Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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... There's reasonable agreement among the non-GFS guidance as it pertains to the synoptic evolution of the Western upper-trough pattern during the medium range period. Elsewhere, the GFS clusters well with the other models. An unusual amount of timing spread exists within the GEFS/ECE/CMCE members for the West Coast system at the beginning of day 3 (12z 04/13). A significant number of ensemble members seem to suggest that the approaching mid-level cut-off low will be slower/displaced to the west compared to their respective means. This diffusion in the ensemble members only worsens as the system moves inland in the subsequent days. On days 3 and 4, a general model blend consisting of the deterministic 00z EC/UKMET/CMC and 06z GFS were used. Weighting favored the 00z models since the GFS is much slower with the cut- off low in the West. The non-Graphcast (GFS based Ai) models produced something similar to that of the 00z operational guidance. The descending trough from western Canada introduces another area of uncertainty on day 5. That being said, the non-GFS guidance continues to cluster relatively well comparitively regarding the timing aspect of the western trough. Despite that the EC diverges a bit from the UK and CMC's longwave energy across the Upper Midwest/Great Lakes. Thus, the 00z ECE was introduced to the blend for day 5. The GFS and UKMET are phased out of the blend on day 6 and are replaced with the 00z CMCE and 06z GEFS. The once Western low moves into the Great Plains and begins to tilt negatively on day 6. There's spread on how quickly this occurs. The GEFS and GFS appear to be a slightly slower and deeper than the Euro and Canadian based guidance with this system, which is reflected in it's enhanced QPF footprint over the Upper Midwest on Tuesday. The inclusion of the GEFS helps to account for areas farther west over Minnesota that may be impacted by heavy rainfall that day. The mid-level trough in the Northwest carries quite a bit of ensemble spread with it, potentially due in part to any interaction it may have with the Plains system. The ECE appears to have the least amount of spread between the ensembles on day 7, with the upper-trough entering the Intermountain West. Therefore, the ensemble means with weighting toward the ECE was preferred on the day 7 blend. ...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. Kebede/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