Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 PM EDT Sat Sep 16 2023 Valid 12Z Tue Sep 19 2023 - 12Z Sat Sep 23 2023 ...Overview... An upper trough lifts out of the Northeast Tuesday as a ridge amplifies over the central parts of the country and a trough from the Gulf of Alaska digs into the Pacific Northwest Tuesday night that likely phases with a low currently off California and closes into a deep low and settles over northern Nevada Thursday. The surface cold front from the eastern trough stalls over the Florida peninsula and northern Gulf of Mexico and provides focus for daily thunderstorms in Florida through the week. Moisture flowing up the Plains brings organized thunderstorm risks Tuesday and Wednesday. In the West, unsettled weather is forecast to develop ahead of this developing trough/low with prolonged precipitation chances across the Northern Intermountain west, Northern Rockies, and eventually the northern Plains. Below average temperatures will spread through the West with this developing system. ...Guidance Evaluation/Predictability Assessment... There was good agreement through Thursday among the 00Z ECMWF/CMC/UKMET on the amplification of the Northwest trough and phasing with the CA low into a sprawling low to use a blend favoring these deterministic models. The 06Z GFS was much quicker with the southward progression of the trough and in ejecting the CA low to be considered an outlier. The 12Z versions of these models have come closer with a bit more progressive trough/low (with the GFS a bit slower), but the 12Z GFS is still the fast outlier. Still, there is good deterministic agreement on the developing low to stall over the interior Northwest or northern Great Basin through Friday before ejecting east (and the ensemble means are generally quicker to eject east) than the forecast blend still leans heavily on deterministics through Day 6 before featuring more ensemble means from the 00Z ECENS/06Z GEFS for Day 7. For QPF, the 13Z NBM provided a good starting point with enhancements made with the 00Z ECMWF/06Z GFS for Days 4/5 (focusing on south-central Plains rain) before leaning heavily on the NBM for Days 6/7. ...Weather/Hazards Highlights... A prolonged fetch of moisture from the western Gulf ahead of an amplifying western trough should allow areas of heavier and possibly organized convective thunderstorms to develop across the south-central Plains Tuesday. The Day 4 Marginal Risk area is maintained for much of Oklahoma and eastern Kansas with a bit of a northeastward expansion into Missouri per the 12Z guidance consensus today. Rainfall should expand both north and south through the Plains Wednesday along a lingering boundary as well as the enhanced moisture transport east of the amplifying trough/low. The ECMWF has focused QPF farther south over Texas where instability and moisture are greater with a separate area of heavy QPF over the Upper Midwest in association with a cold front crossing the northern Plains, while the GFS has favored heavy rain repeating over Oklahoma through Iowa along with a similar separate area over the Upper Midwest. For now, the Day 5 Marginal Risk incorporates both approaches with expanded coverage from northeast Texas through eastern Iowa and then back around the northeastern Great Plains through northern Minnesota. A strong poleward surge of moisture up the Plains is anticipated, but the heavy rain areas will likely become more focused through the coming days. A stalled frontal boundary will maintain a fairly wet pattern over the Florida Peninsula through next week, including the risk of low pressure development over/east of the Peninsula. However, confidence in where that would focus is low enough to not raise an excessive rain outlook area in Days 4 or 5 at this time. The amplifying trough into a low over the West by midweek should bring some prolonged precipitation to the northern Intermountain West and Northern Rockies starting by Wednesday night. There is a general flooding risk for the terrain onto the high plains with higher elevations in the northern Rockies likely to see snow that the extent to which depends on the position and depth of the upper trough/developing low. An area of max temperatures generally 10-15F above normal will shift east with the broad ridge axis from the Northern Plains on Tuesday through the Great Lakes by Thursday and eastern Canada into next weekend. The upper trough forecast to amplify into a low over the West through the middle of next week looks to bring a cooling trend across much of the West starting Tuesday, with high temps around 15F below normal forecast from Montana through California for Thursday and Friday. Otherwise, most temperatures from the South to the East will be near seasonable values through much of the week. Jackson 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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