About Jeff Masters
Dr. Masters cofounded wunderground in 1995. He flew with the NOAA Hurricane Hunters 19861990. Coblogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 1:44 PM GMT on August 02, 2013
After a long trek over the Atlantic Ocean from the coast of Africa, the remains of Tropical Storm Dorian (now called Invest 91L) have finally arrived at the shores of North America. ExDorian is nearly stationary, and is situated over the Northwestern Bahama Islands, just off the coast of Southeast Florida. Satellite loops and Melbourne, Florida radar images show that exDorian has only a limited amount of heavy thunderstorms, which are not wellorganized. There does appear to be a surface circulation center trying to form just north of the storm's heaviest thunderstorms, about 70 miles east of Vero Beach, Florida. However, dry air to the northwest, as seen on water vapor satellite loops, is inhibiting development. WInd shear is moderate, 10  20 knots, but is expected to rise to the high range, 20  30 knots, by Saturday morning. ExDorian is expected to move slowly northwards and then northnortheastwards on Saturday. This motion will get exDorian tangled up with a cold front that extends from Northern Florida northeastwards, just offshore from the Southeast U.S. coast. Before it merges with the front, exDorian has some potential for regeneration into a tropical depression, and in their 8 am Friday Tropical Weather Outlook, NHC gave exDorian a 30% chance of becoming a tropical cyclone by Sunday. ExDorian will likely bring heavy rains to the Northwest Bahamas on Friday, and these heavy rains may also clip the coast of Southeast Florida. However, the bulk of exDorian's rains should stay offshore.
Figure 1. Morning radar image of exDorian from the Miami radar.
Jeff Masters
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Dr. Masters cofounded wunderground in 1995. He flew with the NOAA Hurricane Hunters 19861990. Coblogging with him: Bob Henson, @bhensonweather
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1178. sar2401
Yes, it is that hard. You're saying that if there's a 50% chance a storm will form in two days and it doesn't, that mean the probability of it not getting high or lower than 50% in the next three days is zero? If so, get rid of the two day and just make it five days. 91L is a prime example. Do you actually think, if it doesn't improve in the next two days, the probability it will improve in the next three days is not going down? This is simply needless confusion with no added information.
1177. FIUStormChaser
Is the center still stationary?
1176. wxchaser97
I bet I could find a year when Mexico didn't get a tropical cyclone, at least not from the Atlantic. ;)
1175. VAbeachhurricanes
First, it's been being used for 2 days.
Second, the probability LITERALLY cannot go down for the 5 day timeframe, compared to the two day one. Once there is a storm with a BETTER chance to form after the third day, you will see the change.
1174. tornadodude
cool
1173. sar2401
I'm sure this sounded like a good idea at the time but, if they are just going to give the same probs for all five days, they haven't made a helpful change. This is the first storm to test the system and it certainly appears no one is willing to go out on a limb and lower the 5 day probability, which makes absolutely no sense.
1172. JrWeathermanFL
Then they'll realize it was a bad idea.
1171. CaribBoy
They get cyclones every year!!
1170. Patrap
1169. hurricanes2018
HOT WATER IN THE GOM!
1168. SPLbeater
It wouldnt bother me so much if it wasnt so frequently....ever since the dualpolarization upgrade, it seems they have an unexpected failure each week. Sometimes twice a week.
And it always happens when there are scattered thunderstorms in the forecast, too.
1167. VAbeachhurricanes
What? this isn't hard. If there is a 50% chance to form within two days, and they don't think it'll increase after day two then the prob. stays the same. Because if it happens within two days, it also is concurrently happening in 5 days.
1166. HurricaneHunterJoe
Have you seen M22 under real dark skies lately? So very nice.
1165. CaribBoy
Where? GOM.
