GFS ENSEMBLES FOR DAYS 3 THROUGH 14
VALID June 28 - July 09, 2007
Days 3-5.
The ensembles are predicting the deepening of a trough over the Eastern U.S. during this period, which will put an end to several days of anticipated heat across this region. The ridge over the Central CONUS will allow for increasing temperatures across the Plains states, while another trough off the West coast implies an extended period of hot temperatures and very dry conditions for the West. The flow pattern predicted for day 5 is similar to what can be expected at this time of year... a trough near the West coast, a mid-continent ridge, and another trough near the East coast.
Days 6-10.
The 6-10 day mean height pattern as shown by the ensembles has a smoothed, climatologically accurate appearance. If the mean trough remains largely offshore, we could see some monsoonal moisture streaming up from Mexico into parts of New Mexico and Arizona. However, it appears that monsoonal moisture into the Southwest will be very limited.
Days 8-14.
The Week 2 weekly mean height pattern predicted by the ensembles shows some retrogression of the wave train over the North American sector. This pattern would suggest hot temperatures across the western states, as well as the northern Rockies and northern High Plains. With widespread northwesterly flow anticipated over much of the CONUS at 500 hPa, precipitation should be very limited across the Nation as a whole.
DEFINITIONS AND TERMS.
The ensemble
mean provides the first order advantage of ensemble forecasting, i.e.,
a simple consensus of a large number of forecasts. Even greater benefits
accrue through the use of the ensembles to assess confidence, or uncertainty,
in the forecast. The spread provides the most direct measure of the likely
skill, and therefore the confidence, of the forecast.
The spread
is the standard deviation among the ensemble members, in units of meters.
It varies both regionally and seasonally, much like the actual standard
deviation of the height field. So, we expect high values in high latitudes
and in the 'centers of action', low values in low latitudes, and moderate
values in other locations. A standardized version, i.e. the spread as described
above, divided by the climatological standard deviation of the height field
appropriate to the date, is under development for the 6-10 day and week
2 average forecast periods. According to Whitaker and Loughe, Mon. Wea. Rev.
Dec 1998, pp. 3292-3302, spread is useful as a predictor of skill when
the spread is either very low (skill is likely to be high) or very high
(skill is likely to be low).
Note that the
spaghetti diagrams are daily out to day 7. Then spaghetti maps for the
five day mean covering GFS forecast days 6-10 and the seven day mean over
GFS forecast days 8-14 are provided.
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