Nearcasting in the Tropics

Wishing you were in the tropics today? Check out this concept that was explored at the Tropical desk during today's experiment...

Currently our tropical forecasters use only text for their Area Forecasts. This week we've been exploring a transition to graphical based products, one of which would highlight potential convective activity. To this end, we looked the Nearcast this afternoon. At the other convection desks we've been looking for areas of instability in the theta-e difference, i.e. upper-level drying over low-level moisture. However, in the case of the tropics, convection generally forms when you've got moisture all the way through the column. For that reason, we pulled up the precipitable water parameters from the Nearcast instead.

The image below is the 1700Z forecast of the precipitable water difference from the 1400Z run. This is a difference between the 700-900mb and 700-300mb layers, and for moisture all the way through the column, values would be at or near zero.

20140815 1700Z Nearcast precipitable water forecast and 1700Z visible imagery

The dark browns and tans indicated zero or near zero differences. Note the large areas over the Gulf and also in the southern Caribbean. However, this could either mean there is moisture all the way through the column, or dry air all the way through the column.

To determine what we were looking at, we explored each layer separately. The first image below is the 700-900 layer and the second the 300 -700 layer.

20140815 1700Z Nearcast 900-700 precipitable water 

20140815 1700Z Nearcast 700-300 precipitable water

In these images we were looking for the higher values. Note that the area in the southern Caribbean had very little preciptable water in both levels, while the area over the Gulf and Florida had a significant amount. From this, it was determined that the latter area, noted above in the red and yellow dashed lines, had moisture all the way through the column and would be the most likely area for convection to form.

The visible image underlay is from the same time period and the forecast. Note the convection that formed in those areas where moisture extended through the column.