Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 16 2024 20:03:24 FOUS30 KWBC 162003=20 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 403 PM EDT Tue Apr 16 2024 Day 1 Valid 1830Z Tue Apr 16 2024 - 12Z Wed Apr 17 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS EASTERN PORTIONS OF THE NORTHERN PLAINS SOUTHWARD THROUGH MUCH OF THE MISSISSIPPI VALLEY... ....1815Z Special Excessive Rainfall Discussion... Slow-moving convection had evolved into a complex that was producing 1 to 2.5 inch rainfall per hour rates over portions of northeast Nebraska into far western Iowa as of early this afternoon. The rates...combined with the fact that the convection has already persisted for a couple of hours and the proximity to mid/upper level cold temperatures...has raised the potential for flash flooding in the area. Have issued a Slight Risk area to cover that potential. Refer to Mesoscale Precipitation Discussion 0159 for additional details. Bann ....16Z Update... Previous forecast was maintained with only minor modifications to the expansive MRGL risk area extending from the northern Plains into the Mississippi Valley over into the central Appalachians. Current radar/sat composite shows our broad surface reflection with SLP center located over central NE. Convective points of interest include the current convective bands located over SD/NE and MO/IA that are producing efficient rates of 1-2"/hr within the banded structures (More info in MPD #0158). This trend will continue as warm front progression lifts north and portions of the central Midwest see an increase in convective coverage after 18z with a progressive propagation to the northeast. The previous discussion outlines the reasoning perfectly as the combo of progressive storm motions and drier antecedent conditions over the impacted area will limit flooding in an areal extent and lean more locally within one or two cells that could over perform given the favorably evolving environment. Generally max QPF of 3-3.5" is the top end forecast given the latest probability fields from both NBM and HREF with majority seeing closer to 1.25-2.25" as per the latest mean(s). Over the south, increased diffluent pattern downstream of the mean trough will initiate a line of convection beginning in the Lower Mississippi Valley over AR, spreading northeast into the Ohio Valley by later in the period. This line will not have as favored an ascent pattern as areas in the Midwest, so the threat for flash flooding leans more on the lower end of MRGL comparatively, but certainly non-zero. Another smaller area over WV has a low-end probability for flash flood concerns as an area of mid-level vorticity advects northward into the region during the time of peak diurnal instability. MLCAPE around 1000-1500 J/kg are forecast in-of central and eastern WV which is enough to spur some local thunderstorm activity within the terrain. Chances again are low, but non-zero with some CAMs being more aggressive with the opportunity, so maintained continuity. Kleebauer ....Previous Discussion... Showers and thunderstorm are expected to align near and north of an east-west orientated warm frontal boundary that is stretched across the eastern portions of the Plains and the Ohio Valley. The highest rainfall rates and accumulation will likely be concentrated along the Minnesota/Iowa border where instability will be most supportive to produce 1-2"/hr rainfall over this area, which will likely exceed 1hr FFG in spots. One limiting factor for flash flooding will be the swift movement of cells, limiting the duration of heavy rates. Training/backbuilding near the warm front could help accumulations to reach or exceed 5 inches. Although there may be some 1hr and 3hr FFG exceedance, the coverage and magnitude of impacts are expected to remain below Slight risk levels. The main threat will be to urban areas, where the intense rates may lead to localized flash flooding. Outside of urban areas the dry ground and limited green vegetation may allow for some excess runoff, however these impacts are expected to remain isolated as well. A Marginal Risk area spans from the eastern Dakotas, Upper/Central Mississippi Valley and across parts of the Mid-Mississippi Valley. Campbell Day 2 Valid 12Z Wed Apr 17 2024 - 12Z Thu Apr 18 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT LAKES, OHIO VALLEY AND WESTERN PARTS OF THE NORTHEAST... ....20Z Update... Main change in the D2 MRGL risk was to trim a bit of the southern edge across the lower Ohio Valley given the latest trends