Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu May 11 2023 01:00:55 FOUS30 KWBC 110100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Wed May 10 2023 Day 1 Valid 01Z Thu May 11 2023 - 12Z Thu May 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN THE ARKLATEX REGION... ....Arklatex...=20=20 The collection of a mid-level impulses moving north over east TX and the LA coast will allow the focus of moisture convergence, divergent flow aloft, strengthening low-level inflow/effective bulk shear, and sufficient instability for organized convection to shift north over the Arklatex overnight. Training and efficient bands of convection are expected to develop in the current activity over east TX near the central border with LA which is in the inflow region of the impulse just north. Recent HRRRs and the 18Z NAMnest agree on a focus for overnight heavy rain through this area along the central and northern TX/LA border with a risk for 3-6" overnight. Surrounding areas should also see some training activity, especially central and northern AR where the Slight Risk is maintained. Farther east over MS/AL, interior areas remain in the warm sector with cyclonic flow allowing some repeating activity and a localized flash flood threat with the Marginal Risk maintained. ....Central High Plains/Northern Plains... A combination of low-to mid-level frontogenesis acting in concert with sufficient instability and strong SWly flow will maintain heavy rainfall over northwest KS, down a convergent light in eastern and an MCV over northeast CO will continue to lift north over the Panhandle of Neb overnight. Farther north in WY, moisture convergence persists over the north-central portions of the state with instability lingering at least through the rest of the evening. Therefore, there remain two areas of Slight Risks - over northwest KS into southwest Neb to the CO border and the Slight Risk over north-central WY was expanded southeast to northeast CO by way of the Neb Panhandle for that expanding threat. Precipitable water values of 0.75" to 1" in the central/northern High Plains are a big deal from a heavy rainfall perspective: 2 to 3 sigma above normal over central/eastern WY and eastern CO. There is a risk for an additional 1.5 to 2" from north-central WY to northeast CO from recent HRRRs. There is a notable risk for 2 to 4" more over northwest KS both from the eastward propagating activity east of Colby and from activity currently over eastern CO spreading east over areas that had heavy rain this afternoon. There remains a localized flash flood risk farther east over the north-central Plains where there is a Marginal Risk. The stationary front lingers over eastern Neb into IA, though activity has not initiated there yet. Jackson Day 2 Valid 12Z Thu May 11 2023 - 12Z Fri May 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE NORTHERN HIGH PLAINS AND LOWER MISSISSIPPI VALLEY... ....2030Z Update... ....Lower Mississippi Valley... The same Mesoscale Convective System (MCS) impacting the region from southeast TN through eastern TX currently will continue slowly drifting eastward over the area on Thursday. Plentiful Gulf moisture bringing PWATs as high as 1.75 inches will continue streaming into the area as well. Pinning down how the convective pattern will evolve continues to be the primary struggle, but as is fairly common, the trend in the ensemble blends has been to favor areas further south with time. Thus, the inherited Slight Risk was shifted south and east, in coordination with JAN/Jackson, MS and LIX/New Orleans, LA forecast offices. While the main forcing will lift off to the north, residual fronts and boundaries from today's convection is likely to be enough forcing to keep the primary focus for convection in the new Slight Risk area. ....Northern High Plains... No major changes were made with this afternoon's update, but several smaller but notable changes were included. The Slight Risk area in MT was expanded westward in coordination with GGW/Glasgow, MT forecast office, as per recent model trends bringing unusually high PWATs into eastern MT on a feed straight out of the Gulf. In eastern MT and western ND, that moisture will collide with a slow-moving frontal boundary which will be the focus for lift of that moisture to produce heavy rainfall across this region. FFGs are low from recent snowmelt across the area, so the threat of flash flooding is considerable. There remains some uncertainty as to how effective the rainfall rates will be, and while there was discussion with GGW and BIS/Bismarck, ND forecast offices of a potential MDT upgrade for a portion of their CWAs along the MT/ND border, the decision was deferred at least another cycle to ensure the upward trends in the forecasted rainfall over the area will continue. Needless to say, this area is on the higher side of the Slight Risk category. ....Central High Plains... The Slight Risk area was largely maintained for northeast CO, western NE, and northwestern KS with this update. A strong negatively tilted trough and associated upper level energy will more northeastward towards the area on Thursday. With PWATs around 3/4 inch, which is as much as 3 standard deviations above normal to