| The following is from an OLD discussion from the NHC (11pm EDT last night, Friday): "The hurricane is moving west-northwestward or 300 degrees at 9 kt. The ridge to the north of Dorian is expected to build during the next 24 hours, and this should cause Dorian's heading to bend westward toward the northwestern Bahamas. After 48 hours, the global models show an erosion of the western portion of the ridge, which is expected to cause the steering currents to collapse and the hurricane to slow down considerably by day 3." From: https://www.nhc.noaa.gov/archive/2019/al05/al052019.discus.027.shtml You can get older NHC discussions here: https://www.nhc.noaa.gov/archive/2019/DORIAN.shtml Focusing on the southeast U.S. for the lower levels: GFS: https://www.tropicaltidbits.com/analysis/models/?model=gfs®ion=seus&pkg=mslp_uv850 Euro: https://www.tropicaltidbits.com/analysis/models/?model=ecmwf®ion=seus&pkg=mslp_uv850 I don't why they handle it differently. I don't know how to analyze that. You would probably want to look at the upper levels in a wider view. Again, this gets beyond my understanding. You might look at these two side by side in different browser windows. One on one side of your screen and the other on the other side. I think these are applicable, but again, this gets to be over my head. Eastern U.S.: GFS: https://www.tropicaltidbits.com/analysis/models/?model=gfs®ion=eus&pkg=z500aNorm Euro: https://www.tropicaltidbits.com/analysis/models/?model=ecmwf®ion=eus&pkg=z500aNorm Or maybe an even wider view to see up into Canada: GFS: https://www.tropicaltidbits.com/analysis/models/?model=gfs®ion=atl&pkg=z500aNorm Euro: https://www.tropicaltidbits.com/analysis/models/?model=ecmwf®ion=atl&pkg=z500aNorm I have included some parts of the NHC discussions down below mentioning some about the turn. You don't have to read them, I was trying to find the right wording. I believe the trough in blue (lower pressure) over the U.S. is forecast to erode the ridge (higher pressure) which would allow Dorian to start turning more northward rather than it's current westward movement. Why the models handle it differently, I don't know. The strength of that ridge is important which is why I have wanted that NOAA G-IV data in the models. (GFS at very least, don't know about what other models dropsonde data gets in) The plane drops a small tube and records not only wind, temperature and dewpoint, but the height of the pressure as well. That gets fed into the models. When you don't have that data out there, the models are probably not going to be quite as good. That is my opinion. As for why the GFS is faster with the storm, again going over my head here, but it looks like the Euro has the ridge not breaking down quite as much by 72 hours. At 72 hours in the GFS image you see a little bit of white to the NE. In the Euro at the time, a little bit of red connects still to the NNE of the storm. So it seems like there is more of a weakness there at that time in the GFS, allowing more movement toward the north. Even at 48 hours you can see it looks like the GFS has more of a weakness. Maybe someone else can explain better. From 11am EDT Saturday: "Most of the global models shift the high eastward and deepens a trough over the eastern United States beyond 2 days. This steering flow would typically favor a gradual turn of the hurricane to the northwest and north, however there is large uncertainty in the exact location and timing of this northward turn." From 11pm EDT Friday: "The ridge to the north of Dorian is expected to build during the next 24 hours, and this should cause Dorian's heading to bend westward toward the northwestern Bahamas. After 48 hours, the global models show an erosion of the western portion of the ridge, which is expected to cause the steering currents to collapse and the hurricane to slow down considerably by day 3. Later in the period, the models have again trended to a more significant weakness in the ridge which allows Dorian to turn northwestward, then northward near the east coast of Florida." From 5pm EDT Friday: "The ridge over the western Atlantic appears to be already building to the north of Dorian and is beginning to force the hurricane on a more west-northwest track or 300 degrees at 9 kt. The ridge is forecast to build even more and steer Dorian on a westward track for the next 3 to 4 days. However, the steering currents will then weaken, and this should result in a decrease in the hurricane's forward speed. Given the collapse of the steering currents, the track forecast by the end of the forecast period is highly uncertain, and any small deviation in the track could bring the core of the powerful hurricane well inland over the Florida, keep it near the coast, or offshore." From 5am EDT Friday: "Dorian is now being steered between a mid-tropospheric high centered near Bermuda and a mid-/upper-level low located over the Bahamas. The high is expected to expand westward, with a blocking ridge becoming established over the western Atlantic during the next several days. With the exception of COAMPS-TC and HMON, the other reliable models are in excellent agreement through day 3, with Dorian turning westward south of the ridge and moving near or over the northernmost islands in the Bahamas Sunday and Monday. After day 3, steering currents collapse as a break develops in the ridge, and Dorian will likely slow down considerably as it approaches the Florida peninsula." From 11pm AST Thursday: "Dorian is expected to gradually turn to the west-northwest on Saturday, and westward on Sunday due to a building ridge over the southwestern Atlantic. While the model guidance generally shows this scenario in a broad sense, there are substantial timing differences among the guidance. The source of the uncertainty can be attributed to challenges in forecasting the strength and orientation of the mid-level ridge over the southeastern United States in a few days, along with exactly where and how large Dorian is by then. As you can imagine, with so many complex variables in play, it is no wonder the models have been having a difficult time nailing down the path of the hurricane." |