Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 14 2024 22:58:40 FOUS30 KWBC 142258 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 658 PM EDT Sun Apr 14 2024 Day 1 Valid 01Z Mon Apr 15 2024 - 12Z Mon Apr 15 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL NEAR THE OHIO- WEST VIRGINIA-PENNSYLVANIA BORDER JUNCTION... A line of convection across PA has been generally progressive and has shown signs of organization due to the degree of effective bulk shear present and upstream instability across OH. It's the upstream instability that's the possible issue as there has been recent signs of some backbuilding showers. While precipitable water values are fairly low -- 1-1.25" at best -- where backbuilding and resultant cell training can manage to occur, 1.5" an hour totals would be possible which would be problematic in rugged terrain and urban areas. Have opted to maintain a portion of the Marginal Risk near the tristate border of WV-PA-OH due to possibility of backbuilding. From an excessive rainfall/flash flood threat,=20 expect the risk to be on the lower end of the marginal risk scale=20 -- i.e. closer to 5%. Due to the modest precipitable water values present, expect CIN to develop after sunset and cause the=20 activity to wane in intensity and coverage, with the expectation=20 that it won't last too far into the very early morning hours on=20 Monday, perhaps 0700 UTC. Roth Day 2 Valid 12Z Mon Apr 15 2024 - 12Z Tue Apr 16 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE PLAINS... ....2000 UTC Update... Minor changes were made to the Day 2 ERO, mainly to nudge the southern flank of the Marginal Risk area a bit farther east across the eastern Plains and Lower MO Valley based on the latest guidance trends. Deepening low slowly pivoting into the Upper Midwest will begin to draw moisture from the western GOMEX later Monday afternoon and overnight, with 850 mb southerly flow and moisture flux anomalies reaching 4 standard deviations above normal per the 12Z GEFS. Pre-frontal quasi-linear convection across the eastern Plains will likely be fairly transient per the latest simulated reflectivity and 1-3 hour QPFs per the guidance. However, with mixed-layer CAPEs of at least 1000-2000 J/Kg over the southern portion of the Marginal Risk area, there will be a higher probability of more intense 1-3hr rainfall rates compared to areas farther north. Higher FFGs (lower soil moisture percentiles lower) over the southern portion of the outlook area will maintain a Marginal or localized flash flood risk. Meanwhile, lower FFGs/wetter antecedent soils over the northern portion of the outlook area, along with a better chance for more prolonged activity along/north of the surface warm front, will be offset by decided lower deep-layer instability (even elevated). So even here, again the flash flood risk remains marginal. ....Previous Discussion... The latest model guidance is in good agreement on the eastward push of the strong closed low that is expected to move inland from central California into the Great Basin day 1, across the Central Rockies into the Central High Plains day 2. A broad Marginal Risk area maintained through portions of the Plains in a region of strengthening low level southerly flow and increasing PW values. PW values expected to increase to 2 to 3 standard deviations above the mean along and ahead of the strengthening north to south oriented frontal boundary forecast to push eastward into the Plains on Monday. This and and a broad region of instability along and ahead of this front will support increasing convection, especially late Monday afternoon into the early hours of Tuesday. There continues to be a large spread with qpf details among the models for the upcoming day 2 period. We kept a fairly broad Marginal Risk area to cover this spread. Two areas of concern continue in this broad Marginal Risk area. 1: For a convective line forming late Monday afternoon into the early hours of Tuesday along and ahead of the front over the Southern to Central High Plains. This line will likely be fairly progressive to the east, limiting flash flood potential. The other area of concern would be with a slow moving comma head/deformation precip area farther to the north to the north of the closed low track. The 850-700 mb moisture flux to the north of this closed low becomes very anomalous Monday afternoon into Monday night/early Tuesday with values 2 to 5 standard deviations above the mean in the strengthening southeasterly flow level flow. There is general consensus for a comma head/deformation max from far southeast MT/far northeast WY into SD. With precip totals generally below average over the past few weeks across large portions of the Plains, and current model qpf spread, the risk level was kept at marginal. Hurley/Oravec Day 3 Valid 12Z Tue Apr 16 2024 - 12Z Wed Apr 17 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS EASTERN PORTIONS OF THE NORTHERN PLAINS SOUTHWARD THROUGH MUCH OF THE MISSISSIPPI VALLEY... ....2000 UTC Update... Very few changes made to the Day 3 Marginal Risk area based on the latest guidance trends. Similar to the day 2 period, we continue to depict a broad marginal risk area ahead of the strong closed low forecast to push from the Central High Plains into the Upper Mississippi Valley during day 3. An axis of anomalous PW values, 1 to 2.5+ standard deviations above the mean, will persist day 3 along ahead of the well defined front forecast to push east through the Central and Southern Plains and Lower Missouri Valley. There continues to be potential for organized convection in this anomalous PW axis ahead of the front. However, this convection will likely be fairly progressive, again limiting the heavy precip potential. This will keep the threat level at marginal for the day 3 period along and ahead of this front. Farther to the north...model consensus is for a greater chance of organized precip along and to the north of the west to east oriented frontal boundary stretching from the Northern Plains eastward into the Upper Mississippi Valley. 850-700 mb moisture flux anomalies rise to 4 to 5 standard deviations above the mean into this boundary where strengthening southerly to southeasterly flow level flow will impact the front. An axis of heavy rains likely to the north of the front, with model consensus for areal average 1-2" amounts. This region has been relatively dry over the past few weeks, resulting in dry soils and below average stream flows. At the moment, we have kept the threat level also at marginal across this area. Hurley/Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7w7UAx4EnkvyFrZctplJgNrJaU2XgykUEXdFwGuV4z3S= 8hpTkeC8bPm8BZMxp1iM2dQSkbreK71oRlTW4DrM_K2fJKg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7w7UAx4EnkvyFrZctplJgNrJaU2XgykUEXdFwGuV4z3S= 8hpTkeC8bPm8BZMxp1iM2dQSkbreK71oRlTW4DrMlnBCctM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7w7UAx4EnkvyFrZctplJgNrJaU2XgykUEXdFwGuV4z3S= 8hpTkeC8bPm8BZMxp1iM2dQSkbreK71oRlTW4DrMjBKP8lM$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .