TO: Tropical fruit growers/managers
FROM: Jonathan H. Crane, Tropical Fruit Crop Specialist
RE: Be on the lookout and prepared for cold/freezing temperatures
Cold weather is predicted for early to mid-next week (Jan. 6-8, 2014). So prepare and monitor the weather.
Summary of Miami-Dade County Cold Protection Workshop (18 Dec., 2013)
For those of you who missed this informative workshop here are some key points.
From Robert Molleda, NWS
1. Weather models give equal chances for generally above, below or normal temperatures this winter. The long-term models suggest more warm than cold but this does not mean we cannot have cold/freezing temperatures, their model suggests trends not day-to-day weather.
2. We are in a neutral year (not El Niño, not La Niña). Most of our devastating freezing events in the past have occurred during neutral winter years.
3. Terms to understand when reading the NWS websites:
a. Freeze outlook – forecast provided 3-5 days before potential freeze
b. Freeze watch – forecast 12-48 hours before potential freeze
c. Freeze warning – forecast 12-24 hours before potential freeze
d. Hard freeze – prediction for 28°F (-2°C) or below for 4 or more hours
4. Frost advisory – if frost is expected within 12-24 hours. Frost conditions include:
a. 33°F-37°F (0. 4°C to 2.8°C) temperatures
b. Clear skies
c. Winds less than 5 mph
d. Sufficient low level moisture to freeze
5. Remember temperatures predicted for the urban areas are generally warmer than in the agricultural areas.
From Jonathan Crane, UF/IFAS/TREC (taken from local long-time experts in Homestead). Is freezing weather a potential?
1. Watch the jet stream – if it is looping/dipping south moving cold/freezing air from the N or NW to the S or SE.
2. Is there a low pressure system over the SE U.S., around the Carolinas or W. Virginia. The high pressure system(s) are over the N or NW (Arctic, Canada, North Central US). Seems a lot of times there is a high pressure system just west of Minnesota.
3. The North Central and Central US is covered in snow and/or freezing temperatures.
4. We experience one or more days when the daytime temperature does NOT exceed 65°F (18°C).
5. Days when the daytime temperatures is at or below 60°F (16°C) by 3:00PM or earlier.
6. We experience two or more days and/or nights of cold but nonfreezing weather; especially if they are accompanied by wind.
7. The forecast calls for low night-time due point temperatures (at or below 30°F).
8. Surface winds are blowing from the north-northwest to the south-southeast.
Just a word about when a freeze or near freeze is predicted. If south Florida/Homestead experiences 60°F (16°C) or less temperatures and low day-time dew point temperatures (at or below 40°F/4°C) and clear skies – at sundown the air temperature can drop really fast (like 2-3 degrees/hour) and instead of reaching freezing after mid-night or early morning we reach freezing or near freezing temperatures prior to 12:00AM (midnight); we’ve seen 10:00PM.
A word about dew point – this is a hugely important weather parameter for cold prediction and protection. Definition - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. In general, the dew point is the lowest temperature the atmosphere will reach; so temperatures stop dropping, at least temporarily when the dew point temperature is reached. HOWEVER, dew point temperatures can change during the night if more cold dry air moves over the area and lowers it. Here is an example – The dew point prediction for a cold night is 33°F (0.4°C). Ok, so the air temperature drops and drops and then hits 33°F and the atmospheric temperatures stops dropping and may even increase a little (let us say to 35°F/1.7°C) – that is because when the atmospheric temperatures hits the dew point temperature the air is saturated with water (moisture) and when water changes from a gas to a liquid heat is given off and the air temperature steadies or warms. HOWEVER, a little later that night more cold dry air moves into the Homestead area – NOW the dew point temperature drops again, only this time to 28°F (-2°C) and NOW the temperatures could drop below freezing (32°F/0°C). So watch and monitor the dew point temperatures closely throughout the night.
There are a number of websites you may want to monitor before and during a freeze or near-freeze event these include:
1. NWS 7-day outlook. Perfect the prediction area by zooming in on the map and clicking your specific area.
http://forecast.weather.gov/MapClick.php?zoneid=FLZ173&zflg=1a. On this same site, scroll down to the Hourly Weather Graph and click on that – it gives the hr by hr weather; you can change it to a table by clicking the graph.
2. NWS graphical forecast page
http://www.srh.noaa.gov/mfl/ 3. The real-time weather in the agricultural area offered by the Miami-Dade County Cooperative Extension Service – 4 weather stations in the ag area
http://miamidadewx.locherenv.com:8080/index.html 4. UF FAWN weather station located at the Tropical Research and Education Center, 18905 SW 280 Street, Homestead. We have relocated our weather station to an area much less likely to be influence by our irrigation systems -
http://fawn.ifas.ufl.edu/ 5. A new website Robert Molleda, NWS made us aware of with current (or near current) and predicted weather (temperature, wind, wind direction, etc.) at
http://raws.wrh.noaa.gov/cgi-bin/roman/raws_ca_monitor.cgi?state=MFL&rawsflag=290a. You will need to scroll down to see FL073: Inland Miami-Dade County data.
Lastly, I’d suggest during a predicted freeze night people occasionally go outside into the grove to see and feel what is actually happening in your grove. Why, because sometimes the predictions and real-time weather data do not show a change in low-cloud cover or slight shift in wind speed or direction or whether your irrigation system is clogged!
Please find enclosed a pdf with a PPT from the workshop. Hope this helps. Jonathan
Jonathan H. Crane, Tropical Fruit Crop Specialist
University of Florida, IFAS
Tropical Research and Education Center
18905 S.W. 280 St.
Homestead, FL 33031-3314
Tel: 305-246-7001 x.290
Fax: 305-246-7003
Email:
jhcr@ufl.eduWebsite:
http://trec.ifas.ufl.edu/craneWebsite:
http://trec.ifas.ufl.edu/fruitscapes