####018006370#### AGUS74 KWCO 281556 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT THU MAR 28 2024 .Synopsis... Rainfall and snowmelt-induced flooding likely for the Northeast... Flooding concerns continue across the Southeast and Coastal Mid-Atlantic... Urban and small stream flooding impacts possible this weekend in Southern California... Recessions continue across Northeast Texas and Lower Mississippi Valley... Rainfall potential next week across the Mid-Mississippi and Ohio Valleys... .Discussion... .Northeast... Periods of steady, moderate rainfall and snowmelt through day 2 (Fri) will likely elicit small stream rises and isolated minor river flooding from northern CT through coastal ME. Ahead of this rainfall, antecedent conditions were already saturated following recent rounds of precipitation, and the current snowpack has significantly ripened in some areas, so this additional rainfall and resultant snowmelt will likely immediately runoff into urban areas and nearby small streams. Ensemble guidance (MMEFS), SNODAS, and the NWM continue to indicate up to 2" of snow water equivalent loss, with locally higher amounts in higher elevations of NH and ME, although models appear to be melting slower than observations in portions of ME, so runoff will likely be more significant there. Soil temperatures across southern NH into much of ME are at or just below freezing, which will also enhance runoff into nearby streams and rivers. Guidance from the HRRR, GFS, and NBM-forced NWM are indicating scattered small stream responses from northern CT through Downeast ME generally within the next 24 hours. Given the lack of enhanced rainfall rates, the most likely impacts will be rapid rises on small streams and rivers draining high terrain, as well as standing water on low-water crossings and poorly-drained areas in urban locations. This increase in flows may cause ice movement along the St. John River in ME. Looking ahead, rainfall returns to the region mid-week next week. While the exact location and magnitude of impacts remain uncertain, this area will need to be closely monitored for additional impacts given the current saturated antecedent conditions. .Southeast and Coastal Mid-Atlantic... Urban and small stream flooding impacts remain possible across eastern NC and southeastern VA through this evening as periods of locally heavy rainfall continue. River flooding is now becoming the primary threat, as minor flooding is now forecast across the region in the wake of the 2 - 3" of rainfall that has fallen over the past 24 hours (MRMS QPE). The HRRR-forced National Water Model (NWM) is indicating isolated small stream responses across this region, suggesting that the resultant urban and small stream flood threat should remain rather localized. In regards to the river flooding, crests are expected late this weekend into early next week, and dry weather after today should allow for those crests and recessions to occur unimpeded. Further south into northern FL and southern GA, renewed rises to minor flooding is ongoing following heavy rainfall yesterday. Since the majority of these rivers are slow-draining, crests aren’t expected until mid-to late week next week. .Southern California... Urban and small stream flooding impacts are possible beginning on day 2 (Fri) and continuing through the weekend as an atmospheric river brings periods of moderate to locally heavy rainfall (3-day totals of 2 - 4", WPC) to the region. Antecedent conditions (outside of the urban centers of LA and San Diego) remain somewhat wet from recent rainfall (NASA SPoRT, USGS, NWM), indicating some vulnerability to small stream responses. Guidance from both the GFS and NBM-forced NWM continue to suggest the potential for scattered small stream responses along the I-5 corridor from Santa Barbara to the International Border and along the foothills of both the Transverse and Peninsular ranges, primarily on day 3 (Sat). Corresponding annual exceedance probabilities (AEPs), per the High Flow Magnitude Forecast, are generally near/at the high water threshold (1.5 - 1.6 ARI), which suggests that most streams should remain within bank. However, some locally enhanced AEPs (down to 4%) are noted along small streams near the San Diego metro area, as well as along the foothills of the Transverse Ranges, indicating some potential for locally significant stream rises. The greatest risk of flooding will primarily be in urban areas and along small streams draining steep terrain, especially if more robust rainfall rates were to materialize. .Northeast Texas and Lower Mississippi Valley... Minor river flooding will continue through at least early next week across these regions as runoff from rainfall earlier in the week routes downstream. Most locations are expected to crest by day 3 (Sat) and recede shortly thereafter. Rainfall returns to the region by day 5 - 6 (Mon - Tue); however, confidence in the exact placement of heaviest rainfall and resultant hydrologic impacts remain uncertain at this time. .Mid-Mississippi and Ohio Valleys... A storm system early next week may bring moderate to locally heavy rainfall to these regions, potentially eliciting at least localized hydrologic impacts. Ahead of this event, current soil moisture content indicates some infiltration capacity for runoff (45 - 60% RSM, 0 - 10 cm NASA SPoRT), and streamflows are generally near annual median values for this time of year (USGS), with the exception of portions of MO and IL, where streamflows are elevated from recent rainfall. Additionally, the lack of green up primarily north of the Ohio River may lead to enhanced runoff concerns if robust rainfall rates were to materialize. While guidance from both the NBM and GFS-forced NWM are indicating scattered small stream responses across these regions, the exact location and magnitude of impacts still remain uncertain given the extended nature of the forecast. //Pritchard $$