Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Wed Apr 10 2024 Valid 12Z Sat Apr 13 2024 - 12Z Wed Apr 17 2024 ...Overview... Most guidance shows a similar large scale pattern evolution during the period. A deep Northeast upper trough should depart after Saturday, giving way to weak shortwave energy that may support a northern tier wave during Saturday-Monday. On the opposite coast, expect a strong upper low to reach California this weekend with a brief period of precipitation and continue tracking inland. The upper ridge to the east will support well above normal temperatures over the central U.S. during the weekend. As the upper low reaches the Plains next week, it will likely develop a strong surface system around Tuesday and continue northeastward into midweek. Upper troughing forecast to dig into the Northwest next week along with its leading cold front will eventually bring a cooler trend to that region and may have some influence on the Plains system that will spread an increasing area of rainfall across the central U.S. ...Guidance/Predictability Assessment... Some timing spread persists for the upper low reaching California and continue eastward/northeastward thereafter. Recent GFS runs have been leaning a bit on the slower side of the spread while some of the other operational dynamical models have been trending a little faster over the past day. 12Z ECMWF-initialized machine learning models (MLs) show some timing spread as well but the majority favors timing close to continuity, between the GFS and ECMWF/other faster models. Most of the new 00Z models are showing somewhat of a faster trend though. Associated Plains low pressure should be deepest around Tuesday with the average of MLs suggesting a strength between the deeper GFS/ECMWF runs and weaker CMC/ensemble means. There is a persistent signal for upper troughing to dig into the West during the first half of next week, but again with spread for the details plus a slightly slower trend for progression of leading height falls and associated cold front. The 12Z ECMWF seemed to have less southwestward extent of troughing by Wednesday compared to most other guidance. The MLs also display a moderate amount of spread but with their average ending up fairly close to the dynamical model/mean consensus. By around next Wednesday there is a question of how the western trough may interact with the system emerging into the Plains. The overwhelming majority of guidance says the two features will stay separate, in contrast to the 12Z/09 GFS that merged them to yield a very deep surface low. Details continue to vary for the wave that is most likely to track from the northern Plains through the Northeast Saturday-Monday. The new 00Z UKMET/CMC have added to the uncertainty by tracking noticeably farther northward, with corresponding significant differences in position of the trailing front. The MLs generally favored a composite of 12Z/18Z guidance. The updated forecast started with a 12Z/18Z operational model composite during the first half of the period to reflect the desired intermediate solution/continuity for significant features. Later in the period, a blend of models already toned down some detail differences within an ensemble mean/ML favored general pattern, so the forecast only needed to incorporate a modest input of the 18Z GEFS/12Z ECens means. ...Weather/Hazards Highlights... The deep upper low reaching near California by the start of the period early Saturday may produce some bands of locally moderate to heavy rain during the day, though moisture anomalies do not appear too extreme with this system. This activity could lead to some runoff issues given fairly high soil moisture anomalies currently existing along the coast. Thus the Day 4/Saturday Excessive Rainfall Outlook maintains a Marginal Risk area along the central/south-central California coast where the best average of guidance suggests potential for rainfall enhancement. A slight eastward adjustment in the model average for the upper low has led to a modest trimming of the northern side of the prior risk area. Farther inland, some precipitation in the Sierra Nevada may be in the form of snow. As the upper low progresses eastward, it should produce precipitation of varying intensity across the Great Basin and Four Corners states, followed by increasing coverage/intensity of rainfall over the Plains/Mississippi Valley during the first half of next week as it develops potentially strong low pressure. Highest precipitation totals with this system should be over the northern half of the Plains/Mississippi Valley. This storm may also produce an area of strong winds with the southern half of the High Plains being one particular area of focus. The upper trough digging into the Northwest during the first half of next week should ultimately increase precipitation over/east of that region. Meaningful snow will be possible over portions of the northern half of the Rockies by around next Tuesday. With decreasing confidence in specifics for the time being, a weak wave/frontal system may produce some rain over parts of the Great Lakes/Ohio Valley into the northern Mid-Atlantic/Northeast during at least a part of the weekend. Upper ridging over the Plains and vicinity will support well above normal temperatures across the central U.S. especially during the weekend when some northern-central areas should see highs up to 20-25F above normal. Expect less extreme warm anomalies to extend through the eastern U.S. from Sunday onward. Meanwhile the upper low tracking into California this weekend will bring a below normal highs to the southwestern U.S. Saturday into Monday and the upper trough/cold front moving into the Northwest plus the developing Plains storm will likely bring below normal highs into the Northwest and northern High Plains by next Tuesday-Wednesday. Parts of the Northeast should see moderately below normal highs on Saturday followed by a warmer trend. Rausch 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