Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 15 2024 09:12:17 FOUS30 KWBC 150911 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 511 AM EDT Sun Sep 15 2024 Day 1 Valid 12Z Sun Sep 15 2024 - 12Z Mon Sep 16 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL GULF COAST INTO THE MISSISSIPPI... ....Central Gulf Coast into MS... The setup that has resulted in a convective bands and locally=20 heavy rainfall the past couple days will continue today, just=20 shifted a bit to the southwest. Lingering mid level energy and low=20 level convergence will result in convection today from coastal AL=20 northwestward across MS and into eastern AR. The persistence of the convergence will allow for some training/backbuilding potential=20 locally increasing rainfall totals. The overlap of strongest=20 convergence and higher instability looks to focus closer to the=20 Gulf Coast today, across portions of coastal MS/AL and the western=20 FL Panhandle. The flash flood risk is likely highest across these=20 areas, with 5"+ HREF neighborhood probabilities as high as 50-80%.=20 HREF guidance indicates one round of potential backbuilding cells=20 this afternoon, with another round possible late tonight into=20 early Monday. Given this signal the Slight risk was expanded into=20 this region, and do consider this area to be a higher end Slight=20 risk, with locally significant flash flooding a possibility given=20 what should be very efficient rainfall producers. The flash flood risk is a bit lower further north across MS and AR given weaker instability and what should generally be more transient convective cells. Nonetheless some training/backbuilding is still possible, and soil saturation is still above average due=20 to rainfall from Francine a few days ago. Thus still think a=20 Slight risk is warranted, with isolated to scattered flash flooding possible. .....Coastal Carolinas... An area of low pressure off the southeast coast may bring heavy rainfall into portions of coastal SC/NC between 06z-12z Monday. The GFS remains a fast outlier in bringing heavy rainfall inland from this system, and thus is not a preferred solution. While there is still some spread amongst the other model solutions, some form of consensus is developing. This consensus would suggest that some=20 bands of locally heavy rainfall may get into coastal portions of=20 the Carolinas early Monday. Still think the greater flash flood=20 risk ends up after 12z Monday, but can not rule out some heavier=20 totals right along the coast by 12z...and thus will introduce a=20 Marginal risk. ....South TX... A small Marginal risk was added to far southern TX. Will likely be an active convective day across northeast MX today, with easterly low level flow advecting in moisture off the Gulf, and westerly mid/upper flow bringing in tropical moisture off the Pacific. Low level convergence should be enhanced in this pattern, and also=20 have some mid/upper level energy to work with as well. In general=20 expect the bulk of convective activity to be over northeast MX=20 where convergence is strongest, however there is at least some=20 threat far south TX can see some activity as well. Guidance=20 indicates that any convection that organizes over south TX should=20 drop back south into MX, however there is an opportunity for some=20 slow moving cells later this morning or afternoon before this=20 propagation takes hold. ....Southwest... A Marginal risk covers portions of AZ and NM today where a=20 localized flash flood risk may exist. This risk area was reduced in size to just cover areas where convective coverage should be a bit greater and instability more pronounced. Rainfall over the=20 Marginal risk area could locally exceed 1" which could produce an=20 isolated flash flood over any more susceptible basins.=20 Chenard Day 2 Valid 12Z Mon Sep 16 2024 - 12Z Tue Sep 17 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CAROLINAS INTO FAR SOUTHEAST VIRGINIA... ....Carolinas into Virginia... An area of low pressure will likely move into portions of NC on Monday bringing a heavy rainfall threat to the area. Confidence is increasing that this system will have a notable core of heavy=20 rainfall near the center and heavy convective banding to the east=20 of the center. The threat of flash flooding across portions of=20 eastern NC appears to be increasing, with locally significant=20 impacts possible. With that said there remains some model spread=20 regarding the location of landfall and the structure/intensity of=20 the system.=20 Like was mentioned in the day 1 discussion, think the 00z GFS remains a less likely fast and west outlier with this system on=20 day 2. After discounting that solution the overall model consensus improves, although not without some notable spread. The 00z NAM=20 and Gem Reg are two of the stronger and further south solutions,=20 with the 00z CMC and UKMET further east over coastal NC. With the=20 high res models you have the 00z 3km NAM and ARW stronger/quicker=20 and further south near the SC/NC border, and the HRRR and ARW2 a=20 bit slower/weaker and north. Overall it appears that the earlier=20 the low begins to deepen the stronger, quicker and further south it moves ashore. Where as the models with a slower developing low=20 keep it a bit weaker and east over NC. At the moment do not really=20 have a strong preference, with things likely becoming clearer today as the low begins to become more well defined offshore. The above described uncertainties will keep us from upgrading to a MDT risk at this time. However do consider this a higher end=20 Slight risk across portions of southeast NC. Wile the details still need to be ironed out, what we can say is that the flash flood=20 risk from this system is increasing and locally significant flash=20 flooding is possible over portions of eastern NC (and possibly=20 northeast SC depending on the track). Just about every model=20 solution brings a low pressure ashore with locally heavy rainfall=20 near and east of the center...however enough spread still exists=20 with the track and intensity of the system to keep the risk level=20 at Slight for now. Once the low develops and confidence on the evolution of the system continues to increase, a MDT risk upgrade may be needed. ....Central Gulf Coast... Some flash flood risk will likely linger across portions of the central Gulf Coast into southeast AR on Monday. However the pattern becomes gradually less favorable with time, and thus currently expecting that only a localized flash flood risk will linger into Monday. ....Southwest to Four Corners... A strong deep layered low moving=20 into the CA will result in an uptick in large scale forcing over=20 portions of the Southwest on Monday. Broad south to southwesterly=20 flow ahead of this feature will also advect moisture into the=20 region, and should have plenty of instability and moisture in=20 place for locally heavy rainfall within convection. Given the=20 increased forcing convective coverage should be notable over the=20 region, suggesting at least an isolated flash flood threat. Cell=20 motions should be fairly quick off to the northeast limiting the=20 duration of heaviest rainfall and the overall flash flood risk.