in the HREF blended mean QPF footprint and associated EAS probabilities for at least 1". We've seen a dip in the probabilities from the previous forecast issuance with a 1-5% probability in-of south- central OH with nothing now showing up across northeastern KY. This is likely due to the slight shift in the progression of the low and a further north push of the warm sector into central and northern OH where the better QPF signal and convective rates are being depicted. Low FFG threshold across eastern OH into western PA are the primary factors for the MRGL risk being maintained anywhere as the QPF maximum will likely only top out between 1.25-2". EAS probabilities for 2" rainfall totals are solidly 0%, so the risk=20 is capped overall. That is where we will maintain continuity, as=20 well as the eastern extent over eastern PA into NJ where multiple=20 rounds of convection throughout the period will lead to totals=20 pushing over 1" within areas of low FFG indices, especially in the=20 3 and 6-hr FFG intervals.=20 A non-zero threat also exists over portions of the Deep South as=20 well tomorrow afternoon and evening. Some weak low-level=20 convergence was noted on CAMs with a few members going more robust=20 on the convective threat in eastern MS over into central AL during peak diurnal instability. There was not enough consistency for one to prompt an additional risk area. Also, the rates expected would=20 not threaten current FFG intervals. Unless we get some significant=20 upgrade in the potential, this will remain outside any additional=20 MRGL issuance's.=20 Kleebauer ....Previous Discussion... Showers and thunderstorms will track eastward across the Ohio=20 Valley and some of these storms will be strong and capable of=20 producing high rainfall rates. Overall, the storm motion will be=20 but should generally be progressive in nature. There is less=20 instability with eastward extent, which should result in lower=20 rainfall rates. But given the above average soil saturation and=20 streamflows over portions of Pennsylvania and New York the Marginal looks good. Campbell Day 3 Valid 12Z Thu Apr 18 2024 - 12Z Fri Apr 19 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND MISSISSIPPI VALLEY... ....20Z Update... Limited changes necessary to the previous issuance of the D3 ERO as guidance remains consistent within the overall synoptic progression of the expected SLP and cold frontal passage over the central plains to Mid-Mississippi Valley. A solid convergence signal ahead of a cold front will lead to locally heavy rainfall over portions=20 of MO down through northeast TX during the period in question.=20 There's a bit more emphasis across MO in the latest model suite=20 with some guidance indicating a bit more organization within the confines of the front bisecting an area of relatively unstable air based on the latest theta-E forecast. This should promote some local totals exceeding 1" with potential of up to 2" based on the recent deterministic. The southern end of the MRGL may have the best threat for higher impacts due to prominent MLCAPE and=20 elevated PWATs ahead of the tail end of the cold front promoting higher rate potential. This should be enough for localized flood=20 concerns during any convective initiation over areas between=20 Austin/DFW and points northeast. Will be monitoring that area closely, along with the Mid-Mississippi Valley for possible upgrades pending future CAM outputs.=20 Kleebauer ....Previous Discussion... Another low pressure system will develop across the Southern/Central Plains and tap into the return flow from the Gulf of Mexico. Convection is expected to fire up ahead of the approaching cold front with areal averages of 1 to 2+ inches across the Missouri Ozarks/Mid-Mississippi Valley and trailing southwest to central Texas. Recent rains have lowered some of the FFG across this region and may have an increased sensitivity to additional=20 rain/heavy rain. A Marginal Risk area covers part of northeast=20 Texas to southern Missouri and western Kentucky. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9y6NrWu9HNO-bxVy-G0_EqAj9z6jnZv9p6n3we1zN44M= Sd9PVODf59-eNx7cPtFZ_hosSkVL-NolO-_DJ6ffObDr6fg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9y6NrWu9HNO-bxVy-G0_EqAj9z6jnZv9p6n3we1zN44M= Sd9PVODf59-eNx7cPtFZ_hosSkVL-NolO-_DJ6ffai-9VAo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9y6NrWu9HNO-bxVy-G0_EqAj9z6jnZv9p6n3we1zN44M= Sd9PVODf59-eNx7cPtFZ_hosSkVL-NolO-_DJ6ff-a8vtX8$=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) .