Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 03 2025 23:48:26 FOUS30 KWBC 032348 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 748 PM EDT Wed Sep 3 2025 Day 1 Valid 01Z Thu Sep 04 2025 - 12Z Thu Sep 04 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN ARIZONA... ....Southwest... A broad area of instability and above normal moisture has led to a localized flash flood risk across portions of the southwestern=20 U.S. HREF guidance indicates activity across southern NV and northern AZ should fade early on, with a better chance of lingering convection near the AZ border with Mexico. The existing Slight Risk overlaps some with where the heaviest rainfall occurred yesterday; left it in place as a precaution, with chances highest for heavy rain prior to local sunset. ....South FL... A wet pattern persists near and north of an old polar front in the FL Straits. Daytime convection has mostly faded. An upper jet to=20 the north along with onshore flow is expected lead to an=20 overnight/early morning uptick in convection, particularly where=20 the flow is most onshore across the Keys and southwest FL. Roth Day 2 Valid 12Z Thu Sep 04 2025 - 12Z Fri Sep 05 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST CALIFORNIA AND CENTRAL AND SOUTHERN ARIZONA... ....Southwest... 20Z Update: Guidance continues to depict a very active period beginning Thursday afternoon, carrying through the first half of the evening with convective development across southern AZ precluding to a large outflow moving westward out of the southern deserts through the south-central and western portions of AZ. The signal is clear within all CAMs leading to a strong probability assessment for 3 and 6hr FFG exceedance intervals, each carrying 50+% over a large area extending from Tuscon all the way to the Colorado River basin between the 18-06z time frame in the period. Rates of 1-2"/hr will be plausible in this setup as PWATs remain ~2 deviations above normal, a signal favoring locally heavy rainfall and heightened flash flood concerns over flashy soil/terrain features. This beginning to shift closer to a higher end SLGT risk scenario, but the mesoscale evolution of the pattern leans towards generally progressive, so this would be a detriment for an upgrade potential. For now, this is sufficient for a SLGT risk forecast and will monitor the setup closely for any changes in the spatial coverage of the risk, and the eventual magnitude of impacts that could necessitate an upgrade. Kleebauer ...Previous Discussion.. Thursday is looking like a more active day across portions of AZ into southeast CA as the area begins to feel the impacts of a stronger moisture surge ahead of Lorena. An uptick in moisture and low level flow should help drive a more organized convective threat. Rather impressive instability for this part of the country is forecast, with values upwards of 2000-3000 J/KG, and this combination of above average PWs and instability should support intense rainfall rates. Always worry about cloud cover impacting instability, which could happen in spots, but the overall model signal certainly favors areas of robust instability. In a part of the country where model QPF struggles, CAPE and PW overlap often ends up being a better flash flood potential indicator. Given the ingredients in place, we went ahead and upgraded to a Slight risk for much of central and southern AZ into southeast CA. Additional expansion into more of southeast CA are possible going forward. Chenard ....South FL... 20Z Update: Little change was necessary from the previous forecast as the repeating pattern of scattered heavy convection across South FL will continue through D2 with the best prospects for flash flooding likely tied to more urban zones. Current indications are for areas of southwest FL and the Lower Keys will have the greatest potential, albeit those areas are tougher to flood due to the sandy soil composition. The MRGL was sufficient for the forecast. Kleebauer ...Previous Discussion.. A wet pattern will persist over south FL through Friday with localized flash flooding possible each day. The overall pattern does not change too much, with a stationary front in the low levels, an upper jet to the north providing some right entrance region ascent, and broad mid level troughing. It seems likely that rounds of convection will occur each day, and with PWs around 2.3", heavy rainfall rates are likely. There is some uncertainty with the axis of heaviest rainfall each day, with some models favoring the Keys, and given the location of the stationary front this may make sense. However it still seems likely that heavier convection will impact the urban corridor of southeast FL at times as well. Uncertainty regarding both the axis and also the speed of convection, whether it tends to propagate eastward quickly or can hang up, dictates keeping the risk at Marginal for now. Although could certainly see the need for a targeted Slight risk along the southeast FL urban corridor at some point over the next 3 days given the potential for multiple rounds of heavy rainfall. Chenard Day 3 Valid 12Z Fri Sep 05 2025 - 12Z Sat Sep 06 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO.... ....Southwest... 