####018006727#### AGUS74 KWCO 161513 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT THU MAY 16 2024 .Synopsis... Considerable flooding impacts expected for East Texas into the Southeast... Considerable flooding impacts remain possible across Hawaii... Isolated flooding impacts possible across the Southern Plains through the Lower Ohio Valley... Small stream flooding expected in New England... .Discussion... .East Texas into the Southeast... Considerable flash and urban flooding impacts are expected today for portions of east TX through central LA, along with considerable river flooding in the Neches, Lower Trinity, and Lower Brazos basins which may continue into the weekend. Should significant rainfall develop across this extremely vulnerable region, localized catastrophic flooding could be possible as well, particularly where areal flooding and moderate to major river flooding impacts are ongoing. Antecedent conditions are primed for hydrologic responses given above normal rainfall over the past two weeks (WGRFC), which have left soil moisture down to the 100 cm layer nearly saturated (55 - 75% RSM, NASA SPoRT) and streamflows well above annual median values for this time of year (USGS). The expected slow moving/training nature of these storms, coupled with the high likelihood of intense rainfall rates, will easily overwhelm current infiltration and channel storage capacities leading to immediate runoff and subsequent flooding impacts. Flooding impacts will also be possible for surrounding portions of the region through this evening, with flash and urban flooding the primary threat, but some small stream flooding cannot be ruled out. As the storm system continues progressing east, flash flooding and minor to isolated moderate river flooding will be possible through day 3 (Sat) across the southeast, but primarily from southern MS into central AL where antecedent conditions are a little more primed and heavier rainfall amounts are expected. The HRRR-forced National Water Model (NWM) Short Range Forecast (SRF) has been signaling the potential for rapid-onset flooding across central and eastern TX with probabilities generally at or below 50%. However, these signals may be underdone due to the HRRR being slow to capture the ongoing convection and responses may be more significant than depicted by the NWM. Signals should continue increasing and expand into LA as the event unfolds and the QPF moves into the forecast window. Corresponding annual exceedance probabilities (AEPs), per the SRF High Flow Magnitude, are variable across TX/LA but generally range from 20 - 50%, with lower magnitude AEPs scattered within. The exception to this is the area between Waco/Bryan, TX, east to Lake Charles, LA, where a large swath of widespread 2 - 4% AEPs have been consistently signaling all morning. Given the higher ARI (3.2) incorporated into the NWM for much of this region, widespread AEPs of this magnitude are rarely seen. The magnitude and extent of these robust signals supports the likelihood of widespread and considerable hydrologic responses, and even suggests the potential for some locally catastrophic impacts. Guidance from both the GFS and the NBM-forced NWM Medium Range Forecast (MRF) continues to suggest rapid-onset flooding potential across MS/AL over the next 48 hours. While rapid-onset flooding probabilities are mostly below 50%, probabilities in southern MS into far western AL are starting to increase. Corresponding AEPs are generally near/at the high water threshold which suggests that most streams should remain in-channel, at least initially. However, some isolated AEPs below 50% support the potential for some locally significant rises. .Hawaii... Considerable flash, urban, and small stream flooding impacts remain possible for Kauai, Niihau, Oahu, Molokai, Lanai, Kahoolawe, and Maui through day 2 (Fri). Antecedent conditions are wet following recent rain, and it is likely that additional heavy precipitation will quickly overwhelm current capacity. Given these wet conditions, landslides are also possible in areas of steep terrain. .Southern Plains through the Lower Ohio Valley... Isolated flash and urban flooding impacts will be possible through day 2 (Fri) due to widespread moderate to locally heavy rainfall. Latest guidance from WPC indicates the axis of heaviest rainfall (2 - 2.5") is expected from about Jonesboro, AR to Paducah, KY. Antecedent conditions are generally near normal to somewhat wet throughout most of the region. These conditions should initially allow rainfall to be absorbed into the system. Rainfall should be relatively progressive which will help to diminish the overall flood threat, but isolated flooding impacts cant be ruled out should heavier rainfall/training occur. Both the NWM SRF and MRF are signaling low-end probabilities for rapid-onset flooding (less than 25%) throughout most of the region. Corresponding AEPs are generally at or above 50%. This suggests most streamflow rises, should they occur, will remain in-channel, but some out-of-channel rises can't be ruled out in smaller, more responsive basins. Nonetheless, any impacts will be dependent on where the heaviest rainfall occurs given the overall progressiveness of the system. .New England... Isolated to scattered small stream flooding impacts are expected for portions of eastern CT, RI, southeast MA. Top-layer soils have significantly moistened (NASA SPoRT) in response to the rainfall overnight (1 - 4", MRMS) and river observations show that streams are beginning to rise as the amount of rainfall becoming runoff continues to increase as soil infiltration capacity continues to decrease. The NWM SRF has been consistently signaling potential for high water flows (1.3 ARI) beginning this morning on smaller creeks and streams in the region. NWM SRF high water probabilities in this region are generally above 50%, which is a fairly impressive response from the NWM for this part of the country. NWM SRF max forecast flows are generally near the 1.3 ARI, with some smaller tributaries in RI potentially seeing flows below the 20% AEP, suggesting an increased potential for small stream flooding impacts in this region. Rivers will remain elevated throughout the day and into tonight. //Freeman $$