Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 AM EDT Tue Oct 29 2024 Valid 12Z Fri Nov 01 2024 - 12Z Tue Nov 05 2024 ...Heavy Rain Threat from the Southern Plains to the Mid- Mississippi Valley and Vicinity to develop this weekend... ...General Overview... An upper trough and moderate surface low will eject through the Northeast and eastern Canada Friday overtop a warm ambient upper ridge held over the Southeast. A cold front will swing through the eastern U.S. to focus lingering showers and usher in moderating post-frontal high pressure. The trailing portion of the front is forecast to slow, but offer some convective focus. An emerging heavy rainfall pattern then eminates up through the south-central U.S. with return flow in a deepening moisture channel between the Southeast upper ridge and the approach of upper trough energy and front from the West. Towards that end, aforementioned upstream upper trough energy will dig sharply into an unsettled West, with moderate rains and elevation snows over the Northwest and uncertainly southward to California along with less organized activity to the Intermoutain West/Rockies. This pattern may lead to closed low development at the base of a main trough. ...Guidance/Predictability Assessment... Latest guidance forecast spread has uncomfortably increased through medium range time scales with systems and energies set to work into and downstream with time from the West, lowering forecast confidence. Given cycle to cycle variances also with machine learning guidance, opted to address the full envelop of plausible solutions with preference to a GEFS/ECMWF/Canadian ensemble mean composite blend along with the compatible National Blend of Models and with an eye toward WPC continuity for the latest round of WPC products. ...Weather/Hazards Highlights... A heavy rainfall pattern seems likely on tap by the weekend over the southern Plains and vicinity. A WPC Day 4/Friday Excessive Rainfall Outlook (ERO) Marginal Risk area is in place to address potential, with the new Day 5/Saturday ERO introducing Marginal and Slight Risk areas given consistent guidance signals. Expect activity to expand and focus along a southwest to northeast axis ahead of additional frontal boundaries this weekend and into early next week as robust upper trough/closed low energy digs upstream into the West Coast to re-amplify the flow over the Lower 48 and vicinity. Multiple days of southerly, moist Gulf flow look to increase the chances of more widespread, heavy rainfall with flooding concerns despite dry antecedent conditions, especially from the south-central Plains to the Middle Mississippi Valley. Precipitation associated with Pacific system approach as enhanced by terrain and moderate long-fetch moisture is forecast to focus and with much uncertainty work southward over the West Coast later in the week and through the weekend, with high- elevation snow reaching into favored terrain of the northern Intermountain West/Rockies and enhanced rains reaching down into California. Regardless, the cold frontal surge and possible closed low development at the southern base of the main upper trough will bring in cooler temperatures to much of the West. 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