AGUS73 KOWP 021947 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa, AL 146 PM CST Wed Feb 02 2022 TEST...TEST..TEST Synopsis... Flooding concerns in portions of the Ohio River Valley and the ArkLaTex into the Southeast... Urban and small stream flooding possible across the Coastal Mid-Atlantic into New England... Flash flooding possible across American Samoa... Discussion... Ohio River Valley... Moderate to heavy rainfall producing rises on rivers across the Ohio River Valley is possible through day 2 (Thu) as a slow-moving cold front moves east across the region. Rainfall amounts of 1 - 3”, combined with potential snowmelt of up to 0.5” water equivalent, may produce isolated minor river flooding by day 3 (Fri, MMEFS). In contrast, confidence in potential hydrologic responses remains low as the GFS-forced National Water Model (NWM) Medium Range Forecast (MRF) signals continue to be lacking given uncertainties surrounding the timing of the changeover from rain to freezing and/or frozen precipitation. Meanwhile, across portions of the lower Ohio Valley, the potential for prolonged moderate to locally heavy rainfall (2 - 4”) through day 2 (Thu) could enhance the potential for rapid runoff, increasing flooding concerns along smaller streams and rivers. The NWM MRF 5-Day Bankfull Probability Forecast continues to show a chance of meeting or exceeding bankfull flow across portions of north-central TN into central and eastern KY. Recent model runs continue to reflect a generally 25 - 50% chance with the highest chances generally in the Tennessee Valley. Farther north, some flooding potential also exists on the Wabash River (IN) and in the Scioto and Muskingum basins in Ohio, but precipitation type uncertainty significantly reduces confidence in that basin. ArkLaTex into the Southeast... From the ArkLaTex into MS, AL, and northern GA, there is a threat of localized rapid onset flooding, as well as potential riverine responses to minor and isolated moderate flooding, today through day 2 (Thu). Precipitation totals will generally be lighter (1 - 1.5”) today over the ArkLaTex with some higher amounts (2”) over MS into AL. On day 2 (Thu), these totals are expected to increase over MS/AL/northern GA with widespread totals of 2 - 3” with localized totals nearing 4”. Taking a look at the soil moisture values from NASA SPoRT (and supported by NASA GRACE), drier soils are in place over the ArkLaTex and gradually becoming wetter moving east into MS, AL, and northern GA. The drier conditions over the ArkLaTex, combined with the lower precipitation totals, should limit any hydrologic responses to mainly localized urban and small stream issues today with the best chance for any impacts over portions of MS and AL. The NWM Short Range Forecast (SRF) is starting to signal over portions of SE MS today where precipitation totals are higher today and soils are more favorable for rapid runoff. As the heavier precipitation totals move eastward on day 2 (Thu), the threat shifts to include not only rapid onset flooding but also riverine responses to minor and isolated moderate over the region. The NWM MRF is signalling the potential for bankfull or higher flows as well as rapid onset flooding over portions of central and northern AL into northern GA. This is in general agreement with current output from the MMEFS, which signals the potential for some action to minor flood stage forecasts in the region. A notable difference is over the Tombigbee River river in MS/AL, where the NWM is not signalling bankfull or higher flows; however, MMEFS output as well as forecasts from SERFC are indicating minor flooding is possible. This is likely due to differences in the precipitation forcings in the models rather than a difference in hydrologic states. Due to these differences in precipitation forcings, confidence is low in targeting any specific river system for flood responses. Coastal Mid-Atlantic into New England... Isolated urban and small stream flooding may be possible days 2 - 3 (Thu - Fri) as 1 - 2” of rain combines with warmer temperatures and higher dew points to produce snowmelt across the Atlantic Seaboard from New Jersey north into New England. Widespread riverine impacts are not expected; however, this runoff may dislodge and move existing river ice, particularly in Pennsylvania and New Jersey, where river ice coverage is extensive in some locations (as indicated by NWC SARRIS analysis). There is the possibility of isolated ice jams in these locations, but overall river stages are low enough to preclude any river flooding. Overall, due to the expected short duration of the warmer conditions and rainfall (before a changeover to winter precipitation occurs), the overall confidence in any potential impacts remains low. American Samoa... Localized flash and urban flooding, along with landslides, remain possible today due to periods of heavy rainfall resulting from rounds of thunderstorms. //RLF/W