Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Apr 12 2025 Valid 12Z Tue Apr 15 2025 - 12Z Sat Apr 19 2025 ...Overview... As the medium range period begins Tuesday, upper troughing in the Great Lakes/Ohio Valley and a couple of surface cold fronts will lead to some eastern U.S. precipitation, with possible snow in the higher elevations of the Interior Northeast in generally cooler than average temperatures. Meanwhile southern stream upper energy in the eastern Pacific and California could produce some light scattered precipitation in California to the Four Corners states and then some modest rain in the central Plains/Middle Mississippi Valley midweek. Farther north, model guidance has shown ample spread in the evolution of northern stream troughing in southwestern Canada midweek, with some models diving the trough/embedded upper low more south and some shifting it more east, leading to significant sensible weather differences. This forecast trended toward the trough deepening southward, allowing for more notable precipitation in the Rockies Wednesday-Friday, and slower progression of a cold front and its broad precipitation shield through the central U.S. Thursday toward the east-central U.S. Friday. ...Guidance/Predictability Assessment... The first trough tracking through the east-central and eastern U.S. Tuesday-Wednesday continues to show good model agreement. Initial weakening troughing in southern California Tuesday and the primary upper low in the eastern Pacific have some west-east spread initially, but overall a multi-model blend worked well. Thus the main model diagnostic concern is regarding the next northern stream upper trough and its placement beyond midweek. Models from a day ago heavily favored this trough skirting eastward, but in the past couple of model cycles more spread has arisen. Overall, there has been a slower trend for the trough moving east in models and ensembles. Some deterministic models dive energy southward through the Great Basin or so Thursday-Friday, enough to form a closed low there. These include the 12Z ECMWF and CMC, as well as the 18Z GFS in a significant flip from the 12Z GFS that had the upper trough/low tracking east like older guidance. However, AI/ML models from the 12Z/18Z cycle did not show the troughing digging as far west as this cluster of deterministic models. They generally showed a central U.S. trough, which was still slower with the trough than some previous forecasts. This forecast issuance was not very confident in the solution presented by the 12Z EC/CMC and 18Z GFS. The forecast blend quickly ramped up the proportion of ensemble means Day 5 and used the 18Z GEFS/12Z ECens means alone by Days 6-7 given the low confidence in the changing forecast pattern. Even this blend of the ensemble means was a notable change from the previous forecast, slowing down the trough and cold front and producing much more precipitation across the Interior West. Now the newer 00Z model cycle has come in with a similar pattern for upper troughing to dive south through the Great Basin or so, and now shows the originally eastern Pacific energy phasing in by late week to form a Southwest upper low. This includes the 00Z AIFS in a big change from its 18Z run. So there is more confidence that this much farther west troughing developing is reasonable. Thus even though forecast changes in this cycle were notable, expect more in the next forecast. It appears the energy that will eventually form this trough/upper low is currently spinning over the Bering Sea and vicinity, so hopefully as it tracks into areas with more observed data for better model initialization soon and continued stability in the forecast. ...Weather/Hazards Highlights... A surface low tracking through southeastern Canada Tuesday and its trailing cold front will support some precipitation across the Great Lakes to Eastern Seaboard Tuesday and lingering across the Interior Northeast Wednesday. On the backside of the low, temperatures could be cold enough for snow in higher elevations of the Adirondacks and Green and White Mountains into Tuesday. Gusty winds are also possible. A weakening upper low reaching the Southwest by Tuesday may produce some very scattered and light precipitation. Then as surface low pressure develops in the central High Plains and moves east, rain is forecast to spread across the central Plains to Middle Mississippi Valley for midweek. Recent models show this rain remaining pretty modest, so no excessive rainfall areas are in place for the Day 4-5 (Tuesday/Wednesday) EROs. Then farther north, this forecast issuance has trended toward showing greater coverage of rain and higher elevation snow over parts of the West/Rockies under the digging upper trough. Precipitation looks to spread from the northern Rockies/High Plains Wednesday a bit south centered over Wyoming Thursday and into Colorado Friday. Possible interaction with the southern stream trough tracking east from the eastern Pacific could support precipitation across parts of California/Great Basin/Southwest. A low pressure/frontal system tracking eastward along with some upper-level support could lead to rain and perhaps far northern tier snow in the Midwest Thursday and shifting into the Great Lakes/Ohio Valley and Northeast on Friday. The frontal system and precipitation have trended slower than the previous forecast. Then depending on if and how troughing/an upper low develops over the Southwest late week, moisture streaming into the southern Plains could allow for increasing rain chances there. Overall, recommend continuing to monitor forecast updates until the forecast stabilizes. Initial mean ridging aloft in the West will allow for above normal temperatures there, spreading into the Plains by midweek. Generally temperatures are forecast to be 10-20 degrees above normal, allowing for highs in the 90s in the Desert Southwest and then across much of Texas by Thursday. Cooler than average temperatures are likely in the Great Lakes and Ohio Valley Tuesday into the eastern U.S. Wednesday and possibly into Thursday. Lows should generally be 5-10F below average and highs around 10-15F below average. After that, the temperature forecast becomes more uncertain dependent on the upper pattern. The Interior West looks to gradually cool into late week, though the Northwest could stay a bit above average with possible upper ridging poking in. The southeastern U.S. may warm to above normal late week. Tate 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