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tom1328732

Bizarre North American Weather and You

tom1328732
7 years ago
last modified: 7 years ago

Late autumn is normally a time for frigid temperatures to begin making lengthy appearances in the northern U.S. This year, however, is decidedly different; most places will see record highs over much of the next two weeks. Why, you ask? The upper-level pattern has shifted into a massive ridge over the western 2/3 of the country--sorry, Northeast--meaning there's a dome of warm air over that area. Personally, I have never seen such a long-lived and resilient dome this late in the season before, and there are no signs it's letting up anytime soon (see bottom of post for data/explanation).

What does this mean for your trees?

If you don't mind temps dipping down into the 40s, it means a longer growing season. I leave my trees outside in the 40s, but I make sure to water them with tap water (after displacing all the cold hose water) every morning the temps dip below 47°F. At this point in the year, tap water is still easily 60°F+ from the heat of summer seeping down into the earth.

With this in mind, we should all be aware of winter leaf drop. This phenomenon occurs when roots are below ~50°F and the leaves heat up in sunshine. Believe it or not, even though the sun currently has a declination angle of less than -15°, the sun still packs a wallop energy-wise and can heat up objects relatively quickly (like leaves). When citrus roots get cold, they simply don't work very well. Leaves that need to transpire without an effective water source leads to leaf abscission. Silica describes this in greater detail in this link. Long story short, if you can brave the cooler nights from greater radiative cooling with a little supplemental root heating, you can still milk the sun's full-spectrum light for a while yet.

Predicting the weather:

Us meteorologists (well, ex-meteorologist in this case) use thermodynamic weather models as a tool to see into the future--crystal balls of sorts. They're definitely not 100% accurate, but they do a good job of predicting large-scale weather patterns (called the synoptic scale) a solid week out into the future. There are a whole slew of models out there, but in this situation, I'm going to use the Global Forecast System (GFS), used by the U.S. National Weather Service and developed in the public domain. Different models have different strengths, but for atmosphere thickness and surface temperature estimations, it does pretty well.

Link to the GFS model runs:
http://mag.ncep.noaa.gov/model-guidance-model-parameter.php?group=Model%20Guidance&model=gfs&area=namer&cycle=20161103%2000%20UTC&param=1000_500_thick&fourpan=no&imageSize=M&ps=model

With model runs, you can choose to view a variety of atmospheric "slices" or parameters. For vegetation-related foci, I'm going to suggest choosing the "SFC-LAYER PARAMS" of "10m_wnd_2m_temp". If you press "Loop All" after selecting it, you'll get an animation that shows you the time at the bottom in UTC and a map showing you wind speeds and directions (the sticks) and temperatures (color contours). Temperatures are listed in °C, and they change in response to solar heating, radiative cooling, and air movement (advection). Anything blue is below freezing at the 2 meter level (~6 ft). You'll see different values in the mostly-white zone, but it is IMPORTANT to remember that this is the 2 meter level, and what you experience will typically be COLDER than what is shown, mainly due to radiative cooling, depending on the topography and emissivity of the area. I'd be leery of leaving any potted citrus tree outside with temperatures below 5°C.

There are a variety of parameters to choose from, but hopefully this knowledge helps you plan your great citrus tree relocation from the outdoors to shelter!

-Tom

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