Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
230 FXUS02 KWBC 241854 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Tue Sep 24 2024 Valid 12Z Fri Sep 27 2024 - 12Z Tue Oct 01 2024 ...Helene to bring impactful heavy rain, high wind, and coastal flooding threat to the Central to Eastern Gulf Coast/Southeast ahead/with landfall as a major Hurricane on Thursday... ...Guidance/Predictability Assessment... The main forecast story is the threat and impacts associated with Tropical Storm Helene. Recent guidance has come into better agreement in depicting a split flow pattern with an upper- trough lifting northward over the Northeast as a cut-off upper-low drops southward over the Middle Mississippi Valley/Ozarks Region. This upper-level low then lingers in the general vicinity into the weekend as upper-ridging moves in to the north over the Great Lakes/southeastern Canada. At the same time, the NHC forecast for Helene brings the system northward across the Florida Panhandle Region as a major hurricane and into the Southeast as it undergoes extratropical transition Thursday into Friday. The 06z GFS/00z ECMWF made for a nice compromise between the more southwest UKMET and more northeast Canadian model with the cold low which Helene occludes back into, so the pressures, fronts, QPF, and winds were weighted towards those pieces of guidance. The remainder of the grids started with the 13z NBM which was weighted more towards the 06z GFS/00z ECMWF solutions. ...Weather/Hazards Highlights... Helen moves northward into the Gulf of Mexico threatening maritime interests and the central to eastern Gulf Coast/Southeast mid-late week, with the system set to move inland Thursday into Friday. Confidence is growing in a large system, due to its interaction with a cold low to its northwest and west, that will bring significant rainfall/winds/surf/coastal flooding for the central to eastern Gulf Coast/Florida, with impactful weather possible through the Southeast, and wrapping back into the Mid-South late week into the weekend. The Day 4/Friday ERO period features its anomalous moisture feed into the southern Appalachians as well as over the Ozark Region as Helene occludes under the cold low, with both regions covered with Slight Risks Outlook threat areas. A similar area depicted for the Day 5/Saturday ERO shows a scaling down ERO version as Helene broadens and weakens. However, ample lingering deep tropical moisture will remain to fuel the elongated Marginal threat area. More certain local focus closer to occurrence would likely lead to Slight Risk issuance upgrades, especially given earlier period heavy rainfall will create saturated soils. A Moderate Risk was considered for portions of the Appalachians for the day 4 ERO, but the main drawbacks shifts in the model guidance with Helene`s track and concern that Helene accelerates faster than the guidance currently advertises. Left the overnight ERO levels as they were. The Washington coastal ranges and Cascades will see enhanced rain late week with Pacific system passages early to mid next week, but the remainder of the West and most of the Plains will be dry. Warmer than average temperatures are expected to translate from the Northern Plains east into the Midwest later this week into the weekend following upper-level ridging, with anomalies decreasing by early next week. Cooler than average temperatures are expected centered around the Ozarks Region under the influence of Post- Tropical Cyclone Helene, also returning closer to average by early next week as the upper- low dissipates. Roth/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 $$