Extended Forecast Discussion NWS Weather Prediction Center College Park MD 219 AM EDT Sun Sep 17 2023 Valid 12Z Wed Sep 20 2023 - 12Z Sun Sep 24 2023 ...Overview... An upper trough lifts out of the Northeast Wednesday as a ridge amplifies over the central part of the country and a shortwave digging into the Pacific Northwest Wednesday. This shortwave should phase with a low currently off California and close into a deep low, settling over northern Nevada Thursday-Friday before getting kicked eastward into the northern Plains next weekend. This keeps weather rather unsettled and cool across the West much of the 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... There was good agreement Wednesday-Thursday on the amplification of the Northwest trough and phasing with the CA low into a sprawling low to use a blend favoring the deterministic models. After this, the UKMET is displaced a bit to the south and east with the upper low, and by next weekend the 12z/Sep 16 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 CMC was not used in the late period blend. However, the new 00z CMC (available after forecast generation time) did come in much more in line with the GFS and ECMWF both of which are faster to kick the system into the northern Plains and are more consistent with the ensemble means. 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. 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. Given the increasing uncertainty in the pattern next weekend both in the West and the Southeast, the WPC blend favored a blend closest to the ensemble means, plus the deterministic ECMWF. ...Weather/Hazards Highlights... A 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 Plains, but there remains a lot of uncertainty and spread in the location of heaviest amounts. The broad marginal risk inherited on the Day 4/Wednesday Excessive Rainfall Outlook was broken up into two areas -- from northeast Texas into Missouri where instability should be strongest, and another area across parts of the Upper Midwest, where moisture anomalies are greatest along a slow moving/stationary boundary. The amplifying upper low over the West by midweek should bring prolonged precipitation to the northern/central Intermountain West and northern Rockies. Rainfall should get heavier with time as the low deepens and there is a general flooding risk for the terrain into the northern Plains. A large marginal risk was introduced on the Day 5/Thursday ERO tonight across this region. 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. 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. 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