Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Apr 17 2023 Valid 12Z Thu Apr 20 2023 - 12Z Mon Apr 24 2023 ...Overview and Guidance/Predictability Assessment... Guidance continues to show some important detail uncertainties for what should be a more agreeable large scale upper trough/anchoring low expected to drift from the West/High Plains into the eastern U.S. These issues temper confidence with some surface specifics as well as precipitation distribution. The blocky pattern forecast to extend from eastern Siberia and Alaska through Canada has been contributing some uncertainty, with a ridge near southern Greenland possibly gaining a role in the forecast late in the period as well. Meanwhile, a split in guidance over the central Pacific during the weekend ultimately leads to opposite ideas for the upper flow pattern over the West by day 7 Monday. There have been some notable trends over the past 24 hours regarding the upper trough/low. Consensus has adjusted the low somewhat northward, now close to the U.S.-Canadian border. Meanwhile, after the GFS/GEFS were fairly well on their own with how much northeastern Pacific energy feeds into the trough, there is now more of a signal for some variation of this to occur. This would lead to possible reforming of the upper low instead of there being one persistent low. Corresponding surface evolution would have a leading system from the Upper Midwest into Great Lakes followed by another wave that tracks from the southern Plains into the Ohio Valley/Great Lakes. As for guidance extremes, the new 00Z CMC strays to the slow side while the 12Z ECMWF showed more elongation of energy over the Northeast in response to its southern Greenland upper high extending farther west into Canada. The new 00Z ECMWF has trended away from that idea. The aforementioned split of guidance over the Pacific leads to the GFS/GEFS holding onto an upper ridge over the West through days 6-7 Sunday-Monday, in contrast to a trough arriving per the majority of other models and ensemble means. This difference has generally persisted over multiple runs. The new 00Z ECMWF now weakens this trough in deference to a stronger trough farther offshore, a trend in the GFS/GEFS direction. Guidance spread/trends recommended an updated forecast employing an operational model blend early in the period and then a transition to 50-60 percent total weight of the 18Z GEFS and 12Z ECens/CMCens means. This approach reflected the most common elements of the upper trough/low and surface evolution, while depicting a moderate western upper trough by the end of the period (now having lower confidence given new 00Z guidance). ...Weather/Hazards Highlights... Locations from the Pacific Northwest into the Rockies will see periods of rain and higher elevation snow during the period, with continued uncertainty over coverage and amounts on a day-to-day basis. Generally expect light to moderate amounts, though some localized enhancement will be possible over the Pacific Northwest if the heavier side of the guidance spread happens to verify. Guidance differences in the upper pattern by early next week further decrease confidence in precipitation coverage and totals at that time. Ahead of the advancing western upper trough, the Mississippi Valley and vicinity will be the focus for areas of heavy rainfall Thursday-Friday in association with the flow of moisture ahead of a couple surface waves and the overall mean front. The days 4-5 Excessive Rainfall Outlooks both depict Marginal Risk areas over this part of the country, with the day 4 (12Z Thursday-12Z Friday) area more expansive than the day 5 (12Z Friday-12Z Saturday) that extends from the Lower Ohio Valley south-southwestward. In both cases, embedded Slight Risk areas may arise once guidance details cluster better. Rainfall of varying intensity should then continue across the eastern U.S., with the possibility of highest totals over the Northeast if there is a period of Atlantic inflow supported by coastal low pressure. Meanwhile, there will be the potential for some snow across the far northern tier. The recent northward trend for the upper low track has reduced the possible snow coverage/amounts, though lower-end probabilities for 0.25 inch liquid in the form of snow still extend from northeastern North Dakota through the northern half of Minnesota to the U.P. of Michigan. As the upper trough initially over the West/High Plains drifts eastward, below normal temperatures over the northern half or so of the West and into the northern Plains on Thursday will spread across much of the central U.S. by Saturday and then into the East by Sunday-Monday. Some below normal anomalies could persist over the most northern/southern parts of the Plains into early next week. Coldest anomalies should be over the northern Plains with some highs 15-25F below normal late this week. Ahead of the front/waves heralding the arrival of this cooler air, the eastern U.S. will see readings up to 10-20F above normal Thursday-Friday. The warmth could persist into Saturday along the East Coast. Meanwhile, locations across California, the Southwest, and southern Great Basin may see temperatures trend slightly above normal from Friday through the weekend. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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