Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Thu Apr 08 2021 Valid 12Z Sun Apr 11 2021 - 12Z Thu Apr 15 2021 ...Overview and Guidance Evaluation/Predictability Assessment... Latest guidance continues to show various significant uncertainties with forecast specifics in a blocky pattern. Upper ridging over and east of eastern Canada with multiple closed lows likely to meander to the southwest/south--between the northern tier states and the Canadian Maritimes--will be a dominant theme from the start of the period. An eastern Pacific ridge that builds into Canada ahead of an amplifying North Pacific trough will add to the blockiness mid-late period, likely supporting some degree of mean troughing over the western U.S. with a possible embedded upper low. As for the details, models have varied with what becomes of energy within a leading compact Great Lakes upper low Sunday-Monday as upstream energy (likely another closed low) approaches--ejecting eastward or becoming incorporated into the flow around the upstream feature. For the upper low coming in from the northern Plains, the past couple ECMWF runs have been weaker and more elongated with the energy than most other guidance. New 00Z GFS/CMC/UKMET runs are still more consolidated with the upper low. The 12Z ECMWF uses some of its western elongated energy to develop a deeper closed low a little west of consensus by day 7 Thursday though. Also late in the period GFS runs become more progressive than consensus with this upper low. GEFS mean runs have been holding the upper low back toward the Upper Great Lakes, closer to the majority scenario, due to greater persistence of the eastern Canada ridge. Meanwhile solutions stray considerably with time for the upper low initially near the Canadian Maritimes. Similar behavior among ensemble members is likely playing a role in the means becoming a lot weaker than most operational runs by mid-period. Preference would be for an intermediate solution with the Maritimes low, keeping it a little east of New England. Specifics over the West are still uncertain as well. There is reasonable consensus for mean troughing to develop from early in the week onward but with a fair amount of spread for depth and existence of a possible embedded upper low. The GFS had been mostly on its own for the deep and upper low scenario but recent CMC runs have been trending in its direction and now the 00Z UKMET shows a closed low for a time as well. Through the 18Z cycle the guidance spread was sufficient to favor holding close to continuity via the ensemble means that have all been depicting an open trough of more moderate depth. Based on 12Z/18Z guidance comparisons and continuity, the updated blend started with an operational model consensus early and then transitioned to nearly all 18Z GEFS/12Z ECMWF means by days 6-7 Wednesday-Thursday due to various low-confidence aspects of individual models. The 12Z ECMWF was just close enough in principle to maintain modest input late in the period. Manual editing improved definition of the Maritimes low at that time. The new 00Z ECMWF has completely changed to the GFS cluster in many respects over and near the lower 48, so depending on how the 00Z ECMWF/CMC means trend there could be a significant adjustment in the most likely scenario. ...Sensible Weather and Potential Hazards... The leading system affecting the East on Sunday will produce areas of rainfall over parts of the Great Lakes/Northeast while the trailing front should support some moderate to heavy rain over Florida. The upstream system emerging from the West will spread precipitation across the northern tier, with some snow possible over extreme northern areas. The combination of the second frontal system as well as the leading one that stalls over the northern Gulf/Gulf Coast region could produce some areas of enhanced rain/thunderstorms from southern portions of the Plains to the Lower Mississippi Valley/Gulf Coast. Initially light and scattered precipitation over the northern-central Rockies should become somewhat heavier and more organized into the central Rockies from Tuesday onward as upper troughing develops over the West and high pressure building southward through the Plains promotes upslope flow over some areas. The best potential for meaningful snow will be over the southern Wyoming into Colorado Rockies and possibly east into portions of the High Plains. With greater uncertainty, some precipitation could extend farther westward. Expect portions of the Mid-Atlantic and eastern Great Lakes/Northeast to see well above normal temperatures on Sunday, especially for morning lows with plus 15-25F anomalies possibly near daily record values if they hold for the calendar day. The Sunday frontal passage over the East will bring temperatures down only to near or modestly above normal levels early next week. A cooler airmass will extend from the northern Rockies through much of the Plains from Sunday into midweek behind another front. Most days should feature some areas with highs 10-20F below normal. This cool air will continue into the Mississippi Valley and portions of the East late in the period but in modified form, yielding only single-digit negative anomalies. Above normal temperatures over the Southwest on Sunday should gradually trend closer to normal as upper troughing develops over the Interior West. On the other hand an approaching Pacific ridge aloft may raise northern California and Pacific Northwest highs up to 10-20F above normal from Monday or Tuesday through Thursday. There are still enough uncertainties with specifics of flow aloft late in the period to lower confidence in temperature specifics over the West. 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml