Extended Forecast Discussion NWS Weather Prediction Center College Park MD 321 PM EDT Sun Sep 17 2023 Valid 12Z Wed Sep 20 2023 - 12Z Sun Sep 24 2023 ...Overview... The ridge over the central U.S. will continue to amplify as the upper trough lifts out of the Northeast on Wednesday. Across the Northwest a digging shortwave trough is expected to phase with a low located offshore the California Coast to become a deep low over the Great Basin by Thursday/Friday. This feature will exit into the Plains by the weekend, keeping cool and unsettled conditions across the West for a majority of the extended period. An upper low settling over the Southeast/Florida later this week brings potential for heavy coastal rains next weekend along the southeast and Mid-Atlantic coasts. ...Guidance Evaluation/Predictability Assessment... The latest runs of the guidance and ensemble means continued to show fairly good agreement on the overall amplification of the large scale pattern and its evolution through the extended period. Like their previous cycles, the UKMET remains displaced a bit to the south and east with the upper low, and by next weekend the CMC is much slower to move the low east into the Northern Plains due to a more blocky pattern upstream across the East Pacific. The WPC forecast continues to favor a weaker frontal wave, which regardless of characterization, has potential to spread heavy rainfall up the Southeast and Mid-Atlantic coasts next week. The preferred model consensus utilized a heavier weighting of the ECWMF but also included the GFS, CMC and UKMET. The inclusion of both the UKMET and CMC were reduced then omitted beyond the middle and later periods and were replaced by the GEFS mean and EC ensemble means. This blend provided a sense of continuity from the previous forecast. By late week, there is decent agreement that an area of low pressure may form off the eastern Florida coast, with much more uncertainty on what, if any, tropical characteristics this may exhibit as it lifts north along the Southeast coast next week. ...Weather/Hazards Highlights... Return flow from the Gulf of Mexico will transport PW values of 1.25 inches northward into the Plains ahead of the amplifying trough over the West. This steady stream of moisture should allow areas of heavier and possibly organized convective thunderstorms to develop across the Plains, but there remains a lot of uncertainty and spread in the location of heaviest amounts. The inherited Day 4 Excessive Rainfall Outlook featured 2 areas highlighted by Marginal Risks. The southern one spans from northeast Texas to northern Missouri where moisture anomalies are greatest along a slow moving/stationary boundary. With the latest trend in the guidance minor expansions were made westward along the Texas and Oklahoma border and eastward along the Missouri and Arkansas border. The northern risk area spans from the eastern Dakotas to the Minnesota/Wisconsin border. No changes were made to this area at this time. The amplifying upper low over the West by midweek should lead to prolonged precipitation across the northern/central Intermountain West and Northern Rockies. As the low deepens the rainfall is expected to increase keeping the threat for excessive rainfall and local flash flooding elevated. The Day 5 ERO has a large Marginal Risk spanning from eastern Nevada northward to Montana and east to northern Minnesota. Higher elevations in the northern Rockies are likely to see snow that the extent to which depends on the position and depth of the upper trough/developing low. Increasing potential for more widespread heavier rain develops next weekend across the northern/central Plains into parts of the Mississippi Valley as the West upper low begins to push into the region. Minor adjustments were made to the western bounds over central Idaho and central Nevada and brought further northeast into northern Minnesota. Meanwhile, a stalled frontal boundary will maintain a fairly wet pattern over the Florida Peninsula through much of the week, though 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 possible surface low off the Coast by Friday could tap tropical moisture, increasing the potential for heavier rains along the Atlantic coast from Florida to the Mid-Atlantic. Amounts and extent inland of the heavier rains remains extremely uncertain at this point. An area of max temperatures generally 10-15F above normal will shift east with the broad ridge axis from the Upper Midwest on Wednesday through the Great Lakes Thursday-Saturday moderating some with time. The upper low over the West looks to bring a cooling trend across much of the West, with high temps around 15F below normal forecast from Montana through California through at least Saturday. Modest above normal temperatures may return to parts of Texas this week, while much of the Eastern U.S. should be fairly near normal. Campbell/Santorelli 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