work with, the trough will have plenty of forcing to wring out much of that moisture in the form of heavy rain as instability rises to near 1,000 J/kg. High snow levels may allow for some of the precipitation to fall as rain into the higher mountains, but the Slight Risk was trimmed out of the mountains with this update to account for many of the highest peaks picking up multiple feet of snow from this system. Meanwhile, the extremely-difficult-to-flood Sand Hills region of north-central NE was carved out of the Slight Risk area with this update. ....Lower OH Valley... The inherited Slight Risk area was downgraded to a MRGL with this update in coordination with the PAH/Paducah, KY forecast office. While the same forcing causing thunderstorms further south will make its way to the Lower OH Valley, confidence has decreased on how much that forcing will translate into heavy rains, particularly those capable of producing flash flooding in this region. The significant disagreement in the CAMs along with a decreasing trend in the forecasted rainfall over this region were other factors contributing to the downgrade. Soils are around average for moisture content in this area, so they are likely to be able to handle much of the forecasted rainfall, which for most areas should remain at or under one inch. Of course, higher totals are likely in any thunderstorms, but these should be relatively isolated and therefore unlikely to produce flash flooding. ....Florida... No significant changes were made to the Marginal Risk area in this region. The front along with abundant Gulf moisture and sea breezes will all combine to produce locally heavy rainfall along the northeastern Gulf Coast. The greatest threat of flash flooding is in urbanized areas, particularly Tampa Bay and Tallahassee. Wegman ....Previous Discussion... ....Northern Plains/High Plains...=20 Moisture advection from the Gulf of Mexico increases ahead of a slowing frontal system over the central/southern High Plains and as the upper trough/low approaches from the West. Precipitable water values (PWs) of at or above 1" remain across eastern MT and northeast WY as a pair of low pressure system try to migrate from the High Plains into the central and northern Plains. A decent bubble of 1.25" PWs exists across the Dakotas within the system's comma head. In the High Plains, widespread heavy rainfall is expected in a favorable upslope pattern with available instability to the east and within a region with decent low- to mid-level frontogenesis, over wet ground conditions due to moderate/heavy rainfall the prior day. The Slight Risk has been reconfigured some in order to account for the latest model guidance QPF output. =20 ....Mississippi Valley/Lower Ohio Valley...=20 Near the ArkLaMiss, heavy rainfall, possibly in training bands, is expected early on/Thursday morning prior to the onset of daytime heating -- the 00z HREF indicates a high risk of 5"+ amounts near Winnsboro LA which appears within reason. A long fetch of ~1.5" PWs with decently strong low-level inflow/effective bulk shear and ample instability due to daytime heating should allow for convection across portions of the MS and Lower OH Valleys. There has been some convergence in the guidance concerning the heavy rain signal near the confluence of the Ohio and Mississippi Rivers, with the guidance generally showing the potential for 2-4" of rain, which could fall quickly, perhaps as much as 2.5" an hour should there be any mesocyclones or short training bands. For both regions, Slight Risk areas were introduced.=20 =20 ....Florida... Convergence near an old front combined with easterly flow across the Peninsula, plenty of instability, and precipitable water values near 1.75" should set the stage for heavy rainfall for central and northern portions of Florida's West Coast. Hourly rain totals to 2.5" and local amounts to 5" (as implied by the 00z HREF) are possibilities in this region. Added a Marginal Risk area to accommodate this potential heavy rainfall threat. Roth Day 3 Valid 12Z Fri May 12 2023 - 12Z Sat May 13 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL TEXAS... ....2030Z Update... ....Texas... A rapidly intensifying LLJ bringing incredible amounts of Gulf moisture into the Rio Grande Valley of south central TX will combine with a westward retrograding upper level low over the desert southwest and a somewhat weak upper level disturbance to focus the maximum lift along a rapidly retreating dry line on Friday. Very few changes were needed from the previous forecast, as that incredible moisture with PWATs potentially exceeding 2 inches is forced up the terrain as the southeasterly LLJ intensifies to close to 50 kt. This combination will produce a rather large and nearly stationary area of heavy rain using the up to 2,500 J/kg of MUCAPE and Corfidi Vectors that are nearly anti-parallel to the base low-level flow. This is a very favorable setup for training convection. To make matters worse, soils in the Moderate Risk area are already fully saturated, so the influx of heavy rain in this area will almost entirely runoff, with little rain soaking into the soil. The inherited Moderate Risk was largely unchanged, but expanded a couple