data taken from the National Hurricane Center 2014 verification report.

Figure 9. Radar image of Tropical Storm Marty taken at 2:41 pm EDT from the Acapulco radar.

Tropical Storm Marty bringing heavy rains to Mexico's Pacific coast
After a 12-hour stint overnight as a Category 1 hurricane, Tropical Storm Marty weakened to a tropical storm with top winds of 60 mph at 11 am EDT Tuesday. Radar images out of Acapulco show that Marty continues to bring heavy rain showers to the coast, and as of 8 am EDT Tuesday, the heaviest 24-hour rains along the coast were in the 3 - 3.5" range. With high wind shear of 30 knots now affecting Marty, continued weakening is likely. Marty may have made its closest approach to the coast, as steering currents are now lining up to drive Marty to the west, away from the coast, beginning on Wednesday morning. Given that Marty is a small storm that is weakening and likely to begin moving away from the coast soon, Mexico should be able to avoid widespread destructive flooding from the storm.

Bob Henson and Jeff Masters


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