A large plume of deep tropical moisture associated with tropical disturbance 91L over the SW Gulf of Mexico will rapidly advance northward today and into the TX coast.
PWS will increase from 1.6 to nearly 2.0 inches by this evening and into the 2.3-2.6 inch range on Wednesday while the air mass becomes nearly saturated. Moisture levels of this magnitude are impressive even for this area in June and can result in intense rainfall rates due to the large vertical depth of the moisture reducing evaporation.
Expect numerous showers and thunderstorms to begin to develop from SW to NE late this afternoon as the moisture begins to arrive into SE TX.
Ingredients becoming increasingly favorable for more sustained heavy rainfall late tonight into Wednesday as low level convergence increases over the area and low level inflow increases off the Gulf of Mexico while upper level divergence increases ahead of the approaching trough over the SW US.
A low level convergence boundary may become established between the US 59 corridor and the coast and help to organize and focus thunderstorms and heavy rainfall.
This pattern remains in place Wednesday and Wednesday night while 91L moves up the TX coast and likely either across SE TX or just offshore over our coastal waters.
WPC has upgraded the area into a Moderate risk for flash flooding on Wednesday and increased the rainfall amounts for the region and this seems reasonable based on the latest guidance. Widespread rainfall amounts of 2-4 inches with isolated totals up to 6 inches will be possible. Storm motions will be fast (15-25mph) in the strong SSW to NNE steering flow, but the pattern strongly favors training of cells in bands and lines, especially near any low level boundary that may develop. Rainfall amounts could certainly be higher than shown given the extreme levels of moisture that will be in place. Will favor the heaviest rainfall along and S of the US 59 corridor at this time, but the entire area is at risk. It should be noted that some of the convection allowing models show a strong heavy rainfall signal tonight into Wednesday, and this will need to be monitored closely today.
Rainfall rates of 2-3 inches per hour are certainly possible in this type of air mass if convection can become sustained which will result in rapid street flooding. Rises on area creeks and bayous will be possible especially in area that see repeated rainfall.
Winds will begin to increase this afternoon and evening over the coastal waters which will help to build seas on Wednesday. Tides are forecasted to reach near 3.5-4.0 ft on Wednesday which may cause minor coastal flooding on Bolivar and low lying coastal area on the west end of Galveston, Surfside, and in SE Harris County. A coastal flood advisory may be required on Wednesday.