Fig. 1: Morning satellite view of the Gulf of Mexico.
Discussion: At 8AM EDT “Potential Tropical Cyclone Four” (NOAA/NHC) was located at 23.2N/96.0W or about 200 miles south-southeast of the Rio Grande River. Top wind is near 35 mph and this system is moving northwest at 14 mph. Climate Impact Company will refer to this system as Tropical Depression 4. USAF RECONN investigates this system late this morning.
TD 4 is a well-organized area of intense thunderstorm activity (Fig. 1) moving over 85-86 water temperatures. A shear axis of 30-35 mph is to the north and east of TD 4 but the immediate area surrounding TD 4 and northwestward toward the northeast Mexico coast shows very light shear. Conditions are ideal for development.
There is one unique factor inhibiting development. The Texas drought is causing dry air entrainment from the northwest into TD 4 slowing the usual development of a system in this location. NOAA/NHC forecasts strengthening to a minimal tropical storm (“Danielle”) prior to making landfall this evening in coastal northeast Mexico (Fig. 2). However, tropical cyclone models (and all operational models) continue to (mostly) doubt that intensification due to the dry air entrainment (Fig. 3-4).
Consequently, the rainfall forecasts for the inland-moving tropical cyclone are held down in the 1-2 in. range by NOAA/WPC (Fig. 5). Climate Impact Company concern is that rainfall associated with Danielle is in the 2-4 in. range in south Texas/northeast Mexico and that buoyant moisture is entrained into a monsoon rainstorm over northern Texas this weekend into early next week making forecast amount of >7 in. higher…near 10 in. for parts of Texas.
The Texas drought features 8-16 in. rainfall deficits ahead of this week-long rainfall. That amount is cut in half (or more) and the monsoon rainfall event is further enhanced by inland-shifting Danielle tonight.
Fig. 2: Latest NOAA/NHC forecast track for T.D. 4.
Fig. 3-4: Latest tropical cyclone model tracks and intensity forecasts.
Fig. 5: NOAA/WPC 5-day rainfall forecast for the U.S. Note limited amount associated with Danielle which could easily be somewhat higher.