Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 26 2024 00:21:53 FOUS30 KWBC 260021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 821 PM EDT Mon Mar 25 2024 Day 1 Valid 01Z Tue Mar 26 2024 - 12Z Tue Mar 26 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN AND NEAR MISSISSIPPI... Convection will be ongoing across portions of LA and MS at 01z, fostered by broad low-level confluence, precipitable water values of 1.5-1.8", ML/MU CAPE up to 1500 J/kg, and low-level=20 inflow/effective bulk shear up to 80 kts. These ingredients support mesocyclones and an overall increasing eastward progression with=20 time. Cell motions will have a fairly large component parallel to=20 the low level boundary early on, which should result in some brief=20 training of cells. Guidance and ingredients available support 1-3" of rain with local amounts of 5" in and near MS. Hourly rainfall will peak in the 2"=20 range. The degree of moisture transport and forcing, combined with=20 the brief window of opportunity for training early on as the wave=20 to the west digs supports an isolated to scattered flash flood=20 risk. Impressive IVT is forecast over MS/AL tonight, with values=20 approaching the climatological max for late March. The low level=20 jet is also impressive, with 850mb wind speeds over the 99th=20 percentile. However, the low-level confluence slowly winnows and=20 the corridor of such progresses slightly quicker to the east which should lead to a reduction of the flash flood threat with time. Given the very strong low level southerly inflow, some brief=20 backbuilding/training of cells is possible on the south and=20 southwest flank of the activity. The progressive nature of the=20 system should keep the flash flood threat from getting any higher=20 than a Slight risk, though it was noted in the 18z HREF guidance=20 that the risk for 5"+ between 00z and 12z was non-zero across=20 portions of MS. The Marginal risk still extends northward into southeast MO and southern IL, where 1-3" of rain is possible from a narrow band of thunderstorms moving through the region. Rainfall rates will=20 decrease with northeast extent over this corridor, where decreasing MU CAPE should limit the heavy rainfall potential. Roth Day 2 Valid 12Z Tue Mar 26 2024 - 12Z Wed Mar 27 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE FLORIDA PANHANDLE INTO SOUTHEAST AL AND SOUTHWEST GEORGIA... ....20Z Update... No changes to the previous forecast as model consensus is for a period of convection across the FL Panhandle and adjacent southern AL/GA later Tuesday afternoon through the early morning hrs Wednesday. Some of the hi-res deterministic is a bit more bullish across the Tallahassee area by Wednesday morning with totals pushing over 2" within the hardest hit zones. Area instability is modest, at best but the environment is very moist with 2-2.5 deviations above normal forecast in-of the outlined SLGT area. Frontal progression will be on the slower side as it trudges east along the Gulf Coast, so general storm motion is slower, thus creating the potential for some training as storms fire Tuesday evening. 12z HREF blended mean is consistent with 1.5-2" located inside the SLGT risk with the PMM significantly higher thanks to a couple CAM members going a little more aggressive with the convective development and associated totals. With a slightly better depiction compared to previous forecast with more CAMs involved, there was no reason to deviate from the previous SLGT risk in place. Kleebauer ....Previous Discussion... The risk of flash flooding does look to decrease as this event moves into Tuesday over the Southeast. The better forcing lifts off to the north and IVT and the low level jet weaken. With that said, convection will likely be ongoing at 12z Tuesday over AL with locally heavy rain, and this activity will move into GA and the southern Appalachians through the day as well. Eventually the front and moisture plume are expected to slow down and/or stall out, so while instability and forcing is weaker by this time, can not rule out some continued locally heavy rainfall. One area where a higher flash flood risk could exist remains over portions of southern AL into the FL Panhandle. Instability is higher here, and so expect intense convection to persist here Tuesday morning into the afternoon hours. Activity should be progressive off to the east, but 1-2"/hr totals are still probable. The front stalls out by later in the day, and while forcing and moisture transport decrease by this time, instability will remain to go along with the lingering low level convergence. So while most guidance is not that aggressive with convective development by later in the day into the overnight, the potential still appears to be there for additional