Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 09 2023 15:55:49 FOUS30 KWBC 091555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1155 AM EDT Tue May 09 2023 Day 1 Valid 16Z Tue May 09 2023 - 12Z Wed May 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN SOUTHEAST TEXAS...=20 ....16Z Update... ....Texas... An absolute mess in the CAMs guidance this morning as to the evolution of the convection across southeast TX for the rest of today through tonight. Most of the guidance has a very poor handling (if one at all) as to the current state of the convection. The poor initialization leads to very poor confidence in their forecasts as to how the MCS will evolve through the day into tonight. What broad pattern can be gleaned is that there is less confidence in flooding rains east of the Houston Metro over to the LA border, and as such the MDT was trimmed in that area to key in on a small area of much lower FFGs from last night's heavy rains in that area. Meanwhile, there is higher confidence that the convection ongoing from Houston south and west will reorganize over western portions of either the Slight or Moderate risk areas, then slowly move south and east towards the coast. Whether this line of storms forms later this afternoon or holds off until tonight is another question. Nonetheless, this round looks to be slower moving and with abundant moisture in place, will likely result in flash flooding in the affected areas. Thus, in coordination with HGX/Houston, TX, EWX/New Braunfels, TX, and CRP/Corpus Christi, TX forecast offices, expanded the inherited Moderate and Slight Risk areas westward to account for this potential. ....Kansas... The Slight Risk area across Kansas was expanded with this update to account for the width of the ongoing squall line of convection in the far northern part of the state along the NE border, and significant uncertainty as to how far north and south the line will track through the day today into tonight, along with potential leading convection which would prime the dry soils in this area for the primary and heaviest storms associated with this main line. While the central portions of the Slight Risk area have the best chances of seeing training convection as leading storms form and are overtaken by the main line, southwestern portions of the Slight have had a notable increase in potential for flash flooding, as these small MCS's tend to grow upscale towards the south with the greater moisture and instability. The northwestern-most portions of the inherited Slight Risk area were trimmed as the heaviest rain is ongoing or already done for the day, removing all but a sliver of southern NE. ....Mid-Mississippi Valley... A front evident on satellite may be the focus for slow moving storms to develop over the Memphis area and points south and east into northwestern AL later this evening. While the storms are not expected to be strong or heavy, the potential for training/slow-movement required a small expansion of the Marginal Risk area into this region. Wegman ....Previous Discussion... =20 ....Texas/Louisiana... A southerly low level flow, backing during the morning and early afternoon hours, combined with an incoming upper level low should lead to ample moisture, instability, and effective bulk shear to support another day of organized convection capable of producing heavy to potentially excessive rainfall though over a broader area, with hourly rain totals expected to peak around 2.5". Model indicate the potential for local amounts in the 3-7" range but dispersion remains, with the best overlap existing across southeast TX, especially within 00z HREF guidance. The highest probabilities of 5"+ from the 00z HREF were squarely over Houston TX, where two rounds of heavy rainfall are hinted at by the HREF -- one from the morning into early afternoon and another late in the period/early Wednesday morning as the low-level jet ramps up.=20 The MU CAPE gradient, currently edging back to the northwest after Monday evening's activity, should recede back to the Gulf Coast during/after periods of heavy rainfall as instability inland exhausts. The best onshore flow appears targeted into portions of the Middle and Upper TX coast, which fits the guidance overlap and conceptual models. Some additional eastward shift was made to the risk areas near the Gulf Coast for the Slight and Marginal Risks, more into southwest LA, and was based on the 00z HREF output.=20 Because of the heavy rainfall on Monday along with the backing 850 hPa flow during the morning and early afternoon, upgraded places in and near the heavy rain footprint in Southeast TX to a Moderate Risk per coordination with the EWX/New Braunfels TX, HGX/Houston-Galveston TX, and LCH/Lake Charles LA forecast offices. =20 =20 ....Central Plains...