Subsidence/Low Relative Humidity Squash North Atlantic Tropics July to Mid-August

Highlight: 99L likely to become a tropical storm heading toward northeast Mexico this weekend. Subsidence in outer tropics suppressing season so far.

Fig. 1: Morning satellite view of the North Atlantic basin.

Discussion: Tropical Disturbance 99L crossed the southern Yucatan Peninsula and gained a circulation yesterday although weakening overnight just prior to entering the Bay of Campeche (Fig. 1). The environment is favorable for 99L to regain strength while drifting northwestward over the weekend. NOAA/NHC indicates a 40% chance of a tropical storm; Climate Impact Company risk is 60%. Expect 99L to become Tropical Storm Danielle arriving on the northeast New Mexico coast early Sunday. Heavy rain is likely for southern Texas/northeast Mexico with this system. Close monitoring is required…forecast confidence increases after 99L reorganizes. Elsewhere for now, the North Atlantic tropics are quiet.

The North Atlantic basin is quiet through mid-August (since July 1^st) due to the subsidence pattern in the deep tropics and subtropics in the vicinity of the Bahamas. The subsidence is well-represented by the low relative humidity in the middle atmosphere (600 MB) in these regions (Fig. 2).

Fig. 2: Relative humidity at 600 MB across the North Atlantic tropics is limited due to sinking air in the atmosphere and suppressing tropical cyclone development.