|FORECASTING TRICK SERIES:|
TIMING A FRONT
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 4: TIMING A FRONT
The time of the day a frontal passage occurs will have a significant influence on the forecast. It will have
an influence on the temperature and precipitation forecast. The high temperature tends to occur in
the afternoon while the low tends to occur in the early morning. A front can alter this tendency. The time
of the day the front passes will determine how much this tendency is altered. Precipitation
character can be influenced by front timing.
The first step to timing a front is to determine the approximate time the front should pass using
each forecast model. Write down the approximate hour that the frontal passage should occur and also
determine the model consensus of frontal passage time. Note how much variation there is in the models. They
may all have a similar passage time or there could be an outlier. Next, read the forecast discussion
from the NWS for information concerning the frontal passage and when they have determined it is most
likely to occur. Once you have determined when you think the frontal passage is most likely then you
are ready to determine how it will influence the high and low temperature.
Here are some rules of thumb with cold fronts:
1. A morning cold front can significantly cool the high temperature from that experienced the previous day.
2. If the cold front moves in during the late afternoon then the front will not cool the high temperature
for that day nearly as much as compared to a cold frontal passage earlier in the day.
3. The second night after a cold front passage tends to be colder than the first night. This is because the
winds tend to be lighter the second night, skies tend to be clear and most of the
Cold Air Advection has
occurred by that point.
4. Temperatures behind a cold front tend to cool most when the front moves in during the late afternoon
5. The greatest temperature drop with a cold front tends to occur immediately after frontal passage. The temperature
drop is more gradual several hours after frontal passage.
As mentioned a front can cause high and low temperatures to occur at untraditional times of the day. In
the case of a cold front the high temperature may occur just before frontal passage while the
low temperature occurs at the very end of the forecast period. If the cold front is not strong enough then
the warming from solar radiation may overtake the cooling from Cold Air Advection. For example the temperature
is 50 F in the morning, the cold front passes, temperatures fall into the mid-40s by noon, in the afternoon
temperatures warm to the low 50's. In this case the high still occurred at the traditional time of the
day but the cold front caused the high to be lower than would be the case without the cold front. In the
case of a very strong cold front then the high will almost always occur just before frontal passage with
the low occurring at the end of the forecast period. Pay close attention to how strong the front is as
it approaches and ask yourself how the front will influence the temperature. MOS will often have a
terrible time handling the temperature changes with a frontal passage.
Here is how the timing of a front can affect precipitation:
1. The time that precipitation occurs is often linked to the time of frontal passage.
2. Storms tend to be stronger when the cold frontal passage occurs in the afternoon or evening. This is
because the air can get more
unstable out ahead of the front (i.e. daytime heating).
3. Rainfall tends to be heavier with a slower moving front. Note how the models are handling the speed
of front movement.
4. More people will tend to notice precipitation if is occurs in the morning rush hour or afternoon
rush hour. If timing of precipitation occurs at this time then give the precipitation more emphasis
in the forecast.
When the next front approaches keep the tips above in mind. The emphasis has been on cold fronts but
warm fronts and occluded fronts can be just as important. Note how a warm front or occluded front will
impact the temperature and precipitation forecast also. Your forecast knowledge will help you
outforecast MOS when fronts move through.