The center of Harvey is located about 25 miles west of Victoria early this afternoon. Forward momentum has slowed to a crawling 2 MPH – and we expect that general ‘stalling’ motion to continue for the next four to five days. High winds continue in proximity to Harvey, but the possibility of widespread damaging winds is lessening. It’ll continue to be quite windy this afternoon across South-Central Texas and into the Coastal Plains. The lessening of the wind threat marks a decrease in one threat from Harvey – but the second and most significant threat has only begun. Now a Tropical Storm, this change does absolutely nothing to the historic flooding event expected in the upcoming days. That is one of the most significant messages to share. Harvey’s winds will have nothing to do with the upcoming flood event. Tropical Depression, Tropical Storm, or Hurricane – the name has nothing to do with the rainfall forecast.
Overview on Multi-Day Devastating Flood Threat
An unprecedented flooding event is just beginning across portions of South-Central Texas, the Coastal Plains, and Southeast Texas. As the graphics above depict a multi-day rainfall event is likely as Harvey stalls out across the region. Latest computer model projections indicate Harvey may not move more than 75 miles in the next 5 days. A strong onshore flow from the Gulf of Mexico will continue bringing in ridiculously high moisture values – supporting prolific rainfall rates in Harvey’s convective bands. It will not rain at any given location all the time over the next couple of days. What we’ll really be watching for is intense thunderstorm training – or thunderstorms that keep moving over the same locations. Intense rainfall rates of 3 to 6 inches per hour are expected with the heaviest storms over the next few days. When all is said and done by about Wednesday – perhaps Thursday or Friday – the heaviest hit areas will likely have received 30 to over 40 inches of rainfall. We’ve already seen some locations in the Coastal Plains and Southeast Texas with 7 to 12 inches of rain from just this morning. Widespread rainfall totals will likely exceed 20 inches across the Coastal Plains and Southeast Texas – especially along and south of Interstate 35. 5 to 12 inches of rainfall is expected in Austin and San Antonio – with I-35 being a demarcation line for a sharp cutoff in rain totals. Lower to the west, much higher to the east. These rain total locations are not edged in stone, so do expect some small changes depending on Harvey’s eventual location in the coming days.
Major Flooding Threat Tonight in Southeast Texas, perhaps into the Brazos Valley
The High-Resolution Rapid Refresh model has been consistent in showing a very heavy band of precipitation training (or moving over the same areas) around Houston (Harris County) and adjacent counties late this afternoon into late tonight. This model produced 2 to 4 inches of rain per hours for several hours – eventually indicating the potential of 12 to 20 inches of rainfall near the Houston metro by 3 AM CT Sunday. I’m not going to be that aggressive in my thinking, but there is clearly a signal that high-impact flooding will become much more likely tonight in Houston and surrounding counties. With heavy rainfall already underway this afternoon the stage is being set for big problems. Heavy rainfall is also likely across most of Southeast Texas extending into the Brazos Valley. Rain may approach as far north as Interstate 20 tonight in East Texas to the D/FW Metroplex tonight – with Central Texas being quite wet as well. Tropical moisture is in place so rainfall rates in even ‘lighter’ showers/storms will be in the 1 to 2 inch per hour range.
I anticipate several large flash flood warnings will be in effect tonight. Those in the Brazos Valley and Southeast Texas should be prepared for major flooding tonight. Be ready to move to higher ground quickly if you live in or near flood prone areas. Follow the advice of local emergency officials as they are keeping a very close eye on the situation.