|FORECASTING TRICK SERIES:|
BEWARE THE DARN CLOUDS
METEOROLOGIST JEFF HABY
This 10 part series will detail forecasting tricks that can be used to try to outforecast MOS. Outforecasting
MOS is an important skill for a forecaster. MOS stands for
Model Output Statistics and they are used
as a guide for
temperature prediction and
precipitation prediction by forecasters. Model consensus is the average of the high
temperatures, low temperatures or precipitation amount predicted by several forecast models.
PART 6: BEWARE THE DARN CLOUDS
Clouds can be very frustrating. They can ruin a forecast due to the enormous influence they have on
the surface temperature. During the day clouds reflect solar radiation back into space. With less
solar radiation reaching the surface there is not as much warming. During the night the clouds
reflect and emit longwave radiation to the Earth's surface. With this increase in radiation the clouds
cause temperatures at the surface to be warmer than they otherwise would be.
In many forecasting situations the cloud cover forecast can be easy. It can be obvious when thick cloud
cover will persist or when the skies will be clear. In other situations though the amount of cloud
coverage and cloud thickness can represent a forecast challenge. The development of
turn a clear night into a cloudy night. Below are some tips to follow when cloud forecasting. Once
you feel more confident in the expected cloud cover then you can be more confident in the temperature
Breaks in clouds during day: There is a big difference between cloudy skies and skies that are almost cloudy. The breaks
in the clouds will allow a portal for solar radiation to pour in. Once clouds start breaking
during the day the surface temperature will often increase significantly.
Watch the satellite: Trust what is actually happening more than the initial cloud conditions provided
by the forecast models. Look at the visible and infrared imagery to see the extent of clouds, how
thick they are and at what elevation they are at. Look at the images in motion to see if the
clouds are increasing or decreasing in coverage.
CL, FW and SC: On the models CL indicates clear skies, FW indicates few clouds (less than 25% cloud coverage)
and SC represents scattered clouds (25% to 50% of sky covered). During the day and night
it will not make much of a difference on the high and low temperature whether the skies are clear, few or
scattered. With so much space between the clouds just about all the solar radiation is still going
to make it in (some directly and some by reflecting off of clouds toward the surface) and just about all
the longwave radiation will escape at night.
BK and OV: On the models BK indicates broken skies (greater than 50% coverage of clouds but not complete coverage) and
OV represents overcast skies (complete coverage of clouds). The difference between the two will have a
huge impact on the high and low temperature forecast. If skies remain OV there will be a full insolation of
longwave energy at night and a strong reflection of solar radiation back to space during the day. Once
the clouds become broken the surface temperatures can change significantly. When 1 or more forecast models
is going BK and 1 or more is going OV, note this can have an important influence on the surface temperature.
Fog: Fog can be a significant forecast bust because of its affect from changing clear skies to cloudy skies
(depending on thickness) and the influence it has on surface temperature. Once fog develops at night, especially
if it is thick fog, the temperature will tend to stop dropping at night if the air is stagnant. The temperature
will not rise much in the morning either if the fog is thick. Once the fog breaks though then the
temperature will often shoot up from solar warming and the air mixing out.
The clouds busted my forecast: Often a temperature forecast bust can be attributed to clouds
occurring when none were expected or no clouds occurring when cloudy skies were expected. The
temperature forecast can be unbelievably off when the cloud forecast goes bad.
can be incredibly challenging at times. Keep a journal and note when the cloud forecast goes bad
and the reasons it went bad. Each local forecast areas has it quirks when it comes to clouds.