|EXTENDED FORECASTING OF WINTER PRECIPITATION
METEOROLOGIST JEFF HABY
A. GFSx MODEL
Large changes are common in the extended period from
one model run to the next especially if the atmosphere is baroclinic. In a baroclinic atmosphere, beyond the
5 day output is very unstable output. Generality and trend is what is often broadcasted for an extended forecast.
Below are some important questions to ask yourself:
How likely is winter precipitation?
Discussion: Forecasting winter precipitation is difficult in the extended forecast. 850 temperatures and the
thickness field can be used to infer if temperatures will be cold enough for winter precipitation while the
precipitation prog is used for likeliness of precipitation. If a polar front is progged to pass forecast area,
try to infer the depth of the cold air mass during the precipitation event. This will help with the precipitation
type forecast. Polar fronts become shallower as they move into lower latitudes. If middle and upper level
winds are moving in the same direction as the polar front, the polar air mass will tend to be deeper.
Once the middle and upper level winds change to a westerly/southwesterly/southerly
direction, the polar air in the lower troposphere will tend to become shallower, modify and erode.
How is the orientation of the jet stream changing over time?
Discussion: The jet stream holds the key to changes in the temperature and precipitation pattern. See
if jet stream is trending toward zonality or becoming buckled (meridional). Large temperature swings from
one day to the next are common in a meridional flow. Timing on the passage of the embedded cyclones in the
jet stream will determine timing of temperature changes.
Where are the air masses moving?
Discussion: Check 850 mb temperature field. Check source regions of air masses.
How is the distribution of highs and lows changing over time and how have they changed from
one model run to the next?
Discussion: Temperature trend and precipitation trend will depend on the placement of the high
and low pressure systems. WAA occurs in advance of low pressure, CAA occurs behind low pressure.
Precipitation is more likely near the position of the low pressures, fronts, and downwind of shortwaves.
B. EUROPEAN MODEL
Models shows 500 mb heights and sea level pressure. As with the GFSx, look at the position and
heights of the trough and ridges along with the position of the low level pressure systems.
C. CLOSING COMMENTS
1. If the short range forecast models experience a significant change in the next model run,
the extended forecast will often experience even more of a change.
2. Do not be afraid to make swift changes to the extended forecast when the new model data is available.
To eliminate some of the dramatic changes a forecaster may have to make to an extended forecast in a
baroclinic atmosphere, it is best not to give forecasts out beyond 5 days.
3. Extended range models trend toward climatological normal temperatures and precipitation chances
(especially after day 5). Extended models do not handle unusual weather events very well.