counties north and south along the Rio Grande River. The signal for heavy rain is actually even higher in this same region for Day 4/Saturday, and the potential is there for needing a High Risk from San Antonio west with future updates as forecast confidence in a slow-moving signal for heavy rain continues. Stay tuned. ....South Dakota & Nebraska... Few changes were needed for this region, as the signal for heavy rain with potential for 2-4 inches remains in much the same region as inherited. The only change of note was removing the Slight from Nebraska's Sand Hills region as flooding is highly unlikely there given the ability for sand to absorb all of the forecast rainfall. The enveloping Marginal Risk includes areas of NE south of the Sand Hills, so the Marginal Risk includes the Sand Hills for simplicity. ....Midwest... In coordination with ILN/Wilmington, OH and RLX/Charleston, WV forecast offices, the Slight Risk area along portions of the Ohio River was downgraded to a Marginal Risk with this update. Signals in the forecast models are inconclusive, and the area is likely to be able to handle the rainfall. While it's possible localized areas may see Slight Risk levels of flash flooding, the signal was no greater along the OH River than other parts of the Midwest, so larger Marginal was decided to be more representative of the current threat for rain in this area. Wegman ....Previous Discussion... TX... An upper low is expected to move out of northern Mexico into TX to the west of the track of the system crossing the state at this time, strongly increasing low-level inflow up the Rio Grande and into portions of the Escarpment towards 60 kts at 850 hPa.=20 Precipitable water values rise to 1.75-2", while CAPE should be 3000 J/kg or so. Convection with heavy rainfall is expected Friday evening which should continue overnight. Conceptually, the initial convective area should take roughly four hours to move -- any veering in the 850 hPa inflow is modest and towards Saturday 12z. Given the available moisture, 2.5-3" an hour totals cannot be ruled out while the convection is stationary -- especially if mesocyclones form -- which could pile up totals quickly. An equally big concern would be if convection formed out ahead of the developing complex -- particularly along the Escarpment -- once a cold pool formed leading to cell mergers as the advancing organized convection moves in to any preceding thunderstorm activity. Other problems would be if a warm advection band formed to its east-southeast which should be the eventual direction of propagation parallel to the 1000-500 hPa thickness gradient or if convection starts training on its western flank -- it's unclear whether that would be in Mexico or South-Central TX closer to the Rio Grande Valley. Some of the model guidance indicate that local amounts in the 10" (00z Canadian Global, 00z NAM12, and 09/12z ECMWF) or even 20" (Canadian Regional) range are possible should the worst case scenarios play out. For now, an upgrade to a Moderate Risk is being made which appears to be a conservative move. Further elevation in the risk level is possible in later cycles should the guidance get less dispersive while continuing to show such significant heavy rainfall potential. The new Moderate Risk area was coordinated with the CRP/Corpus Christi, EWX/New Braunfels TX, and SJT/San Angelo TX forecast offices. Northern Plains... Heavy, convective rains within a comma head of a low are expected to drop heavy rainfall in and near southwest SD due to ample moisture and the available instability. The guidance shows a good signal for local 2-4" amounts, which when added to rainfall on the previous day should be becoming problematic across the area. A slight risk was added on this issuance. Ohio Valley... Moderate low-level inflow bringing precipitable water values of 1.5-1.75" up the Ohio River could lead to heavy, excessive rainfall in and around the Upper Ohio Valley. Any training convection would likely occur during the afternoon and evening when the greatest instability is available. While most of the guidance shows local amounts in the 2-3" range (outside of the dry 00z GFS), the 00z Canadian Regional was much more emphatic across much of KY/the Ohio Valley. While there's still a bit of dispersion in the guidance on location, the ingredients suggest that the 00z UKMET, 00z Canadian Regional, and 00z NAM may be on the right track for where the heavy rain axis would be favored. A small Slight Risk was introduced in this update covering where heavy rainfall is expected to fall over low to modest flash flood guidance values. Hourly rain totals to 2" and local amounts to 4" would be possible wherever any short training bands set up. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4gAKV_AeX2N7zDLMZ7Ii-9nxHeWjZDSev7kAkICNGQYY= ksQ55L1XRBlvUWo005DWoeFIdu348ulY3yePGlRq7BvjDLc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4gAKV_AeX2N7zDLMZ7Ii-9nxHeWjZDSev7kAkICNGQYY= ksQ55L1XRBlvUWo005DWoeFIdu348ulY3yePGlRq0KvH8Jk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4gAKV_AeX2N7zDLMZ7Ii-9nxHeWjZDSev7kAkICNGQYY= ksQ55L1XRBlvUWo005DWoeFIdu348ulY3yePGlRqLVRI_lc$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .