Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Nov 24 2022 00:50:12 FOUS30 KWBC 240050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 749 PM EST Wed Nov 23 2022 Day 1 Valid 01Z Thu Nov 24 2022 - 12Z Thu Nov 24 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Northeast Texas... The guidance indicates that ahead of a digging trough across the Rockies that moisture surges northward, with precipitable water values surging towards 1.25-1.5" within a region of 850 hPa wind convergence. Low-level inflow/effective bulk shear appear sufficient for cell organization, should enough instability become available. NAM guidance from 18z does show a small pool of 1000-1500 J/kg of MU CAPE to develop between the Heart of TX and TX Big Country/Northwest TX, so organized convection is possible over a small area. The 18z HREF signal is quite small for 2-3" of rain overnight into Thursday morning; an isolated flash flood risk cannot be ruled out based on the ingredients available. Making the flash flood task more difficult will be recent dryness, as rainfall over the past week in northeast TX has been ~10% of normal. While a risk of flash flooding cannot be ruled out in urban areas of northeast/North TX, the possible area with a Marginal Risk appears too small for a probabilistic depiction at this time. Roth Day 2 Valid 12Z Thu Nov 24 2022 - 12Z Fri Nov 25 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY... ....2030Z Update... Few changes needed overall to the previous Slight Risk area. The axis of heaviest precipitation continues to slowly drift southward. Thus the changes that were made included expanding the area just a bit towards the south and west down the TX Coast, and more significantly...trimming the northern extent of the Slight Risk area out of AR (mostly) and northern MS. Instability (or lack thereof) was the primary driving factor, as there's good agreement that the instability needed to generate the rainfall rates needed to potentially induce flooding will be confined closer to the coast. Future updates may need to continue trimming the northern extent of the Slight, at least closer to the I-20 corridor. Wegman ....Previous Discussion... A widespread multi day heavy precipitation event developing late day 1 and continuing into the day 2 and 3 periods ahead of the developing closed low sinking south through the Southern Rockies into the Southern High Plains at the beginning of the day 2 time period. There are some timing differences after this with the southeastward push of the closed low day 2 into the Southern High Plains with the GFS on the more progressive side compared to the EC, NAM and UKMET. Despite the differences, models agree that low level flow off the western Gulf will continue to strengthen from the south and then south southwest on day 2, spreading anomalous PW values...2 to 2.5+ standard deviations above the mean across the risk areas. This will support the first round of heavy rains spreading east northeastward Thursday into Thursday night from East Texas into the Lower MS Valley. Overall, there is good agreement on this but with both timing differences with the downstream push of the convection and differences with the latitude of the heaviest totals. With respect to the latitude of the heaviest amounts, consensus is to favor the more southern max qpf axes solutions which would fit better with the farther south axis of instability and position of the west to east oriented frontal boundary expected to strengthen along the Gulf coast from the Upper TX coast, eastward along the LA coast. This would be closer to the farther south qpf axes of the FV3LAM, EC and UKMET.=20 Timing of the downstream push of convection is less certain.=20=20 Overall, however, there will be potential for training of cells in a general west southwesterly to east northeasterly direction across the risk area with totals in excess of 5"+ in areas of training. The previous slight risk area was trimmed on the north end to reflect favoring the farther south qpf solutions. The slight risk was expanded farther to the southeast on the south end to cover southeast LA and far southern MS as the EC, EC mean and NAM all suggest a potential axis across these area. Oravec Day 3 Valid 12Z Fri Nov 25 2022 - 12Z Sat Nov 26 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA... ....2030Z Update... Unlike Day 2/Thanksgiving, there has been a dramatic shift in the total precipitation on Friday. A corridor of heavy rain is expected to develop especially early in the day from coastal southeast Texas northwest into central TX and even portions of the Permian Basin before the precipitation changes over to snow in far west Texas into NM. There is significant model disagreement on where this axis of heavy rain will set up. However, there has been a very notable southwestward/slower shift in the location of the heaviest rain in nearly all of the model guidance. Normally these large shifts in the models are limited to just one or two, but when almost all of them latch on to this same trend, it's worth noting and adjusting the forecast accordingly. Thus, the previous Slight Risk has been shifted well to the west of previous. Therefore, much of MS and LA are likely to be dry or only have light rain on Friday, while TX gets nearly all of the heaviest rainfall. The Slight Risk was redrawn to focus mainly on where the greatest instability and moisture will be, so it's over much of southeast TX, including Houston, San Antonio, Corpus Christi, and Austin. Areas in the marginal risk over central TX will be getting the "leftovers" of the convection, which will include steady rain, but is generally not expected to fall heavily enough to result in flooding except in isolated instances. With the continued southwestward trend in the models, additional shifts in the Slight Risk are probable with future updates. Wegman=20 Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!92HzfTK0Anpsf_glGErfJ9D27-Pk81CJ4MAvmjVboJBl= el6jO3gS_Vp4gD5FxPTaNnA3_05MGGrToDso9nqvAds_maY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!92HzfTK0Anpsf_glGErfJ9D27-Pk81CJ4MAvmjVboJBl= el6jO3gS_Vp4gD5FxPTaNnA3_05MGGrToDso9nqvfVNWzOQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!92HzfTK0Anpsf_glGErfJ9D27-Pk81CJ4MAvmjVboJBl= el6jO3gS_Vp4gD5FxPTaNnA3_05MGGrToDso9nqvbS7gD-w$=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) .