Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Tue Jun 11 2024 Valid 12Z Fri Jun 14 2024 - 12Z Tue Jun 18 2024 ...Heatwave to spread from the Southwest to parts of the central and eastern U.S. during the period... ...Excessive Rainfall Threat for Florida through late week and perhaps the central Gulf Coast early next week... ...Guidance/Predictability Assessment... Guidance remains in good overall agreement through most of the period with respect to the overall pattern, featuring mean ridging over the West progressing eastward through the period over the central and then eastern U.S., and with mean troughing settling in over the West in the wake. Guidance indicates a southern stream shortwave over the Southwest will lift northeast rounding the building ridge with additional embedded waves translating the northern tier as the mean troughing begins to take hold over the West. Uncertainty increases late this weekend into early next week as an initial wave/potential closed low lifts northeastward over the northern Plains/south-central Canada, with the 12Z CMC the greatest outlier in terms of weaker intensity and timing. The 00Z ECMWF/CMC and 00Z ECens mean are then initially more aggressive than the 06Z GFS/GEFS mean with another embedded wave digging further southward over the West. The suite of machine-learning models from the EC encompass this envelope of solutions, reinforcing the uncertainty, though again with the overall pattern generally well agreed upon. The updated WPC forecast featured a composite blend of the deterministic guidance initially, with a lower contribution from the CMC compared to the ECMWF/GFS given noted differences. The latter half of the period includes a contribution from the ECens/GEFS means, 50% of the blend by day 7 (Tuesday), as timing/evolution differences in the West increase. Meanwhile, an upper-level weakness initially remains over Florida, slowly retrograding west towards the end of the period. This keeps a stormy pattern in the forecast for Florida and then the central/potentially western Gulf Coast, with the ridge transitioning over the East also helping to funnel tropical moisture northward. Forecast QPF amounts remain the most uncertain, with the latest GFS/GEFS mean guidance along the central Gulf Coast and the ECens mean and particularly ECMWF favoring more of the western Gulf Coast. The GFS would also favor a potentially more significant rainfall event. For QPF, opted for a composite guidance blend with some continuity, which still tends to favor at least locally heavy amounts over the central Gulf Coast for now but includes some more moderate amounts into the western Gulf from the EC guidance. ...Weather/Hazards Highlights... A protracted period favorable for heavy rains should continue through later week for the Florida Peninsula as well above normal tropical/Caribbean moisture pools along and south of a wavy stalled front with aid from upper level impulses/energy back through an unsettled Gulf of Mexico. There remains high confidence in heavy rainfall in Florida with locally heavy rainfall rates, but lower confidence on placement of any extreme convective amounts on any particular day. Storm total rainfall (including what falls before the medium range period even starts) of 7-10 inches, with locally higher amounts to 15 inches, is forecast. WPC Excessive Rainfall Outlook Slight Risk area is in place for the central to southern Florida Peninsula into Day 4/Friday given the preceding days rains, as uncertainty with the potential totals from convection remains. Heavy rain may last into Saturday across Florida where a Day 5 Marginal Risk has been introduced for lingering activity. However, as the weekend progresses and into early next week, there is a growing guidance signal that moisture influx and possible heavy rainfall may refocus and increase into the central Gulf Coast and vicinity, which will need to be monitored for runoff issues given the favorable deeply moist pattern. Much uncertainty remains, particularly with the potential amount of rainfall and with westward extent into the western Gulf. Meanwhile, leading and dynamic northern stream upper trough energy approach and passage is expected to fire convection along/ahead of a wavy/pooling surface front over the Northeast into Friday where a Marginal Risk area remains in place as the parallel upper flow and convective axis may lead to repeat cells and runoff issues. Elsewhere, an increasing convective precipitation threat will emerge across the central and northern Plains/Upper Midwest late week into the weekend as well defined southern stream troughing ejects northeastward toward the region from the Southwest to focus lift/instability. A WPC ERO Marginal Risk area remains in place for Day4/Friday from the southern High Plains to the central Plains, except for the Sandhills of Nebraska that are not as sensitive to heavy rain. A Day5/Saturday Marginal Risk area has been maintained from the central Plains to the Upper Midwest given system progression and continued unstable flow. As for anomalous temperatures, an amplified upper ridge slated to steadily/broadly shift from the Southwest through the south- central U.S. to the East will produce much warmer than average temperatures and the threat for some record heat into next week. Temperatures will initially already be rather hot over portions of the South into Mid-Atlantic Friday before the even broader region of heat expands into the Midwest/Ohio Valley as the upper-level ridging builds in. Highs are forecast well into the 90s, potentially low 100s, and with lows in the 70s providing little relief from the heat overnight. The Climate Prediction Center continues a heat threat into week 2. Putnam/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