=20 A second focus for storms should extend from NE across KS into northwest AR in proximity to the surface boundary where any mid-level capping inversion appears weak. Morning activity is expected to bisect KS after moving out of southwest NE, with the 00z HREF hinting at an afternoon round on the western flank of any outflow boundaries thrown out by the first round in the vicinity of central KS. Another round is possible late in the period across southwest NE as the low-level jet ramps up overnight into the early morning hours, but at the moment the guidance doesn't forward propagate it into KS by 12z Wednesday. Enough moisture, low level inflow/effective bulk shear, and instability should exist to allow for convection with hourly rain totals to 2" and local amounts of 3-9" per the available global and mesoscale model guidance. Because of some dispersion on the latitude of the heavy rain axis, the HREF probabilities of 3" are under 40%, though there are non-zero probabilities of 5"+ in the mesoscale guidance that compose the HREF. Portions of southern NE have seen heavy rainfall over the past few days, but soil saturation per NASA SPORT appears minimal. While KS has been dry over the past week, the best signal for 3"+ and 5"+ is over central portions of the Sunflower State. Either way, urban areas would see the largest impact of the heavy rainfall. Because of the high potential noted, the shift of the 00z UKMET southwest to better agree with the other available guidance, and the experimental Colorado State ERO first guess based on the 00z GEFS indicating a high-end Slight Risk near the NE/KS border, added a Slight Risk for portions of southern NE and northern KS. =20 Roth Day 2 Valid 12Z Wed May 10 2023 - 12Z Thu May 11 2023 ....A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR SOUTHEAST TEXAS...=20 =20 ....Plains...=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. Where those ingredients exist the most prolonged period of time is in and near the ArkLaTex, where a Slight Risk area remains. There is a general signal in the global guidance for local 3-5" amounts somewhere across the eastern half of TX within the Slight Risk area. The better signal in general heavy rainfall was where ingredients suggest it should be across eastern OK and western AR, which remain in the Slight. The mesoscale guidance shows that portions of the Upper TX Coast should be receiving heavy rainfall at the beginning of the period in an area where a CAPE gradient should have developed due to recent heavy rainfall per conceptual models near the base of the mid level low. Based on the pattern showing low-level confluence south of the upper level low, it's possible that this would be in the form of training bands coming in off the Gulf of Mexico which can be highly efficient so long as the effective bulk shear can remain at or above 25 kts. The 00z HREF shows high probabilities of 5"+ in 12 hours early in the period. Combined with Monday's and Tuesday's expected rainfall, a Moderate Risk upgrade was made on this cycle per coordination with the EWX/New Braunfels TX, HGX/Houston-Galveston TX, and LCH/Lake Charles LA forecast offices. =20 Across the Central Plains, northeast CO, eastern WY, and southeast MT, a combination of upslope flow and some low- to mid-level frontogensis acting in concert with sufficient instability should lead to heavy rainfall. Heavy rain -- both which has occurred and is expected on Tuesday/Tuesday night -- keeps areas in and around southern NE and 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. Closer to the 850 hPa low in WY, the guidance has a strong signal for local 2-4" amounts. A new Slight Risk area was added for southeast MT, northeast WY, and western SD.=20 =20 Roth Day 3 Valid 12Z Thu May 11 2023 - 12Z Fri May 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN HIGH PLAINS... ....Northern Plains/High Plains... Moisture advection from the Gulf of Mexico increases ahead of a slowing frontal system over the central/southern High Plains and the upper trough/low approaches=20 from the West. Precipitable water values (PWs) of at or above 1" remain across eastern MT and northeast WY as a low pressure system slowly migrates through the central Plains. A decent bubble of 1.25" PWs exists across North Dakota within the system's comma head. 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.=20=20 The Slight Risk area has been maintained from continuity over parts of eastern Montana into northeastern Wyoming and western portions of the Dakotas with some reconfiguration. ....Mississippi Valley/Lower Ohio Valley... A long fetch of ~1.5" PWs with decently strong low-level inflow/effective bulk shear and ample instability should allow for another day of convection across the ArkLaTex which is expected to shift into portions of the MS and OH Valleys. There is significant dispersion in the heavy rain signal here, with the 00z UKMET over AR and MO, and the 00z GFS the most northerly in and near IL/IN where heavy rain fell recently. Not helping matters is the deep layer cyclone pulling away from CO into SD, which is leading to warming at 700 hPa for locations closer to the ArkLaTex which could lead to a stronger mid level capping inversion down to the south. PWs show a subtle decrease on Friday when compared to Thursday. Due to the dispersion in the guidance and the less favorable environment for heavy rainfall down south, left the risk level as Marginal. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6bwWeNEzqRqLIeuiCtpapWH09YutAzB8A02pY4n6J7HE= 6GnOVqvytW3WW6nv4kABj0Tt1w9yR8PvubDQAuua7nicwy8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6bwWeNEzqRqLIeuiCtpapWH09YutAzB8A02pY4n6J7HE= 6GnOVqvytW3WW6nv4kABj0Tt1w9yR8PvubDQAuuaLeWQLzc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6bwWeNEzqRqLIeuiCtpapWH09YutAzB8A02pY4n6J7HE= 6GnOVqvytW3WW6nv4kABj0Tt1w9yR8PvubDQAuuaT5zVqYM$=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) .