Tropical Depression Four developed this weekend in the southern Gulf of Mexico and gradually became better organized. Monday morning, it was upgraded to Tropical Storm Danielle.

Less than 60 miles east-southeast of Tuxpan, Mexico, the storm is poised to move ashore later Monday just south of Tampico and unload extremely heavy rain.

Tropical storm warnings have been issued from Laguna Verde to Rio Panuco, Mexico.

Danielle is the earliest fourth named storm in the Atlantic basin on record, breaking the previous record set by Debbie in 2012 on June 23.  Note that weak storms were likely missed before weather satellites in the mid-1960s.

The system’s upgrade to tropical storm status Monday arose from improved appearance on satellite imagery as well as measurements from an Air Force reconnaissance plane.

Danielle is currently over very warm waters in the Bay of Campeche exceeding 84 degrees (29 C), which are favorable for additional strengthening. But the storm is also experiencing about 15 mph of wind shear out of the west. While not prohibitively strong, these levels of vertical wind shear can still limit strengthening by disrupting the storm’s circulation. The storm’s proximity and increasing interaction with land should further impede significant intensification.

Since the tropical cyclone is still fairly weak, the primary threat from Danielle is going to be heavy rain and potential mudslides. Danielle’s slow forward speed has the potential to exacerbate the heavy rain threat, as it is currently moving toward the west at less than 10 mph.

The latest model output from the Global Forecast System paints a bullseye of precipitation over the mountainous terrain near Danielle’s landfall point.

The latest National Hurricane Center advisory states that there is the potential for 6 to 10 inches of rainfall across portions of the Mexican states of Veracruz, Tamaulipas, San Luis Potosi, Queretaro, Hidalgo and northern Puebla.

Even higher rainfall accumulations are possible over the higher terrain of the southern Sierra Madre Oriental mountain range. Some of the pockets of higher terrain seen in the Google Maps plot below exceed 2,000 meters (6,500 feet) in height.

It should be noted that while the Atlantic hurricane season is off to a fast start in terms of named storms, early season activity does not necessarily correlate with overall basin-wide activity.

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For example, using data since 1950, seasons that had two or more named storms form prior to July 1 actually had less overall basin-wide activity, as measured by Accumulated Cyclone Energy (an index that blends intensity, frequency and duration of tropical cyclones into one number), than did seasons with fewer than two named storms by the end of June. This is likely due to the fact that early season storms do not typically form from purely tropical systems. Most major hurricanes form from tropical cyclones that begin in the deep tropics.

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