Extended Forecast Discussion NWS Weather Prediction Center College Park MD 446 PM EDT Mon Apr 17 2023 Valid 12Z Thu Apr 20 2023 - 12Z Mon Apr 24 2023 ...Overview and Guidance/Predictability Assessment... An upper low atop the High Plains across the U.S./Canada border extending troughing into the central U.S. Thursday is agreeable in the guidance with showing a shift eastward through the weekend into early next week. But guidance continues to show some important detail uncertainties that 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. In the past couple of days, guidance has generally stabilized with the position of the the first upper trough/low Thursday, but questions remain regarding how much northeastern Pacific energy feeds into the trough late week, possibly leading to 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. By day 6/Sunday models vary with if/where the low remains closed, with recent GFS runs more open over the CONUS possibly having a closed low farther north in Canada. Tended toward the non-NCEP closed solutions to some extent. Over the Pacific, guidance seems to be trending toward a solution of holding most energy off the coast and keeping ridging across the West. The GFS/GEFS has had this type of evolution more persistently, but the EC suite trended toward this in the 00Z runs and the 12Z deterministic run has maintained it. But the CMC mean and NAEFS mean (at least through the 00Z runs) maintain a solution with troughing coming into the West along with the deterministic CMC. While this aspect of the forecast remains uncertain, the better consensus of the EC and GFS/GEFS suites seemed to suggest a ridgy pattern in the West in a change from the overnight forecast. Guidance spread/trends recommended an updated forecast employing an operational model blend early in the period and then a transition to including some GEFS and EC ensemble means late. ...Weather/Hazards Highlights... 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. This cycle of the WPC forecast embeds a Slight Risk into the larger Marginal from northeastern Texas and southeastern Oklahoma into much of Arkansas on Day 4/Thursday-Thursday night. This area could see the front stalling and a surface low along the front to enhance support for heavy rain, with possibly multiple hours of high rain rates. By Day 5/Friday, high rain rates will likely continue, but there is much more uncertainty in an area of focus, plus potential for the front to be moving faster, leading to less focus of the high rain rates over any particular area. Thus maintained a Marginal Risk across the south-central U.S. into the Mid-South and Ohio Valley on Day 5/Friday, but an embedded Slight Risk is certainly possible in the future. 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, while most snow looks to focus in Canada, there will be the potential for some snow across the far northern tier of the U.S., particularly from North Dakota through the northern half of Minnesota to the U.P. of Michigan. A possible but low-probability round of snow may also affect Lower Michigan around Saturday. 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. 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. Tate/Rausch Hazards: - Heavy rain from the lower to mid-Mississippi Valley into portions of the Tennessee and Ohio Valleys as well as the Midwest, Thu-Fri, Apr 20-Apr 21. - Heavy snow across portions of northern Minnesota, Thu-Fri, Apr 20-Apr 21. - Severe weather across the eastern portion of the southern Plains to just west of the lower to mid-Mississippi Valley, Thu, Apr 20. - Severe weather across portions of the lower Mississippi Valley and interior South, Fri, Apr 21. - Flooding possible from the northern Plains to the upper Midwest. - Flooding occurring or imminent across portions of the northern Plains and the upper Midwest. - Flooding likely across portions of the northern Plains and the upper Midwest. 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