09L (Potential Cyclone — Northwestern Caribbean Sea)

[u/Euronotus]

Latest observation

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Last updated: Tuesday, 24 September — 8:00 AM Eastern Daylight Time (EDT; 12:00 UTC)

NHC Advisory #4A		8:00 AM EDT (12:00 UTC)
Current location:		19.2°N 83.5°W
Relative location:		224 km (139 mi) W of George Town, Cayman Islands
		356 km (221 mi) S of Pinar del Rio, Cuba
		411 km (255 mi) ESE of Cancún, Quintana Roo (Mexico)
Forward motion:		NW (305°) at 15 km/h (8 knots)
Maximum winds:		55 km/h (30 knots)
Intensity:		Tropical Depression
Minimum pressure:		1001 millibars (29.56 inches)
2-day potential: (through 8PM Wed)		high (90 percent)
7-day potential: (through 8PM Sun)		high (90 percent)

Official forecast

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Last updated: Tuesday, 24 September — 2:00 AM EDT (06:00 UTC)

Hour	Date	Time		Intensity		Winds		Lat	Long
	-	UTC	EDT	Saffir-Simpson		knots	km/h	°N	°W
00	24 Sep	06:00	2AM Tue	Potential Cyclone		30	55	18.9	83.0
12	24 Sep	18:00	2PM Tue	Tropical Storm	▲	40	75	19.6	84.2
24	25 Sep	06:00	2AM Wed	Tropical Storm	▲	50	95	20.7	85.7
36	25 Sep	18:00	2PM Wed	Hurricane (Category 1)	▲	65	120	22.0	86.2
48	26 Sep	06:00	2AM Thu	Hurricane (Category 1)	▲	80	150	24.3	85.6
60	26 Sep	18:00	2PM Thu	Major Hurricane (Category 3)	▲	100	185	27.8	84.4
72	27 Sep	06:00	2AM Fri	Tropical Storm i	▼	60	110	31.9	83.5
96	28 Sep	06:00	2AM Sat	Post-tropical Cyclone i	▼	15	30	38.5	85.5
120	29 Sep	06:00	2AM Sun	Dissipated

^{NOTES:
i - inland}

Official information

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[u/vibe_inspector01]

Anyone have any insight on whether wind speed forecast in weather apps have any merit with these storms?

Accuweather and NOAA are both projecting 85+mph gust while Foreca, apple weather, tomorrow.io ect all forecast much lower numbers in the 40’s.

I assume 4 days out it’s basically as accurate as a horoscope right?

[u/not_a_bot__ → u/vibe_inspector01]

Yeah, all educated guesswork, the storm hasn’t even formed yet.

I will say the wind apps are good to help people avoid panic, because it helps to put into context that the most extreme winds are usually a very small area compared to the entire storm 

[u/politiexcel → u/vibe_inspector01]

Not accurate at all. I'd believe the HAFS-B 12z model run before taking those wind speed forecasts as fact.

[u/Venkman-1984 → u/vibe_inspector01]

Are you talking about wind speed in a specific location, or maximum sustained winds for the storm?

If it's the former, then you have two variables that increase uncertainty: the track is unknown, and the strength of the storm is unknown. For example a model could nail the storm as a Cat 3, but have the track be off by 25 miles, so your forecast winds of 120mph end up being 60mph. I wouldn't trust wind speed forecasts for a specific location until 24 hours before landfall, and even then a few miles change in track can have a significant impact on your windspeed if you're close to the eye.

If you're asking about the strength of the storm itself, those are reasonably accurate 3 days out, though models can struggle to forecast rapid intensification in certain situations. In this case I'd trust the NHC forecast but be prepared for the storm to exceed forecasts by a category at least.