Day 2
For about the past week, the numerical weather prediction (NWP) models that meteorologists use to make predictions have been suggesting that a major cold air outbreak will overtake most of the US beginning the middle of next week. Below is the forecast temperatures valid at 12PM, Thursday 7 January 2010 taken from the mid-day (18 UTC) run of the Global Forecast System (GFS) NWP model. This model is run 4 times a day (00, 06, 12, 18 UTC) and produces a forecast valid out to 384 hours into the future for the entire globe. (I would be remiss if I didn’t say that anything past about 180 hours is almost pure fantasy.)
What is interesting about this forecast is that a vast majority of the contiguous United States (CONUS) is at or below 30 degrees Fahrenheit. (The 30 degree Fahrenheit temperature line [isotherm] is marked in white.) As this cold air outbreak continues to push south and east, citrus growers in Florida will need to pay close attention to how far south the freezing temperatures extend. A hard freeze would destroy the citrus crop (which is in full swing), and a hard freeze is certainly within the realm of possibilities for northern Florida. A general rule of thumb, the stronger the high pressure coming south out of Canada (the big blue H on the map), the farther south the freezing temperatures will push – especially near and east of the high pressure (in the Northern Hemisphere). (This tends to be because the winds around a high pressure are clockwise in the Northern Hemisphere. This results in a northerly component to the wind east of the high pressure. A southerly component to the wind would be expected to the west of a high pressure in the Northern Hemisphere.)
(Image courtesy of the Hoot Project (a division of the Oklahoma Weather Laboratory [OWL]).
