Hydrometeorological Discussion
Issued by NWS Tuscaloosa, AL
Issued by NWS Tuscaloosa, AL
744 AGUS74 KWCO 131523 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT THU JUN 13 2024 .Synopsis... Considerable to locally catastrophic flash and urban flooding in Florida... River flooding continues in East Texas... Potential for isolated flooding impacts in the Central Plains and the Upper Midwest... Heavy rainfall next week along the Western and Central Gulf Coast... Snowmelt-induced rises continue across California, Intermountain West, and Rockies... .Discussion... .Florida... Ongoing heavy showers and storms are expected to persist across the southern half of the peninsula into the weekend. Considerable flash and urban flooding impacts have already occurred and are likely to continue into tomorrow. Outside of urban areas, the top soils are at or near saturation (80 - 90% RSM, 0 - 10 cm NASA SPoRT) following prolonged heavy rainfall of 4 - 8", widespread, with localized amounts over 20" (48 hrs, MRMS). With the expectation of prolonged and slow moving heavy showers and storms again today and tapering off into the weekend, remaining soil infiltration capacity will likely be overwhelmed and rainfall will immediately runoff into nearby canals, drainage systems, and urban areas, generating large areas of standing water on roadways, overflowing of canals, storm drains and culverts, and flooding of poorly drained and low-lying areas in urban centers, with Naples and West Palm Beach down to Miami being most at risk. Locally catastrophic impacts are possible in these urban areas. Small stream flooding is possible, though not as likely, given the underlying karst topography and streamflows on a regional scale being normal to below normal for this time of year (USGS). However, rapid rises on small streams and canals are possible where heavy rainfall persists. The HRRR-forced National Water Model (NWM) supports this idea, as it depicts low to medium probabilities (26 - 50%) of rapid stream rises in urban areas along the I-75 and I-95 corridors. Corresponding annual exceedance probabilities (AEPs) supports a high likelihood of considerable impacts, as most AEPs are below 10%, with isolated responses as low as 2%. The greatest impacts are expected in urban areas and these impacts will largely be dependent on rainfall intensity. .East Texas... Moderate to major river flooding will continue across the Sabine, Trinity, and Neches river basins into the weekend. Crests are expected late this week as the flood wave routes downstream. Little to no precipitation through day 4 (Sun) should allow for unimpeded recessions across the area. .Central Plains and the Upper Midwest... Persistent rounds of storms through the Central Plains and the Upper Midwest starting day 2 (Fri) and continuing through day 7 (Wed), may cause isolated instances of urban, flash, and small stream flooding. Much of eastern MN and WI are vulnerable to hydrologic impacts due to wet soils (65 - 80% RSM, NASA SPoRT) and normal to much above-normal streamflows (USGS). Additionally the area has ongoing minor flooding, particularly on the Rock, Little Sioux, and Minnesota rivers. In contrast, the Central Plains has drier soils and generally normal streamflows for this time of year. Cumulative rainfall totals for days 1 - 3 (Thu - Sat) are between 1 - 2", with locally higher amounts, (WPC) and will likely generate instant runoff in eastern MN and WI regions, flowing into already swollen streams and possibly contributing to new and renewed rises. However, for the Central Plains, early rainfall will prime the area for later rounds of anticipated rainfall. Due to it being a convective season, later forecast periods are subject to much uncertainty in magnitude and placement. If days 4 - 7 (Sun - Wed) QPF materializes, subsequent urban, flash, and small stream flooding are likely to occur for both regions. The NWM Medium Range Forecasts (MRF), GFS, and NBM are signaling the potential for rapid-onset flooding. However, the GFS-forced NWM indicates more widespread responses, particularly in eastern NE and much of IA. These are more southeastern than the NBM signals due to differences in QPF placement. Regarding the overall magnitude of the streamflow responses, the GFS-forced MRF High Flow Magnitude Forecast (HFMF) is signaling widespread low annual exceedance probabilities (AEPs, 10% or lower) for much of central and southern NE into extreme south western IA, while the NBM-forced MRF HFMF has generally higher AEPs (50% or more), suggesting only modest rises. Discrepancies between model guidance adds to a lower confidence in magnitude and placement of the prolonged rain event. .Western and Central Gulf Coast... Heavy rainfall is forecast days 4 - 7 (Mon - Thu), bringing with it potential for flooding impacts. There is much uncertainty about the intensity and westward spread of the rainfall, as well as its potential flood-related impacts, due to the convective season and the timing of these events being later in the forecast period. Antecedent conditions are generally not conducive to hydrologic impacts, except for southeast TX. Portions of southeast TX are especially vulnerable to additional rainfall, with many reservoirs greater than 90% full and ongoing moderate flooding. Elsewhere in the region, soils are on the drier side (20 - 50% RSM, 0 - 10 cm NASA SPoRT) and streamflows are normal to much below normal (USGS). The NWM MRF has been signaling potential for rapid-onset flooding, primarily in southern LA and TX, and further east across southern MS and AL. However, due to significant model variability, the location and amplitude of impacts still remain uncertain. .California, Intermountain West, and Rockies... Periods of scattered rainfall will continue to promote increased runoff across much of the Intermountain West, north/central Rockies, and the Sierra Nevada this week. Isolated minor river flooding in northern NV, extreme eastern ID, and western CO is ongoing or expected to crest this weekend into early next week. Area streams, creeks, and rivers will continue to see rises as these conditions persist. //Bliss/Wood $$