BORING!!! Where are the CAPE VERDE STORMS!! XD
1164. TropicalAnalystwx13
The MDR has been dry so far this season because the high has suppressed the Intertropical Convergence Zone far to the south. When this lifts northward, we should see an increase in atmospheric moisture. Aboveaverage sea surface temperatures favor upward motion across this part of the Atlantic, and we've actually seen that so far this year.
Compare the 200mb velocity potential map for July 2013:
to that of the averaged map for the top ten ACE year for the same month:
1163. Patrap
By Dr. Jeff Masters, Director of Meteorology
The behavior of the atmosphere is governed by physical laws which can be expressed as mathematical equations. These equations represent how atmospheric quantities such as temperature, wind speed and direction, humidity, etc., will change from their initial current values (at the present time). If we can solve these equations, we will have a forecast. We can do this by subdividing the atmosphere into a 3D grid of points and solving these equations at each point. These models have three main sources of error:
1) Initialization: We have an imperfect description of what the atmosphere is doing right now, due to lack of data (particularly over the oceans). When the model starts, is has an incorrect picture of the initial state of the atmosphere, so will always generate a forecast that is imperfect.
2) Resolution: Models are run on 3D grids that cover the entire globe. Each grid point represents of piece of atmosphere perhaps 40 km on a side. Thus, processes smaller than that (such as thunderstorms) are not handled well, and must be "parameterized". This means we make up parameters (fudge factors) that do a good job giving the right forecast most of the time. Obviously, the fudge factors aren't going to work for all situations.
3) Basic understanding: Our basic understanding of the physics governing the atmosphere is imperfect, so the equations we're using aren't quite right.
Types of hurricane forecasting models
The best hurricane forecasting models we have are "global" models that solve the mathematical equations governing the behavior of the atmosphere at every point on the globe. Models that solve these equations are called "dynamical" models. The four best hurricane forecast models—ECMWF, GFDL, GFS, and UKMET—are all global dynamical models. These models take several hours to run on the world's most advanced supercomputers.
There are also dynamical models that cover just a portion of the globe. These are less useful, unless the hurricane happens to start out inside the domain the model covers and stay there. Hurricanes moving from outside the model domain into the model domain are not well handled. An example of this kind of model is the NAM model covering North America and the surrounding waters, run by the National Weather Service (NWS).
Another type of hurricane model is a statistical model. These models do not try to solve mathematical equations on a grid. The advantage of these statistical models is that they are fast to run and can provide output in a few minutes. There are also hybrid statistical/dynamical models, and simple trajectory models.
A full list of all of the tropical cyclone track and intensity models can be found on the National Hurricane Center's website.
A summary of the top six models:
ECMWF: The European Center for MediumRange Weather Forecasting (ECMWF) model is the premier global model in the world for medium range weather forecasting in the midlatitudes. In 2006, the ECMWF made improvements that starting producing very accurate hurricanes forecasts.
GFS: The Global Forecast System model run by the NWS. Excellent graphics are available on the web from the National Center for Environmental Prediction. Wunderground.com also has GFS plots. I like the Tropical Atlantic imagery. If you select "Shear" from the "level" menu, then click on "Add a Map", you'll get plots of the wind shear that I talk so much about.
GFDL: The NWS/Geophysical Fluid Dynamics Laboratory model. The GFDL and HWRF models are the only models that provide specific intensity forecasts of hurricanes. Wunderground.com makes these graphics available on Wundermap. More detailed GFDL graphics are available at NOAA/NCEP. See the "GHM" model under the heading, Hurricane Graphics.
UKMET: The United Kingdom Met Office model. Data from this model is restricted from being redistributed according to international agreement, and graphics from the UKMET are difficult to find on the web. Only paying subscribers are supposed to have access to the data.