20Z Update: The latest 12z guidance continues to lend uncertainty in the exact track of the circulation from Lorena as the storm makes approach into the CA Baja before sliding into Sonora. Some of the model guidance remains particularly aggressive with holding the center of circulation to the International border, just south of southeastern AZ leading to multiple solutions (GFS/RRFS/NAM/HAFS suite/HWRF) indicating a swath of heavy rainfall impacting at least portions of southeast AZ into southwest NM. ECMWF/CMC and mean ensemble output remains not as aggressive as the previous models referenced putting a little more emphasis on the low degrading prior to reaching northern Sonora leading to the remnant moisture field not advecting further north. Assessment of the guidance compared to reality is important in this scenario as major discrepancies early on would very well inhibit model outputs through the course of the run. As of now, the cluster with the greater outputs over AZ/NM are handling the system a bit better which trends more in favor of the wetter solutions as we move forward in time. Ensemble bias corrected QPF was also particularly wet aiding to a little better consensus on what could transpire across the high deserts near the border. Ensemble totals between 1.5-3" with locally higher are being forecast in those more substantial outputs, a range that would cause some flash flood concerns, especially when you add in the more effective warm rain processes being depicted when looking at forecast soundings over parts of the aforementioned area. PWAT anomalies closer to 3 deviations above normal would be common in the current expected evolution, plentiful for more locally significant impacts. Considering the increasing concern and marginally favorable consensus out of the 12z suite, a SLGT risk was implemented over southeast AZ into southwest NM for the threat. There is the potential for an even higher risk overall, but didn't want to make commitment until we have a stronger consensus on the potential as higher risks necessitate more agreement at these leads. Will be monitoring the situation closely as even smaller deviations in track/evolution can play a significant role on the overall forecast. The SLGT will suffice, for now given the recent trends. Elsewhere across the West the large Marginal risk area stretches northward into OR and as far east as CO. The overall synoptic pattern favors isolated to scattered convection over this entire area, and the forecast PWs and instability would support locally heavy rainfall rates. At the moment not seeing enough coverage to justify any Slight risk areas, but we will continue to monitor. Sometimes we can get a more organized flash flood risk develop on the edge of the deeper tropical moisture, where cloud cover is low enough to allow for destabilization, but moisture is still high enough for heavy rates. Kleebauer/Chenard ....South FL... 20Z Update: Very little deviation in the previous forecast as the 12z model suite remains consistent in its interpretation of convection expected across southern FL. Locally heavy precip between 2-4" are forecast in the period, however its location will certainly be important with the urban centers the most favorable for at least scattered flash flood prospects. As we move forward in time, the output via CAMs will provide better insight on the threat, so wanted to hold off on any upgrades until we have a look at the high-res suite that could resolve smaller mesoscale factors a bit better than the global deterministic and smoothed ensemble mean. MRGL risk remains. Kleebauer ...Previous Discussion.. A wet pattern will persist over south FL through Friday with localized flash flooding possible each day. The overall pattern does not change too much, with a stationary front in the low levels, an upper jet to the north providing some right entrance region ascent, and broad mid level troughing. It seems likely that rounds of convection will occur each day, and with PWs around 2.3", heavy rainfall rates are likely. There is some uncertainty with the axis of heaviest rainfall each day, with some models favoring the Keys, and given the location of the stationary front this may make sense. However it still seems likely that heavier convection will impact the urban corridor of southeast FL at times as well. Uncertainty regarding both the axis and also the speed of convection, whether it tends to propagate eastward quickly or can hang up, dictates keeping the risk at Marginal for now. Although could certainly see the need for a targeted Slight risk along the southeast FL urban corridor at some point over the next 3 days given the potential for multiple rounds of heavy rainfall. The large scale pattern becomes more meridional by Friday, with upper level flow becoming more southerly over south FL and the upper jet orientation favoring a bit more upper level divergence. Thus could potentially see a pattern that may favor more of a stationary or training convective threat along the urban corridor of southeast FL. However the location of the front by then will play a role in this, and so not confident enough to go with a Slight risk at this point. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4hSTYoxIhuEqKDkuPLXgR6khfiHVinvoj84j5Xq91C7I= AAXnU8NmZ412qghSyPFtJTWQa11vCjAyEBSG9k_vBZHlf70$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4hSTYoxIhuEqKDkuPLXgR6khfiHVinvoj84j5Xq91C7I= AAXnU8NmZ412qghSyPFtJTWQa11vCjAyEBSG9k_vlF7vs_I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4hSTYoxIhuEqKDkuPLXgR6khfiHVinvoj84j5Xq91C7I= AAXnU8NmZ412qghSyPFtJTWQa11vCjAyEBSG9k_vXxXP2EI$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .