Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 22 2023 01:08:37 FOUS30 KWBC 220108 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 907 PM EDT Fri Jul 21 2023 Day 1 Valid 01Z Sat Jul 22 2023 - 12Z Sat Jul 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW MEXICO, SOUTHEAST/MID-SOUTH, AND NORTHEAST/MID-ATLANTIC... ....Northeast/New England... The combined effect of eastward translation by the area of convection in the Northeast U.S. and the loss of daytime heating allowed for a shrinking of the Marginal and Slight Risks in New England. Dynamics remain strong enough to keep showers going a couple more hours but the deepest instability has been shunted out of the area with a corresponding decrease in the rainfall rates.=20 ....Southeast... Initial focus will be with on-going convection across parts of the Southeast U.S. as an MCV continues to propagate south and eastward into portions of Georgia. Within the Slight risk area...greatest potential for excessive rainfall looks to be across the north where flash flood guidance was the lowest...and where heaviest rain has already fallen today. Felt some eastward expansion of the Slight Risk from the current position was necessary but did not extended too far into the higher flash flood guidance.=20 Models are not quite as bullish with the potential for a west to east training from Arkansas eastward into Mississippi and Alabama due to the development of convection which grows upscale into another MCS. Despite somewhat reduced QPF which would normally preclude an outlook area due to high FFG...this may be round two for some areas so covered the possibility with a Marginal Risk. ....New Mexico... On-going convection over northern New Mexico will continue to be supported by a 50 kt upper level jet max aloft and sufficient moisture in the low levels to continue producing localized maximum rainfall rates around an inch into the late evening...with flooding concern coming from the potential for upwind development with a lower...but non-zero risk...as activity begins pushing off to the southeast towards the TX border. Bann Day 2 Valid 12Z Sat Jul 22 2023 - 12Z Sun Jul 23 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST, CENTRAL PLAINS, SOUTHEAST, AND SOUTH FLORIDA... ....21Z Outlook Update... The ongoing forecast (with discussion below) is generally on track. Several areas of thunderstorm development are expected across the Gulf Coast, Southeast, southern Rockies, and central Plains especially from Saturday afternoon onward. Main changes were to expand Marginal flash flood potential 1) into central South Carolina, where newer guidance depicts 1+ inch rainfall totals with likely deep convection in that area, 2) across western Kansas where thunderstorm development is expected across areas of wet soils from prior rainfall, and 3) across central/southern Arizona where easterly mid-level flow will allow for a few areas of thunderstorms to propagate westward off higher terrain and produce spots of heavier rainfall on rocky/susceptible terrain. A Marginal area was considered across Maine, although current guidance is generally uncertain and totals in most models are relatively light. Still, ground conditions are susceptible for flash flooding given recent rainfall, and any trend toward heavier or more widespread rainfall will likely necessitate an upgrade in later outlooks. Cook ....Previous Discussion... ....Southeast... A continued amplification of the eastern CONUS longwave trough will drive subtle height falls southward, pushing a cold front towards the Gulf Coast. This front will slow or stall in a generally west to east fashion Saturday as it becomes entrenched within more zonal flow at the base of this broad cyclonic flow. Convergence along this boundary aligned with right entrance region upper jet dynamics will be enough to support and sustain convection Saturday into Saturday night over the Southeast. Instability along and south of the front will be substantial, generally over 2000 J/KG, and PWs should generally be near or over 2"...thus hourly rainfall over 2" is certainly supported. There remain some questions regarding convective mode and evolution through the period. Organized convection may be ongoing at 12z Saturday, with additional development through the day and overnight probable as well. The flattening of the trough atop the front will result in nearly unidirectional low and mid-level flow, becoming more parallel to the front, which could result in more significant training of convection. On the other hand the gradual southward progression of the front and convective propagation could push activity more steadily off to the south, which would limit the degree of training. Overall do think the setup is favorable enough that we will see some areas of flash flooding evolve, but confidence on exactly when and where remains limited. FFG is very high over this region, and thus do not really think a large Slight risk would verify. Instead will probably have a smaller area where things end up coming together, and so will hold off on any upgrade at this time and continue to monitor. As we get more into the