HWRF: The NWS/Hurricane Weather Research Model. HWRF is a nonhydrostatic a coupled oceanatmosphere model, will utilize highly advanced physics of the atmosphere, ocean and waves in one prediction system, providing unparalleled understanding of the science of tropical cyclone evolution. Its output gives meteorologists an analysis of the hurricane in threedimensions from realtime airborne Doppler radar. It will make use of a wide variety of observations from satellites, data buoys, and hurricane hunter aircraft. No other hurricane model accesses this wide of a range of meteorological information. The GFDL and HWRF models are the only models that provide specific intensity forecasts of hurricanes. Detailed HWRF graphics are available at NOAA/NCEP. See the "HWRF" model under the heading, Hurricane Graphics.
NOGAPS: The U.S. Navy's Navy Operational Global Prediction Center System. Graphics are available at the Navy web site. This model has been performing poorly in recent years compared to the other global models, so it has been removed from the consensus models that the National hurricane Center uses as of 2011.
One other model worth looking at, but not as good as the other six is the Canadian GEM model.
Nonglobal models
The BAMM model (Beta and advection model, medium layer) is included on Wundermap. The BAMM is a simple trajectory model that is very fast to run, and did the best of any individual model at 35 day track forecasts in 2005. Since this model is always available, we have included it along with the "big four". In general, one should not trust the BAMM model for the 12 day time period when output from "the big four" are available. "The big four" are generally not available for tropical disturbances, and for these situations we post plots of a number of other nonglobal models such as the LBAR, A98E, etc. All of these models are described in detail on NHC's web site.
Model performance
So which is the best? The best forecasts are made by combining the forecasts from three or more models into a "consensus" forecast. Over the past decade, NHC has greatly improved their forecasts by relying on consensus forecast models made using various combinations of the GFS, GFDL, NOGAPS, UKMET, HWRF, and ECMWF models. If you average together the track forecasts from these models, the NHC official forecast will rarely depart much from it, and the NHC forecast has been hard to beat over the past few years. The single bestperforming model over the past two years has been the ECMWF. This model outperformed the official NHC forecast in 2010 for 3day and 4day forecasts, and in 2009 for 4day and 5day forecasts. You can view ECMWF forecasts on our Wundermap with the model layer turned on. 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 our computer model forecast page that plots positions from the other major models. As seen in Figure 3, over the past two years, the GFS and GFDL model have been the next best models, with the UKMET model not far behind. Last year, the NOGAPS model did very poorly, forcing NHC to come up with some new consensus models this year, the TCOA and TVCA, that do not include the NOGAPS model. For those interested in learning more about the models, NOAA has a great training video (updated for 2011).
1162. sar2401
Yeah, Ft. Rucker AL radar (KEOX) was down for two days. I tried to warn them when they took the low bid for maintenance from Bubba's Radar and CB Shop there would be problems, but did they listen.....? :)
1161. TerraformingMaster
1160. AussieStorm
Yeah, we need clarification. The NHC said THIS SYSTEM HAS A MEDIUM CHANCE... 50 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS...AND A MEDIUM CHANCE...50 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE NEXT 5 DAYS. But they also said... THE LOW IS EXPECTED TO ENCOUNTER STRONG UPPERLEVEL WINDS ON SATURDAY WHICH WILL NOT BE CONDUCIVE FOR DEVELOPMENT...AND IT IS EXPECTED TO MERGE WITH A FRONTAL TROUGH OFFSHORE OF THE EASTERN UNITED STATES ON SUNDAY. So how can they give 91L a 50% chance in 5 days when in 3 days they say it is expected to merge with a frontal trough.
1159. Patrap
1158. sar2401
I'm sure you're not making anything up, I'm just not sure what you're hearing is actually how you interpret it. Just look at the current model runs. They are absolutely unanimous in sending 91L NNE and out to sea. There is no fading front. 91L is leaving. It's not going into Florida, it's not going into the Gulf. This play is in the final act.
1157. SPLbeater
000
NOUS62 KRAH 022147
FTMRAX
Message Date: Aug 02 2013 21:47:19
KRAX IS DOWN DUE TO MALFUNCTION...RETURN TO SERVICE TIME IS UNKNOWN.
