Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 07 2024 00:50:47 FOUS30 KWBC 070050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 750 PM EST Wed Mar 06 2024 Day 1 Valid 01Z Thu Mar 07 2024 - 12Z Thu Mar 07 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHEAST... ....Northeast... Low pressure organizing over eastern North Carolina this evening will deepen as it tracks north/northeast toward southern New England overnight, interacting with a stationary boundary currently offshore New Jersey to Long Island. A strengthening low level jet characterized by 850 mb winds exceeding 50 kts will bring a surge of higher moisture toward southern New York and southern New England with high probabilities of PWs exceeding 1.25" across much of southern New England. Meanwhile, an mid-level shortwave evident in latest water vapor imagery over southwest Virginia, will quickly lift northeast as well, and provide the large scale forcing for ascent. The latest guidance remains locked on the area for greatest rainfall totals through 12Z to be over southern New England including much of CT, RI, western to east-central MA and far southern VT/NH where totals are expected to be 2-3". Hourly rain totals are likely to top out in the 0.5-0.75" range as there is a lack of instability that will keep rain rates in check (18Z HREF probs for 0.5" in an hour are generally at or below 50 percent). However, the longer duration of moderate to locally heavy rainfall will still produce widespread totals of 2-3" (18Z HREF probs show 60-80 percent chance of exceeding totals of 2") and will likely result in instances of flooding, particularly given the very wet ground conditions in place. The Slight Risk was trimmed on the western/back edge as main precipitation is expected to be over New England tonight, but until the shortwave energy and front clear through, widely scattered / isolated showers remain possible through the Mid-Atlantic where the Marginal Risk remains in place. ....Southern California... Convective showers ahead of a strong cold front and low pressure moving onshore southern California will continue to produce isolated rain rates up to 0.5-.75"/hr through the late evening hours where some local enhancement due to the terrain (particularly over the Transverse Range) may result in some locally higher rain totals (1"+). Rain rates are expected to wane after about 04Z once any remaining instability diminishes and the frontal passage pushes through. Until then, some isolated flash flooding will remain possible. Taylor Day 2 Valid 12Z Thu Mar 07 2024 - 12Z Fri Mar 08 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS THROUGH THE MID-MISSISSIPPI VALLEY... ....2030Z Update... The previously issued broad Marginal Risk remains on track for the period, with a messy convective evolution. Did give consideration to embedding a couple of Slight Risk areas within it. First, hi-res and global models still paint an axis of heavier QPF near the Kansas/Missouri border, and HREF probabilities for multiple inches of rain are lighting up. However, soil moisture is rather dry, and the elevated nature of the convection could play a role in limiting rain rates. HREF probabilities for actually exceeding FFG are quite low. Though PW will likely be above the 90th percentile for this time of year, this only equates to around an inch for PW, which may cause rain rates to suffer. Additionally, there is some chance that these northern areas of the system could underperform with the QPF because of convection to the south "stealing" the better moisture and instability, and some AI models are indicating this. This does point to another area of concern across north-central Texas ahead of the dryline. Hi-res models have a max in QPF near the Dallas-Fort Worth Metroplex with possibly a couple of rounds of convection. If these amounts do arise in the urban area, this could cause Slight Risk-level impacts there, but perhaps not if the heaviest amounts are offset from the Metroplex. With some spread in the guidance on the placement, held off on any upgrade in the southern areas of the Marginal as well. Tate ....Previous Discussion... ....Southern and Central Plains through the Mid-Mississippi Valley... Very few changes were needed once again to the broad Marginal Risk area across the center of the country. The guidance remains remarkably consistent on the convective evolution across the area. With the benefit of CAMs guidance, the overall structure will be for greater instability and storms on the southern side of the precipitation shield associated with the low, while most areas get a long-duration steady rain with embedded convection. Otherwise the pattern remains largely unchanged. There remains some potential for a Slight Risk being needed, but with remarkable consistency and a rather underwhelming convective signature across the area, it appears the most likely flash flooding will occur with highly localized areas of very persistent convection, and are highly unlikely to impact larger areas. Thus, it appears the potential for a future Slight upgrade has decreased. Certainly should any persistent convection occur over an urbanized area (KC the most likely city to be impacted), then impacts will exceed Marginal criteria. A developing surface low over Oklahoma Thursday will draw increasing Gulf moisture northward into the Marginal Risk area as a 160 kt southwesterly jet moves overhead. Further, clashing air masses are expected over northern portions of the Marginal Risk area as a polar air mass pushes south across the Dakotas, intersecting the warm, moist air mass across the southern Plains. The result of all of these various ingredients coming together will be widespread thunderstorm development over the Marginal Risk area from northeast Texas north across all of Arkansas and Missouri through Thursday night. The storms will have plenty of instability and moisture to be capable of heavy rainfall rates locally. Generally dry soil conditions will diminish any flash flooding threat broadly, but portions of northern Arkansas and southeast Missouri saw locally heavy rains overnight last night, which could help support flash flooding in these isolated areas by Thursday. Guidance has sped up just a bit from previous forecasts, so the Marginal has been shifted east a few