high res guidance window we should be able to better pin down potential risk areas. ....Southwest... The inherited Marginal risk was adjusted only cosmetically to account for new guidance, but remains generally unchanged for portions of AZ and NM. The mid-level ridge will gradually expand and intensify Saturday, but lobes of vorticity rotating around the periphery will continue to produce waves of enhanced ascent. This lift will occur within still favorable thermodynamics as PWs are slightly above normal and MUCAPE reaches 1000+ J/kg. This will result in scattered, primarily aftn/eve, convection with rainfall rates possibly exceeding 1"/hr. Although organization and intensity may be somewhat muted compared to Friday, storm motions will remain quite weak and variable, so any heavy rain producing cell could yield more than 1" of rainfall, which could result in isolated instances of flash flooding, especially if these cells move across more vulnerable areas. ....South Florida... Anther day of afternoon thunderstorms is expected Saturday as an overlap of impressive instability of 2000-3000 J/kg and PWs around 2" support diurnal convection. Forcing for ascent will increase once again as the longwave trough across the eastern CONUS amplifies further to produce at least modest height falls into Florida, while the associated upper jet streak shifts off the Southeast coast to leave modest upper diffluence over the peninsula. The overlap of this ascent and favorable thermodynamics will produce scattered to widespread thunderstorms with rainfall rates of at least 2"/hr at times. Although at the moment model guidance is generally less aggressive with convective organization compared to Friday, slightly stronger westerly low level flow may favor localized stationary cells along the urban east coast of the state. Thus see no reason to deviate from the inherited Marginal risk. ....Central Plains... A Marginal risk was introduced across portions of KS/NE and western MO with this update. Subtle shortwave energy riding over top the impressive western CONUS ridge will interact with an area lower level convergence to trigger convective development Saturday afternoon/evening over NE/KS. Organized convection will tend to drop southward, and PWs are only around climatological average values. So certainly not a high end or widespread flood threat. However we'll have plenty of instability to work with, and the persistent convergence may allow for some initial slow cell movement. This pattern has generally been pretty productive in supporting some intense convective clusters, so do tend believe something closer to the wetter 00z GFS solution. Chenard Day 3 Valid 12Z Sun Jul 23 2023 - 12Z Mon Jul 24 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST AND SOUTHEAST... ....21Z Outlook Update... The overall forecast philosophy is on track, with only minor changes made to portions of the Southeast to account for increased thunderstorm potential across portions of central South Carolina Sunday afternoon. A Marginal area was considered for eastern Missouri, although uncertainty regarding the development of a small convective cluster in that area precludes an areal delineation at this time. See the previous discussion below for more details. Cook ....Previous Discussion... ....Southeast... Generally a continuation of the pattern described in the day 2 discussion as we head into Sunday. Will still have a front in place with high PWs and instability along and south of it, will still have right entrance upper jet dynamics, and unidirectional westerly flow could favor some training potential. The front should be further south by this time, focusing the greater risk closer to the Gulf Coast. There is some question regarding the exact frontal position and instability/PW gradient by this time. Some chance the heaviest activity ends up just offshore, but also a nearly equal chance it remains right along the coast. The setup certainly continues to bear watching, as there is Slight risk potential. But given the high FFG and above mentioned uncertainties, we will stay with the Marginal risk for now. Antecedent rainfall on day 2 may also end up playing a role in exactly where the greater flood risk ends up on day 3. ....Southwest... Not much change here either compared to day 2. The better moisture does migrate westward by this time, so the risk should be lower over NM. But marginally above average PWs over AZ will persist, with another day of isolated to scattered diurnally driven convection. Any storms could drop a quick inch of rain, which could drive a localized flash flood risk within any more susceptible basins. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5PEvyKRz8VVlRkFL-VEKIwISuhcpYocmMeKImaFxXv6b= th3xXxvTYdmpS6IXxFtLU0TtAwlgux2nXr0Z59gIi5uVaQk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5PEvyKRz8VVlRkFL-VEKIwISuhcpYocmMeKImaFxXv6b= th3xXxvTYdmpS6IXxFtLU0TtAwlgux2nXr0Z59gIuKl5pjQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5PEvyKRz8VVlRkFL-VEKIwISuhcpYocmMeKImaFxXv6b= th3xXxvTYdmpS6IXxFtLU0TtAwlgux2nXr0Z59gIlgb8lqs$=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) .