Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 09 2023 20:30:43 FOUS30 KWBC 092030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 PM 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.=20 ....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 2 Valid 12Z Sun Dec 10 2023 - 12Z Mon Dec 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHEAST... ....2030Z Update... With a subtle eastward shift in the QPF across portions of the Northeast United States...which implied that colder air would also be drawn a bit farther eastward with a corresponding expansion of the snow area...adjusted the western boundary of the Marginal risk. With at least some potential for melting of snow due to warmer rain falling on the snowpack...did not entirely remove the overlap between WWDs snowfall graphic and the Marginal Risk area.=20 Given a consistent signal for the 2+ inch rainfall amounts to the east and south of there...saw no reason to make many adjustments to the Slight Risk area. The placement of the southern boundary of the Marginal risk area was nudged north given model trends.=20 Overall...though...the big-scale picture remains similar to the 0830Z ERO. ....Pacific Northwest... The previously-issued Excessive Rainfall Outlook was supported well by the 09/12Z model runs and their QPF generally in the 1 to 3 inch range. For that reason...few changes were needed. Bann ...0830Z Excessive Rainfall Discussion... ....Eastern U.S... A very dynamic system will impact the East Coast Sunday into Sunday night, continuing to advance downstream from the Midwest and central Gulf Coast from Saturday. A cold front is forecast to be progressive across the southern U.S. as the base of the upper trough becomes negatively tilted as it swings from the Lower Mississippi Valley to the East Coast through the period, with a deepening surface low forecast to track from the Mid Atlantic into the Northeast. A Slight risk was maintained across portions of the Northeast...generally including northern NJ, into southeast NY, CT, central/western MA and southern VT/NH. Rainfall amounts of 2-3" are likely over a good portion of the Slight risk area, with most of the deterministic guidance indicating localized swaths as high as 3-4". Limited instability will keep rainfall rates from getting too high, but the negative tilt of the mid level trough and strong low level convergence along/near the low track should be enough to result in embedded heavier convective elements within the broader rain shield...with pockets of greater than 0.5"/hr probable. The increasingly negative tilt of the trough and associated surface low deepening will prolong the duration of rainfall over this area...which is responsible for the localized 3"+ totals that are anticipated. This is still a borderline areal vs flash flood threat given the sub 1"/hr rates that are expected. However soil information from NASA SPoRT and CREST suggests that after some initial infiltration a good chunk of this rainfall will become runoff. Thus would expect to see the potential increase for some flooding of streams and other low lying or flood prone locations...including poor drainage and urban areas. If we end up with 0.5"/hr plus rainfall rates and total rainfall over 3" over an urban or other flood prone area then even flash flooding is certainly a possibility. There remains some spread in the axis most favored for these higher rates and totals...but overall think the Slight risk area encompasses the most likely location at the moment. A big wild card continues to be snow melt over northern New England. The 50+ dewpoint air gets quite close to areas of deeper snow pack and higher SWE over central/southern VT into central NH. At this point it appears like there will be some SWE loss adding to the flood potential over VT and possibly NH, but probably not a long enough overlap of 50+ dewpoints and higher SWE for there to be a significant contribution. Nonetheless something to continue to monitor. Over the Mid Atlantic generally dry soil conditions and below average streamflows should be more of a limiting factor for flooding compared to areas of the Northeast described above. However still expecting a swath of 2-3" of rain, and potentially a more narrow swath of 3-4" across this region as well...with some hourly rates getting over 0.5"/hr. In general exceedance of the current 1,3, or 6hr FFG appears unlikely...however would still anticipate there to be some excess runoff concerns that could develop...especially where total rainfall begins to exceed 3". The threat of true flash flooding remains low given the the only modest rainfall rates...but some urban and areal flood impacts are certainly a possibility. With the flash flood risk lower than over the Northeast, a Marginal risk should still suffice. Will need to keep an eye on portions of NC into southeast VA, where there may be some overlap in higher instability and longer duration rainfall. Farther south, limited instability and the progressive nature of the front continues to warrant the gap in the Marginal Risk across portions of Georgia and South Carolina. However we will maintain a risk across the eastern Florida Panhandle into southern Georgia where guidance continues to indicate some potential for training, which could yield localized flash flooding. ....Northwest... Little change made to the existing Marginal Risk over western Oregon given the atmospheric river event will still be ongoing at 12Z Sunday. While IVT values are expected to wane through the period, localized high rainfall rates (near 0.5 in/hr peak values) will continue for the first 6-12 hours of the period prior to the moisture plume weakening and the Pacific front moving through the region. At this time it appears that the heaviest rainfall Sunday will be south of the areas with the most rain on Saturday as the AR shifts southward. Given that, and the overall weakening of the IVT plume with time, we think a Marginal risk should still suffice...with a general 1-3" of rainfall expected. Chenard Day 3 Valid 12Z Mon Dec 11 2023 - 12Z Tue Dec 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MAINE... ....2030Z Excessive Rainfall Discussion... No changes needed to the on-going Excessive Rainfall Outlook. Bann ....0830Z Excessive Rainfall Discussion... The event described in more detail in the day 2 discussion will be ongoing Monday morning across portions of ME. More uncertainty exists with the forecast by this time...both with regards to ptype and additional QPF magnitudes. The model consensus favors mainly rain over eastern ME Monday, but a transition to snow is certainly possible over northern ME. The GFS is most aggressive with the mid level energy and surface low...and thus drops an impressive 4-7" of rain over ME for a 48 hour total. On the other hand the ECMWF and UKMET are more in the 2-4" range, with the official WPC forecast also 2-4". The PQPF 75th percentile 3-4" and the 90th percentile is more in the 3-5" range. Thus it becomes apparent that the 00z GFS is an outlier and lower probability outcome. We can not completely rule it out, and a solution like the GFS would likely pose a more significant flood risk over ME, but for the moment it appears like a lower probability outcome. The more probable rainfall of 2-4" would still pose a flood threat, but likely more at the Marginal risk level. Snow melt is a factor to consider and keep an eye on, although the more substantial SWE is located over northern ME...and at the moment not expecting the 50+ dewpoint air to make it that far north. So while snow melt may play some factor in the flood risk over ME, at the moment not expecting it to have a significant contribution to any flash flood risk. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_w_FzyW14ut40KsuUWSG-CcQ5zwLKtYIOvosnTq93Ub7= UHzwhdT9RUZ9ofhbG4pXUFnwFN8Hys2-3_wFePGxuMwu9vM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_w_FzyW14ut40KsuUWSG-CcQ5zwLKtYIOvosnTq93Ub7= UHzwhdT9RUZ9ofhbG4pXUFnwFN8Hys2-3_wFePGxIx106Rc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_w_FzyW14ut40KsuUWSG-CcQ5zwLKtYIOvosnTq93Ub7= UHzwhdT9RUZ9ofhbG4pXUFnwFN8Hys2-3_wFePGxYp1OWgU$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .