####018004028#### AGUS74 KWCO 031523 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1023 AM CDT WED APR 3 2024 .Synopsis... Small stream flooding impacts expected across the Mid-Atlantic through Wednesday night... Widespread river flooding across the Ohio River Basin / Midwest... Potential for flooding impacts early next week across the ArkLaTex... Snow melt across the Intermountain West... .Discussion... .Mid-Atlantic... Potentially widespread small stream flooding impacts, especially in the vicinity of quick responding headwater streams draining the Appalachians in central PA, are possible through tonight as multiple periods of moderate rainfall (1 - 3" totals through day 1 (Wed), locally higher amounts) are expected to persist across PA, northern VA, MD, DE, NJ, and southern NY. Wet soils (70 - 85% RSM 0 - 10cm, NASA SPoRT) and normal to much above normal climatological streamflows (USGS) favor quick and efficient runoff and rapid responses on smaller creeks and streams, respectively. This potential for small stream flooding impacts is supported by the National Water Model (NWM) Short Range Forecast (SRF) signals. Across western PA where 1 - 3"+ of rain has fallen over the past 12 hours (RFC QPE), the NWM SRF is indicating widespread basin average probabilities of 75 - 95% for high water flows (1 year ARI flow) through this afternoon. The region between Pittsburgh and Harrisburg may potentially see flows with annual exceedance probabilities (AEPs) of 2 - 10% on smaller headwaters, suggesting potential for widespread small stream flooding impacts. Additionally, MMEFS suggests potential for moderate river flooding across this region, with low potential for a couple rivers reaching major flood as well in the Lower Susquehanna Basin. Further east in PA, and elsewhere across northern VA, MD, DE, NJ, and southern NY, NWM SRF basin average probabilities for high water flows (1.3 year ARI flow) range from 50 - 90%, however, flows struggle to exceed the 50% AEP across this region. This is suggesting that while widespread small stream response can be expected, the areal coverage of tangible flooding impacts may be a bit more limited in comparison to central PA given the lower magnitude flows being forecast by the NWM SRF. The HRRR, which forces the NWM SRF, seems to have a fairly representative handle of the QPF into this afternoon, increasing confidence in the rises being forecast from the NWM across the entire region. Hydrologically significant precipitation is not expected for the next week after this system wraps up on day 2 (Thu), so flows will be able to route and recede unimpeded. .Ohio River Basin / Midwest... Widespread minor and isolated moderate river flooding is ongoing and forecast to continue through the end of the week in response to widespread 2 - 4" of rainfall over the past 2 days. Wills creek at Cambridge (OH) will crest in major flood late day 1 (Wed). Hydrologically significant precipitation is not expected for the next week, so flows will be able to route and recede unimpeded. .ArkLaTex... There is potential for heavy rainfall and subsequent flooding impacts on days 6 - 7 (Mon - Tue) from eastern OK/TX through AR/LA into MS. While there is very little confidence in the specifics of any impacts, this region has been fairly wet over the past 30 days (RFC QPE 1 - 5"+ positive departures from normal precipitation) and the NWM is signaling river rises across portions of the region, so trends will be monitored closely. .Intermountain West... Snow melt is expected to continue through the end of the week (SNODAS), with additional precipitation expected late this week through the weekend. This will likely produce rapid rises on mountain streams with very cold water. //JEC $$