1156. guygee
Yes it is a rare event when all the models agree and turn out to be wrong. I really do not expect that to happen this time. But a col is like a meteorological Twilight Zone, and this thing at my front door, so I am thinking in devil's advocate mode until it clears out. I'm watching it until it's gone.
1155. Patrap
1154. HadesGodWyvern (Mod)
Tropical Cyclone Advisory #35
TROPICAL STORM JEBI (T1309)
9:00 AM JST August 3 2013
===================================
SUBJECT: Category One Typhoon Named Cyclone In Gulf Of Tonkin
At 0:00 AM UTC, Tropical Storm Jebi (985 hPa) located at 20.9N 107.9E has 10 minute sustained winds of 45 knots with gusts of 65 knots. The cyclone is reported as moving west northwest at 16 knots.
Gale Force Winds
===============
180 NM from the center in east quadrant
100 NM from the center in west quadrant
Dvorak Intensity: T3.0
Forecast and Intensity
======================
24 HRS: 23.7N 103.2E  Tropical Depression Overland southern China
Additional Information
=======================
JEBI will move west northwestward for the next 12 hours
Cyclone will be downgraded to a tropical depression in 24 hours
Cyclone will weaken due to landfall expected within 6 hours over Vietnam
1153. CosmicEvents
.
If it's day 15 and not day 35 then we're left guessing about what the probability is day 35. 40/40 could mean 40/0, 40/10, 40/20, 40/30 or 40/40, or 40/50 even if it's a close call.
.
They need to clarify the point for the prob and wx geeks.
1152. Tropicsweatherpr
I am beggining to think the CV season will be below average and most of the developments will take place west of 60W because of the low instability in MDR between Africa and Lesser Antilles in other words more homegrown than CV systems.
1151. chrisdscane
open
1150. Patrap
1149. wunderweatherman123
1148. FIUStormChaser
I agree with you, and you may be correct. Just think there are alot of players in this equation and anything can happen with this blob. Lol it should have died along time ago.
1147. sebastianflorida
1146. sar2401
I keep looking but, with all the different bands of convection around, and now with more convection over land than over the ocean, I can't tell what's moving where. The latest satellite frames show offshore convection fading away at an amazing rate.
1145. moonlightcowboy
Good observation, technique. I mentioned earlier that the ULL in the northwest Caribbean was still significantly extorting steering away from the weak frontal trough.
1144. sar2401
I don't know why, Cody, since I almost never trust the GFS this far out, but I think this may actually happen. It's the area where things are the least bad for storm formation, if nothing else, and the Yucatan has been amazingly quiet this year so far.
1143. Pallis
1142. sar2401
Anything in particular or just life in general?
1141. TropicalAnalystwx13
1140. sar2401
I sure don't know of one, but there are many better hurricane historians here than me. Methodologically, it would have to be one heck of a TW or TD to have something like that happen.
1139. sebastianflorida
1138. beell
I'm struggling with it.
All I can figure is the NHC is giving a 50% chance that 91L will still be a viable INVEST on day 3, lol.
1137. moonlightcowboy
Wait, who's on first? ;)
1136. sar2401
OK, it's going to pick up speed to 200 mph and be over Bermuda tomorrow. I can do wishcasting as good as the next guy. :)
1135. TerraformingMaster
1134. Methurricanes
1133. sar2401
Every single dynamical model has it ENE and out to sea. As much as I'm not a firm believer in models, I'm not foolish enough to believe every single one of them is wrong. In terms of future storms, it's not going to be much different until the AB moves north and the SAL dust starts to clear up. When that happens, it will start to be nailbiting time.
1132. RascalNag
I won't be going over it most likely, since my destination is in the midAtlantic.
1131. beell
Well, that certainly make sense! Thanks.
Gotcha, Cos!
1130. guygee
1129. EyEtoEyE
1128. vis0