Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 PM EDT Thu May 9 2024 Valid 12Z Sun May 12 2024 - 12Z Thu May 16 2024 ...Excessive Rainfall/Flooding Threat for the Southern Plains, Gulf Coast and Southeast... ...Guidance/Predictability Assessment... The latest models and ensembles show reasonably good agreement that by the start of the period on Sunday, the pattern should trend more progressive featuring a series of troughs/shortwaves migrating from the Central to Eastern U.S. as upper level ridging tries to build back in over the West. There was good enough agreement in the guidance for a general blend of the deterministic guidance the first half of the period as troughing shifts out of the Northeast and another potent shortwave moves through the Central Plains to Lower/Middle Mississippi Valley. By Day 6/Wednesday, there are some timing differences with this shortwave as it shifts into the Mid- South and Southeast which has critical implications on resulting QPF across this region. There are also some more noticeable differences with an amplifying shortwave from central Canada into the Plains, and how strong ridging builds back in across the West. Over the past couple of guidance runs, the CMC remains the strongest with that ridge, while the GFS and ECMWF (to varying degrees) are a little weaker due to stronger shortwave energy British Columbia next Thursday. There is a lot of run to run inconsistencies between models too, so it seems prudent at this point the blend trends increasingly more towards the ensemble means late in the period to mitigate/smooth these uncertainties. Did continue to include some ECMWF just for a little extra system definition. The approach maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... A general split flow pattern will persist over the lower 48 states into next week. This pattern will be highlighted by an amplified southern stream upper trough slated to eject slowly but steadily east- northeastward from the Southwest Sunday and through the Mid- South/South to offshore the Mid-Atlantic/East Coast next Thursday. This system and associated lead wavy front will increasingly tap and feed inland a return of deep Gulf moisture and instability under favorable upper trough and right entrance region upper jet support to fuel a potentially significant heavy rainfall and runoff/flooding period. The WPC Day 4/Sunday Excessive Rainfall Outlook (ERO) depicts a Marginal Risk area spread south/southeastward from the central-southern Plains to the lower Mississippi Valley. Within this, a Slight Risk area remains in place for Southeast Texas into Louisiana given moist soil/streamflow sensitivities from recent heavy rainfall. The overall risk should shift east on Day 5/Monday as highlighted by the ERO with a marginal risk across the Middle/Lower Mississippi Valley into the Gulf Coast/Southeast. An expansive slight risk exists from Southeast Texas into the Florida Panhandle. Heavy rain and a threat of runoff issues is also expected to spread across the Southeast through next midweek and possibly build back into the southern/central Plains as additional shortwave energy moves into the region. Meanwhile, several amplified upper troughs and surface systems are set to work over the northern tier of the nation from the Northwest and north- central U.S. through the Northeast and focus mainly modest moisture and precipitation. Santorelli/Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw