Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 10 2023 20:29:51 FOUS30 KWBC 102029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 PM EDT Wed May 10 2023 Day 1 Valid 16Z Wed May 10 2023 - 12Z Thu May 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ALONG THE TEXAS/LOUISIANA BORDER AND THE ARKLATEX REGION... ....16Z Update... A few notable edits were made to the Moderate Risk region with today's midday update. The mesoscale convective system (MCS) currently impacting the Gulf Coast is slowly shifting north and east across far eastern TX and southwest LA this morning. For the most part, rainfall rates are at 1/2 inch per hour or less, though a few embedded, but rather quick-moving and small cells have occasionally been producing rates as high as 1.5 inches per hour. There is reasonably good agreement in the near-term CAMs guidance that the area of rain will lift north up the Moderate and Slight Risk areas through the afternoon and this evening, occasionally producing cells producing rainfall rates to 1.5 inches per hour, but most areas will remain at a half inch per hour or less. This initial round of rain will still produce several hours of light to moderate rainfall, over areas with saturated soils. The soils are particularly close to saturation on the Texas side of the border. Late tonight, as an embedded vortex moves north, a renewed round of convection is likely to develop over northeast TX and slowly move eastward with time. While this second round won't necessarily produce heavier rainfall rates than the first round, the fact that it will move over many of the same areas already hit this afternoon is likely to be what finally tips the bucket and produces more widespread flash flooding, especially if the heavier rainfall occurs over particularly sensitive areas. Current 1-hour FFGs over northern portions of the Moderate Risk area are generally hovering around 2.5 inches/hour, but after the first round of rain moves through later this afternoon, these values are expected to decrease in time for the second round. The Moderate Risk area was trimmed on the western side out of Houston and points west with this update. Rainfall rates with the current MCS are generally staying below 1/2 inch per hour. Showers and storms are likely to form late this afternoon in this area, especially if a period of sunshine can occur early this afternoon along the Gulf Coast southwest of Houston, but those storms are likely to be tied to any sea breeze front that pushes inland in the broad onshore flow, and will therefore be fairly isolated and form along a line and move parallel to each other, eliminating much chance of training. For this possibility of isolated flash flooding, the Marginal Risk was left in place, with the Slight remaining in Houston proper due to locally lower FFGs there due to the urbanization. Finally, generally light showers have developed over southern MS and eastern LA, so the Marginal Risk area was expanded eastward a bit for the possibility of nuisance urban flash flooding in Baton Rouge and New Orleans. ....Northeastern WY... Only cosmetic changes were made to the Slight Risk area with today's update. The focus for the strongest convection looks to be primarily confined to WY and far western SD, so the Slight Risk area was trimmed out of MT and off the higher peaks of north central WY where snow will be the dominant precipitation type. Low level easterly flow will direct a bit of the Gulf moisture plume advecting north up the Plains over eastern NE and SD westward into the Slight Risk area, where the combination of some forcing from a rather weak upper level wave and the anomalous moisture raising PWATs to between 3/4 and 1 inch (which is as much as 2.5 standard deviations above the climatological norm, will support strong to potentially severe storms tracking NNE across northeastern WY. There is good agreement that most of this activity will happen at or after sunset this evening. Backing flow will allow storm motions to turn more towards the NNW with time, which will increasingly promote training convection across this region. The storms will weaken as the press northward into MT, but they will also congeal into a larger area of light to moderate rain, which will prime the soils in the area for the potential flash flooding on Day 2/Thursday. Wegman ....Previous Discussion...=20 =20 ....Western Gulf Coast across AR...=20=20 The combination of a mid-level low moving north out of TX into the Midwest and an incoming closed cyclone moving through the West should set the stage for good moisture transport through the Plains, divergent flow aloft, strengthening low-level inflow/effective bulk shear, and sufficient instability for organized convection. To the south of the lifting upper level low, training and efficient bands of convection are expected from the Middle and Upper TX coast north-northeast through the Sabine River into far southwest AR. The reconfigured Moderate Risk area was coordinated with the HGX/Houston-Galveston TX, LCH/Lake Charles LA, SHV/Shreveport LA, and LZK/Little Rock AR forecast offices. The mesoscale guidance shows that portions of the Middle and Upper TX Coast should be receiving heavy rainfall at the beginning of the period in an area where a CAPE gradient should be lifting north within a warm advection pattern behind the departing upper low. The 00z HREF shows non-zero chances of 3"+ in an hour and 8"+ in 24 hours within the Moderate Risk area, which should be a morning event primarily in Southeast TX and occur during daytime heating into the overnight hours for places farther north. The Slight Risk area has been expanded across much of AR -- local Moderate Risk impacts are possible Wednesday night into early Thursday where any efficient, training bands manage to form. =20=20 ....Central High Plains/Northern Plains... A combination of upslope flow and some low- to mid-level frontogenesis acting in concert with sufficient instability should lead to heavy rainfall. Heavy rain which has occurred the past several days keeps portions of southern NE and northwest KS in the Marginal Risk. Precipitable water values near or above 1" in the High Plains are a big deal from a heavy rainfall perspective.=20 Closer to the 850 hPa low in WY/MT, the guidance has a strong signal for local 2" amounts...local amounts to 4" can't be ruled out. There has been some reduction to the size of the Slight Risk to follow the trend seen on the most recent guidance. =20 Roth=20 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7vkEs6PJvVBLdSFiSm0YdJdwC-REZzJ6xRZvOdZuI7-B= F6Glh6Ynl17O2_BkPTsptkQ1VWwnCt2BXhG_Hng-ltz4y1E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7vkEs6PJvVBLdSFiSm0YdJdwC-REZzJ6xRZvOdZuI7-B= F6Glh6Ynl17O2_BkPTsptkQ1VWwnCt2BXhG_Hng-tPdQOiM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7vkEs6PJvVBLdSFiSm0YdJdwC-REZzJ6xRZvOdZuI7-B= F6Glh6Ynl17O2_BkPTsptkQ1VWwnCt2BXhG_Hng-FbHbe94$=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) .