counties to include more of the Mississippi River, and less of the central High Plains. With several urban areas in the Marginal Risk, the local flash flooding threat will be higher in these areas should storms become persistent. Should high-resolution guidance come into agreement on storm structure and location with future forecasts, a local Slight may be needed somewhere in the current Marginal Risk area. Without the benefit of that guidance at that time, confidence is low on where that may be, and as such the Marginal Risk remains for now. ....Coastal New England... There is no change to the forecast reasoning this morning. The very last of the rain will end after 12Z Thursday for Cape Cod and the eastern tip of Maine, but the rain is not expected to amount to much after 12Z and will not cause additional flooding. Thus, the area remains with no flash flooding risk with this update. Wegman Day 3 Valid 12Z Fri Mar 08 2024 - 12Z Sat Mar 09 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY INTO THE SOUTHEAST... ....2030Z Update... The forecast still looks on track for a notable heavy rain event in the southeastern U.S. Friday as near max PW and moisture flux/IVT values are forecast. The main change to this forecast issuance was to expand the Marginal and Slight Risks on their western side in the Lower Mississippi Valley/southern Plains as well as the Slight's northern side based on the new WPC QPF and model trends a bit slower and farther north with the convection. This was an expansion rather than a shift, to maintain a nod toward the ECMWF and persistence with its central Gulf Coast heavy rainfall. Exact positioning of the heaviest amounts remains in question and will depend on how all the synoptic and mesoscale surface and upper level features evolve. The waffling models and the uncertainty in the convective evolution the day before precludes any update to a Moderate Risk at this point, but this remains a higher-end Slight. Machine learning first guess EROs based on the GEFS also support this enhanced Slight. But perhaps with decreasing lead time and getting into the hi-res model timeframe, confidence in the heaviest rainfall axis and its impacts may increase. Tate ....Previous Discussion... ....Southeast... A potent positively tilted longwave trough will eject out of the Rockies and across the Plains into the Mississippi Valley by Saturday morning. A vigorous 150 kt southwesterly jet will lead ahead of the trough as a shortwave at its base drags the entire trough eastward. A low will develop near the Arklatex Friday afternoon, then track northeastward to the Tennessee Valley by Saturday morning. The low will support the development of a strong low level jet emanating from the Gulf and into the Southeast Friday night. The LLJ will intensify both in intensity and depth of moisture as it tracks eastward across the Southeast. The mid-level low is expected to remain south of the surface low, which will support the development of a mid-level front somewhere over central Alabama and eventually extending into Georgia. This front will become the primary forcing mechanism which will force the warm, moisture-laden air mass associated with the LLJ upward, becoming the boundary along which numerous storms are expected to develop and then train towards the northeast, with continuous backbuilding expected. The upper southwesterly jet streak and mid-level low will gradually shift the axis of this front from east-west to a more southwest to northeast orientation. Thus, the streak of heaviest rainfall is likely to focus in a southwest to northeast oriented band through the Slight Risk area from southwest Mississippi to northwest Georgia. The LLJ is expected to be strongest and most persistent across Alabama, but the boundary will be the focus for training thunderstorms across all three states. While these kinds of events aren't all that uncommon, unfortunately the mesoscale nature of the boundary makes it notoriously difficult for the guidance to resolve. There's very good agreement that given this Day 3 forecast that there will be an area of very heavy rainfall that persists for an extended period of time, there is enough uncertainty as to whether the heaviest and most persistent rains impact an urban area, which would greatly enhance the flash flooding risk and resultant impacts. While even rural areas would be greatly impacted by as much as 6 inches of rain (locally higher) in 12 hours in any area, the high FFGs and relatively dry conditions across the Slight Risk area should require those heavy rains to occur in order for impactful flash flooding to result. Thus, the corridor from east-central Mississippi, central Alabama, and northwest Georgia is considered under a higher-end Slight, and it's probable a Moderate Risk may be needed in this area with future updates with CAMs guidance and better agreement where the heaviest rains will be. Coordination with BMX/Birmingham, AL and FFC/Peachtree City, GA forecast offices resulted in agreement that the area remains in a Slight, but all acknowledge the ingredients are coming together for a potential Moderate Risk with future updates. Areas that pick up 6 inches or more of rain in 12 hours (or less) could see numerous instances of flash flooding with considerable flash flooding likely. The area will continue to be closely monitored. ....Mid-Mississippi and Lower Ohio Valleys... On the cold frontal side of the low, interaction with the stronger forcing associated with the baroclinic interaction will result in a local maximum of heavier rains from south-central Missouri through east-central Illinois. Instability will be greatly lacking in this area, so any isolated flash flooding will occur as a result of a long-duration heavy rain before much colder air moves in and ends any flash flooding threat. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_glGT1pW538fp5qsaFOv9dv39nZRL8N1yiK-iQfqSp0b= OhthZ8o2-a2Vdaeu0VHyF1Mz1Di4s3JT9VuMxo5GNkBXHeA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_glGT1pW538fp5qsaFOv9dv39nZRL8N1yiK-iQfqSp0b= OhthZ8o2-a2Vdaeu0VHyF1Mz1Di4s3JT9VuMxo5Gq79p6wo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_glGT1pW538fp5qsaFOv9dv39nZRL8N1yiK-iQfqSp0b= OhthZ8o2-a2Vdaeu0VHyF1Mz1Di4s3JT9VuMxo5GhBwrFFo$=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) .