Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 14 2023 18:51:11 FOUS30 KWBC 141851 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 250 PM EDT Sun May 14 2023 Day 1 Valid 16Z Sun May 14 2023 - 12Z Mon May 15 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST TEXAS... ....16Z Update... ....Texas... A few small changes worth noting: The Moderate Risk area was nudged eastward a row of counties or 2 with this update. 12Z CAMs guidance has come into much better agreement that there will be additional convective redevelopment within the Moderate Risk area early this afternoon, which will persist for much of the rest of the day. These thunderstorms will still have 1,000-1,500 J/kg of CAPE to work with, PWATs to 1.75 inches, and a stalled out frontal boundary guiding that convection north. There is some disagreement on where it redevelops, though regardless it's likely the Houston metro will be impacted. While the main trajectory of the storms will be northward, there will be a slow eastawrd progress to the line as well. By the time the line reaches east of Galveston Bay and the Slight Risk area, it should both be weakening and becoming more progressive, reducing the flash flood threat. The surrounding Slight Risk area was expanded northward with this midday update following both radar trends and CAMs guidance. The convection is likely to make it to the Dallas-Fort Worth Metroplex, where it may cause additional flash flooding problems due to the lower FFGs there. For the area north of the Slight Risk, the storms should be either weak enough or more widely scattered as to only cause an isolated flash flooding concern. With good agreement that all significant convective redevelopment will occur over eastern Texas, the western portions of all of the ERO risk areas were trimmed eastward with this update. ....KS through IL... No changes were made to the Slight Risk area for this region. The main event has yet to begin as a cold front approaches from the north, and guidance is still highlighting the Slight Risk area for scattered showers and thunderstorms that may be capable of producing flash flooding rains. Wegman ....Previous Discussion.. ....Texas... The Slight risk was trimmed back even more across TX with this issuance, however a relatively narrow Slight risk remains across portions of southeast TX. Observational trends at 08z show a well defined vort max offshore the TX coast, with this feature expected to move inland today and likely help focus a flash flood risk. There remains some question as to the strength of this feature as it moves northward across east TX, and thus also the degree of convective organization. Recent observational trends seem to suggest this feature is pretty well defined, and recent HRRR runs have generally been trending upwards with QPF. The environment is supportive of efficient rainfall, with high PWS, sufficient instability and wind profiles supportive of some backbuilding of cells on their southern flank. So these observational and HRRR trends are a bit concerning, and after discussion internally and with HGX, we made a late call to upgrade a portion of this area to a Moderate risk. Overall there is pretty good agreement on the axis amongst HREF members, recent HRRR runs and the Gem Reg, so not much uncertainty there. The uncertainly deals more with the magnitude of the event. Whether cells stay disorganized and generally transient in nature, or if they grow deeper, become more organized and then show some backbuilding characteristics. We are leaning towards the latter scenario, hence the MDT risk upgrade, and some locally significant flash flooding could evolve from a=20 setup like this. ....OK/KS into the Mid MS Valley... A Slight risk was introduced across portions of northeast KS into central/northern MO and southwest IL. An interesting setup here, with what is pretty much a backdoor cold front dropping south southwest into the region through the day. Meanwhile weak vort centers lifting northward in the deep layer southerly flow should interact with this front and help organize convective development. One piece of energy is the remnant MCV currently over the TX Panhandle (as of 08z) and the other is the strung out pieces of vorticity streaming north out of TX and OK. Convection should break out this afternoon within the Slight risk area, with some merging of cells possible as south to north moving cells move into the frontal axis. Most of the KS and MO portion of the Slight risk has below average soil saturation and streamflows, so FFG is high. Thus flash flooding here may tend to be more isolated in nature for the afternoon/evening hours. However there is a decent signal for potential continued training convection overnight over portions of northeast KS into adjacent areas of MO. In fact the 00z HREF shows 5"+ neighborhood probabilities of over 30%, and 3 hour FFG exceedance probabilities get over 30% as well. This type of nocturnal convection that is being depicted by several of the models tends to be very efficient warm rain driven. Confidence