Ohio and Tennessee Valleys...
Confidence remains high in significant rainfall-related impacts on
Saturday and Saturday Night with model guidance in excellent
agreement depicting a swath of 3-6 inches of rainfall in the 24-hour
period across the region. Although models vary slightly with the
placement, they almost unanimously depict a corridor of heavy rain
about 750mi long centered somewhere in TN/KY. This same region has
generally seen above average precipitation in recent weeks, with the
14-day precip in much of KY, N TN, WV, and W VA at least double the
normal values. The confluence of these factors should make the
region especially vulnerable to flooding from the predicted
rainfall. For additional context on the current hydrologic picture
across the region, refer to the National Hydrologic Discussion from
the National Water Center at water.noaa.gov.
Convection may be ongoing at the beginning of the forecast period
and should become increasingly focused along a developing warm front
to the east of a surface low that will be emerging out of the Plains
on Saturday. The front will be positioned on the nose of very strong
SW, or even WSW, inflow, which should favor backbuilding and
training and a corridor of very heavy rainfall. The environment will
be atypical of mid-February with PWATs reaching around 1.5 inches
and MUCAPE of 500-1000 j/kg. For example, the Nashville sounding
archive exists back to 1948 and they've only ever reached 1000 j/kg
MUCAPE once in February, and 1.5 inch PWATs a few times. The
environment will support rainfall rates in excess of 1 inch per
hour, and given the potential for training and backbuilding, these
rates may be sustained for several hours in some locations. These
would be the areas of greatest concern for flash flooding -- not
only in terms of likelihood but severity -- particularly if the
higher rain rates are sustained over urban areas or vulnerable
terrain for extended periods of time
It's worth noting a couple additional things. First, the amount of
rain forecast from E KY into S WV and W VA in particular is quite
unusual for the heart of the winter season. Calendar day precip
records across that area for January and February are generally in
the 2-3 inch range, and NBM probabilities indicate it is likely (60+
percent chance) we will see these types of 24-hour rainfall totals
in those areas. Second, this relatively rare winter rain event would
be delivered to an area that has been quite wet recently, and in
some areas has snow on the ground. And third, the high-end scenarios
in much of the Moderate Risk area would be significant by historical
standards. The NBM 90th percentile from N TN into S/E KY and S WV is
on par with the top several wettest days (for any season!) in the
station records in the area. This corridor is where the greatest
concern for severe impacts exists, and it's possible an upgrade to
High Risk may be needed for portions of the area in subsequent
updates.
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