Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jun 19 2023 19:58:16 FOUS30 KWBC 191958 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 357 PM EDT Mon Jun 19 2023 Day 1 Valid 1944Z Mon Jun 19 2023 - 12Z Tue Jun 20 2023 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ALONG THE GULF COAST FROM FAR SOUTHEASTERN MISSISSIPPI INTO FAR WESTERN FLORIDA, INCLUDING THE CITIES OF MOBILE AND PENSACOLA... 1945 UTC Update -- Based on the current observational (composite radar, satellite, etc) and mesoanalysis trends, have included a targeted Moderate Risk area long a small portion of the central Gulf Coast (on either side of Mobile Bay), while also extending the Slight Risk area farther west into eastern LA (east of Lake Pontchartrain). Low-mid level flow (bulk shear) remains quite conducive for organized, widespread convection and cell training. This along with the highly-robust degree of deep-layer instability (mixed-layer CAPEs of 2500-4500 J/Kg) will make for potentially several hours of convection with hourly rainfall rates of 2-3+ inches within the Moderate Risk area. Hurley Previous discussion below... ....Southeast... A closed mid level low over southern IL as of 06z Monday will intensify and drift eastward today into KY, before slowly dropping south into TN later tonight. This feature will be responsible for heavy rainfall and potential flash flooding across much of the Southeast portion of the country today into tonight. The area with the highest end potential appears to be across portions of the western Carolinas into adjacent areas of the southern Blue Ridge in VA. Persistent anomalous 850mb upslope south southeasterly flow into the terrain is expected...with this low level flow strong enough and oriented in a way to result in pretty weak Corfidi Vectors. These weak propagation vectors suggest some backbuilding/training potential within the persistent SSE low level flow. 00z HREF data supports 1-4" of rainfall across this region, with neighborhood probabilities of exceeding 3" 50-90%, and exceeding 5" 20-50%. Thus while most places will probably be in that 1-4" range...isolated to scattered 4-7" amounts seem possible through 12z Tuesday. HREF mean instability ranges from around 500 J/KG over far western NC to around 1500 J/KG over west central NC...enough to support some heavier convective rainfall rates. Antecedent conditions are dry over most of this area...so streamflows are running below average and FFG is high. Absent these dry conditions...the environmental setup and model output would potentially support a Moderate risk over portions of western NC into the southern Blue Ridge of VA. However given the dry conditions, it seems likely that at least initially most of the rainfall will be absorbed and not result in flash flooding. In fact, even some of the more aggressive HREF QPF solutions are struggling to exceed 1,3, or 6hr FFG. With time do suspect that the potential training/repeat convection and persistent upslope flow will be enough to overcome these dry conditions...and do expect some flash flooding potential today into tonight across this area. But at this time think the coverage may stay low enough keep the risk at a Slight level. This is a favorable setup though and would consider this a higher end Slight...so certainly something to keep an eye on. The eastern extent of the Slight risk is more of a lower end Slight as we don't have the orographics in play...however higher instability here and a favorable backbuilding setup does suggest that rainfall rates could at least locally be enough to overcome the dry antecedent conditions here as well. A second Slight risk is being carried across portions of the central Gulf Coast into southern GA. The convection currently over portions of MS and AL should drop south into this region today. Initial convection may tend to be progressive...but there's a signal for potential redevelopment and/or some eventual backbuilding of the initial activity. The general environment should remain conducive with strong deep layer westerly flow and persistent left exit region upper level support from the southern stream jet. So if early morning convection does not push outflow through the area and into the Gulf then the setup would be there for additional convection and possible training within the westerly flow. Portions of this corridor has seen above average rainfall of late, suggesting at least some increased susceptibility to additional flooding. Slow moving convection closer to the center of the mid level low is expected across portions of KY and TN today. Should end up with pretty good coverage of convection today, as indicated by HREF 1" EAS probabilities over 50%. This does raise some concern about the ability to maintain instability long enough to flash flood...but on the other hand the scattered to numerous coverage does increase the chance of local cell mergers and enhanced rainfall duration. HREF neighborhood probabilities of exceeding 3" are over 40%, so seems probable we'll have at least some localized rainfall amounts getting into that 2-3" range. Dry antecedent conditions are resulting in high FFG over the area...so the plausible looking HREF is really only showing localized instances of FFG exceedance...which aligns well with the Marginal risk probabilities.