Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Jun 8 2024 Valid 12Z Tue Jun 11 2024 - 12Z Sat Jun 15 2024 ...Hot temperature shift from the West/Southwest to the Plains next week as downstream heavy convective rainfall threats affect the south-central states and from the Gulf of Mexico into Florida... ...Overview... There is now better agreement in the guidance that an elongated upper low over southern-eastern Canada will split, sending a trough through the Northeast into Tuesday and a secondary shortwave through the northern Plains/Great Lakes Tuesday-Wednesday. Slow moving southern stream energy will keep unsettled conditions and heavy rainfall potential particularly across the southern Plains and lower Mississippi Valley early-mid next week, with an inherent stronger threat signal for a multi-day wet period across the Florida Peninsula along with building energies and convection to monitor over a warmed Gulf of Mexico. Upper ridging over the West/Southwest will gradually work into the south-central U.S. with summer heat as a closed low works slowly to the Southwest from the tropical East Pacific. Meanwhile, the pattern will favor multiple but mostly dry Pacific upper trough and cooling frontal intrusions across an unsettled Northwestern U.S.. ...Guidance/Predictability Assessment... Recent guidance forecast spread and run to run continuity issues continue to improve compared to the past day or two of run cycles for at least the first half of the upcoming medium range forecast period. This bolsters forecast confidence back to near normal levels overall, albeit still with standard and widespread warm season variances in the details. The greatest region of uncertainty remains over the Gulf of Mexico as lower heights aloft move in to the region. The details of how this interacts with an increasing moisture feed into/towards Florida remains highly uncertain. Confidence is increasing towards a heavy rainfall event for Florida, but exactly where the rainfall axis sets up is unknown and highly variable. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of reasonably well clustered guidance from the 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian models valid for Tuesday into Thursday. Growing forecast spread seems manageable but sufficient to favor a switch to more inclusion of the ensemble means to mitigate the details differences. This approach maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... A wavy front across the south-central Plains and vicinity will combine with slow moving upper energy to continue a threat for heavy rainfall into Day 4/Tuesday from wettened Oklahoma to northern Texas, with additional threat shifting into the lower Mississippi Valley into Day 5/Wednesday. WPC Excessive Rainfall Outlook (ERO) Marginal Threat areas are depicted for each day. A multi-day period favorable for heavy rains will also develop across the Florida Peninsula starting Tuesday as well above moisture pools along/south of a the wavy/stalling front with some aid from weak upper level impulses/energy through the Gulf of Mexico. There is a lot of uncertainty in the details and much of the state is in a drought. Still, some threat for high rain rates exists, and its possible much of the threat may manifest for more susceptable urbanized areas. Day 4/5 WPC ERO Marginal Risk areas have been continued to include the central and southern Peninsula to account for the uncertainty, with a looming threat into later next week as well there and with any activity/moisture feed eminating over the Gulf. Increasing precipitation threat across much of the central and northern Plains emerges late period with troughing shifting northeastward into the region from the Southwest. An amplified mean upper ridge slated to slowly shift from the West/Southwest to the south-central U.S. will continue to cause much warmer than average temperatures. The highest temperature anomalies of 10-20F above normal are forecast for the Great Basin, warming a bit and expanding eastward as the week progresses into the Rockies and Plains. Temperatures will also rise a few degrees in the Desert Southwest to bring some temperatures back above 110F, with heat risk threat levels mostly at moderate to major levels. Heat should build again across south to south Texas as well, with heat indices back up to around 110. 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