Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jul 21 2023 16:00:30 FOUS30 KWBC 211600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Fri Jul 21 2023 Day 1 Valid 16Z Fri Jul 21 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... 1600 UTC Update -- Minor changes were made to the Day 1 ERO based on the latest observational/mesoanalysis trends and incoming 12Z CAM guidance (including the 12Z HREF exceedance probabilities). We did pull the back edge of the Slight Risk area farther east across eastern NY-NJ. Meanwhile, lingering MCS/MCV across AR and southern MO will continue to get rejuvenation from the more unstable airmass downstream across the Slight Risk area over the Lower TN Valley (mixed layer CAPEs currently 1500-2500 J/Kg). Hurley Previous discussion below.. ....Northeast and Mid-Atlantic... Convection is ongoing as of 08z across portions of eastern PA/NY and should continue to push eastward early this morning. It should gradually weaken with eastward extent, although the southern portion of the line is maintaining longer than most guidance indicated. This does raise some question with regards to how quickly instability will recover this morning behind this line from DE/NJ north towards NYC. Not expecting much of a flash flood risk with this initial line, with the threat primarily expected with the redevelopment later this morning into the afternoon/evening hours. A slowing surface front and persistent upper level divergence will support the potential of multiple convective rounds today across portions of the Northeast. Mean flow is out of the west or west southwest at about 20-30 kts...which would support rather swift eastward movement of cells. However with the front slowing, lower level convergence will be rather persistent, which will support a few rounds of storms, and may also support some brief anchoring of cells along the convergent axis. Both of these would locally extend rainfall duration and increase the flash flood risk. PWs are only modestly above average, so not expecting the most efficient rainfall producers. But we are in mid summer, and so PWs are still high enough for heavy rainfall rates within any convection...and the potential brief training should be enough to at least partially overcome the modest PW values. HREF data supports decent coverage of 1" in an hour rainfall, with more isolated areas around 2". Storm total rain should approach 3" in spots, and can not rule out an isolated 5" amount. Do expect that we will see some flash flooding today across portions of NJ northward into eastern NY, VT and western MA/CT. If antecedent conditions were neutral this would be a solid Slight risk, but the fact that soil and streamflow conditions are above average does begin to push this event towards the higher end of the Slight risk probability range. Gave some consideration to a MDT risk upgrade, but the coverage and magnitude of impacts will probably stay just below MDT risk levels. Will consider this a higher end Slight risk though, and can not rule out localized higher end impacts if the wrong basin or urban area gets impacted by a quick 2-3" of rain. ....Mid-South/Southeast... An MCS will likely be ongoing across portions of OK at 12z this morning. The complex should remain rather progressive as it moves across OK and AR, however heavy short duration rainfall rates are likely, with a quick 1-3" of rain expected. The forward speed of the complex will probably keep the flash flood risk isolated in nature for the most part. While some weakening is possible by later this morning, do expect the complex to maintain to some extent as it moves into southern TN and northern MS/AL. As daytime heating increases, we should see increasing instability out ahead of the feature, along with enhanced moisture convergence along its track, especially where interaction with the southward dropping cold front occurs. Thus additional development along and ahead of the complex appears likely by this afternoon. This should allow for some cell merging and/or training along the track of the complex, and supports an isolated to scattered flash flood risk extending east southeast across southern TN, northern MS/AL and eventually northwest GA. Additional development is also likely Friday night as convergence along the southward dropping front is progged to increase. Some west to east training is initially possible with this development across portions of AR/MS. Activity may then grow upscale into another MCS that would track southeast across portions of MS/AL. While this would probably be progressive...this may be round two for some areas after what transpires this morning and afternoon. Add all this up, and despite the high FFG, think a broad Slight risk is warranted from OK into western GA...accounting for convection this morning/afternoon and for what develops later tonight into early Saturday. ....New Mexico... Weak shortwave impulses embedded within an amplifying ridge over the Desert Southwest will produce ascent within an environment of increasing heat and instability. HREF guidance suggests MUCAPE will rise to around 2000 J/kg during peak heating...with strengthening low level upslope flow and convergence along/behind a southward dropping front helping initiate convective development. The region will also be clipped by the right entrance region of an upper jet to the northeast, enhancing divergence aloft and supporting a maintenance of deep convection. HREF data supports a high likelihood of 1" an hour rainfall, with isolated/localized 2" within an hour possible over northeast NM. Convection will initially be slow moving near the terrain, but should eventually build upscale enough to propagate southeast along the instability axis into the western TX Panhandle. Expect that some flash flooding will occur over northeast NM where convection initiates and is initially slow moving, and then a lower but non-zero risk will exist as activity begins pushing off to the southeast towards the TX border. ....Southeast Florida... Not much change to the inherited Marginal risk across southeast FL. Another day of scattered afternoon air-mass type thunderstorms is likely across the FL peninsula, but some slightly greater organization and coverage is possible compared to the past few days. PWs around 2" will combine with MUCAPE reaching 3000 J/kg to provide extreme thermodynamics to support rainfall rates of 2-3"/hr within convection. Although most of the development will occur along the typical sea breeze/gulf breeze with additional storms occurring along outflows, a weak boundary dropping south across the peninsula may help slightly increase convective organization. The 00z HREF guidance still supports localized 3"+ rainfall amounts, which if occur over a sensitive urban area, would cause some flooding concerns. With generally weak low level flow, the latest HREF does focus the highest rainfall over inland areas, with a bit less of a threat along the urban coastline. So the risk of flash flooding is probably a bit lower, but still can not rule out some heavy amounts along the coast and will thus maintain the Marginal. Chenard 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... ....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... ....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!6DEC-fDSgvexFio2VDQBBkAgy1U-zRqat27nHS1bmYl8= 67YmLUyL5AsMgkY1-Dl9Gj-VZVrpJMExp4M1ulm3r3wba_s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6DEC-fDSgvexFio2VDQBBkAgy1U-zRqat27nHS1bmYl8= 67YmLUyL5AsMgkY1-Dl9Gj-VZVrpJMExp4M1ulm3sknflMM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6DEC-fDSgvexFio2VDQBBkAgy1U-zRqat27nHS1bmYl8= 67YmLUyL5AsMgkY1-Dl9Gj-VZVrpJMExp4M1ulm3eb267Lg$=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) .