Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 1:30 AM GMT on July 27, 2013
Tropical Storm Dorian continues to move across the central Atlantic:
Wind: 45 mph, with higher gusts
Location: 17.8°N 45.8°W
Movement: W at 22 mph
Pressure: 1010 mb
The strong subtropical ridge in place over the Atlantic has claimed another victim in 2013. Convection has essentially vanished, with only a few patchy showers remaining in a broken squall to the north. An earlier ASCAT pass around 1200 UTC suggested that Dorian was still carrying winds of tropical storm force to the north of the circulation, and the fast forward motion of the cyclone may actually assist in maintaining those winds for a little while longer, even in the absence of deep convection. As is expected to be the case with a westward-moving system at or above 20 mph, the pass showed an almost complete lack of wind to the south side of the center; several scatterometer passes throughout the day give fairly convincing evidence that Dorian no longer possesses a well-defined surface circulation, and last light visible imagery a couple hours suggested that if a circulation still does exist, it is extremely tight.
Figure 1. Latest infrared satellite image of Tropical Storm Dorian. Image credit: NOAA's Satellite Services Division (SSD).
Satellite pictures show arc clouds emanating westward away from the center, which suggests that Dorian is still ingesting dry air. Water vapor imagery suggests that Dorian has essentially reached the axis of the driest air in the Atlantic basin, and if it somehow -- and quite miraculously -- survives for the next 24 hours, it will move into a more moist environment. I was tempted to forecast almost immediate dissipation, but I am not confident enough to do that, and I would rather maintain some continuity with my previous forecast. In about 72 hours, the global models still show an increase in shear as Dorian moves north of Puerto Rico; the mid-oceanic trough responsible for this is clearly visible on water vapor imagery, as it has been for the last several days. While upper-level winds could improve if Dorian reaches the Caribbean or the Bahamas, they will still not be ideal, and even if they were, there would likely be very little left of the system by that time to assume advantage in such an environment. I am forecasting Dorian to dissipate in about 96 hours, although it is likely to occur much, much sooner.
Dorian remains south of a well-established mid-level ridge. Although an upper-level low pressure system is in the process of amplifying over the Great Lakes, a solution well-advertised by the global models over the last few days, the cyclone is too weak to follow the mid- to upper-level flow and respond to this weakness. A general westward to west-northwestward motion is anticipated throughout the remainder of the forecast period as Dorian is steered primarily by the low-level flow.
5-day intensity forecast and positions
INITIAL 07/26 2100Z 17.8°N 45.8°W 40 KT 45 MPH
12 hour 07/27 0600Z 18.3°N 49.2°W 35 KT 40 MPH
24 hour 07/27 1800Z 18.6°N 52.5°W 30 KT 35 MPH
36 hour 07/28 0600Z 18.9°N 56.3°W 30 KT 35 MPH
48 hour 07/28 1800Z 19.4°N 60.4°W 30 KT 35 MPH
72 hour 07/29 1800Z 19.5°N 67.6°W 30 KT 35 MPH
96 hour 07/30 1800Z...DISSIPATED
5-day track forecast
Figure 2. My 5-day forecast track for Dorian.
Tropical Storm Flossie continues to move toward the central Pacific:
Wind: 60 mph, with higher gusts
Location: 16.5°N 134.1°W
Movement: WNW at 20 mph
Pressure: 999 mb
There has been little change to the cloud pattern over the last several hours, and an ASCAT pass around 1800 UTC showed a wide swath of believable 40 to 45 kt winds, primarily in the northern portion of the circulation; given the well-documented low bias of this instrument, Flossie is likely still producing winds of around 50 kt.
Figure 3. Latest infrared satellite image of Tropical Storm Flossie. Image credit: NOAA's Satellite Services Division (SSD).
Flossie will soon cross the 26C isotherm, and while convection may decrease some thereafter, the global models do not show westerly shear increasing until Flossie is well into the central Pacific. I don't see much evidence of westerly upper-tropospheric flow getting ready to bite on the system in water vapor imagery, which gives some credibility to the model forecasts of a later arrival of vertical shear. While a steady weakening is forecast throughout the forecast period, waters actually warm the closer you get to Hawaii, so a rare landfalling tropical cyclone on those islands seems like a distinct possibility, and it is what I will forecast at this time pending evidence to the contrary.
Satellite and microwave fixes this evening indicate that Flossie is pretty much on track with the 21Z NHC forecast, moving west-northwest to the south of a strong subtropical high. The model consensus has not changed appreciably, and Flossie is still expected to turn westward at longer ranges as the ridge strengthens, probably in the vicinity of 145W. This pattern favors an eventual landfall on the Big Island of Hawaii. There are some forward speed differences, with the GFS being faster and the ECMWF being slower. My forecast favors the former, and is also strongly in line with the forecast from the National Hurricane Center.
Tropical storm watches and warnings may be required for portions of the Hawaiian Islands during the next 24 hours, and interests there should monitor the progress of Flossie.
Even if Flossie does survive as a tropical cyclone and goes on to impact those islands, the primary threat will be heavy rains capable of causing flooding, high surf, and tropical storm force winds, particularly along higher elevations. These winds could cause power outages.
5-day intensity forecast and positions
INITIAL 07/26 2100Z 16.5°N 134.1°W 50 KT 60 MPH
12 hour 07/27 0600Z 17.3°N 137.3°W 50 KT 60 MPH
24 hour 07/27 1800Z 18.0°N 140.5°W 45 KT 50 MPH
36 hour 07/28 0600Z 19.0°N 144.0°W 40 KT 45 MPH
48 hour 07/28 1800Z 19.6°N 147.5°W 40 KT 45 MPH
72 hour 07/29 1800Z 19.6°N 153.6°W 35 KT 40 MPH
96 hour 07/30 1800Z 19.6°N 157.9°W 30 KT 35 MPH
5-day track forecast
Figure 4. My 5-day forecast track for Flossie. My forecast points do not go out to 120 hours because the map I use to draw my forecast maps don't exceed to the more westward longitude Flossie would arrive at in 120 hours.
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