Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 AM EDT Wed Apr 19 2023 Valid 12Z Sat Apr 22 2023 - 12Z Wed Apr 26 2023 ...Overview and Guidance/Predictability Assessment... Latest guidance shows an amplified pattern during the weekend as a negatively tilted upper trough moving into the East closes off an upper low near the Upper Great Lakes while a ridge drifts into the West. Then the mean flow should become flatter and broadly cyclonic by midweek, as the Great Lakes upper low continues into Canada while the elongating trough reaching into the Northeast steadily weakens underneath a blocking ridge that builds halfway between Hudson Bay and southern Greenland, and Pacific shortwave energy reaches into the western/central U.S. The two main precipitation areas of interest will be over the eastern U.S. ahead of the amplified trough and then from the Pacific Northwest through the central U.S. in association with the early-mid week trough energy. Within the initial eastern trough, models are still in the process of refining the shortwave details that will be important for resolving surface low details from Saturday into Sunday as well as timing of the trailing front that ultimately pushes into the Northeast. In the 12Z/18Z guidance, the UKMET was the notable extreme in being much slower with the primary surface low so the updated forecast did not use that particular solution. The 00Z GFS jumped to being slightly on the fast side after the 18Z run was just a tad slow, while the 00Z CMC/UKMET fit the majority cluster well. Overall the 00Z model trends so far are toward faster progression of the front reaching the Northeast, lowering QPF totals. Meanwhile the guidance has been showing significant multi-day oscillations for the Pacific/western U.S. pattern for early next week. Most solutions are settling on the idea of Pacific shortwave energy progressing into the western-central U.S. by next Monday-Wednesday, though with a fair amount of spread for amplitude and timing. The 12Z ECMWF and latest CMC runs are more amplified versus the GFS with a somewhat farther south track of a possible embedded upper low. Ensemble means reflect this mean pattern as well, but individual member spaghetti plots still become widely dispersed by that time frame--reflecting the various possibilities that guidance has been showing over recent days. The fact that all the operational models maintain the closed upper low tracking across the central into east-central Pacific through day 7 Wednesday versus the means that open it up earlier would provide support for maintaining a meaningful operational component late in the period. Based on the above considerations, early part of the period incorporated the 18Z GFS and 12Z ECMWF/CMC followed by a transition toward half models and half total 18Z GEFS/12Z ECMWF means, to account for lower confidence in details but some valid signals in the operational runs over the Pacific. ...Weather/Hazards Highlights... Ahead of the eastern U.S. upper trough/Great Lakes-Canada upper low, the cold front reaching the East Coast will provide some focused rainfall but recent model runs are showing that the duration and totals will be very sensitive to small scale shortwave details. The new 00Z model runs have trended noticeably faster, thus lowering rainfall amounts. The Day 4 (12Z Saturday-12Z Sunday) Excessive Rainfall Outlook maintains the Marginal Risk area from North Carolina into parts of West Virginia but with decreasing confidence especially for the western part of the area. Then as the front directs Atlantic flow into the Northeast by Day 5 (12Z Sunday-12Z Monday), latest trends are clustering toward only a fairly small area of significant totals, most likely in the vicinity of southwestern Maine. Given the small nature of the area, low predictability in light of model variability thus far, and fairly dry ground, no risk area has been proposed at this time. On the other hand, there is a Day 5 Marginal Risk area proposed over southern Texas where a signal exists for some locally heavy rainfall with time combination of easterly low level flow and weak shortwave energy. Moisture from the Pacific will initially bring some rain and higher elevation snow into the Northwest/Northern Rockies, with moderate amounts. Then precipitation should progress east-southeast into the central Rockies while rain increases in coverage and intensity over central-southern parts of the Plains/Mississippi Valley. Specifics of location and amounts by Monday-Wednesday have lower confidence as they will be determined by upper shortwave/embedded upper low details that the guidance is not consistently resolving yet. Some areas of heavy rainfall could develop over the central U.S. toward Tuesday-Wednesday with the potential contribution of Gulf moisture. A broad area of below normal temperatures reaching the Plains to just west of the Appalachians as of Saturday will push eastward Sunday-Monday, with decent coverage of minus 10-20F or so anomalies during the weekend. Warmth ahead of the leading cold front should maintain above normal temperatures through Saturday over the northern Mid-Atlantic into far eastern Great Lakes. Expect moderately above normal temperatures over the Southwest/California during the weekend, continuing to some extent into next week. Elsewhere the overall flattening of the mean pattern aloft by midweek will support a steady trend toward more normal temperatures. 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