Extended Forecast Discussion NWS Weather Prediction Center College Park MD 430 PM EDT Thu Apr 22 2021 Valid 12Z Sun Apr 25 2021 - 12Z Thu Apr 29 2021 ...Heavy snow possible for the Sierra Nevada Sunday... ...Heavy rain threat for parts of the Southern Plains into the South next week... ...Overview... The dominant feature of the period will be an eastern Pacific/West Coast upper trough that should make its way through the West during the first part of next week and reach the central U.S. by next Wednesday or so. The low pressure/frontal system and broad flow of moisture ahead of the trough will spread precipitation over a large portion of the West, including appreciable to perhaps significant snow in higher elevations of the Sierra Nevada. Meaningful totals should also extend into the Great Basin and Rockies. Once the upper trough reaches far enough east, expect southerly flow from the Gulf to strengthen and add moisture to an unstable environment--leading to the potential for severe weather and heavy rainfall from portions of the Southern Plains eastward. The rest of the east-central U.S. may see somewhat less extreme but still possibly significant rainfall. Corresponding to guidance differences for specifics of energy within the upper trough and potential influence of upstream Pacific flow, there is still a fair degree of uncertainty in the details of the rainfall threat. However confidence is higher in the general theme of a heavy rainfall event. This system will separate well above normal temperatures to its east and chilly but slowly moderating readings in its wake. Meanwhile early in the period strengthening low pressure will track from near the Mid-Atlantic coast into the Canadian Maritimes and affect the Northeast into early next week. ...Guidance/Predictability Assessment and Preferences... The primary forecast consideration was to continue leaning slower than most recent GFS runs for the main system of interest, especially by the latter half of the period. The 00Z GFS was somewhat closer to existing consensus from the 00Z models and 00Z/06Z ensemble means so that run provided the modest GFS input for the blend. Trends over the past couple days have leaned slower, and in fact the new 12Z CMC/UKMET close off a slower/southward western upper low to extend this recent trend. The 12Z ECMWF upper low over the West also wobbles southward by late Tuesday but ultimately is not far from continuity the rest of the way. Guidance has been having difficulty in resolving the specifics of upstream Pacific flow for multiple days, leading to wide variance in when/how much shortwave energy and moisture may reach the Northwest and having some influence on the western-central U.S. system. Latest trends from 12Z guidance support a stronger ridge moving into the West. A consensus operational model blend with somewhat more ECMWF input early in the period, followed by a transition to a model/mean mix tilting more to recent ECMWF runs/00Z CMC/00Z ECMWF mean relative to the 06Z GEFS/00Z GFS represented preferences before arrival of 12Z solutions. The system affecting the Northeast early in the period has trended a bit weaker/faster over recent days. Among latest guidance the 12Z CMC is the oddball in pulling the upper energy and surface low well south of consensus. ...Weather/Hazards Highlights... The system affecting the Northeast early in the period will wrap around showers (some snow in higher elevations) on Sunday into Monday. Some enhanced rainfall is possible over New England with windy conditions region-wide as the low deepens and tracks into the Canadian Maritimes. Moderately below normal temperatures will prevail immediately behind this system. In the West, the incoming Pacific upper trough/low and surface front will likely push through California Sunday and the Great Basin Monday. This system will spread lower elevation rain/mountain snow focused over southwestern Oregon down into northern and central California, especially along the Sierra Nevada. Moderate rain/snow will also fall across the Great Basin and central Rockies with lowering snow levels behind the front. Expect highest totals with this system to be over the Sierra Nevada where over a foot of snow is possible. Lesser amounts of moisture should extend to the north and south of this corridor. Also some leading precipitation may stream across the extreme northern tier into the Upper Great Lakes, with a little snow possibly included near the Canadian border. Once the system moves to the Plains by Tuesday or midweek, potential will increase for severe weather (see SPC for details) and areas of heavy rainfall. The best signal for highest rainfall totals currently exists over/near Arkansas into the Lower Ohio Valley but a much broader area from the Plains into Lower Mississippi Valley/Tennessee Valley could see at least some heavy rainfall in this event. There is good confidence in a significant rainfall event overall but the details remain in question. Ahead of the initial western system, an area of above normal temperatures will progress from the central-southern Plains into the East over the course of the period. Warmest anomalies in the plus 10-20F range should extend from the southern two-thirds of the High Plains through the Midwest, Great Lakes/Ohio Valley, and Northeast. Some areas may approach or reach their warmest readings so far this season. Cooler air behind the western cold front will likely bring high temperatures down to 10-20F below normal over parts of the southern half of the West early next week. In modified form (with decreasing coverage of double-digit anomalies) the cool air will reach the Plains by midweek or so. The West Coast into western Great Basin should rebound to above normal by next Thursday as an upper ridge builds in. Expect parts of the Northern Plains and vicinity to be on the chilly side into Tuesday before warming to near or above normal by Wednesday or Thursday. Rausch/Fracasso 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 Hazards: - Heavy precipitation across portions of the Central Great Basin and California, Sun, Apr 25. - Heavy rain across portions of the Central/Southern Plains, the Middle/Lower Mississippi Valley, the Tennessee Valley, the Ohio Valley, the Great Lakes, the Southern Appalachians, the Southeast, and the Mid-Atlantic, Tue-Thu, Apr 27-Apr 29. - Heavy rain across portions of the Middle/Upper Mississippi Valley and the Great Lakes, Wed, Apr 28. - Heavy rain across portions of the Northeast, Sun, Apr 25. - Severe weather across portions of the Central/Southern Plains, Tue, Apr 27. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - High winds across portions of the Central Great Basin, the Southwest, the Central/Southern Rockies, the Central/Southern Plains, and the Northeast, Sun-Mon, Apr 25-Apr 26. - Enhanced wildfire risk across portions of the Southern Rockies, the Central Great Basin, and the Southwest, Sun, Apr 25. - Enhanced wildfire risk across portions of the Southern Rockies, the Southern Plains, and the Southwest, Mon, Apr 26. 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