Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 AM EDT Sat Apr 18 2020 Valid 12Z Tue Apr 21 2020 - 12Z Sat Apr 25 2020 ...Another Excessive Rainfall/Severe Weather Threat over the South Mid-Late Week... ...Overview... Continue to expect a fairly progressive pattern across the lower 48 while long-term negative height anomalies aloft persist over northern Canada. Strong dynamics over southeastern Canada and Great Lakes/Northeast will bring a vigorous surface low/frontal system across this area Tue-Wed. Meanwhile there is still a decent consensus that an upper trough emerging from the southwestern U.S. will support low pressure that should track from the Southern Plains to near the East Coast Wed into Fri. This system may focus another episode of heavy rainfall and strong to severe convection across the South during the mid-late week time frame. Significant rainfall could also extend farther northward over the central/eastern states. A frontal system and shortwave energy will bring an episode of precipitation into parts of the West. Specifics of this energy aloft and possible upstream energy, along with the associated surface reflection, become more uncertain after day 5 Thu--leading to lower confidence in sensible weather effects over a large part of the country by Fri-Sat. ...Guidance/Predictability Assessment... Based on guidance through the 12Z/18Z cycles, from day 3 Tue into day 5 Thu an operational model blend provided a good consensus and/or intermediate representation depending on the system of interest. Specifics of the southern tier system expected to reach the Lower Mississippi Valley by around early Thu will hinge on smaller scale shortwave details that have low predictability that far out in time. This favors a solution somewhere between the weakest and strongest extremes for the surface low. The updated blend provides good continuity from previous cycle. In the new 00Z guidance the GFS deepens the core of the upper system, leading to a stronger/northward surface low over the Plains/Mississippi Valley while the CMC strays from consensus much earlier than its prior run did. Over the past couple days the CMC has been very inconsistent and compared poorly to consensus for this system. The 00Z ECMWF has trended stronger/slower but remains south of the GFS track (more similar to the UKMET). So not counting the disagreeable CMC there is now a stronger trend for this system. Solutions are still diverse with a wave that may reach into the Upper Mississippi Valley/Upper Great Lakes Wed-Thu, and confidence is low given the weak nature of shortwave energy aloft. Clustering is better with the leading vigorous system affecting the Great Lakes/Northeast though typical detail uncertainties remain. After early Thu a composite of recent GFS/ECMWF runs and their means offered fairly good consistency for the Plains system expected to lift northeastward into the eastern U.S. and western Atlantic. The forecast eliminated the 12Z CMC component after Thu due to becoming more suppressed. Farther upstream there is decent agreement with the shortwave energy reaching into the western states by early Thu but then models/ensembles rapidly diverge for specifics. A couple possibilities include onward progression of this energy followed by another shortwave that reaches the west-central U.S. (per the GFS, while the GEFS mean ultimately dissipates the lead shortwave after Thu), or a phasing of energy into one trough (12Z and 00Z/17 ECMWF as well as ECMWF mean). Evolution of energy within the one or two shortwaves is yet another question mark. The progressive nature of the overall pattern appears to support continued eastward motion of the wave(s)/frontal system reaching the Plains/Mississippi Valley by day 6 Fri. This consideration favored including some ideas from recent GFS/ECMWF runs and ECMWF mean with an area of low pressure tracking into the Ohio Valley or vicinity by early Sat. Meanwhile teleconnections relative to the core of negative height anomalies forecast over northern Canada further reinforce the idea of a progressive mean pattern across the lower 48--but offer the potential for a modest dip in the flow at least over the southwestern U.S./northwestern Mexico. Thus the GFS/GEFS scenario over the western half of the country cannot be fully discounted. Combining the above led to trending the forecast toward multi-run operational input (due to low confidence details in any particular cycle) and among ensembles slightly more 12Z ECMWF mean than 18Z GEFS mean. Highlighting the uncertainty, the new 00Z ECMWF has completely changed to favoring a slower trough. ...Weather/Hazard Highlights... The system crossing southern portions of the West on Tue will bring mostly light to moderate precipitation to parts of the Four Corners states. Around Tue or Tue night rainfall should develop over portions of the southern half of the Plains with some activity locally moderate/heavy. Then during Wed-Thu expect a potential for some areas of heavy rainfall to continue eastward across the Lower Mississippi Valley and into the Southeast. Uncertain details of system evolution temper confidence in precise axis of highest rainfall totals, as well as northern extent of meaningful rainfall which could reach as far north as a Central Plains/Mid-Mississippi Valley to New England axis. This uncertainty also affects the ingredients relating to strong/severe convection. Consult the latest Storm Prediction Center outlooks for latest information regarding severe threats. Rainfall that has recently occurred over the past week and additional rain with a system this weekend will lead to high sensitivity to any more rainfall over portions of the South. Elsewhere, shortwave energy and moisture reaching the West will bring a period of terrain-enhanced rain and higher elevation snow of varying intensity from the northern half of the West Coast through the northern-central Rockies. As the energy aloft and associated surface system progress beyond the Rockies after Thu, some areas of rainfall should spread across parts of the central/eastern U.S. but specifics are very uncertain at this time. The system affecting the Great Lakes/Northeast early in the period will bring mostly light to moderate precipitation, with any snowfall most likely to be over interior New England and northern New York. The deep upper trough crossing the Great Lakes/Northeast Tue-Wed will bring a brief period of below normal temperatures to areas as far south as the Ohio Valley through Mid-Atlantic. Expect coolest readings of at least 10-15F below normal to be centered over the Great Lakes and interior Northeast. Meanwhile parts of the Northern Plains will see highs of 10-15F above normal into midweek. Specifics become increasingly uncertain late week into the weekend but the general tendency toward low amplitude mean flow should promote moderate temperatures over most areas during that time. One exception may be over/near California and the Southwest where proximity of a flat eastern Pacific-West Coast ridge aloft could support some localized plus 10-15F anomalies. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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