Predecessor Rain Events and Tropical Storms Gamma and Delta

Tropical Storm Gamma's moisture streams over the Florida peninsula on October 4, 2020.

Several prominent features are setup cross Central Florida today setting the stage for a possible Predecessor Rain Event.

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First, a coastal front/trough stretches from the Cape to Southwest Florida.  This is evident in the surface observations which show ESE/SE winds along the coast from the Cape to Southeast Florida, shirting to N/NNE from Daytona Beach south/west to the Naples area.

Tropical Atlantic makes for rainy Florida, October 2020.

Second, exceedingly deep moisture from Tropical Storm Gamma continues to stream across Florida.  PWATs from evening soundings were 2.20" and 2.35" in Tampa and Melbourne respectively.  That is a super-moist atmosphere.

Third, sufficient divergence pattern aloft remains in place, especially along the I-4 corridor.  The warm front that passed through yesterday has become mostly diffused, though a small hint of it remains in the thermal gradient seen from the Suwannee Valley to the Nature Coast (temperatures in the 60s to the northwest and 70s to the southeast).

All of these features set the stage for a Predecessor Rain Event (PRE). Cote (2007) first defined the term predecessor rain event (PRE) in his 2007 thesis (Predecessor rain events in advance of tropical cyclones. M.S. thesis, Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, 200 pp.) to describe meso- and subsynoptic-scale regions of high-impact heavy rainfall that occur well in advance of recurving tropical cyclones (TCs) over the eastern third of the United States. 

Hurricane Delta genesis?
East of Jamaica on October 4, 2020

PREs pose a difficult forecasting challenge because operational models can have difficulty properly representing mesoscale regions of heavy rainfall in space and time, and the mesoscale features that serve to focus the heavy precipitation can often be under analyzed or missed altogether. 

Tropical Sunset on Lake Theresa, October 2020.

Often the storm causing the PRE can be located great distances from the rain event it spawns.  In Central Florida today, we are currently located about 1000 km from the center of Tropical Storm Gamma (617 miles), and yet the moisture is being streamed directly over the Florida Peninsula.

Read more about PREs in Galarneau, Bosart, and Schumacher (2010) Predecessor Rain Events ahead of Tropical Cyclones.

Model intensity (above) and possible track forecasts for potential Hurricane Delta are updated every 6 hours.  Go to tropicaltidbits.com and click on "Current Storms."  

Currently Potential Tropical Cyclone 26, forecast to be a strong 100 mph Hurricane Delta at landfall on Friday, October 9, 2020.

Hurricane Delta will be called "Potential Tropical Cyclone 25" until winds reach tropical storm strength.

So far this PRE has not produced huge amounts of rain over large areas though some areas of the Treasure Coast have seen up to 6.00-inches in the past 24 hours.  The moisture from T.S. Gamma is forecast to continue to stream over Florida during the coming week and be enhanced by the passage of Tropical Storm Delta in a few days, potentially spawning a second PRE.

This is unusual October weather for Central Florida as generally October is very dry and very hot as we transition from wet season into dry.  After years of dry native Floridians welcome the rains of October 2020.

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