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Hydrometeorological Discussion
Issued by NWS Tuscaloosa, AL
Issued by NWS Tuscaloosa, AL
501 AGUS74 KWCO 301513 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1013 AM CDT SUN JUN 30 2024 .Synopsis... Flooding impacts possible across Southern New England this afternoon/evening... Renewed flooding possible this week for the Upper and Middle Mississippi Valley.... Daily convection across the Four Corners Region... Heavy rainfall potential Monday and Tuesday across Puerto Rico as TS Beryl tracks south of the region... Increased snowmelt and glacier runoff in Alaska... .Discussion... .Southern New England... Isolated flash, urban, and small stream flooding is possible this afternoon and evening as multiple rounds of convection progress across the region. Soils remain wet due to recent rainfall (50 - 65% relative soil moisture (RSM) 0 - 10 / 0 - 100 cm NASA SPoRT), therefore successive rounds of intense rainfall may be able to overcome soil infiltration capacity and lead to excessive runoff and subsequent flooding impacts. The HRRR-forced National Water Model (NWM) Short Range Forecast (SRF) is consistently signaling potential for rapid onset flooding (ROF) across CT, southern MA, and RI this afternoon into the evening hours, with basin averaged probabilities for ROF increasing above 25%. Probabilities for high water flows (1.3 ARI Flows) are above 50%, and rises on small headwater tributaries could be locally significant as suggested by isolated flows with AEPs 10% being forecast (NWM SRF). Antecedent normal to below normal climatological streamflows (USGS) should help mitigate initial riverine response, and flooding impacts will be contingent on basins receiving repeated rounds of heavy rainfall. .Upper and Middle Mississippi Valley... Moderate to major river flooding is ongoing across far southeast SD, southern MN, and northern IA and is forecast through this week along the Missouri (moderate) and Mississippi (moderate to major) River mainstems. Rain returns to the region beginning day 2 (Mon) and lingers through day 6 (Fri). Ample deep layer soil moisture remains in place across eastern SD, southern MN, IA, and WI (60 - 80% RSM 0 - 100 cm NASA SPoRT), suggesting that the already elevated/flooded rivers across the region will likely respond fairly efficiently to the expected additional rainfall/runoff. Generally 1 - 3" inches of rain is expected by the end of the week, which is likely not enough to cause widespread new and renewed river flooding, however some basins could see some new and renewed river rises/flooding (NWM Medium Range forecast (MRF) High Water Probability) if heavier totals (3 - 4"+) materialize. At the very minimum, delayed recessions of ongoing flooding and isolated flash flooding can be expected. New river flooding impacts will be possible, with some new and renewed rises into minor and moderate flood being possible (HEFS, CR Ensemble QPF hydrographs) if the heavier aforementioned rainfall totals come to fruition. The NWM MRF (NBM and GFS forced) continue to indicate potential for widespread new and renewed river rises by mid week in response to this rainfall with peak flows mostly on days 3 - 6 (Tue - Fri) across southern MN, western WI, and IA. The magnitude of response is still uncertain, but the NWM MRF is indicating potential for 4 - 20% AEP flows on many smaller tributary rivers across the aforementioned region, so trends will need to be closely monitored heading through the week. .Four Corners Region... Daily convection will continue the potential for isolated significant flash flooding in arroyos, slot canyons, dry washes, urban, and recently burned areas through the end of the week. Locations that have received multiple rounds of rainfall over the past week are most susceptible to new flooding impacts. .Puerto Rico... Moderate to heavy rainfall (1 - 4") Monday through Tuesday across southeast Puerto Rico will bring the potential for isolated flooding impacts. Streamflows across this portion of the island are generally normal to above normal in comparison to both the daily and annual climatological normals, suggesting soil capacity is nearing saturation in response to recent rainfall. This increases the potential for rapid runoff and flooding concerns, however any flooding impacts are expected to remain isolated given the small areal coverage of heavier rainfall and lack of significant rainfall totals. .Alaska... Recently warm temperatures continue to lead to increased snowmelt and glacier runoff, leading to rivers running high in areas across southern AK. Minor flooding on the Chilkat River near Klukwan continues through at least the weekend. Also, elevated flows and localized flooding is possible along the Skwentna, Yentna, and Nuyakuk Rivers. //JEC $$