is only average, but the ingredients do appear to be there for a more focused flash flood threat by tonight. Over the eastern edge of the Slight risk FFG is lower, and 3 hr FFG exceedance probabilities in the HREF are actually over 50% from far eastern MO into southern IL. So scattered FFG exceedance seems pretty likely here, which should translate into at least some flood impacts within any more susceptible basins/urban areas. The Slight risk area aligned well with the overlap in convergence and the highest PW anomalies (approaching mid May max values). It is also aligned with the highest HREF probabilities and a Slight risk area depicted by our GEFS based CSU machine learning ERO. So all in all seems to be pretty good agreement on this area experiencing some flash flood risk today/tonight. This Slight risk is surrounded by a broader Marginal risk. Ingredients do support a localized heavy rainfall threat across much of OK/KS into portions of the MS valley not in the Slight risk. The Slight risk was placed where the best organized threat appears to be, but the risk of localized flash flooding exists with the Marginal risk as well...just expected to be more isolated in nature. Chenard Day 2 Valid 12Z Mon May 15 2023 - 12Z Tue May 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN TEXAS AND THE MO/OK OZARKS REGION... ....19Z Update... ....Texas... Few changes needed for the Slight Risk across south Texas. It's notable that this is probably the weakest signal for rainfall all week, so this would certainly qualify as a low-end Slight, which could very conceivably be downgraded with future updates. Of course, the highly favorable antecedent conditions are a factor that can't be ignored. As is typical with these summer-like weather patterns, it will all depend on where the convection develops as relates to where the soils are most favorable. The inherited Slight Risk area was expanded as there's a nominally better signal for heavy rain across central TX. ....Ozarks Region... Few changes needed here as well. The Slight Risk area was simply adjusted and reoriented onto more of a southwest to northeast axis, following much of the length of I-44 from Tulsa east. The meteorology otherwise remains largely the same, and the previous discussion provided below remains valid. ....FL Panhandle Region... Much of the guidance suggests enhanced amounts of rainfall will be seen across the FL Panhandle into far southern GA Monday. While plenty of Gulf moisture and instability will be present, the area remains in a Marginal Risk (largely for flood sensitive areas such as urban Tallahassee) as FFGs in this area are very high, so flash flooding should be isolated at best. Wegman ....Previous Discussion... ....Texas... The inherited Slight risk over TX was shifted southeast with this update to follow model trends. This portion of the forecast remains pretty low confidence at this point. Moisture and instability is expected to be in place,...just a matter of where any boundaries and/or vort centers will be to help drive convective development and any organization. South TX appears to be the most likely location for this...both with potential convection moving in from Mexico, and with any convective vort center moving inland out of the Gulf of Mexico. These features are all pretty subtle in nature and will be hard to pin down at this lead time. In general the overall forcing looks lower compared to Sunday, so some chance that convection ends up pretty disorganized and transient...in which case the risk may be more of a Marginal level. However following ensemble QPF probabilities supports south TX as still having some risk for flash flooding. Whether it is a lingering boundary/vort near the coast, or activity moving in from MX or developing in the Hill country...hard to say. But all of this is a possibility, and so will continue to maintain a Slight risk overlapping the area of greatest threat at this time. ....Mid/Lower MS valley... A Slight risk was introduced across southern MO, northern AR and immediate adjacent areas. This is a pretty big change from continuity...but there has been a generally unanimous shift in how the guidance is handling both the front dropping in from the north and the subtle shortwaves moving east out of the Plains. The expectation is that the front that will drive some flash flood risk Sunday over central/northern MO and vicinity will drop south into southern MO and northern AR by Monday. Meanwhile some of the shortwave energy that has been streaming northward over the Plains should eject eastward and interact with this front. This interaction should help drive a more organized convective threat. The most noteworthy feature to watch is actually the vort center that is off the TX coast now (at 08z Sunday) and will then move northward across east TX today. It is this feature that is forecast to be the main potential piece of energy that interacts with the front. Those models that are stronger with this feature (00z 3km NAM and Gem Reg) are the heaviest with QPF over this