DISCUSSION: With the peak of the Atlantic hurricane season less than a month away, it should be expected that tropical systems begin to appear on model guidance more frequently. Today's 12z run of the GFS model provided a great example, as it showed two important areas to monitor a week from now. The map above, courtesy of Tropical Tidbits, shows high and low pressure areas next Sunday from each of the GFS model's 21 ensemble members. The two-digit numbers represent the last two digits of each system's pressure, so 67 means 967mb and 12 means 1012mb.
In this model forecast, there's a fairly potent low pressure area just to the north of Puerto Rico and a less notable low pressure area farther to the east. The reason I used a forecast for just seven days away, instead of farther out in time, is because ensemble forecasts tend to spread out as you move deeper into the forecast period, therefore showing more uncertainty. Even at just seven days, we can see the spread in the low pressure near Puerto Rico, as there are suggestions the storm may move either right over Haiti and the Dominican Republic or move away further to the north and east at that time.
Since many of the individual GFS ensemble members are showing these areas of low pressure, the main idea to take away from an ensemble forecast like this is that there is a decent chance for activity in the eastern tropical Atlantic over the next seven days. Meteorologists use ensemble forecasts not necessarily for exact location, but to judge the certainty of a particular event to occur. If conditions remain ideal for both areas to develop into tropical storms, the one near Puerto Rico would be named Harvey and the area further east would be named Irma.
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©2017 Meteorologist Jake Spivey