Hydrometeorological Discussion
Issued by NWS Tuscaloosa, AL
Issued by NWS Tuscaloosa, AL
255 AGUS74 KWCO 201529 HMDNWC National Hydrologic Discussion - EXPERIMENTAL NWS National Water Center - Tuscaloosa AL 1015 AM CDT THU JUN 20 2024 .Synopsis... Lingering flooding expected in Texas... Locally considerable flooding is possible in the Northern Plains, Upper Midwest, and Upper Great Lakes...Flash flooding possible in the Four Corners region...Localized urban flooding possible in the Northeast... Snow-melt induced flooding in Alaska... .Discussion... .Texas... Lingering flooding from Tropical Depression Alberto will continue across southern TX today. Although conditions are expected to gradually improve as we head into the weekend, there is the potential for the wet weather pattern to continue through at least early next week. Antecedent soil conditions have moistened due to the rain across south TX with a relative soil moisture of 65 - 80%. While not saturated, there is still some infiltration and in-channel storage capacity available but with the continuation of rainfall and given antecedent conditions, additional flooding may be possible.. The HRRR-forced National Water Model (NWM) Short Range Forecast (SRF) rapid-onset flooding probabilities continue to be fairly low-end (less than 50%) with scattered annual exceedance probabilities (AEPs) below 20% from the High Flow Magnitude Forecast (HFMF). These signals align with current high water flows likely through normally dry streams and arroyos across south TX. .Northern Plains, Upper Midwest, and Upper Great Lakes... Flash, urban, and river flooding, with the potential for locally considerable impacts, are expected this week and through the weekend from a multi-day storm system bringing repeat rounds of heavy rainfall across the region. The first round of heavy rainfall is forecast starting today and will continue through day 3 (Sat), peaking on day 2 (Fri). A brief lull is then expected on day 4 (Sun) before additional rounds on days 5 - 7 (Mon - Wed), although this rainfall should be less significant. Current guidance places the axis of heaviest rainfall (7-day totals) across eastern SD, southern MN, and central WI (3 - 5", locally higher possible). The region remains hydrologically active and vulnerable having received 150 - 200% of its normal rainfall over the past 30 days (CPC). USGS streamgages are reporting widespread above normal flows both historically and for this time of year. Soil moisture down to the 100 cm layer remains on the wet side as well (65 - 80% RSM, NASA SPoRT), suggesting limited infiltration capacity. Given these conditions, it will not take much rainfall to generate immediate runoff and subsequent flooding of small streams and creeks, as well as low-lying and poorly drained locations in urban areas. Each additional round of rainfall will serve to further compound the flood potential. Additionally, isolated moderate to major river flooding is ongoing or forecast across the Minnesota, Upper Mississippi, and Crow river basins in southern MN, with ensemble guidance (HEFS 30%) indicating additional rises along the Mississippi and any tributaries affected by this heavy rain. Many locations along the Mississippi River will approach flood stage towards next weekend. Both the NBM- and GFS-forced NWM are responding accordingly with widespread responses and the potential for rapid-onset flooding. Both models show good agreement with the placement of the rainfall, however, the GFS is much more aggressive with the overall magnitude of the responses. . Four Corners... Locally heavy rainfall, bringing the potential for flash flooding in states surrounding the Four Corners region, will be of concern today into day 2 (Fri). Areas most at risk include flooding on recent burn scars, in slot canyons and dry washes. .Northeast... The potential exists for localized flooding impacts in primarily urban areas on days 2 - 4 (Fri - Sun) as showers and thunderstorms develop across the region. Outside of sensitive urban areas, near normal relative soil moisture values (50 - 60%, 0 - 10 cm, NASA SPoRT) and near to below mean annual streamflows (USGS) suggest sufficient infiltration capacity overall to capture runoff. These factors, combined with modest rainfall amounts (less than 2"), will help to mitigate any widespread hydrologic responses and limit the threat to urban, low-lying, and poor drainage areas. .Alaska... Warm daytime and overnight temperatures are causing significant snow and ice melt at high elevations, leading to minor flooding on the Chilkat River near Klukwan, which is expected to persist due to diurnal temperature trends, and localized flooding in the Yetna and Skwetna river basins. Similar flooding is anticipated along the Nuyakuk River. These warm conditions are projected to continue through early next week in southwest, southcentral, and southeast areas of the state, further increasing the rate of snowmelt runoff into rivers. //Kirkpatrick $$