North Atlantic To Become Active – But Less Active Than Previously Forecast

Tropics Will Activate But Just How Much?
08/30/2018, 10:59 am EDT
Seasonal Canada Outlook for Meteorological Autumn 2018
09/02/2018, 9:55 pm EDT
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A robust tropical system is developing off the West Africa Coast soon to become Tropical Storm Florence. A tropical disturbance is developing in the vicinity of the Dominican Republic and Puerto Rico. The environment to support tropical development is improving. However, a dramatic improvement forecast earlier this week has eased a bit as both NCEP and ECWMF do not shift MJO-like convection to the North Atlantic tropics opting instead for stalling in the East Pacific. Therefore activeness of the North Atlantic tropics for the first third of September is likely less buoyant than indicated by forecast models earlier this week.

Support for an active North Atlantic tropical regime eases back

Fig. 1-2: The NCEP and ECMWF 14-day MJO forecast indicate support of enhancement for tropical cyclone risk is now tilted more toward the East Pacific rather than the North Atlantic.

Discussion: Madden Julian oscillation forecasts are in agreement that the enhancement of tropical convection over the next 2 weeks is most likely in the tropical East Pacific rather than the tropical North Atlantic (Fig. 1-2). The forecast is a change from earlier in the week when the MJO progression the tropical North Atlantic was forecast. The forecast adjustment by both NCEP and ECMWF is important as to the “activeness” of the North Atlantic tropics for the first 2 weeks of September.

Currently, there are 2 axis of upper shear across the deep tropics…one just north of the South America coast and another across the northern Caribbean Islands (Fig. 3). The shear axis have been present most of summer and preventing tropical waves to form. Note the upper shear axis fades in the eastern North Atlantic tropics where Tropical Disturbance 90L is located (and likely to become Tropical Storm Florence within 1-2 days). Presence of the MJO or MJO-like atmospheric conditions will ease the upper shear.

Fig. 3: The upper shear axis analysis across the North Atlantic basin.

If the MJO influence is confined to the East Pacific, tropical events will increase in that basin (already occurring with Miriam, Norman and another storm to follow). When the East Pacific basin is active the western half of the North Atlantic tropical basin is inactive.

Operational models already show the influence of what is described as both the GFS and ECMWF backed off developing a Gulf tropical system early next week and because the North Atlantic shear is maintained Florence is a “fish storm” turning north before reaching 50W (Fig. 4). Other systems may follow Florence in the outer Atlantic but also struggle to make eastward progress if the shear pattern remains in-place because MJO influence failed to materialize.

Fig. 4: The North Atlantic basin satellite view from earlier today.

Fig. 5: Tropical cyclone models 5-day forecast track of 90L (which will become Florence).