Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1055 AM EST Sat Dec 09 2023
 
Day 1
Valid 16Z Sat Dec 09 2023 - 12Z Sun Dec 10 2023 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF 
THE PACIFIC NORTHWEST...

...Pacific Northwest...
An Atmospheric River will begin to impact the Pacific Northwest 
from later this morning and afternoon and persist into the 
overnight hours.  Forecast IVT from the 00Z guidance supports peak 
values of 500-700 kg/m/s. With flow relatively orthogonal to the 
coast, some heavier rainfall totals are expected along the Coastal 
Ranges into the Cascades. Model guidance generally remains in good 
agreement, focusing the highest rainfall totals across northwest 
OR into western WA. A lack of instability should generally keep 
rainfall rates below 0.5"/hr, however HREF guidance indicates up 
to a couple hundred j/kg of CAPE offshore to right along the coast 
later today, which may allow for a few areas of embedded higher 
rates to make it onshore. HREF neighborhood probabilities of 
exceeding 0.5"/hr do get up towards 50%, suggesting that localized 
rates exceeding that threshold are probable. Most of the region 
will pick up 2-3" of rainfall, although localized 3-5" totals are 
likely in the more favored west to southwest facing slopes. 
Flooding impacts will be increased within the Slight Risk area 
given the saturated soil conditions and above average streamflows 
over this region from the atmospheric river event from earlier in 
the week.  

...OH/TN Valley into the Southeast...
On-going convection over portions of central/eastern Arkansas has 
assumed a more northwest to southeast orientation...which has 
allowed for more of an eastward progression and reduced the 
potential for training and excessive rainfall in the short term.

By early this afternoon...a fairly substantial instability plume 
for the time of year develops from the Gulf Coast all the way into 
KY...with values exceeding 1000 j/kg with northern extent, and 
over 2000 j/kg farther southwest. This will be plenty of 
instability to support convective development along the eastward 
moving cold front from LA into KY. In general this activity should 
stay progressive enough off to the east to limit the overall flash 
flood risk. 09/12Z HREF guidance still shows high probabilities of 
rainfall exceeding 0.5"/hr, with lower probabilities of 1"/hr. 
This seems reasonable, with the instability supporting briefly 
intense rates, but the eastward motions limiting the duration of 
these higher rates. Through 12Z Sunday...most areas should see 
total rainfall of 1.5" or less as this system progresses off to 
the east...although do suspect that the 09/12Z HREF is probably 
under-doing max QPF values to some extent over portions of 
MS/AL/TN where more robust instability should support localized 
2"+ totals. Nonetheless even with these locally higher totals any 
flash flood risk is expected to remain isolated in nature.

Still maintaining a closer eye is in the vicinity of the southern 
Appalachians...generally from northern GA into southeast TN and 
southwest NC. The 09/12Z HREF shows 3"+ storm total neighborhood 
probabilities exceeding 40% across this area, with showery 
activity this morning and afternoon (with some upslope component 
in play as well), giving way to what should be a more intense 
convective line by tonight where hourly rainfall may push 1". This 
type of rain may begin to push us close to Slight risk 
levels...but with soil saturation and streamflows well below 
average, it seems like this rain probably won't be enough to cause 
anything more than isolated flood issues. Thus will keep the risk 
level at Marginal, but continue to monitor. 

Chenard/Bann

 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt