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Over the past 24 hours, much of our area has received between 3 to 5 inches of rainfall with some isolated amounts nearing 6 to 7 inches. Due to the rainfall, we have seen rises in many of our local creeks and streams along with an initial 5-foot jump on the Brazos River. After today, the extended forecast has overcast to clear skies through the middle of next week giving us a brief time to drain out while we wait to see what will occur on the Brazos River. The image below is a summary of the rainfall over northern Fort Bend County as of 10:47 AM from the Harris County Flood Warning System.
With the significant rainfall finished, we will focus on the Brazos River gages as water makes its way from upstream of Hempstead through Richmond. The 9:27AM forecasts show Hempstead peaking at Gage 50.4 on Monday afternoon/evening, San Felipe peaking at Gage 121 on Tuesday morning, and Richmond hitting Gage 44.4 on Thursday morning. Please note that the Richmond forecast does not show a peak; therefore, we could see higher elevations than the current forecast shows. Based on historically flooding events, when Hempstead has hit near Gage 50, we have seen Richmond hit between Gage 48 and 50 a few days later. We do not have as many records with San Felipe, but this matches what we have seen the past 3 years at the San Felipe Gage.
Based on the levels predicted, gravity drainage will be impacted for the next week or two. While the Brazos River is elevated, the extended forecast is currently showing another ½ inch or so of rainfall starting on Wednesday. Even though it appears that we could avoid any significant flooding from the Brazos River, we need to continue to watch the extended forecast for any additional rainfall that could impact our region.
The WGRFC has issued a forecast for the majority of the gage locations in the lower Brazos River Watershed. The 7:44 AM published forecast shows the Brazos River in Richmond hitting Gage 42 feet on Wednesday, December 12 around 6 AM. As shown on the graphic, the elevation of 42 feet is at the end of the current forecast window. This means that the current forecast does not necessarily represent the final peak that could occur in Richmond. The final peak will depend on the actual amount of rainfall that will occur in the watershed. We will continue to watch the local conditions in Fort Bend County, but also the conditions upstream. Currently, the WGRFC is forecasting Hempstead to hit 52.1 and San Felipe to hit 120.6.
As of this morning, the NWS is continuing to show heavy rain hitting the Greater Houston Area starting later today. The region has the potential to receive between 2” to 5” with isolated amounts between 8” to 10”. With these totals and the short duration, we have an increased risk of street ponding with local rises in our creeks and streams occurring Friday night. This is particularly the case in areas with a history of ponding and/or poor conveyance.
Based on this morning’s forecast maps for precipitation, we could see between 3 to 6 inches of rainfall within Fort Bend County while the upper portions of the Brazos River watershed, especially the Brenham area, could see closer to 5 to 6 inches of rainfall. There is the possibility of isolated amounts of 8 to 10 inches. With these totals and the short duration, we have an increased risk of street ponding with local rises in our creeks and streams occurring Friday night. This is particularly the case in areas with a history of ponding and/or poor conveyance.
Currently the Brazos River in Richmond is at Gage 18.85 feet with no river forecast published. Based on the precipitation values we could see a rise in the Brazos River through Fort Bend County; however, until we have more certainty on the location of these isolated higher amounts, the actual impacts to the watershed will change. The WGRFC typically uses “past precipitation and the precipitation amounts expected approximately 12 hours into the future from the forecast issuance time” to produce forecasts on the Brazos River. Based on this timing of this event, we could potentially see a forecast published as early as tomorrow morning or afternoon. We will provide additional updates as necessary as things develop.
Below is the update from HCFCD on this weekend’s storms.
Powerful storm system will bring high impact weather to TX starting later today.
Flash Flood Watch has been issued from noon Friday until noon Saturday for all SE TX counties.
Flash Flooding of streets along with significant rises on area bayous, creeks, and rivers to flood levels is possible.
Powerful upper level storm system moving into S CA and NW MX this morning will track generally eastward and into TX over the next 48 hours. SW flow aloft is already spreading clouds across the area with currently best moisture return focused across SC/C TX where scattered showers have already developed. Activity will increase today from the coastal bend northward along the I-35 corridor where favorable moisture transport overlaps with short wave energy aloft. Some of this activity will likely affect our far western and southwestern counties this later this morning into the afternoon hours.
