|FORECASTING TRICK SERIES:|
OCEAN AND LAKE INFLUENCE
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 5: OCEAN AND LAKE INFLUENCE
There are many forecast areas that border ocean areas, the Great Lakes or other smaller water bodies. The
influence of the water will have a huge impact on high and low temperature. Below are several set-ups
to be aware of:
Water colder than land: This is common during the day along the west coast of the U.S. and along the Great Lakes in
late spring and summer. A breeze from the water will keep temperatures cooler than they otherwise would be,
especially the high temperatures. The cold water tends to be a stabilizing influence to the air. A marine
layer can develop over the water and move inland. On the west coast the clouds and cool temperatures
from the marine layer can be much colder and a great contrast in weather to the areas further from the
coast not within the marine layer.
Water warmer than land: This is common along the Gulf coast in summer at night and along the Great Lakes in late fall
and winter. This situation tends to cause the air to fall to saturation at night. With increased
moisture in the air and a higher
dewpoint, the temperature can have trouble cooling off
significantly at night. The warm waters help the low temperature to be warmer than it otherwise
Moisture: Note if advection is brining in moisture from the water body. Higher dewpoints advecting in
will often spell warmer low temperatures. The overnight low will tend to not fall more than 5 degrees F
below the evening dewpoint in stagnant air. This is more true the higher the dewpoint is.
Fog: Fog can ruin a temperature forecast. If fog moves in from the water body expect the temperature
to not be able to cool off as much at night and to not be able to warm as much during the day. Once
the fog breaks during the day the temperature can shoot up once solar radiation starts reaching
the surface and the air mixes.
How close the station is to water: How close the station is to the water can make a big difference. The water
influence reduces with distance away from the shore. The range between high and low temperature
tends to be less at the shore. Note how strong the advection is from the water or even if the
air is advecting from the water. If the winds are blowing from land toward water the weather will be
influenced at a minimum amount by the water. If the winds are blowing weakly toward the land then
there will be a significant influence near the shore but that influences will reduce rapidly
with distance from the shore. If the wind is advecting strongly from the water toward land the
water influence on weather will be significant far inland.
Lake-effect precipitation: Moisture advection from lake will tend to add to precipitation totals. The lake
can also be the source of all the precipitation that occurs in a particular event. When
very cold air advects over a much warmer lake the chances for clouds and precipitation increases.
Lake-effect snow can occur in this situation.
If you conclude MOS is not picking up on the ocean or lake influence as much as it should then
adjust the temperature forecast. Keep a journal to determine over time which model's MOS does best
with the water influence in certain weather situations.