...General Overview...
The center of former Hurricane Harvey will continue to meander around (with a very slow, erratic movement) in the area to the South of I-10 generally 50-75 miles to the East/Southeast of the San Antonio through Sunday afternoon. Most computer models then suggest that later on Sunday afternoon into Sunday evening, the system will start to turn back toward the Southeast and move slowly back toward the coast through Tuesday (but obviously as a much, much weaker system vs. the first time around).
We say "most" models suggest this because there are some that do not. The other models show the steering flow will remain very week, and the system will continue to move little to nowhere through early next week. Uncertainty in this regard has increased, therefore our confidence meter has been lowered from "Moderate to High" to "Moderate" as of this update. It will simply require more time to resolve these issues and observe the movement of the system until a more dominant steering pattern takes over and finally ejects it out of the region. Hopefully these trends will become more clear once again during the day on Sunday as additional inland data is gathered.
Regardless of how the above ultimately works out, here is what we can expect from now through Sunday evening: Widespread rain will continue across the local area. Most of the widespread rain will be light to moderate in intensity overall, but there will be periods (as we have seen today) of locally heavy rainfall mainly through Sunday afternoon/early evening. Where the exact bands of heavier rain showers tend to set up from time to time will determine who receives the most rain, as has been the case so far today.
In general, we still expect the highest rainfall and greatest potential for significant flooding on a widespread basis to remain just to the East of the area, further to the East of I-35, with the main concern (with regard to flooding) locally being low lying/poor drainage areas, specifically the typical problem low water crossings. This will especially be an issue near and to the East of I-35 based on present trends (which is really not a significant change from earlier updates).
Gusty East/Northeast winds will continue especially into early tonight before decreasing on Sunday morning.
Please see below for additional details.
...Local Rainfall Forecast and Potential Flooding Impacts...
At this time we continue to forecast average total rainfall through late Sunday evening (since pre-dawn Saturday) of 3-8 inches across the local area. The lower end of the range will be in far Western sections, with the higher end in far Eastern sections (see map at the top of this bulletin). Generally speaking, the highest totals in the immediate local area are expected to occur immediately along and to the East of I-35, generally from near/North of San Marcos through Kyle and Buda and then into the central third of the Austin Metro. In that narrow band (shown in red on the above image) we are continuing to forecast 6-8 inches of rain on average, but this is highly dependent on exactly where heavier bands of precipitation continue to set up from time to time through Sunday. It is possible that some locations within that band will receive less than the indicated amount, and it is also possible that other locations within the same band may receive slightly higher than the indicated amount, depending on where the heavier precipitation bands set up.
To the East of the aforementioned line - further to the East of I-35 - widespread 8-10 inch (and higher) amounts are expected (as shown in purple on the above image), as we've forecast for several days now.
At this time we continue to expect that the primary flooding concern for the majority of the local region will be in/near low lying and poor drainage areas (especially low water crossings and the like) at times through Sunday. Any parts of the area that receive repeat episodes of locally heavy downpours would be particularly vulnerable to such flooding, especially at low water crossings and similar areas. This would be even more likely to occur closer and to the East of I-35, as described in greater detail above (and much as we have seen so far today).
...Uncertainty In the Rainfall Forecast Beyond Sunday Evening...
After Sunday evening is where the certainty level begins to erode even further, with regard to precipitation. Earlier today it seemed reasonably certain that the more widespread rain would taper off for a time by late Sunday evening (Midnight Sunday night), with the lull continuing into Monday. (We still expected to see scattered showers during that time, but not as widespread and generally not as heavy as we saw over the weekend).
The main question now goes back to the comments near the top of the page, in that there is uncertainty as to whether the system will turn back toward the South later Sunday as we've been expecting, or whether it will remain stationary for another 24-36 hours before moving again. This uncertainty has increased during the day today, as computer model solutions have diverged and the system itself has shown a continued erratic movement.
If the original scenario proceeds as expected, then rains will decrease on Sunday evening and we will have a general lull in the more widespread/occasionally heavy activity as we move into Sunday night and Monday. If, however, the system continues to remain stalled out, then additional periods of more widespread rainfall, including some locally heavy, would be possible further into Sunday night and also into Monday. If this latter scenario were to occur, we could potentially expect an additional 3-6 inches of rainfall on Sunday night and into Monday, with locally higher amounts possible especially near and East of I-35. This would be on top of the amounts already forecast through Sunday evening in the same areas.
Another important point to consider: Even if the rain does generally decrease for a time from later Sunday into Monday, there are indications that we will receive additional waves of moisture (both directly and indirectly from Harvey) again by Tuesday into Thursday of next week. Any additional rain that falls during that time period on top of what will have already been received could cause additional problems as we move into the mid-week period next week.
We will continue to monitor computer model trends as well as observations associated directly with Harvey and attempt to resolve all of the above issues as soon as it is reasonably possible. This time on Friday we had hope to accomplish this by today, but it now appears that we need to move into Sunday before these issues may hopefully be resolved. This is an unusually complex situation for this type of dynamic system, with many variables at play (some of which are still unknown at this time).
...Local Wind Impacts...
Regardless as to how the rain portion of the system works out, the wind portion should be somewhat more straight forward. With the center of the system over land for some time now, the wind field will continue to decrease in both size and intensity.
Winds will generally decrease into the 15-25 mph range by 12 Noon on Sunday. In the meantime, look for sustained winds of 25 mph with gusts of 30-40 mph this evening into early tonight, decreasing to 15-20 mph with gusts of 20-30 mph later tonight into Sunday morning.
We will continue monitoring the situation and issue updates as conditions warrant.
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