=20=20 ....UT through eastern ID, western WY, and eastern MT... The upper level trough will slowly pivot to be less positively tilted with time, meaning the southern end of the trough will gradually track east, while the northern edge remains largely stationary. Embedded shortwave disturbances will pivot northeastward ahead of the low, and act as the forcing for additional shower and occasional thunderstorm activity over the region. The axis of heaviest rain will stretch from the Great Salt Lake in northern UT through the northeast corner of MT. Once again the heaviest rainfall totals will be along southwest facing slopes along that axis, particularly through the Grand Teton and Yellowstone N.P. areas. MUCAPE values approaching 500 J/kg should allow convective elements to be embedded within any areas of showers, locally enhancing rainfall rates. Overall likely a lower end Marginal risk,but can not rule out an isolated instance or two of flash flooding, so did not want to drop the risk. Chenard Day 2 Valid 12Z Tue Jun 20 2023 - 12Z Wed Jun 21 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR MUCH OF NORTH CAROLINA, NORTHERN SOUTH CAROLINA, NORTHEAST GEORGIA, AND PORTIONS OF SOUTHWEST AND SOUTH-CENTRAL VIRGINIA... 1900 UTC Update -- Minor changes made to the Day 2 ERO based on the latest (12Z) guidance and trends. Based on the current radar loops (solid line of convection moving in), along with the guidance trends (higher Day 2 QPF per HREF mean), have pulled the eastern edge of the Slight Risk through the Triangle and into portions of the NC Coastal Plain. This is consistent with the latest (12Z 6/19) CSU first-guess fields, as both 2022 versions (FVSGEFS and UFVS-FVSGEFS) support the eastward expansion of the Slight Risk area. The 12Z guidance does show an expanded footprint of heavy rainfall, with elongated bands of convection moving ashore from the Gulf Stream on the far outer (eastern) periphery of the mid-upper low. This would be in a favored region dynamically (upper diffluence) and thermodynamically (along an axis of highest or 2.00+ inch PWs). The aforementioned CSU first-guess fields do indicate a more 'enhanced' or 'higher Slight' Risk area for Day 2 from parts of Upstate SC into west-central NC and south-central VA. This would correspond to a 25%+ neighborhood probability of exceeding FFG, i.e. an enhanced Slight Risk yet not quite Moderate (which is at least a 40% neighborhood prob). As noted in the previous discussion, the subsequent excessive rainfall risks will be predicated on the antecedent conditions, so again cannot rule out a more targeted ERO update to 'Moderate', depending on the convective evolution today-tonight. Hurley Previous discussion below... ....Central Gulf Coast-Eastern TN Valley-Southeast-Lower Mid Atlantic... The mid/upper low over the Southeast will continue to be a focus for heavy rainfall and potential flash flooding into Tuesday and Tuesday night. The more southerly position of the system compared to day 1 will result in a couple of important changes to the setup. We should see a tightening of the gradient between this low and a high pressure to the north...resulting in even stronger easterly low level flow into the terrain of western NC and southwest VA. This will enhance the upslope component to the rainfall, but should also act to increase stability...with a low level boundary and instability gradient likely pushing south into NC or SC with time. Thus for the portions of northwest NC and southwest VA that should be hit hard with rainfall Monday...it will be a battle between increasing orographic ascent and decreasing instability on Tuesday. Ground conditions will likely be more sensitive by this time, but the question of instability and rainfall rates keeps the risk at a Slight for now. To the south there will likely be a favorable setup for training/backbuilding along the aforementioned low level boundary/instability gradient, which should become pretty stationary in nature by later Tuesday. Of course there is still uncertainty to where this exactly sets up...but at the moment somewhere from central/western SC into central to west central NC seems most likely. Environmental ingredients solidly support a Slight risk across these areas (northeast GA, northern SC, central/western NC, southern VA)...with plentiful moisture/instability and a low level boundary to help focus some potential repeat convection. We very well could end up with a Moderate risk somewhere within the current Slight risk...however would like to wait and see how activity evolves Monday...as that will help determine which areas are most susceptible...while also allowing for a few more model runs to help pin down where instability will be strong enough to support higher rainfall rates. Surrounding this Slight risk is a road Marginal risk. The pattern will remain conducive for heavy rainfall...but areas outside of the Slight risk currently have