Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 25 2024 07:59:57 FOUS30 KWBC 250759 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 359 AM EDT Mon Mar 25 2024 Day 1 Valid 12Z Mon Mar 25 2024 - 12Z Tue Mar 26 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST... The inherited Slight risk still looks in pretty good shape. Convection will be ongoing at 12z across portions of northeast TX up towards the OK/AR border. As we go through the morning hours we should see an uptick in convective intensity as this line moves east into AR and southern MO. Increasing forcing from the west is=20 expected to tighten and strengthen low level convergence helping=20 intensify convection. Activity is expected to have an eastward=20 movement, however cell motions will have a fairly large component=20 parallel to the low level boundary for a period of time this=20 morning and afternoon, which should result in some brief training=20 of cells.=20 Guidance supports 1-3" of rain along an axis from western to=20 central AR into southern MO, with both the HREF and experimental=20 REFS ensembles showing a 20-60% chance of localized amounts=20 exceeding 3". Hourly rainfall will likely exceed 1", but 2"/hr=20 probabilities are low...with most high res guidance peaking hourly=20 rates around 1.5". Limited instability and a limited duration of=20 heavy rainfall are both minimizing factors for flash flooding.=20 However do think the degree of moisture transport and forcing,=20 combined with the brief window of opportunity for training as the=20 wave to the west digs, all support an isolated to scattered flash=20 flood risk here. There may be a corridor with a relative min in QPF over the eastern half of AR, before activity ramps back up again over MS during the overnight hours. Impressive IVT forecast over MS/AL tonight, with values approaching the climatological max for late March. The low level jet is also impressive, with 850mb wind speeds over the 99th percentile. These factors, combined with the strong mid/upper=20 forcing, should be enough to overcome the marginal instability in=20 place and result in strong organized convection capable of heavy=20 rainfall rates.=20 High res models easily support 1"/hr rainfall with this activity,=20 with localized swaths exceeding 2"/hr appearing likely. Thus rates=20 will be there for flash flooding...with the main question being=20 whether the duration is there as this convection will likely be=20 moving off to the east at a decent clip. However, given the very=20 strong low level southerly inflow, would expect to see some brief=20 backbuilding/training of cells...especially on the south and=20 southwest flank of the activity. Given the impressive moisture and=20 forcing in play, suspect that rainfall magnitudes from this event=20 will verify on the higher end of the model envelope over far=20 northeast LA into central MS and western AL. Of the two high res=20 ensembles, the experimental REFS is wetter than the HREF, and thus=20 am leaning in that direction, which would support a swath of 3" of=20 rain (REFS neighborhood probabilities of exceeding 3" are 40-60%).=20 This should be enough to result in isolated to scattered instances=20 of flash flooding. In the end the progressive nature of the system=20 should keep this from getting any higher than a Slight risk event. The Marginal risk still extends northward into northeast MO, IL=20 and southern WI...where 1-3" of rain is expected. Rainfall rates=20 will decrease with northeast extent over this corridor, which is=20 why the risk level is Slight in southern MO, but just Marginal=20 further north and east. Chenard 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... The risk of flash flooding does look to decrease as this event moves into Tuesday over the Southeast. The better forcing lifts=20 off to the north and IVT and the low level jet weaken. With that=20 said, convection will likely be ongoing at 12z Tuesday over AL=20 with locally heavy rain, and this activity will move into GA and=20 the southern Appalachians through the day as well. Eventually the=20 front and moisture plume are expected to slow down and/or stall=20 out, so while instability and forcing is weaker by this time, can=20 not rule out some continued locally heavy rainfall. One area where a higher flash flood risk could exist remains over=20 portions of southern AL into the FL Panhandle. Instability is=20 higher here, and so expect intense convection to persist here=20 Tuesday morning into the afternoon hours. Activity should be=20 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=20 moisture transport decrease by this time, instability will remain=20 to go along with the lingering low level convergence. So while most guidance is not that aggressive with convective development by=20 later in the day into the overnight, the potential still appears to be there for additional development. Overall this Slight is lower=20 confidence than day 1. Current QPFs from the models support more of a Marginal risk...however given the setup and ingredients, there=20 is some potential for convection to over perform near this slowing=20 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... ....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=20 than the 00z deterministic guidance would indicate. Either way,=20 confidence is indeed growing for a swath of 1-3" of rainfall=20 across this corridor.=20 Increasing forcing from the west will move over a stalled out=20 front...likely resulting in cyclogenesis and a swath of heavy=20 rainfall. PWs are forecast near or above the climatological 90th=20 percentile, so we will likely have the moisture in place for=20 excessive rainfall...and the duration also looks to be there given=20 the stalled nature of the front. The main uncertainty comes down to the degree of instability and rainfall rates. At the moment it=20 would appear like instability may be lacking over most of this=20 corridor, which may act to temper rainfall rates...and thus even=20 1-3" of rainfall would probably just be worthy of a Marginal risk.=20 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=20 convective QPF out of the GFS. Thus tend to think this specific=20 forecast is overdone.=20 The best chance of more robust and persistent instability is probably closer to the Gulf Coast. It is here where some better=20 moisture coming in off the Gulf may sustain enough instability for more vigorous convection. Even this is uncertain to a degree, but=20 confidence is a bit higher here for higher rainfall rates and the=20 potential for 3"+ total rainfall. Thus for now think the best=20 strategy for the ERO is to start small with the Slight risk, and=20 keep it confined to where confidence is higher on better rainfall=20 rate potential. Then keep the rest of the corridor in a Marginal=20 risk and continue to monitor trends over the coming days and=20 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=20 with some weak instability working into the backside of the cold=20 front. Still look on track for 1-2" of rain, with localized 3"=20 amounts. Considered removing the risk, but soil saturation and=20 streamflows are running above normal over the area, so seems=20 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!8v-VcMo7DxjIKS2Eg1t0g-EL_oa61SmC2RtOlPCe2N4K= Wpg6Y-nUis4kK9AnzediiNhR1DqtylmvMh8aYDbeSq4I2ho$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8v-VcMo7DxjIKS2Eg1t0g-EL_oa61SmC2RtOlPCe2N4K= Wpg6Y-nUis4kK9AnzediiNhR1DqtylmvMh8aYDbe8kN3Voo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8v-VcMo7DxjIKS2Eg1t0g-EL_oa61SmC2RtOlPCe2N4K= Wpg6Y-nUis4kK9AnzediiNhR1DqtylmvMh8aYDbeRfmLoUU$=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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