Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 08 2024 20:11:32 FOUS30 KWBC 082011 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 411 PM EDT Mon Apr 8 2024 Day 1 Valid 16Z Mon Apr 08 2024 - 12Z Tue Apr 09 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS AND ADJACENT SOUTHERN MISSISSIPPI VALLEY... ....16Z Update... Maintained continuity from previous forecast with issuance of the SLGT risk across the Arklatex with emphasis along the I-20 corridor from Dallas through the AR/LA border. Latest CAMs are in agreement with warm front positioning in-of the tri-state area of TX/AR/LA, riding an east-west alignment into the mid-south that will be the focal point of heavy rainfall this evening into the end of the period. 12z HREF blended mean remained steadfast from the previous run with a general 2-3" swath extending from eastern of the DFW metroplex through to the Mississippi River along that I-20 corridor with embedded 4+" totals across parts of east TX and northeast LA. The most aggressive signature comes from the HREF EAS indicating a large area of 20-40% exceedance probability of 3" with a bullseye of 40-45% located over northeast LA. The signal is likely due to the morning convection coupling with the later convective development that allowed for general totals to be higher in that corridor. The main deterrent for an upgrade was general capping of the hourly rates between 1-2"/hr with some local cells potentially exceeding 2"/hr based on the latest probabilities (10-25%). This area of the CONUS is typically an area that needs significant rates to induce greater flood concerns and is well reflected into the FFG indices of 2.5-3"/hr for exceedance. Majority of flooding will be from the cumulative rainfall factor over 3 and 6-hr increments, which will be enough for widespread flooding with major concerns likely confined to the urbanized areas where runoff potential is highest. Regardless, the setup is aligned for widespread heavy rainfall given a modest theta-E advection regime, backing flow ahead of a progressing upper trough out west, and a focal point from a warm front bisecting the above areas. Some discrepancy in exact placement of the boundary could waver the elevated risk area south or north, but general consensus is within the I-20 confines with a heavy rain footprint extending 75-100 miles either side. Further west into the rolling plains of west TX, increased large scale ascent ahead of the upper trough over the Great Basin will allow for convective development off the eastern Caprock, advancing east-northeast within the general steering pattern. Locally heavy rainfall within the initiation point through the western rolling plains will have the opportunity to drop a quick 1-2" of rain that could induce some localized flooding concerns within any smaller town settings and low-water crossing thresholds around the area. There was enough consensus for the threat to expand the MRGL to include the above areas. Kleebauer ....Previous Discussion... The potential for heavy to excessive rainfall looks to develop over portions of the southern U.S. as shortwave energy diving southward over the Intermountain region helps carve out a deep layer southern stream trough. Flow aloft over the Slight Risk area backs and begins to accelerate Monday evening and Monday night, with increasingly divergent flow aloft and increasing low level moisture transport helping to set the stage for convection capable of producing downpours. Toward the latter half of the day one period (Monday night), coupled upper jet dynamics will lead to more favorable deep-layer lift, with an increase in low-level theta-e/moisture transport from the western GOMEX into a quasi-stationary surface boundary (which is maintained by the persistent FGEN within the right-entrance region of the northern jet streak across the Lower MO Valley). This is when the moisture and kinematic anomalies begin to increase more appreciably per the GEFS and SREF, with 850 mb southerly flow and moisture flux anomalies nearing +3 standard deviations above normal by 12Z Tue. Meanwhile, current NAEFS forecast IVT (integrated waver vapor transport) increases above 750 kg/ms late Monday night as well. WPC has maintained the Slight Risk area that was inherited from yesterday's Day 2 ERO. Inhibiting a more enhanced risk is the relatively lack of deep-layer instability later in the period when the dynamical forcing would be increasing, as implied with the latest HREF probabilities (40-60% chance of exceeding 1"/hr rates, but only 20-30% probs of exceeding 2"/hr). The other factor is the relatively dry (at least for now) antecedent soils, with the top meter soil depth moisture percentiles generally 30% or less per the latest NASA SPoRT analysis. This will undoubtedly change going into the day 2-3 period (more below). Hurley Day 2 Valid 12Z Tue Apr 09 2024 - 12Z Wed Apr 10 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN MISSISSIPPI VALLEY... ....20Z Update... The premise for the D2 outlook remains the same with increased convective organization due to the initiation of a SLP center over the south-central plains along with increased upper forcing thanks to the approach of a stout mid and upper trough. Trough axis will pivot eastward out of the Southwestern US into west TX by Tuesday afternoon with amplified flow out ahead of the mean trough. Guidance remains persistent with an initial wave of heavy rain in the morning across northern and central TX, expanding east into the ArklaMiss before settling by late-afternoon. A secondary enhancement of the convective scheme will initiate by the early evening as the trough axis pivots to the northeast and begins to tilt negative as it enters the Red River. This will generate considerable backing of the flow in-of the Lower Mississippi Valley with a strengthening LLJ advecting deep, moisture out of the GoM through much of the Southeastern US into the Tennessee Valley. A shortwave ejection ahead of the mean trough will allow for a rapid development of convection over east TX, blossoming into an organized heavy rain signature across LA/AR/MS as we roll into the evening.