FGUS72 KJAX 072000 ESFJAX Probabilistic Hydrologic Outlook National Weather Service Jacksonville FL 300 PM EST Thu Mar 7 2019 ...Spring Flood Potential Outlook for Southeastern Georgia and Northeast Florida... ...Above normal potential for river flooding across the region this Spring... ...Existing Conditions... In the Altamaha, Satilla, and St Marys Basins... Rainfall since the start of the water year in October 2018 has been above normal, particularly over the Satilla River Basin. Upstream of the Altamaha River, rainfall has been considerably above normal in the Oconee and Ocmulgee River Basins. 90 day precipitation has been near normal across these basins, with shorter term precipitation at 30 and 60 day intervals running 2 to 4 inches below normal. Even with the shorter term deficits, streamflow conditions across these basins remain at normal ranges for this time of year. Streamflows above Macon and Milledgeville on the Ocmulgee and Oconee, respectively, are running slightly above normal. In the Upper Suwannee and Alapaha River Basins... Longer term rainfall since October 1st, 2018 is much above normal throughout these two basins, particularly so within the Alapaha Basin, where significant flooding occurred in December from heavy rains at the start of that month. In the Suwannee, longer term rainfall across the Suwannee Basin has been above normal, with significant rains occurring in December, that resulted in a basin wide flood event. Flooding in the upper Suwannee reached higher end moderate to low end major flooding. Rainfall through the last 60 days has been generally below normal in the upper portion of the basin. Interestingly, groundwater measurements across the Suwannee Basin are much above normal, into the 95th percentile in some places. This is important heading into the Spring, not only from a river flood perspective, but also because the areal flood risk is clearly increased given high soil moisture content. In the Santa Fe Basin... Longer term rainfall since October 1st, 2018 is well above normal across the entire Santa Fe Basin. Heavy rains in December also contributed to basin-wide flooding. The combination of routed flows down the Santa Fe and Suwannee in December resulted in an extremely lengthy period of flooding on the lower Santa Fe Basin. Even with 30- 60 day rainfall being near to slightly below normal, this basin remains especially vulnerable to flooding, as even relatively light rainfall amounts could produce flooding in the lower portion of the basin, as would a scenario that features increased flows moving through the Middle Suwannee Basin later this Spring. Groundwater observations are also similar to those in the upper Suwannee, showing some wells in the 95th percentile in some places. The areal flood risk is considerably increased in the Santa Fe Basin. In the Black Creek Basin... Longer term rainfall from October through much of December was above normal in the Black Creek basin, but after December, rainfall was generally at or just below normal, with the last 30 days being fairly dry. Even with the generally dry conditions over the last month or so, streamflows on both the north and south fork components of Black creek are at normal levels. ...Long Term Precipitation Outlook... The three month precipitation outlook for March, April, and May indicated an increased chance for above normal precipitation across the region. The one month outlook for March suggests and increased chance for above normal precipitation across much of Southeastern Georgia. For reference, the monthly normal precipitation for select stations across the region appear below: Location Normal Precipitation --------------------------------------------------------------- Waycross, GA (Mar) 3.64 in. Waycross, GA (Apr) 2.75 in. Waycross, GA (May) 2.30 in. --------------------------------------------------------------- Jacksonville, FL (Mar) 3.95 in. Jacksonville, FL (Apr) 2.64 in. Jacksonville, FL (May) 2.48 in. --------------------------------------------------------------- Lake City, FL (Mar) 4.79 in. Lake City, FL (Apr) 2.92 in. Lake City, FL (May) 3.01 in. --------------------------------------------------------------- High Springs, FL (Mar) 4.38 in. High Springs, FL (Apr) 2.83 in. High Springs, FL (May) 3.01 in. Typically, April and May are drier months across the region, and in the absence of significant rains in the month of March, river flooding becomes less likely after mid April across the region as fewer storm systems impact the region. ...Spring Flood Outlook Summary... Despite the recent dry conditions across much of the region from late January to present, area streamflows remain at least near normal levels, with some portions of the Santa Fe Basin above normal. With ground water observations from the Suwanee and Santa Fe Basins indicating conditions well above normal and other reports from around the hydrologic area indicating increased ground water levels, conditions are favorable for river flooding this spring, provided heavy rainfall. Thus all basins across Southeast Georgia and Northeast Florida have an above normal risk for river flooding this Spring. While the overall flood risk is above normal across the region, given current conditions, the Upper Suwannee and Santa Fe Basins are especially vulnerable to river flooding this Spring. $$ Godsey