Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 08 2023 08:29:32 FOUS30 KWBC 080829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 AM EST Fri Dec 08 2023 Day 1 Valid 12Z Fri Dec 08 2023 - 12Z Sat Dec 09 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 2 Valid 12Z 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 impact the Pacific Northwest Saturday, with IVT values upwards of 500-700 Kg/m/s forecast along the coast by Saturday evening before beginning to taper off Sunday morning. With flow relatively orthogonal to the coast some heavier rainfall totals are expected along the coastal ranges. Most model guidance has come back south a little bit with the core of the AR, but things are still generally on track from the previous forecast. Maximum rainfall totals are expected over northwest OR into western WA...with the global model consensus of 2-3" of rainfall. Once we get into more of the high res model time frame suspect we'll see QPF totals peaking more in the 3-5" range over this area..which will probably begin to cause some flooding impacts given the saturated soil conditions and above average streamflows over this region.=20 ....OH/TN Valley into the Southeast... A broad southwest-northeast axis of heavy rainfall is forecast later in the day Saturday through early Sunday from the the Ohio/Tennessee Valleys southwestward into the Mid-South and the Lower Mississippi Valley/central Gulf Coast as Gulf moisture returns northward ahead of an upper-trough/surface cold front pushing eastward. The upper forcing and surface front are pretty progressive off to the east, which will be a limiting factor for excessive rainfall amounts. With that said, there will likely be a swath where upper flow will be more parallel to the front, with storm orientation along the front allowing for at least some training. Where this occurs a slightly higher flash flood risk could evolve, although still some uncertainty on where this may end up. The better instability will be focused over the southwestern extent of the risk area. Meanwhile the mid level trough generally stays positively tilted through this period, which may tend to favor a slightly further south max QPF swath as well. Thus at the moment thinking the best alignment of higher instability (supporting more intense rainfall rates) and brief training along the front will be from northeast LA into central/northern MS, northern AL and central TN...with the 00z Gem Reg and UKMET best aligning with this thinking. Both of these models would support localized swaths of 2-3" of rainfall generally between 21z Sat and 06z Sun, which may begin to approach or locally exceed FFG. Still can not rule out a targeted Slight risk over this corridor as we see more high res model data...especially if 3"+ probabilities begin to increase. Either way the progressive nature of the event and dry soil conditions/low streamflows would keep this as a low end Slight risk at best...but something to continue to monitor. For now a broad Marginal risk was maintained. With time we may be able to trim some of the southern and northern extent of the Marginal risk...but the area was generally left unchanged to account for some lingering uncertainty in the expected max QPF swath. Chenard Day 3 Valid 12Z Sun Dec 10 2023 - 12Z Mon Dec 11 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE EAST COAST AND THE PACIFIC NORTHWEST... ....Eastern U.S... A very dynamic system will impact the east coast Sunday into Sunday night. Any instability will generally be confined to areas from NC to the Gulf Coast, and thus highest rainfall rate potential will be there as well. However the progressive nature of the system and high FFG should keep any flash flood risk localized in nature at best over these areas. Despite the lack of instability, the greater hydrologic concerns will probably be from the Mid Atlantic into the Northeast. The mid level trough should become negatively tilted as it crosses this region, with a strong upper jet to work with as well. Thus despite the lack of instability this forcing should be enough to generate some heavier rainfall rates of ~0.5"/hr and 1"+ per 3 hours, with good model agreement for event total rainfall of 1-3" across this region. In fact most of the deterministic guidance would support localized swaths over 3" from the Mid Atlantic into the northeast. Overall there is moderate to high confidence in these rainfall totals and rates...however lower confidence in how this rainfall translates to hydrologic impacts. With generally dry soil conditions and below average streamflows over the Mid Atlantic, would tend to think most of this rainfall will be beneficial in nature. However if and where we start getting rainfall up over 2-3" there will probably be at least some runoff impacts. The threat of true flash flooding seems low given the the only modest rainfall rates...but some urban and areal flood impacts will be a possibility. Things are even more complex over the Northeast...generally portions of Upstate NY into New England. The meteorology and rainfall forecast is similar here compared to the Mid Atlantic...but the hydrologic sensitivity is likely greater. Both NASA SPORT and CREST soil saturation over this region suggest less of this rain will infiltrate the soil compared to the Mid Atlantic...meaning more runoff and the potential for more flood impacts. With that said, the lack of more intense rainfall rates probably still keep this as more of an areal flood instead of true flash flood event. A big wild card will 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 more likely that there is not much overlap of long duration 50+ dewpoints and higher SWE, and thus while snow melt will play some factor, it may not significantly increase the flood threat. With that said, it will be a close call, and a warmer trend in the guidance could result in more significant SWE loss, in which case the expected 1-3" of rain would have even a greater impact. With the ERO more flash flood focused, and this event more of an areal to river flood threat, we opted to keep the risk at the Marginal level. With that said, if the system trends warmer over areas of snow pack and/or higher rainfall rates come into play...then a targeted Slight risk upgrade is possible over portions of the Northeast with future updates. ....Northwest... Introduced a Marginal risk over western OR with this update. The AR event from day 2 should still be ongoing into Sunday, with some continued flood impacts possible. IVT is forecast to be weaker by this time, but saturated conditions from the event earlier this week and some possible overlap with the day 2 rain...suggest that some flood risk may continue. It should be noted that this forecast is of lower confidence than the day 2 forecast. Model guidance has not been very consistent with the AR and QPF forecast for this period...both with respect to the magnitude and location of the event. It makes sense that the focus will be south of what happens on day 2 as the IVT plume drifts southward...and it makes sense that the rainfall magnitudes will be lower than day 2 as IVT magnitudes decrease. But model runs have been fluctuating between western WA and OR as a focus...and fluctuating between a 1-2" event and a 2-3" event. Would expect to see continued fluctuations in the guidance, but hopefully we'll begin to see a solution lock in over the next few model cycles. For now went with a Marginal risk over area most likely to be impacted...but some adjustments to this are likely with future updates. Should some of the more aggressive model members pan out then a Slight risk is a possibility given the increased sensitivity over the area...but for now think a Marginal risk best depicts the threat given the aforementioned model spread and uncertainty. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4d4PvSzaVvE61xHjH9ilYkUP52WvXMq0lZwq-kNTaPiz= iTcKFM20e8GRwGgpo8pQjN8SlO_mcLn1X2SqZ-ixVtQZJ_c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4d4PvSzaVvE61xHjH9ilYkUP52WvXMq0lZwq-kNTaPiz= iTcKFM20e8GRwGgpo8pQjN8SlO_mcLn1X2SqZ-ixsVg6YsM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4d4PvSzaVvE61xHjH9ilYkUP52WvXMq0lZwq-kNTaPiz= iTcKFM20e8GRwGgpo8pQjN8SlO_mcLn1X2SqZ-ixKJBdM4M$=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) .