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Florida Freeze an Interesting Mesoscale and Ag Forecast Case Study

By Joe D'Aleo
Friday, January 30, 2009

The Freeze on Wednesday and Thursday, January 21-22, 2009 was a very interesting case of several mesoscale local factors at work.


Tuesday night and Wednesday morning saw the cold dry air drive down the peninsula as a sharp trough with leftover arctic air from the big record setting cold outbreak earlier in the weak moved over Florida. Some areas reached freezing. This was an advective freeze morning. Note the low dewpoints in the tends to the central areas.

Thursday though was the killing freeze morning as high pressure settled in overhead under a cold trough and with very dry air and clear skies. Very light to in places calm winds allowed for strong radiational cooling. Temperature which Wednesday failed to rise above the 40s as far north as Orlando dropped to the 20s and even a few teens (15 at Archibold) in the coldest spots.


Vegetable growers suffered severe losses in southern areas away from the coast. Citrus growers Thursday used wind machines and irrigation in some areas to successfully stave off the cold.  Despite the efforts, there were reports of localized crop damage and leaf-burn because of the freeze, Florida citrus experts said. It is still early in the discovery process, however, and growers may not know for days how much damage actually occurred. “There's going to be some damage - we expect that. Quantifying it is difficult," said Andrew Meadows, spokesman for Florida Citrus Mutual in Lakeland, Fla.


WSI AgTrader had forecasted the potential the prior week and in the detailed Sunday morning forecast. See WSI AgTrader for a trial service and access to full model and COMPLETE ECMWF operational and ensemble data.



The state of Florida is a unique challenge for forecasters because it is a peninsula surrounded by warm water. It is flat except for a slight ridge in the central part of the state, and lower flatlands on either side, has a huge lake in the south. It has cities mainly on the coasts and in the central (Orlando) that produce urban heat.

Before the killing freezes of the 1980s, orange groves were found up to Orlando. They were moved south after the devastation that occurred there with temperatures into the teens. Further south groves were less subject to advection freeze though still vulnerable to radiational frost if winds go calm after advective cold shot.

One of the challenges for the remote forecasters and market traders is that the traditional observing stations are coastal cities and not representative of the readings where they grow crops.. On Thursday if you looked at them, you would have thought the state escaped serious issues.


The 6:43 am EST map courtesy of RAP shows the temperatures and winds mainly for the major cities with their urban heat island warmth. You see land breezes that develop in a weak pressure gradient situation as the cold land develops high pressure relative to the warmer surrounding oceans in all coastal areas. This keeps the low clouds off the coast.



Just inland, the winds are calm and temperatures were many degrees colder as this higher resolution plot from the Florida Automated Weather Network (an agricultural network).  You can see warmer temperatures in the central ridge and near Orlando but cold on either side.

Freeze forecasters know timing, airmass, timing and delivery are the keys to a Florida freeze. Arctic air is critical. This freeze was surprisingly not the main arctic thrust but remnant arctic air. The first primary thrust went east, staying north of Florida. The coldest driest air needs to be overhead during the night not during the day and the delivery has to be down the peninsula. Any kind of onshore flow will bring moderation from the water and often clouds. An upper jet stream may bring cirrus clouds which hold temperatures up. This situation featured all three factors although the arctic was no longer as extreme as it was a few days earlier.


With high latitude blocking developing under a strong stratospheric warming, another threat could develop before the season is over.


Meanwhile with La Nina conditions, the dry winter will stress the citrus trees further. Precipitation is well below normal.


The citrus crop has had to battle hurricanes, citrus diseases, drought and now freezes.

Citrus trees were damaged by the 4 hurricanes in 2005. It takes a few years for new citrus trees planted to become productive. Citrus is subject to citrus canker which can’t be controlled easily (trees must be removed). Citrus in Florida and Brazil suffering from SDS – Sudden Death Syndrome, where the plants wilt and die. To make matters worse, orange juice demand diminished because of the low carb diets.

Competition from other countries, especially Brazil has reduced the amount of citrus in the United States. Citrus in Brazil does not have freeze threat but is also fighting diseases.