This series of Haby Hints investigates problems that cause a forecast to bust. A bust occurs when a certain weather parameter is expected but one or more factors cause the forecast to be wrong. This particular Haby Hint will focus on how fronts cause forecast problems.
A front is a boundary between two distinct air masses. Often very contrasting temperatures and dewpoints will occur on each side of the front. In order to make a correct forecast it is important to have the best idea when the front will pass through a forecast area.
Fronts are one reason extended forecasts can be wildly off. For example, the 5 day forecast may have a high temperature in the 70s F for 5 days out. However, a cold front moves through earlier than expected causing the actual high to only be in the 50s F. It can be very difficult to trust forecasts that are 5 and more days out especially during the time of the year that fronts are common.
Even a short term forecast can be wildly off because of fronts. If the timing of a front is a few hours off it can have a significant impact on the high and / or low temperature for that day. For example if a front moves through at 8 am instead of noon the high temperature can end up being significantly cooler than forecasted.
Fronts may cause highs and lows to occur at untraditional times during the day. In association with a strong cold front, the high will occur before frontal passage and the low will occur at the end of the forecast period for that day (assuming CAA and temperatures continue to decrease throughout that day's forecast period). The timing of the front is critical in determining what the high will be before the front passes and how much CAA will occur when the forecast period ends.
The timing of when precipitation will begin and end will often depend upon when the front passes. Here are a few relevant generalities to this issue:
a. Thunderstorms and severe thunderstorms are more common before a cold front passage as compared to after.
b. Precipitation tends to be heavier before a cold front passage as compared to after.
c. POP values generally decrease after frontal passage (especially for cold fronts and drylines).