906 FOUS30 KWBC 111557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Sat May 11 2024 Day 1 Valid 16Z Sat May 11 2024 - 12Z Sun May 12 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN HIGH PLAINS INTO PARTS OF CENTRAL AND SOUTH TEXAS... ...Eastern NM/southeastern CO into portions of Western and Central TX... A vigorous upper level low over the Four Corners this morning will continue to move eastward this afternoon and early evening, with an influx of deep-layer forcing and moisture across the southern High Plains into much of the TX Cap Rock-Rolling Plains/Hill Country into parts of South Texas. Upper diffluence will increase as height falls approach the area east of the Rockies in tandem with an increase in moisture via S/SE flow along/east of the dryline. 850mb moisture flux anomalies peak around +2 to +3 sigma this afternoon/evening from southeast CO southward and eastward to around 100W, which forms the bulf of the outline for the Marginal Risk area. Forward motion will remain progressive, but sufficient CAPE/ML-CAPE (up to ~1000J/kg north and ~2000J/kg south) and precipitable water values (>90th percentile... ~1" north and 1.5" south) will favor max rain rates of around 1-1.5"/hr and 1.5-2"/3hr beneath stronger cores esp over TX where FFG values are higher. Farther north, lower FFG values along with lower rates (generally <1"/hr) extend into southeastern CO where the Marginal outline was tugged northward. Isolated risk of flash flooding summarizes the threat over the area. Farther east and later in the period (06-12Z Sun), advancing warm front into southeastern TX will promote rainfall into the region with potential for some modest rainfall rates over some sensitive areas that have seen much more rainfall than normal (>200%) north of Houston toward Dallas. Greater threat will carry into the D2 period, but is non-zero before 12Z. Fracasso/Hurley Day 2 Valid 12Z Sun May 12 2024 - 12Z Mon May 13 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF CENTRAL AND EASTERN TEXAS INTO THE LOWER MISSISSIPPI VALLEY... ...Summary... Mid-upper level trough traversing the Central-Southern Plains north of the Red River will lead to a surge of deepening instability and moisture from the western Gulf of Mexico as a warm front pushes into eastern TX and southwest LA. Favorable dynamic and thermodynamic profiles will result in widespread convection across central-eastern portions of TX into parts of the Lower MS Valley, where a high-end Slight Risk of excessive rainfall is anticipated Sunday and Sunday night. ...Central-Southern Plains and Portions of the Lower Mississippi Valley to the Mid-Upper TX and western LA Gulf Coast... The latest guidance (00Z 5/11 model cycle) showed a bit of an uptick in the exit region forcing ahead of the approaching mid- upper level trough, at least in terms of the low-level ageostrophic response across central-eastern TX into the Lower MS. Valley. More specifically, the ensembles now show 850 mb southerly flow anomalies getting to 2-2.5 standard deviations above normal (a little more robust compared to yesterday's guidance). Meanwhile, at the same time 850-700 mb moisture flux anomalies top out between +2.5-3.0, which is also an uptick from yesterday's model progs. The duration of the best moisture transport/anomalous moisture flux within the lower-mid levels remains rather transient however, while the degree of kinematic and thermodynamic response east of the upper trough does seem to be mitigated by the absence of a more robust subtropical jet streak. An uptick in low-level frontogenesis along the warm front is still anticipated however via the right- entrance region of a 90-100 kt upper jet streak traversing the Southern Plains and TN Valley. This will also help retard the warm front's east-northeastward progression. Largely elevated, convection will become more widespread downwind of the warm front in a favorable deep-layer warm/moist advection pattern off the western Gulf of Mexico. The lingering low-level frontogenesis/slow progression of the surface front will allow for some cell training, especially across parts of central-eastern TX which had received quite a bit of rain over the past week. Over this region eastward through western LA, believe the ERO risk will be more of a "high end" Slight (i.e. at least 25% risk of rainfall exceeding FFG within 25 miles of a point). Within the Slight Risk area, 00Z models show pockets of 3-5+ inches of rain during day 2 within this area, with the 00Z RRFS noting a narrow band of 6-8" totals just south-southwest of the ArkLaTex, including Longview TX and Shreveport LA. For now, given the areal spread in the guidance in terms of the heavy rainfall footprints, have opted to not hoist a Moderate Risk. Later shifts can re-evaluate the need for a targeted Moderate Risk based on the most recent observational and model trends. Hurley Day 3 Valid 12Z Mon May 13 2024 - 12Z Tue May 14 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE GULF COAST REGION TO SOUTHERN PORTIONS OF THE MID-SOUTH FROM EASTERN TEXAS TO THE FLORIDA PANHANDLE... ...Summary... The aforementioned mid-upper level trough and corresponding surface low/frontal systems are expected to pivot slowly into the Mid Mississippi Valley by Monday night will continue to foster a heavy rainfall -- which by Day 3 (Mon-Mon night) will shift a little farther east to the central Gulf Coast region. Compared to yesterday's Day 4 ERO, only minor changes were made to the Slight and Marginal Risk areas. ...Lower-Mid Mississippi Valley, Parts of the Midwest, Much of the Gulf Coast Region, Parts of the Southeast... As with the Day 2 ERO, anticipate at least a high-end Slight Risk, which for Day 3 would be across the Gulf Coast region from eastern LA and southern MS-AL to the western FL Panhandle. Buckling upper flow ahead of the trough will help to bolster the low-level frontogenesis for a bit ahead of the advancing cold front, and while while the anomalies and percentiles have come up per the global guidance, forecast confidence for a prolonged period of strong low-level southerly inflow/moisture transport off the Gulf, which helps justify a Day 3 Moderate Risk ERO, remains low. Latest CSU ML first guess fields support that thinking as well. Could certainly see at least 2 rounds of more organized convection; one more elevated ahead of the advancing warm front, with the next perhaps a QLCS ahead of the cold front. However in either case given the shear profiles, the storms should be moving relatively swiftly given the degree of forward propagation inferred with westerly Corfidi Vectors averaging 20-30 kts through the Day 3 period. Model guidance 1 and 3 hour QPF progs Mon-Mon night all show this. Also, there's a legitimate chance that the second round of heavier rain (ahead of the cold front) may be farther south than the first round. Suspect ultimately there could be a Moderate Risk hoisted over a relatively small/targeted area, but given the aforementioned considerations along with the areal spread in the guidance QPFs, the forecast confidence to hoist a Moderate Risk at this point (Day 3 forecast) is too low. Hurley Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt