Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 26 2024 15:55:28 FOUS30 KWBC 261555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1155 AM EDT Tue Mar 26 2024 Day 1 Valid 16Z Tue Mar 26 2024 - 12Z Wed Mar 27 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST... 16Z Excessive Rainfall Discussion... Few changes needed to the on-going Excessive Rainfall Outlook. Satellite imagery this morning shows a plume of moisture streaming northward from the Gulf of Mexico while a cold front was advancing westward from the Lower Mississippi Valley. Think the combination of the two could focus enough forcing to get localized downpours into the early afternoon. However...widespread cloud cover should=20 limit the instability and associated rainfall rates. Still=20 expecting a downward trend in a convective trends and associated=20 risk of flash flooding later today...with the possibility that the risk convection redevelops later tonight near the front in southern Georgia and the Florida panhandle with a corresponding low-end risk of excessive rainfall. Bann 08Z Excessive Rainfall Discussion... Convection will be ongoing at 12z across portions AL, with the heaviest rainfall rates likely over southern AL into the far western FL Panhandle. Still expect the potential for 1.5"/hr rainfall into the morning hours, but convection should be steadily pushing off to the southeast by this time...and a weakening convective trend is expected by mid morning. For these reasons the flash flood risk should be on a downward trend as well, with only isolated issues possible. Further north, convection over AL should already be weaker by 12z as it outruns the instability and overall forcing/moisture transport declines. Showers will move into GA and the southern Appalachians this afternoon and evening. Limited instability and decreasing forcing all suggest only modest rainfall amounts, and indeed areal averaged rainfall is expected to stay in the 0.5" to 1" range. However with the slowing front later today, and some weak instability present...most guidance does indicate some periodic embedded heavier convective cells. Still not looking at anything extreme, but localized 2" swaths are possible. We will have to keep an eye on potential convective redevelopment later tonight over portions of the FL Panhandle into southern GA near the stalled boundary. Low level moisture transport and mid/upper forcing gradually increase, with an uptick in instability also expected. FFG is quite high across this area, and at the moment no guidance is really coming all that close to exceeding this FFG through 12z Wed. So while there is some opportunity for convection to over perform given the stationary boundary and increasing instability, tend to think any flash flood risk will remain localized in nature through 12z Wed. Chenard Day 2 Valid 12Z Wed Mar 27 2024 - 12Z Thu Mar 28 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA... ....Southeast... A swath of heavy rainfall appears likely Wednesday into Wednesday night across portions of the FL Panhandle, into southeast GA and northward into the central/eastern Carolinas. Increasing forcing from the west will move over a stalled out front...likely resulting in cyclogenesis and a swath of heavy rainfall. PWs are forecast near or above the climatological 90th percentile, so we will likely have the moisture in place for excessive rainfall...and the duration also looks to be there given the stalled nature of the front. The main uncertainty comes down to the degree of instability and rainfall rates...along with the exact axis of heaviest rainfall. Model guidance has been bouncing west and east with each run, and at the moment not quite ready to say we have any consistent trend. A good chunk of the 00z models trended east...such as the GEM reg, ECMWF, UKMET and RRFS. However the GFS, 3km NAM and FV3LAM remain west. The 00z experimental RRFS ensemble and 06z HRRR also are favoring a further east solution. Given all the available data, we are leaning more towards the eastern model camp, but this is not a lock, as additional model fluctuations are anticipated. It will really come down to where the front stalls, and thus where the low track ends up. Instability and rainfall rates are the next piece of uncertainty. The best chance of more robust and persistent instability is probably closer to the Gulf Coast. It is here where some better moisture coming in off the Gulf may sustain enough instability for more vigorous convection. Even this is uncertain to a degree, but confidence is a bit higher here for higher rainfall rates and the potential for 3"+ total rainfall. Thus for now still think the best strategy for the ERO is to start small with the Slight risk, and keep it confined to where confidence is higher on better rainfall rate potential. The Slight risk aligns well with the higher QPF signal seen in the 00z ECMWF and UKMET, as well as in the experimental 00z RRFS (which is forecasting a swath of 3-5" of rainfall). Portions of the eastern Carolinas into southeast VA may eventually need a Slight risk upgrade, but the combination of uncertainty on the exact rainfall axis along with questions regarding the degree of instability, suggest keeping the Marginal risk is the way to go for now. Plus soil saturation and streamflows are generally around or slightly below average, resulting in relatively high FFG. We will continue to reassess on future shifts. ....Northwest CA into southwest OR... We will maintain a Marginal risk across these areas, although this is probably a lower end risk. IVT is not all that strong, and the system is progressive, both of which will be limiting factors for excessive rainfall. However upper forcing is pretty good, and it does look like we may be able to generate 0.5"/hr rainfall rates with some weak instability working into the backside of the cold front. Still look on track for 1-3" of rain, and with soil saturation and streamflows running above normal over the area, it seems plausible that any embedded higher rates could cause some localized flooding concerns. Chenard Day 3 Valid 12Z Thu Mar 28 2024 - 12Z Fri Mar 29 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE COASTAL MID-ATLANTIC INTO COASTAL NEW ENGLAND... An area of low pressure moving up the coast will result in a heavy rain threat to portions of the eastern Mid-Atlantic into eastern New England Thursday into Thursday night. As described in the day 2 discussion, there remains uncertainty with the track of this low and and thus also with the axis of heaviest rainfall. The 00z non- NCEP guidance has trended east with this cycle, with the GFS and NAM remaining west. While they do not go out into day 3, the 00z experimental RRFS ensemble and 06z HRRR are more in line with the eastern model solutions at the end of their runs. Thus we are favoring something closer to the more eastern solutions at this time...although as is typically the case, something somewhere in between the two is probably most likely. Regardless, with the ERO being a probabilistic product, we did hedge a bit west of the WPC deterministic QPF for the western extent of the Marginal risk. Heavy rainfall is likely along and just inland of wherever the low does track. An argument could be made for a Slight risk over eastern NC into southeast VA given two day rainfall in the 3-5" range in the WPC forecast and in several deterministic models. However, do think that by Thursday this rain will likely be on the cool side of the developing low, and thus should be more stratiform in nature by this period. For that reason, and the described lingering uncertainty on the exact axis, think maintaining the Marginal risk is the way to go for now. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_ig3ov9ZysZeqhiFofNVfK8c8UOya87U10o67JTQcvoJ= pQOqphz2gC8GuXeSCPm0s9Zdq4-iu_epac9oeK-6W9MXMCw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_ig3ov9ZysZeqhiFofNVfK8c8UOya87U10o67JTQcvoJ= pQOqphz2gC8GuXeSCPm0s9Zdq4-iu_epac9oeK-6uIF2Vsw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_ig3ov9ZysZeqhiFofNVfK8c8UOya87U10o67JTQcvoJ= pQOqphz2gC8GuXeSCPm0s9Zdq4-iu_epac9oeK-6micduzw$=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) .