a lower threat of any more organized flash flooding. Instead things look more localized and less predicable in nature. One interesting feature that has been persistent in guidance is a northwest to southeast axis of convection across portions of LA into the northern Gulf and then into the west coast of FL. Certainly an abnormal setup...but does seem favorable for intense convection along what looks like a pretty sharp boundary and instability gradient in the model fields. Most of this is currently progged over the Gulf, but portions of southeast LA should also be impacted. Activity looks fast moving given the strong flow...but storms could move parallel to the boundary...so some flash flood risk is possible in and around New Orleans. ....Northern Plains... A Marginal risk was maintained across portions of central ND/SD with this update. A narrow corridor of very strong instability is forecast along a low level convergence axis. Subtle shortwave energy and some right entrance upper jet dynamics should help aid in convective development along this boundary Tuesday afternoon. The narrow nature of the instability pool does suggest cells may not have too much longevity as instability gets eroded fairly quickly. However deep layer shear does support some convective organization...so tend to to think cells will maintain along/near the boundary just long enough to drop some local 2-3"+ amounts. This part of the country is not that susceptible to flash flooding, so think a Marginal risk should suffice, with localized issues possible. Chenard Day 3 Valid 12Z Wed Jun 21 2023 - 12Z Thu Jun 22 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND HIGH PLAINS... ....Southeast... The closed mid/upper level low persisting over the Southeast will bring another day of heavy rainfall and potential flash flooding to much of the region. Hard to get into too much detail by this time...as the area of greatest flash flood potential will be heavily dependent on which areas have the most saturated conditions by this time, and exactly where the better forcing/instability overlaps. The closed low will have lobes of embedded energy rotating around it, and we should still have a low level boundary and instability gradient in the area...but trying to predict these features at this lead time is difficult. So continue to think the best course of action is to maintain a rather broad Marginal and Slight risk across this region. The Slight risk stretches from the FL Panhandle into southern VA...and is where the better instability/moisture/forcing looks to have a chance of overlapping. There has been a northward shift in the QPF over portions of the Mid-Atlantic, but lack of instability becomes a concern the further north you go, so was a bit tighter with the northern edge of the risk areas. As we get closer in time we may be able to shrink/focus the Slight risk and/or have to add an embedded MDT risk...as soil conditions and streamflows should become elevated over at least some of this area by Wednesday given the long duration favorable heavy rainfall pattern in place. ....Plains... Stronger mid/upper level forcing moving over a stationary boundary will trigger convective development from northeast CO into ND on Wed. Over 2000 J/KG of CAPE should be present near the boundary, with enough height falls ahead of the approaching trough and pretty substantial upper level divergence helping trigger convective development along the low level convergence axis. PWs are also forecast upwards of the climatological 90th percentile...so plenty high enough to support a heavy rainfall threat. While heavy rainfall is possible anywhere along the boundary, there appears to be a pretty good signal for excessive rainfall over portions of northeast CO into western NE and southwest ND. To start, mean winds will be parallel to the front supporting some training...and with time southeasterly 850mb flow increases into the boundary and persists. This will support some slow eastward propagation of convection as it grows upscale in nature. The persistence of the 850mb moisture transport into the stationary front should be supportive of a backbuilding and/or training area of convection, which may be enough to result in some area of flash flooding...and supports a Slight risk. The Marginal risk was extended south into portions of north TX as well. A trough will extend southward into this region...and while weaker forcing should keep convective coverage lower than further north...slow moving cells near the boundary may still lead to a localized flash flood risk. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!54mQEGwmVgE-NuV-pTyOxT8NfdtMekRGqeJUS-em-8Gi= pjeRzcdtspN-OIbmrVC-gvvo-MrEE3vWaGctfK3NIobMmpk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!54mQEGwmVgE-NuV-pTyOxT8NfdtMekRGqeJUS-em-8Gi= pjeRzcdtspN-OIbmrVC-gvvo-MrEE3vWaGctfK3NAyvuhk4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!54mQEGwmVgE-NuV-pTyOxT8NfdtMekRGqeJUS-em-8Gi= pjeRzcdtspN-OIbmrVC-gvvo-MrEE3vWaGctfK3NZiTjgzI$=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. 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