A mid-level, low-pressure system clearly noted in radar data from Houston and Corpus Christi continues to spin over the NW Gulf of Mexico about 150 miles south of Palacios.
Numerous rainbands have formed in the last few hours across the NW Gulf of Mexico from Galveston Bay to over 200 miles off the upper TX coast and are rotating inland as far as Downtown Houston this morning.
Brief heavy rainfall is likely with these bands this morning. Overall, expect an increase in the frequency of these bands today and some potential for cell training in the Flash Flood Watch area which could produce a quick 1-2 inches of rainfall through this afternoon.
A mid-level, low-pressure system and weak surface low/trough will begin to slowly lift northward into SE TX tonight through Wednesday night with numerous showers and thunderstorms expected. Best estimate track for this feature is inland over Matagorda/Brazoria Counties and then slowly NNE into Fort Bend and Harris Counties Wednesday and NNE toward Montgomery and San Jacinto Counties on Thursday.
Global guidance has shifted ever so slightly eastward overnight with the track of this feature. Heavy rainfall both near the core or center of the low-pressure area and within bands on the eastern and southern flank of the system will be likely.
This becomes especially true late tonight into Wednesday as the 850mb moisture transport increases along with good low-level inflow off the Gulf of Mexico. Moisture values will be extremely high during the period with PWS ranging from 2.35-2.65 inches which will support intense rainfall rates in any deep and sustained convective developments.
Widespread rainfall amounts of 5-7 inches are expected over much of the area with isolated totals of 10-12 inches. Deep tropical moisture in place will support excessive hourly rainfall rates in deeper convection with rates of 1-3 inches possible. While grounds are dry, rainfall today will start to result in gradual saturation of the soil conditions and as rainfall totals increase over the next 24-36 hours run-off will also increase. Flash flooding will be possible under any areas of sustained training or cell clustering. Some of the higher resolution guidance is showing some indications of banding and potential for cell training late tonight into much of Wednesday over the region.
While watersheds are currently running below base flow, expected rainfall amounts will almost certainly lead to significant run-off once grounds become saturated. Rises on creeks and bayous to bankfull or above will be possible leading to more significant flooding. High short duration rainfall rates will lead to street flooding at times. Think most of Harris County can currently handle 5-6 inches of rainfall before significant bayou and creeks flooding would be a concern. However, this greatly depends on the intensity of the rainfall rates and any breaks between the intense rainfall cells. Given the complexities in where any sort of training bands may become established yields little confidence on where any higher totals and larger flood threat would be.
WPC has placed portions of SE TX in a moderate risk for flash flooding today, Wednesday, and Thursday.