region, and the weaker models generally have less QPF. Hard to say how it will actually evolve at this point, but may tend to lean towards a slight stronger solution given observational trends Sunday morning, but admittedly this is low confidence. Either way the combination of the front and plentiful moisture/instability will drive convection and a possible flood risk. But if the stronger wave scenario does evolve then convection would likely be even more organized in nature. Overall think there is enough going for it to warrant the introduction of a Slight risk with this update. There remains some latitudinal spread with this QPF axis...with the 00z EC and UKMET north, and the 00z 3km NAM, GEFS and Gem Reg south. For now tend to favor the further south solutions...and so the Slight risk is more aligned with this scenario. But this is only average confidence and will have to continue to monitor trends. Chenard Day 3 Valid 12Z Tue May 16 2023 - 12Z Wed May 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO AND TENNESSEE VALLEYS TO THE CENTRAL AND SOUTHERN APPALACHIANS... ....19Z Update... ....OH & TN Valleys and the Central and Southern Appalachians... Unlike on Day 2/Monday, significant changes were needed from the inherited forecast for Day 3/Tuesday. In much of the guidance, most dramatically the last couple runs of the EC, the front drifting southward from the Great Lakes has slowed down significantly, allowing the rainfall footprint in the guidance to shift north quite a bit. With plenty of instability (over 2,000 J/kg, and PWATs to 1.5 inches to the south of the front, there will be plenty of moisture, instability and forcing for heavy rain. As such, introduced a Slight Risk in coordination with PAH/Paducah, KY, LMK/Louisville, KY, and JKL/Jackson, KY forecast offices. ....Mogollon Rim of Arizona... A Marginal Risk area was introduced for the Mogollon Rim region of Arizona. For a couple days now the models have been hinting at abnormally high amounts of atmospheric moisture, as high as 4 sigma above normal in this region, though there was no forcing to help get storms to form. Finally now on Tuesday looks like some weak forcing will allow showers and thunderstorms to develop, which could produce heavy rain and isolated flash flooding, especially on any old burn scars, slot canyons, and dry washes. ....Intermountain West... A Marginal Risk area was also introduced from the Okanogan Highlands of northeast WA east to the western MT Hi-Line. A strong shortwave trough will skim well northeast of the area, but the combination of plenty of antecedent moisture, topography helping with forcing, and 500-1,000 J/kg of CAPE should be enough for some isolated flash flooding, especially along the mountains. The area around Great Falls, MT is currently in the bullseye for the most rainfall, but that is subject to change. Wegman ....Previous Discussion... For similar reasons as described in the day 2 discussion...some pretty large changes were needed to the day 3 ERO as well. We will carry a broad Marginal risk from portions of the southern Mid Atlantic, the TN Valley, the Southeast and along the Gulf Coast. Models are now in pretty good agreement on how things should evolve Tuesday, with a stronger cold frontal push helping drive the convective threat. In addition to frontal convergence we will have some upper jet support to work with by this time, with the right entrance region moving across the region, and broad mid level troughing. This enhanced lift should result in robust convective development within the Marginal risk area. Moisture will remain well above average, and we should have plenty of instability to work with as well. The front looks pretty progressive in nature and most cells should be quick moving given the increased deep layer flow with the digging troughing. This should result in generally less of a heavy rainfall duration, and is keeping the threat of flash flooding at a Marginal level. Something to keep an eye on is potential shortwave/MCV activity moving in from the MS Valley...generally the same energy that is mentioned in the day 1 and 2 discussions. If this energy can remain well defined and move into this environment to go along with the broader increase in synoptic support already mentioned...then there is potential for a more organized flash flood threat. This evolution remains unclear, not just in whether this energy is still well defined, but also where it would track. So will need to monitor trends over the coming days, but for now think a Marginal risk handles the threat. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4HAGjDQmbYaNOJZTEP6DacR6-tPW6y81KUwb2BMzsvqH= AG5GTMdowiGQ1XxitinSWhNxLcz6AuZuARgwwyRyqINLEhY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4HAGjDQmbYaNOJZTEP6DacR6-tPW6y81KUwb2BMzsvqH= AG5GTMdowiGQ1XxitinSWhNxLcz6AuZuARgwwyRyw0832yI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4HAGjDQmbYaNOJZTEP6DacR6-tPW6y81KUwb2BMzsvqH= AG5GTMdowiGQ1XxitinSWhNxLcz6AuZuARgwwyRyympXJvs$=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) .