Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 15 2023 00:50:51 FOUS30 KWBC 150050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 850 PM EDT Sun May 14 2023 Day 1 Valid 01Z Mon May 15 2023 - 12Z Mon May 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF EAST TEXAS AND THE UPPER TEXAS COAST AND FROM THE LOWER MISSOURI VALLEY INTO THE MID MISSISSIPPI VALLEY... ....Texas into Western Louisiana... 2230 UTC Update -- Made notable modifications to the Day 1 Excessive Rainfall Outlook across this region, based on the current observational/mesoanalysis trends as well as with the latest high-res guidance (particularly with the last several rapid-refresh runs along with the 18Z HREF deterministic and probabilistic output). The upshot was removing the Moderate Risk, while shrinking the Slight to areas along and west of the SE TX-SW LA border. Elongated, S-N oriented upper level shear axis/vort lobe west of the upper level ridge is continuing to funnel a narrow ribbon of higher theta-e air across a small portion of the western Gulf into eastern TX/western LA. Mixed-layer CAPEs of 2000-2500 J/Kg along with PWs of 1.7-1.9" will continue to support rainfall rates up to 2-3" per hour...or 1-1.5" per half hour...within a relatively narrow corridor within the Slight Risk area. The good news is that the axis of current and forecast heavy rainfall is expected to remain east of where the highest rain totals were observed over the past 24+ hours, thus not expected to overlap with the areas of reduced FFG. This was the main reason for expiring the Moderate Risk. ....OK/KS into the Mid MS Valley... 2230 UTC Update -- Based also on observational trends (radar/satellite/mesoanalysis), have made some alterations to the Slight Risk across portions of the Lower MO Valley into the Mid MS Valley, while removing the northern portion of the Marginal Risk across northern IA-IL and southern MN-WI. Meanwhile, expanded the Slight Risk a little farther south and southeast, now including portions of southwest IN and western KY, along the axis of most favorable thermodynamics with mixed-layer CAPES 2000-2500 J/Kg and PWs of 1.7-1.8". Maximum hourly rainfall rates of 1.5-2.5" are anticipated underneath the strongest cells within the expanded Slight Risk area, which is supported by the 18Z HREF 40km neighborhood probabilities. Through 06Z Monday, HREF probabilities of 3-hour QPF exceeding current FFG peak between 30-60% across parts of northeast KS into northern MO, and from 40-70% across portions of northeast MO into central-southern IL. Hurley 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!_v3JlE2KSpxPwC7RrCaGv_JPyMLGVJn2dUptE10brzwX= ftB76hbWDvH7b-fXlI-pfsdH0WDb0L-kxvNGYWNJ-5N41jI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_v3JlE2KSpxPwC7RrCaGv_JPyMLGVJn2dUptE10brzwX= ftB76hbWDvH7b-fXlI-pfsdH0WDb0L-kxvNGYWNJsHALMPA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_v3JlE2KSpxPwC7RrCaGv_JPyMLGVJn2dUptE10brzwX= ftB76hbWDvH7b-fXlI-pfsdH0WDb0L-kxvNGYWNJNwC3U8I$=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) .