Extended Forecast Discussion NWS Weather Prediction Center College Park MD 313 AM EDT Mon Jun 10 2024 Valid 12Z Thu Jun 13 2024 - 12Z Mon Jun 17 2024 ...Heat focus from the West/Southwest to the Southern Plains this week as a Multi-Day Excessive Rainfall Threat looms for Florida... ...Guidance/Predictability Assessment... Model and ensemble forecast spread has overall decreased over the past few guidance cycles over much of the lower 48 with a series of progressive northern stream systems and within a more flow separate southern stream, slowly bolstering forecast confidence. Guidance solutions also seem a bit better clustered with an overall evolution of lowering heights, and highly anomalous moisture axis, and energy over the Gulf of Mexico into Florida, but important differences remain to offer uncertainty. Tropical system development is not forecast by the latest from the National Hurricane Center, but regardless, confidence in an excessive rainfall threat for central and southern Florida is increasing. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of the 18 UTC GFS and 12 UTC ECMWF/Canadian/UKMET for Thursday-Saturday. Opted for smaller input from the models in lue of more consistent GEFS/ECMWF ensemble means at longer time frames amid growing uncertainties. The blending process tends to mitigate variance by limiting detail consistent with individual feature predictability. This also acts to maintain good WPC product continuity that remains generally in line with a similar blend of latest 00 UTC 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 moisture pools along and south of a wavy/stalling front with aid from upper level impulses/energy back through an unsettled Gulf of Mexico. Uncertainties remain, but there is increasing confidence in the looming threat with any activity/moisture feed emanating over the Gulf. Storm total rainfall (including what falls before the medium range period even starts) of 7-12 inches, with locally higher amounts, is possible. WPC Excessive Rainfall Outlook Slight Risk areas remain in place for the central to southern Florida Peninsula. Enhanced rainfall and local runoff issues may subsequently lift/work over the Gulf to the Central Gulf Coast and vicinity over time to monitor. Meanwhile, an increasing precipitation threat across much of the central and northern Plains emerges late period as amplified southern stream troughing ejects northeastward into the region from the Southwest to focus lift/intstability. SPC shows a threat for severe weather and a WPC Day 5/Friday ERO Marginal Threat area has been introduced for the central High Plains. Elsewhere, leading and dynamic northern stream upper trough energy approach and passage is expected to fire convection along/ahead of a wavy/pooling surface front from the Midwest through the Northeast later this week. WPC introduced ERO Marginal Threat areas from the vicinity of Chicago Thursday through northern New England Friday as the parallel upper flow and convective axis may lead to some repeat cells and local runoff issues. As for anomalous temperatures, an amplified mean upper ridge slated to slowly shift from the West/Southwest to the south-central U.S. and parts of the East will continue to produce much warmer than average temperatures. The highest temperature anomalies of 10-20F above normal are forecast for the Great Basin, shifting eastward as the week progresses into the Rockies and Plains. It will be especially hot across the Desert Southwest and South Texas given elevated Heat Risk and Heat Index threat values. 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