=20 12z HREF probabilities for rainfall in excess of 3" is pretty elevated with a large footprint of at least 20% and a max of 50-70% located over the northeast LA/southeast AR border where all hi-res deterministic has the highest totals for the 24 hr period. A secondary max is popping south of the DFW metroplex with the morning convective pattern as some guidance is very bullish on the prospects of 3-5" locally within a band of thunderstorms forming just south of the warm front. This certainly has merit considering the 5H evolution and the alignment of the boundary over north TX. The highest probabilities will yield to the Moderate Risk in place with an adjustment in size to encompass the HREF blended mean of at least 2" with a wide swath of 3+" over northern LA into the ArklaMiss. That area will lean the higher side of the Moderate threshold and has an opportunity to upgrade to a High risk pending the convective evolution tonight into tomorrow. For now, have maintained continuity for that area, but emphasized the potential.=20 The Slight Risk was extended all the way back across the TX Panhandle into the Sacramento Mountains in NM as another round of strong thunderstorms with the capability of heavy rainfall will lead to a threat of localized flooding within the Caprock out to the terrain in southern NM. The priming from today's rainfall will also play a role in the potential, thus wanted to cover the bases given guidance increase in regional rainfall up to 1-2" in the aforementioned area.=20 Kleebauer ....Previous Discussion... Heavy to potentially excessive rainfall should be on-going at the beginning of the outlook period on Tuesday morning that persists into the daytime hours. Flow aloft should be increasingly diffluent east of an approaching deep-layer trough and closed low while moisture from the Gulf continues to be drawn northward. Meanwhile, the surface front to the north of the ArkLaTex to ArkLaMiss corridor will drift slowly southward, which given the aforementioned FGEN support from the northern stream jet streak, will maintain widespread rainfall with repetitive convection and heavier rainfall rates across many of the same locations. Helping to boost the rainfall rates will be the uptick in deep- layer instability as well, with MUCAPEs climbing between 500-1000 J/Kg across the Moderate Risk area (and closer to 1500+ J/Kg along the Gulf Coast). Low-mid level moisture and kinematic anomalies remain impressive (around +3 standard deviations) ahead of the upper trough, so given the boost in instability, along with the priming of soils with the rainfall during day 1, WPC has maintained the Moderate Risk area inherited from yesterday's Day 3 ERO. Hurley Day 3 Valid 12Z Wed Apr 10 2024 - 12Z Thu Apr 11 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE MID-SOUTH TOWARDS THE GULF COAST... ....20Z Update... The Moderate Risk from the previous forecast was maintained, but the areal extent changed quite a bit from the previous forecast. In coordination with local WFO's, given the recent progressive nature of the convective pattern in the latest 12z guidance, there was more merit to keep the Moderate over the area that will most likely see significant rainfall during a larger portion of the forecast period. Deterministic trends have pinpointed the area along I-20 in MS up through I-59 in Alabama as the primary axis for heavy precip with totals over 2" becoming more likely given the=20 latest NBM and GEFS/ECENS probabilities exceeding 40% for at least=20 2" which is pretty bullish at range. The large scale forcing across the Southeast will be plentiful ahead of the amplified longwave pattern, and with established cold pool propagation from nocturnal convection, we'll see a decent period of organized thunderstorm activity capable of dropping upwards of 2"/hr over the area along and south of the I-20/59 interchange. The question becomes how far south and east will the convection last during the period in question. As of now, there's a chance for heavy rains to expand all the way into GA with a northern extension of heavier precip into the southern Appalachians of SC/GA.=20 The heaviest precip will remain within the axis of greatest=20 instability which is trending towards southern AL/GA thanks to a=20 tongue of modest theta-E's and higher dew points given the proxy to the GoM. In coordination with local WFO's across the central Gulf=20 coast, have compacted the previous Moderate down into an area=20 encompassing a small part of the Mississippi Delta, through much of central AL, and southeast into the GA/AL border north of the FL=20 Panhandle. Pending the propagation of the convection on Wednesday,=20 an expansion is possible, but there's enough consensus in ensembles and ML prognostications to maintain the MOD over the above areas.=20 Kleebauer ....Previous Discussion... Per collaboration with the WFOs, have upgraded to a Moderate Risk across the ArkLaMiss region through much of MS-AL, southwest TN, and the far western portion of the FL Panhandle. Still quite a bit of spread among the 00Z models, however most of them continue to show multiple (2 to 3) footprints of heavy rain, to the tune of 3-6+ inches. Over northern portions of the Moderate (including the western TN Valley), the multi- day rainfall in Days 1-2 (and Slight- Moderate ERO risk areas) factored in terms of maintaining a Moderate Risk on Day 3, even though WPC leaned toward depicting the heaviest QPF farther south toward the Gulf Coast. Farther south, the FFG going into the event will be notably higher given the drier antecedent soils, however the more favorable thermodynamic profiles (especially greater deep- layer instability) will allow for more intense short- term rainfall rates, as noted per the swath of 3-6+ inches of rain per the global models. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!443pwRuXPtW38dXlbMhvxkoL4JyLOTZz3fkQcrfvQwy1= CpTkwcW-71wjbDXylb0LuppKB0lR-m6M9bSgwwfcf8yxZdc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!443pwRuXPtW38dXlbMhvxkoL4JyLOTZz3fkQcrfvQwy1= CpTkwcW-71wjbDXylb0LuppKB0lR-m6M9bSgwwfcygKuLao$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!443pwRuXPtW38dXlbMhvxkoL4JyLOTZz3fkQcrfvQwy1= CpTkwcW-71wjbDXylb0LuppKB0lR-m6M9bSgwwfcKaSob7E$=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) .