Update from Jeff:
Heavy rainfall is likely over parts of SE TX this afternoon into the evening hours.
Weak surface boundary noted in surface observations this morning is stalled roughly along a line from Eagle Lake to Katy to Humble to Cleveland as noted by dewpoints in the mid and upper 60’s to the north of this line and low 70’s to the south. Additionally, winds to the north of this boundary are from the ENE/NE and ESE south of the boundary. With the approach of a trough out of the SW US by early afternoon, expect lift and convergence near this boundary to increase and the development of thunderstorms. There appears to be decent agreement that storms will develop along this boundary from Colorado County to southern Austin/Waller Counties into northwest Harris and Montgomery Counties as early as early afternoon but more likely mid to late afternoon. Additional activity possibly in the form of an organized thunderstorm complex may approach from central Texas this evening.
It is this initial development of thunderstorms that has the concerning heavy rainfall threat as these storms look to anchor near the low level boundary over the area, initially move slowly and potentially exhibit cell training for a period of time before eventually moving toward the east and weakening into the late evening hours.
Factors are in place for heavy rainfall including near maximum amounts of moisture for this time of year, sustained lift this afternoon and evening, a slow moving/quasi-stationary low level boundary, favorably aligned low level inflow off the Gulf of Mexico, and enhanced divergence aloft. The combination of these factors point toward a heavy rainfall event north of I-10 later today into tonight. It should be noted that there is going to be a strong rainfall gradient over the area with likely several inches of rainfall occurring to the north of I-10 and potentially very little rain near the coast. It is possible that locations into north and west Harris County (Katy, Cypress, Tomball, The Woodlands) see several inches of rain while areas over southeast Harris County (Clear Lake, Baytown, Pasadena) see very little rainfall.
Rainfall Amounts:
Widespread amounts of 2-4 inches is likely north of I-10 with isolated totals of 5-7 inches or higher. HREF guidance continues to show some fairly impressive isolated totals…and given the setup and moisture in place I would not discount the higher amounts. South of I-10 rainfall amounts will be less than an inch and locations near the coast may see no rain at all.
Additionally, rainfall rates of 2-3 inches per hour will be possible in the stronger storms and this will cause rapid onset flash flooding. This is certainly the setup where everything is fine and in 1-2 hours flash flooding is occurring with intense short duration rainfall rates. This will be especially true in any urban areas where primary street drainage systems can be quickly overwhelmed with such rainfall rates.
Hydro:
Depending on where exactly the corridor of heavy rainfall develops will determine which watersheds are most impacted. Current thinking is from the Trinity across the San Jacinto into the Brazos and San Bernard basins and would not rule out the creeks in NW Harris (Spring, Cypress, Little Cypress, Willow). Depending on totals and location, significant rises overnight will be possible on some of these basins.
Jeff Lindner
Director Hydrologic Operations Division/Meteorologist
Harris County Flood Control District
9900 Northwest Freeway | Houston, Texas 77092
346-286-4000 (main) | 346-286-4165 (direct) | 281-924-2091 (cell)
jeff.lindner@hcfcd.org | Twitter: @jefflindner1