development. Overall this Slight is lower confidence than day 1. Current QPFs from the models support more of a Marginal risk...however given the setup and ingredients, there is some potential for convection to over perform near this slowing front. Thus think maintaining the Slight risk is the best option at this time. Chenard Day 3 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... ....20Z Update... Minimal changes were made to the previous forecast as the current SLGT and MRGL risk areas remains on track. Across the Southeast and Southern Mid Atlantic, ascent will be maximized over the east thanks to the progression of a potent shortwave trough exiting out of the southern plains into the Deep South. Large scale forcing within a diffluent area of the upper trough will initiate a round of heavy rainfall across much of the Carolinas with totals of 2-3" with local to 4" increasingly likely within the D3 period. Fortunately, despite the rainfall totals in question, much of the precip will be from stratiform with some isolated, embedded low- topped convection limiting the threat of rates breaching much above 1"/hr at any point during the rainfall. With agreement from the local Carolina WFO's, decided to refrain from any upgrades in the MRGL risk with more time to assess as we march forward in time. The best rates will be closer to the Gulf Coast where a prevailing theta-E wedge will be located over the Big Bend of FL extending inland into southern GA. This is where the best prospects for significant rainfall rates in excess of 2"/hr are plausible as instability crosses within a stout 2-2.5 deviations above normal PWAT anomaly focused over the Southeast into the Mid Atlantic. Latest global deterministic has ramped up the prospects for heavy rain within that corridor of instability with totals now exceeding 3" among multiple members with an associated increase in the accompanying ensembles. GEFS probability for 2" of precip is now up to 60% across the eastern portions of NC and northern SC with 60-70% now located along the Big Bend of FL. This is sufficient at range to keep continuity with the previous forecast SLGT. Out west, only a minor adjustment to the northern extent of the MRGL risk was made to correlate with ensemble mean QPF which are below the threshold necessary for even a Marginal within the southwest edge of the Portland CWA. QPF trends still remain on track further south where the IVT plume is most likely to provide a period of enhanced rainfall into an area that has seen plenty of the past several weeks. The Marginal risk is still at the lower end of threshold thanks to the forecasted rates, but persistence works in this pattern. Kleebauer ....Previous Discussion... ....Southeast... A swath of heavy rainfall appears likely Wednesday into Wednesday night across portions of the FL Panhandle, into southeast GA and north into the central/eastern Carolinas. At the moment the models are in pretty good agreement with the axis of this swath...although given we are 3 days out the uncertainty is probably a bit greater than the 00z deterministic guidance would indicate. Either way, confidence is indeed growing for a swath of 1-3" of rainfall across this corridor. 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. At the moment it would appear like instability may be lacking over most of this corridor, which may act to temper rainfall rates...and thus even 1-3" of rainfall would probably just be worthy of a Marginal risk. The 00z GFS is outputting 4-6" of rain over portions of SC/NC...but instability is limited, and only a small portion of this rain is convective QPF out of the GFS. Thus tend to think this specific forecast is overdone. 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 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. Then keep the rest of the corridor in a Marginal risk and continue to monitor trends over the coming days and adjust as necessary. ....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 1-2" of rain, with localized 3" amounts. Considered removing the risk, but soil saturation and streamflows are running above normal over the area, so seems plausible that any embedded higher rates could case some localized flooding concerns. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_cBpCutdPfwszZdGMkb4VsRnRL4dZ04PSwnVBQBQpl3U= o7jagAqzq5pOBydkLQs2FAw9GycYoz2iAzopWSJD-2VXzI0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_cBpCutdPfwszZdGMkb4VsRnRL4dZ04PSwnVBQBQpl3U= o7jagAqzq5pOBydkLQs2FAw9GycYoz2iAzopWSJDZ0PgShY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_cBpCutdPfwszZdGMkb4VsRnRL4dZ04PSwnVBQBQpl3U= o7jagAqzq5pOBydkLQs2FAw9GycYoz2iAzopWSJDWP4oWCg$=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. 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