Extended Forecast Discussion NWS Weather Prediction Center College Park MD 143 PM EDT Sat Apr 20 2024 Valid 12Z Tue Apr 23 2024 - 12Z Sat Apr 27 2024 ...General Overview... The forecast period begins on Tuesday with an amplifying upper trough over the Great Lakes and then over the Northeast U.S., and this will propel a cold front across much of the Eastern U.S. for the middle of the week. Surface high pressure then builds in across the Midwest and then across the East to close out the week along with cooler temperatures. Meanwhile, a synoptic scale trough develops across the Western U.S. by Friday and into Saturday with a couple of shortwaves ejecting out across the Plains, and supporting surface cyclogenesis across the western High Plains. This will likely result in developing showers and storms for the Central U.S., with some heavy rain and strong storms possible by the end of the week. ...Guidance/Predictability Assessment... The two main forecast challenges are centered on the approach of two shortwaves in the northern stream, potentially phasing and closing off in the Nortehast by the middle/late portions of next week. The GFS and its ensemble mean continue to support a stronger/more phased system that features a deep closed low over New York by 00Z Thur while other operational guidance like the CMC offer a less phased, quicker, split system that pushes through more subtly into Wed/Thur. The EC AIFS ML model tends to lean toward the more progressive solution and lines up well with the other non-GFS solutions. The other area of concern is across the West with the next puece of shortwave energy moving into SoCal, Desert Southwest and eventually ejecting out into the Plains by D6-7. The 00Z/06Z GFS (and now the 12Z run too) offer a much faster solution (and somewhat stronger) compared to the non-NCEP guidance (and EC AIFS ML model). For the WPC model blend preferences and subsequent grids, the preference was for the slower system and incorporating more of the ECMWF compared to the GFS. ...Weather/Hazards Highlights... Upper trough and frontal passages through the Midwest/Great Lakes and then the Northeast will be accompanied by some widespread light to moderate rains early-mid next week. Given the overall progressive nature of this storm system, and the lack of appreciable instability and QPF most places in this period, no flash flood threat areas are currently warranted for the WPC Excessive Rainfall Outlook for Tuesday. On Wednesday, warm air advection across Texas and Oklahoma will bring a return to scattered showers and storms across this region, with some of the model guidance suggesting the potential for some local one inch rainfall totals. However, the guidance is not locked in on the location of these scattered QPF maxima, and therefore it seems prudent to hold off on any risk areas for the Day 5 outlook on Wednesday until a better model signal develops for placement. More widespread showers and storms are then expected to develop across the Plains and Midwest states into later next week as moisture and instability increase with surface low pressure system development, and the potential exists for some strong storms ahead of the dryline in the Southern Plains, so this will continue to be monitored. Initial upper level ridging out West should support above normal temperatures through about Wednesday, and then a return to near or slightly below normal levels for much of the West Coast and Intermountain West starting Thursday. Warmer temperatures and increasing humidity are forecast for the south-central U.S. by next Friday and beyond as the upper ridge axis builds back, and southerly flow from the western Gulf advects moisture northward ahead of a developing surface low. Highs could get over 100 degrees for portions of southern Texas by Saturday. Hamrick/Taylor 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