A storm in the Free State on Sunday 2 October
Yesterday we spoke about the potential for a tropical cyclone formation in the Mozambique Channel over the next few days. Over the past 18 hours the area of disturbed weather in this region has undergone significant organization, and is now an easily identifiable tropical disturbance on satellite imagery.
The JTWC (Joint Typhoon Warning Center) has begun discussing the disturbance, and echoing some of the environmental factors that we touched on in yesterday’s article. Sea surface temperatures are high, but dry air is often a factor with storms moving towards the African continent. Dry air can be seen on satellite imagery to the south west of the storm, and according to the JTWC, it has been the cause of convection problems in the ‘SW quad’ of the disturbance.
Here is their full advisory from 20:00 this evening.
ABIO10 PGTW 121800
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/SIGNIFICANT TROPICAL WEATHER ADVISORY FOR THE INDIAN
1. NORTH INDIAN OCEAN AREA (MALAY PENINSULA WEST TO COAST OF
A. TROPICAL CYCLONE SUMMARY: NONE.
B. TROPICAL DISTURBANCE SUMMARY: NONE.
2. SOUTH INDIAN OCEAN AREA (135E WEST TO COAST OF AFRICA):
A. TROPICAL CYCLONE SUMMARY: NONE.
B. TROPICAL DISTURBANCE SUMMARY:
(1) THE AREA OF CONVECTION PREVIOUSLY LOCATED NEAR 20.7S
39.6E, IS NOW LOCATED NEAR 21.0S 40.1E, APPROXIMATELY 505 NM NORTHEAST OF MAPUTO, MOZAMBIQUE. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS A CONSOLIDATING LOW-LEVEL CIRCULATION CENTER (LLCC) WITH CONVECTIVE BANDING THAT HAS THINNED OUT OVER THE PAST THREE HOURS. A 121619Z SSMIS 91GHZ IMAGE REVEALS AN ORGANIZED LLCC WITH BROKEN BANDING WRAPPING IN FROM THE NORTHEAST. UPPER LEVEL ANALYSIS INDICATES A FAVORABLE ENVIRONMENT WITH NEAR-RADIAL OUTFLOW AND LOW VERTICAL WIND SHEAR. ADDITIONALLY, SST VALUES OF 30C ARE VERY FAVORABLE. HOWEVER, THE SYSTEM IS BEING INFLUENCED BY THE DRY AIR LOCATED TO THE SOUTH WHICH IS WEAKENING THE CONVECTIVE STRUCTURE. GLOBAL MODELS SHOW A SOUTHWEST TRACK WITH STEADY DEVELOPMENT OVER THE NEXT TWO DAYS. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 30 TO 35 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 996 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS REMAINS HIGH. SEE REF A (WTXS21 PGTW 121300) FOR FURTHER DETAILS.
(2) NO OTHER SUSPECT AREAS.//
Please Note: Discussions within are based off the assumption of model guidance being accurate, something that is often not the case. We expect this data to change at least in some form over the next 48 hours and will share our thoughts accordingly as the storm progresses, remember – right now we don’t even have a named storm.
Our Discussion and Analysis
Always focus on the releases from the JTWC above our own, as they are official meteorologists at warning centers and we’re merely severe weather enthusiasts with experience in tracking storms as a hobby.
The forecast of intensity becomes tricky using public model data. The 12z run of the global GFS model has the central pressure initiated at 1008mb, a much higher value than that of the JTWC forecast which estimates a current central pressure of 996mb, this is usually due to lower resolution models being provided to the public. Looking at the storm on satellite imagery, 996 seems far more reasonable — so instead of focusing on values, we shall instead focus on the relative change in the GFS MSLP forecast.
The GFS model forecasts the storm to linger in the channel over the next couple of days, as the JTWC mentioned – moving southwest steadily. In terms of intensity, it seems to suggest that the storm will slowly deepen over this time period, and actually shows impressive deepening just prior to landfall. This indicates that the storm may encounter favourable conditions as the forecast period goes on. In fact, the 108 hour forecast, shows landfall with the lowest pressure that the model had shown during its period in the channel.
The Euro (ECMWF) model seems to suggest the same as the GFS model, moving the storm SW and then undergoing intensification as it approaches the coast in a few days time.
Looking at the upper air environment overall, it seems that this storm is almost certain to develop into a tropical cyclone in the short term and find itself in what should mostly be a favourable environment.
It is worth mentioning that the storm is moving quite a lot slower on the GFS forecast with landfall forecast to take place at this stage later on Wednesday evening by the EURO and as late as mid-day on Thursday by the GFS. This is quite a bit difference in timing, and as mentioned before I would highly recommend on waiting for an official warning from a tropical cyclone agency before placing any bets on landfall time or location.
Having tracked hurricanes in the Atlantic for the past 14 years I have learned to never underestimate a storm, but also to be fully aware that a storm can show massive potential to become a dangerous, life threatening event. Only for it to die out due to changes in environmental conditions that were not expected.
What It Means For South Africa
At this point, things continue to look as though this storm and potential tropical cyclone will have a direct effect on South African weather. Despite the storm looking to most likely make landfall (should it form as forecast) in Mozambique, the effects could be felt as early as Wednesday or even late Tuesday if the Euro is correct in its timing. The first effects are likely to be showers associated with the outer bands. Rough seas would also likely be witnessed early on along coastal areas of northern KZN.
If the storm approaches the coast as currently forecast, it could bring with it heavy showers by the 17th, to the extreme northern parts of KZN. Though the real rain is likely to occur as the storm weakens as moves further inland, bringing with it the potential for heavy rain and flooding to Limpopo later in the day. Rainfall values displayed by the GFS model after landfall are extremely impressive, with forecasts of more than 9 inches (228mm) in a 36 hour period.
There are a lot of variables at play here, but there is the real chance of this being one of the first storms in quite a few years to effect South Africa in such a way. We will be monitoring it closely and should you be potentially affected by this forming storm, we recommend you follow the official warnings as they are released.