WCO issues Area Hydrological Discussion (AHD) at Dec 18, 6:12 AM CST620
AGUS74 KWCO 181217
AHDNWC
Area Hydrological Discussion #318 - EXPERIMENTAL
NWS National Water Center - Tuscaloosa, AL
612 AM CST Wed Dec 18 2024
WHAT: Urban and small stream flooding. Isolated minor river flooding
possible in western KY.
WHERE: Portions of western and middle TN, much of KY, and extreme
southwestern OH. Includes metropolitan areas such as Memphis,
Nashville, Louisville, and Cincinnati.
WHEN: Urban flooding is possible through mid-morning. Small stream
and river flooding likely through early this afternoon.
CONFIDENCE: High confidence in isolated small stream and minor river
flooding.
FORECAST RAINFALL AND ANTECEDENT CONDITIONS...
QPF 1 - 3" (HRRR/WPC)
Rainfall Rates up to 1.5"/hr (HRRR/WPC)
Soil Moisture: 55 - 75% RSM across the entire region, up to 80% RSM
near the OH/KY border (0 - 10 cm, NASA SPoRT)
Streamflows (USGS): Above to much above normal streams are most
elevated across northern/central KY
DISCUSSION...
A line of showers and storms will continue to slowly progress across
the area of concern through late morning, bringing enhanced rainfall
rates and the potential for urban, small stream, and isolated minor
river flooding, especially across western KY. Antecedent conditions
are wet following recent rainfall (2 - 4", past 72 hours, MRMS QPE),
indicating lowered infiltration capacity in soils, which will likely
lead to immediate runoff into nearby creeks/streams/urban drainages.
Overflowing of ditches/culverts/storm drains, as well as rapid stream
rises, particularly in urban areas, are possible. The latest guidance
from the HRRR-forced NWM has been consistently indicating widespread
small stream responses from near Memphis to Cincinnati, with peak
flows expected to occur by late morning (7 - 9 hours). The coverage
of the responses from the NWM are likely overestimated across much of
the Lower OH Valley given known statistical biases and is
unrepresentative of the expected isolated nature of the impacts.
Despite the widespread streamflow responses, the magnitude of these
rises are not expected to be significant, as most of the annual
exceedance probabilities across the region indicate modest out of
bank responses (AEPs of 50% and greater). However, there are isolated
AEPs as low as 20% across western KY, which coincides near the areas
where minor river flooding is expected. As such, areas of heavier
rainfall may have locally impactful small stream rises across the
region, particularly within the Green (KY), Lower Cumberland (TN/KY),
and Obion (TN) basins.
GRAPHICAL REPRESENTATION weather.gov/owp/operations-ahd
Additional National Water Center products are available at
weather.gov/owp/operations
//JDP
ATTN...WFO...ILN...MEG...PAH...LMK...JKL...RLX...OHX
ATTN...RFC...ORN...TIR...WPC
Source:
WCO issues Area Hydrological Discussion (AHD) at Dec 18, 6:12 AM CST---------------
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