Excessive rainfall event likely Friday afternoon-Saturday morning resulting in flooding.
A surface cold front currently moving southward is now expected to reach our NW counties around College Station early Friday and then slowly move across SE TX during the day. This front will act as a strong lifting feature to promote widespread showers and thunderstorms within a very favorably moist air mass. In fact models continue to suggest near record/record moisture levels will be in place by Friday afternoon with PWS of 1.8-2.0 inches over much of the area. Forecast soundings show little instability, but loaded with plentiful moisture form the surface into the upper levels indicating convection will be very effective at heavy rainfall production. Unfortunately it appears the slow moving surface front will be nearing the US 59 corridor around the time that maximum parameters for heavy rainfall focus over SE TX. This includes a very strong 45-55kt low level jet which will transport copious moisture into the frontals slope. Additionally, winds aloft become increasing divergent helping to aid lift and vent near surface lift. Add in a slow moving surface boundary and a nearly “tropical air mass” and you pretty much have everything needed for excessive rainfall rates.
Widespread rainfall amounts of 3-6 inches appear likely over nearly all of the area with isolated totals of 8-10 inches. It is still somewhat uncertain where those higher isolated totals may occur, but areas along and NW of US 59 may have a slightly greater risk than areas S of US 59. Much of the higher totals will tend to focus closely with the surface front and where it slows at times as it crosses the region. In addition to the overall totals, high hourly rainfall rates of 2-3 inches per hour will be possible which will lead to rapid urban flooding of street systems.
Forecasted rainfall amounts and the widespread nature of the rainfall pattern strongly support significant rises on area watersheds. Think most basins can handle 6 hour totals of 3-4 inches, but anything greater than about 4 inches in 6 hours is going to likely be cause for concern. Creek, bayou, and river flooding will be possible especially in the San Jacinto, Trinity, and Brazos basins. For Harris County, all of the creeks and bayous will likely see significant rises and exceeding of flood stage levels is possible on some of the watersheds depending on exactly where the heaviest rains fall. Greatest concern at the moment is for the watersheds over the northern half of Harris County, but think all watersheds have about equal chances of seeing significant rises and possible flooding. Usual common flooding locations on Little Cypress Creek and lower South Mayde Creek will likely experience flooding. Some structure flooding will be possible if watersheds exceed their banks or intense rainfall rates overwhelm street systems.
Timing: Friday morning College Station area, mid to late afternoon Houston area through Saturday morning
Amounts: 3-6 inches widespread isolated 8-10 inches
Watersheds: flooding of creeks, bayous, and rivers is possible
Street Flooding: high threat for street flooding
Severe: a few storms late Friday night may become severe near the coast with a tornado threat.
Forecasted Rainfall Amounts:
Due to rainfall upstream of the District in the Brazos River watershed, we are anticipating elevated water levels in the Brazos River and possibly Ditch H. The current forecast is estimating the Brazos River level at the Richmond gauge to be 39.2 feet on Sunday, October 21, 2018. It is possible we could see Richmond gauge at or above 40 feet. The District’s operator and engineer are monitoring and will continue to monitor the Brazos River levels and update the District’s website as necessary.
For additional information regarding current Brazos River levels, you may visit the website of Fort Bend County Office of Emergency Management at http://fbcoem.org.
With the current rain throughout Texas and in our area, river projections have began to increase on the Brazos. Right now, projections are 38.4′ which doesn’t pose any threat to District’s levee systems. Based on what we are seeing and with the continued releases upstream, we will probably see a river crest in the low 40s which does seal off gravity flow to all pump stations. After speaking with the County, they believe it will probably stay up for an extended period of time (a week or so) until water levels throughout Texas can drain out. The District and LMS are preparing for a 40’ elevation on the river and going through our Action Stage protocol. Should the river reach 40′, we will pump any rainfall that occurs to make sure we are always at full storage internally. All pump stations are fully operational and ready to go should more rainfall come to the District. There are projections of 2-4” of rain over the next week, which is well under what our pump stations can handle. The District doesn’t anticipate having to run pumps for a long period of time to catch up on internal water levels.
We will continue to monitor the upcoming river heights and give provide updates on this site should things change.