=20 However with the persistent forcing in place areas could see=20 multiple convective cells/clusters move through over the period=20 adding to rainfall totals. ....NV/ID... An axis of higher areal averaged rainfall is likely closer to the stronger forcing underneath the deep layered low over portions of=20 northern NV, southeast OR and into ID. On Monday morning there is=20 some signal for potential training rainfall bands over portions of NV, although instability looks rather weak, suggesting rainfall=20 rates may struggle to get high enough for flash flooding.=20 Nonetheless will need to be monitored and some areas of 1-2" of=20 rainfall are possible, which for some of this area could be around=20 a 5yr ARI event. Thus even without higher rates some areal flooding is a possibility. By Monday afternoon we should see some increase=20 in instability over the broader region likely allowing for some=20 heavier convective cells. However currently expecting these cells=20 to be rather quick moving. Thus overall think a Marginal risk=20 should suffice, with localized flooding concerns a possibility. ....ND/MN... A Marginal risk was added across portions of ND and MN. Model=20 guidance shows a lifting warm front, mid level vort energy moving=20 through, and divergent flow aloft between two jet streaks...likely enough forcing to get convective development Monday into Monday=20 night. Convective details differ from model to model, but the=20 overall ingredients are similar and instability should be=20 plentiful. Overall the threat is there for an axis of training=20 and/or backbuilding convection within the Marginal risk area, and=20 some flash flooding could evolve. Chenard Day 3 Valid 12Z Tue Sep 17 2024 - 12Z Wed Sep 18 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-ATLANTIC, FLORIDA PANHANDLE, IDAHO AND MONTANA... ....Mid-Atlantic... The coastal low pressure system described in the day 2 discussion=20 will move inland across the Mid-Atlantic on Tuesday. Just like on=20 the previous days the GFS is an outlier and not a preferred=20 solution. Taking a non-GFS consensus yields a broad area of heavy=20 rain potential over the Mid-Atlantic. Despite areal averaged QPF of 1-3" in the WPC QPF, and several models showing even higher=20 totals, the risk level was left at Marginal for now. The system=20 should be weakening fairly quickly as it moves inland, so there=20 remain some questions with regards to the structure of the system=20 by Tuesday. Expect the heavy rainfall near the center of the low to lose intensity as the low weakens and the area of rain moves away from=20 the better instability. However if the low moves towards the=20 central Appalachians then upslope enhancement could make up for=20 this to some extent and drive a flash flood risk. Thus can not=20 rule out a Slight risk upgrade over portions of western/northern=20 VA, but questions with the Storm track, structure and instability=20 suggest keeping things at a Marginal level is best for now. We should still be seeing some convective banding east of the=20 center across the coastal plain from eastern VA into MD and DE.=20 However, still some questions regarding the degree of instability=20 present and how transient the convective bands will end up being. Overall think a broad Slight risk might be overkill at the moment given these questions. At least some flash flood risk likely=20 exists and the Marginal risk covers that threat. As confidence=20 increases upgrades may need to be considered near the center of the low (if instability ends up persisting and/or terrain influences=20 are maximized), and another area somewhere along the coastal plain=20 if enough instability and persistent banding look more likely.=20 ....FL Panhandle... The risk of flash flooding across the Southeast continues to decrease on Tuesday as forcing weakens and dry air wraps in behind the Atlantic coastal low. However some risk of backbuilding=20 convection may persist along portions of the FL Panhandle.=20 Confidence is lower by this point, but enough of a threat to=20 continue with the Marginal risk. ....ID and MT... The strong deep layer low that was over CA/NV on Monday will shift northeastward into ID and MT on Tuesday. Overall looks to be impressive forcing over the region and expect a rather widespread=20 1-1.5" rainfall. However most of this rainfall will more than=20 likely be stratiform in nature as the low deepens and forcing=20 persists. The lack of more intense rainfall rates should limit the flash flood risk, although some areal flooding concerns are=20 possible given the widespread nature of the moderate to heavy=20 rainfall. The better instability and risk of higher rates will be=20 over eastern MT, but expect convection will tend to be quick=20 moving here. Thus this area should see lower coverage of heavy=20 rainfall, but a better chance of localized flash flooding given the higher rate potential. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_xl7HvISLHsAkmd9HDx5JuHoqH_E-tOhDUnllHu7Jd-q= fdc1CqdN5zZ5sNBe4rPdxj4d1rMaEzwAX91sT_5SvIK_LQQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_xl7HvISLHsAkmd9HDx5JuHoqH_E-tOhDUnllHu7Jd-q= fdc1CqdN5zZ5sNBe4rPdxj4d1rMaEzwAX91sT_5SPSQM914$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_xl7HvISLHsAkmd9HDx5JuHoqH_E-tOhDUnllHu7Jd-q= fdc1CqdN5zZ5sNBe4rPdxj4d1rMaEzwAX91sT_5SIHKTlYY$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .