The National Weather Service is warning Spring-area residents to prepare for heavy rain and possible flash flooding starting Wednesday, Sept. 30, with peak rain chances on Thursday and Friday.

The Weather Prediction Center's Excessive Rainfall Outlook places northwestern counties in the Houston-Galveston region under a "slight risk" designation, one step above the "marginal risk" covering the rest of the area. Spring sits in that higher-risk zone.

NWS Houston said on X on Sunday, Sept. 27, that an upper-level disturbance moving north and east will drive the rain. The agency's Area Forecast Discussion calls for 1 to 3 inches of rainfall across the region, with storms persisting through at least Saturday, Oct. 3.

Polo's moisture adds fuel

The same forecast discussion notes that an upper-level trough crossing the southern Rockies will interact with the remnants of Hurricane Polo, pulling deep tropical moisture from the Pacific into the region. Polo reached Category 5 strength in the eastern Pacific earlier in the week before weakening, the Houston Chronicle reported.

That extra moisture is expected to push precipitable water values to the 90th-to-97.5th percentile of normal for this time of year, according to the NWS forecast discussion. The highest percentiles are forecast near the western counties of the Houston-Galveston area.

Confidence still low on totals

The weather service cautioned that confidence on rainfall totals and the location of the heaviest rain across Southeast Texas remains low. The Houston Chronicle reported that the NWS puts the probability of more than 1 inch of rain at 50% to 60% for Thursday and 30% to 40% for Friday. The chance of exceeding 2 inches on Thursday is 30% to 45%.

The timing is still uncertain.

The rain arrives after weeks of dry conditions. More than 42% of Southeast Texas is classified as abnormally dry, and severe drought expanded from 4.8% to about 9.7% of the region in the most recent U.S. Drought Monitor update, the Chronicle reported. A burn ban remains in effect for unincorporated Harris County, according to Ready Harris.

How to stay informed

The Harris County Flood Control District operates a real-time Flood Warning System that tracks rainfall and water levels in bayous and major streams. The NWS said finer details will come as confidence increases. Residents can monitor updated forecasts at weather.gov/hgx.