566 FOUS30 KWBC 140823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 422 AM EDT Wed Apr 14 2021 Day 1 Valid 12Z Wed Apr 14 2021 - 12Z Thu Apr 15 2021 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF CENTRAL LOUISIANA AND FAR SOUTHWESTERN MISSISSIPPI... ...Lower MS Valley... Mid-level shortwaves will continue to ride along the fairly zonal flow atop a surface boundary draped from the Southern Plain into the Lower MS Valley. The first shortwave has triggered convection overnight which should be ongoing into portions of Eastern TX and northern LA at the start of the forecast period. Given enough moisture, instability and shear, this convection will become organized (likely into an MCS) and drop south through the morning/early afternoon while progressing east. Uncertainty on the evolution of this precipitation activity continues even within the near term. Regardless, most models signal heavy rain atop saturated soils leading to the potential for more widespread significant flash flooding. Precipitable water values will try to surge north again to around 1.75+ inches to just south of the frontal boundary aided by 30+ knot southerly low level flow; this is over 2 standard deviations above the mean. Instability will climb to around 2000-3000 J/kg this morning with the best instability across southeast TX and western LA. As the shortwave interacts with better instability and moisture, convection will continue to blossom along the front across north/central LA this morning. With enough shear, expect this convection to organize into an MCS and ride south along the instability gradient. While the convection should be fairly progressive as it moves east, the outflow boundary may be slow to sink south allowing for more east-west training of convection. This could lead to higher hourly rain totals around 2-3 inches/hour. As a result, areal average precipitation could range between around 2-5+ inches across central/southern LA into portions of southern MS. As previously mentioned there is a bit of uncertainty with respect to the evolution of this next round of convection. Much of southeastern LA is saturated with any additional heavy rain leading to runoff concerns. One possibility is for the heaviest rain to sink farther to the southwest where the environment is less overturned and training may occur along the southwest flank of the outflow boundary. Regardless, there seems to be enough confidence in the potential for heavy rain across portions of south-central LA where some locations received 3-5+ inches of rain over the past 24 hours. Therefore, after careful coordination with the local offices, a small Moderate Risk area was introduced with the expectation that more widespread and significant flash flooding may occur within this region. The Slight and Marginal Risk areas were slightly modified at the update to account for the QPE and 00Z model guidance. There is another shortwave that will approach the Southern Plains tonight triggering another round of convection across eastern TX/OK. This activity will becoming better organized along the aforementioned front over central LA into MS. Given recent rainfall yesterday and what is expected this morning, this region could see another round of heavy rain and flash flooding by late tonight/early Thursday. We will continue to monitor this potential keeping in mind the ongoing activity this morning, the surface front position and the latest model guidance. ...Southern Mid-Atlantic... A deepening mid/upper level low will gradually meander east over the Great Lakes region with ridging along the east coast shifting offshore. Impulses riding the base of the trough axis will traverse the region and interact with a lee trough and approaching front. As a result, a weak surface low will develop by late afternoon. In response, precipitable water values will climb to around 1.25 inches (which is over 1.5 standard deviations above the mean) aided by strengthening low level southwesterly flow. With some diurnal heating this afternoon, this should help sharpen the lee trough and create more surface convergence. A lot of uncertainty surround the amount of instability with MUCAPE values struggling to get to 1000 J/kg. Regardless, there should be enough moisture and boundary layer lift to promote convection that could drop 1-2 inches of rain per hour. This activity should be fairly progressive, but backbuilding/training is possible along the front as propagation vectors align with the mean wind. Thus, areal average precipitation could exceed 2-3+ inches in some locations within a few hours. While much of this region has observed around normal precipitation as of late, there are some locations that have seen above 200% of normal with recent convection. Therefore, a Marginal Risk was introduced with the potential for FFG being exceeded or heavy rain occurring over an urban corridor. Most of the precipitation activity should move offshore by late Wednesday night allowing the potential for flash flooding to diminish after that point. Pagano Day 2 Valid 12Z Thu Apr 15 2021 - 12Z Fri Apr 16 2021 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A SMALL AREA ALONG THE CENTRAL GULF COAST... Highlighted a small area, mainly confined to parts of Louisiana, where there appears to be a marginal risk of excessive rainfall on Thursday. In the broad scale picture, deepest moisture/instability and the potential for heaviest rainfall rates should be moving out to sea early on Thursday and there should be a relative lull in activity. However, the models keep some rainfall lingering over a region that already had more than 300 percent of normal rainfall in the previous week before localized 2 or 3 inch rainfall amounts fell on Tuesday...with the potential for more heavy rain in the short-term again today. This area would be particularly sensitive to flooding from even modest rainfall amounts on Day 2. As a result, placed a Marginal Risk area that was roughly co-located with the placement of the Moderate Risk area on Day 1 despite rainfall amounts that would generally not prompt a Marginal Risk area. Once there is less uncertainty on how quickly rain shuts down following the passage of a cold front, it is possible that the area can be removed in subsequent outlooks. Bann Day 3 Valid 12Z Fri Apr 16 2021 - 12Z Sat Apr 17 2021 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE CENTRAL GULF COAST REGION FROM FRIDAY INTO EARLY SATURDAY MORNING... ...Louisiana to Southern Mississippi/Alabama... After a relative lull in convection capable of producing heavy rainfall on Thursday, the return of a boundary from the Gulf of Mexico combined with the approach of more shortwave energy from the west should result in increasing risk of excessive rainfall on Friday across an area already susceptible to flooding. Low level flow should begin to back and strengthen across eastern Texas and western/southern Louisiana on Friday in response to shortwave energy approaching from the north and west. As it does so, the low level flow will be riding up and over a quasi-stationary boundary across the region. This should foster a growing precipitation shield in the broad isentropic lift/warm advection pattern...with convection becoming increasingly able to produce heavier rainfall rates Friday night and into the pre-dawn hours on Saturday as precipitable water values increase to between 1.6 and 1.8 inches. The orientation of the boundary and the southwesterly low-level flow suggest that cell training is possible. The numerical guidance is in generally good agreement with this scenario. One area of concern was with the 14/00Z GFS Friday evening over southern Louisiana. The very large magnitude of vertical velocities over a large part of southwest Louisiana is a signature of convectively-induced grid-scale feedback. The real concern is not so much that the GFS formed convection where it did but how far downstream the low level thermal and wind fields impact the model QPF. Thus, was more inclined to follow a non-GFS solution for placement of the Slight Risk area at this time. Bann 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 $$
