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 or evening.
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.