7-day MREF Precipitation Forecasts

Diagnostic tools for the precipitation forecasts at 7 days are presented in the graphics below. First is the probability of exceedance for 24-hour precipitation exceeding thresholds of 0.1", 0.25", 0.5" and 1.0" for the 00 and 12 UTC MREFs valid at the time of interest (a total of 23 ensemble members). The probabilities of exceeding the thresholds are as indicated in the color bar at the bottom of the graphic.

Note that probability diagrams from the 00 UTC ensemble run seven days prior to the time of interest indicate there is only a 5-35% chance of 24-hour precipitation amounts exceeding 0.25" over the Mid-Atlantic states for the period, but higher probabilities (>50%) over the southern tier of the U.S. from TX to SC.

EPA Initial Times: 00 and 12 UTC 29 Nov 2002
Probability of 144-168 hour Accum. Precip. Exceeding Threshold Inidicated

Threshold = 0.1"	Threshold = 0.25"
Threshold = 0.5"	Threshold = 1.0"

As a companion graphic, the spaghetti diagram for 24-hour precipitation of 0.25" or more from the 12 UTC run used above, is shown below. We note that one member has precipitation exceeding 0.25" in the Mid-Atlantic states; all others are further south and west. This ensemble member also has:

• The -2°C isotherm through the Mid-Atlantic
• A sharper 500-hPatrough over the Midwest than the other ensemble members, and
• A 500-hParidge rather than a trough over the western Atlantic.
• A southwest-to-northeast jet maximum over eastern North America at 250-hPa, with the Mid-Atlantic states in a favorable (right entrance region) area for precipitation.

The other member with a tendency to have an jet streak with similar orientation has the 0.25" isohyet to the south and west of the Mid-Atlantic states, but with a suggestion that the precipitation could move to the north and east subsequent to the valid time.

Click Question 1 in the menu to the left to answer a question based on the 7-day MREF forecasts.