Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Mon Apr 20 2020 Valid 12Z Thu Apr 23 2020 - 12Z Mon Apr 27 2020 ...Excessive Rainfall/Severe Weather Threat over the South Mid-Late Week... ...Overview... Latest guidance continues to be similar for the expected large scale evolution aloft, featuring a transition toward a western ridge/eastern trough pattern by next weekend and early next week. Incoming eastern Pacific energy will try to erode the western ridge somewhat while the upper trough heads into the East. The western ridge erosion may be temporary based on model/ensemble forecasts beyond day 7 Mon. Within this agreeable large scale scenario there are still a number of uncertainties regarding how individual shortwaves within the developing eastern trough will evolve/interact as well as for the multiple shortwaves that may flow into western North America from the Pacific during the weekend and early next week. On the positive side there is at least some improvement in general clustering for depiction of the surface pattern associated with the eastern trough. Earlier in the period there are persistent strength and timing differences for the system tracking from the south-central Mississippi Valley through Mid-Atlantic into the western Atlantic. The associated heavy rainfall/severe threat across the South should be ongoing at the start of the forecast period and extend at least through Thu, while meaningful rainfall will extend farther north and continue into Fri. ...Guidance/Predictability Assessment... For the system affecting the eastern U.S. late this week, the 12Z/18Z cycles of guidance generally trended a little faster than runs from 12-24 hours ago. Similar to some prior runs, the 12Z CMC/UKMET began to lag other models/means by day 4 while interestingly the 12Z ECMWF actually strayed to the faster side of the envelope by Fri. Following a majority emphasis of the GFS/ECMWF and their means appeared reasonable, with a small input of CMC/UKMET whose slower timing could be considered to provide some account for previous consensus continuity. New 00Z runs maintain the uncertainties seen thus far with important shortwave subtleties likely to remain unresolved until the short range time frame. The UKMET/CMC continue to defined the slow side of the envelope. Behind this system there is still considerable spread for the exact details of trough energy reaching the western/central U.S. late this week and pushing across the East this weekend into the start of next week. Thus far operational model runs do not provide any well-defined trends as timing and location changes of any possible embedded upper low (and thus corresponding surface evolution) appear fairly random so far. Ensemble means have shown a steadier trend over the past day toward a deeper trough reaching the East and better defined surface low pressure. The deeper trend aloft ultimately leads to a somewhat faster timing for the surface low/frontal system. Looking at the models and ensemble means as a whole, best probability for low pressure should translate from within the Ohio Valley to East Coast region early day 6 Sun to near or just offshore the New England coast by early day 7 Mon. Confidence in the specific 12Z/18Z GFS and 12Z ECMWF/CMC solutions is low but incorporating their ideas with the latest GEFS/ECMWF means best represented the most common aspects of the guidance while accounting for detail uncertainty. New 00Z model runs do not appear to provide any improved detail agreement. The theme of shortwave uncertainty continues upstream as eastern Pacific energy flows into western North America Sat-Mon with no clear consensus among latest guidance and poor continuity among runs from the prior couple of days. Among individual solutions that could be a little more questionable include the new 00Z GFS that is noticeably amplified with its shortwave reaching the West Coast Sat-Sun and the 18Z GEFS mean which is strongest with its trough that reaches the West Coast by early Mon. The shortwave reaching the Plains on day 7 Mon in the 12Z ECMWF did not compare well to other guidance either, and the new 00Z run has trended favorably. Preferred a model/mean blend to reflect the most agreeable aspects of the large scale pattern while deferring to future runs to provide some detail improvement. Based on the combination of system considerations, the updated forecast from Thu into early Sat started with greater emphasis on the 12-18Z GFS/12Z ECMWF with modest weight of the 12Z CMC/UKMET and 18Z GEFS/12Z ECMWF means. A more balanced model-mean weight by days 6-7 Sun-Mon yielded the desired intermediate solution over the East and minimized potentially adverse influence of the GEFS mean near the West Coast late in the period. ...Weather/Hazard Highlights... The system affecting the eastern U.S. late this week will spread a broad shield of rainfall to the east of the Mississippi River. Expect the threat of heavy rainfall and potential for some severe weather over the South to continue into Thu. Monitor Storm Prediction Center outlooks for the latest information regarding severe threats. Meaningful rainfall should reach farther north Thu-Fri, through the Ohio Valley/Mid-Atlantic and possibly parts of New England. Guidance spread for strength/track and timing of this system is still great enough to lower confidence for determining the coverage and intensity of rainfall over the northern Mid-Atlantic and New England in particular. The shortwave energy crossing the West late this week will bring a period of rain and higher elevation snow to the northern-central Rockies and vicinity. As this energy continues eastward and amplifies late week through the weekend there may be some areas of rainfall over the Plains with a broader shield of moisture spreading over the eastern states. There is still enough uncertainty over details at the surface and aloft to keep confidence fairly low in determining location and magnitude of the highest totals. Best ensemble probabilities for significant precip currently highlight an area encompassing the Mid-Atlantic/Appalachians/Northeast with a somewhat lesser signal for parts of the Ohio Valley/Great Lakes. Some operational model runs have other ideas though. Depending on exact evolution at the surface and aloft, there could be enough cold air present for some higher elevation snow near the Canadian border. A succession of shortwaves rounding the western U.S. upper ridge may bring periods of rain and possibly high elevation snow to parts of the Pacific Northwest into the extreme northern Rockies from the weekend into early next week. Guidance varies considerably on the timing and amplitude of individual shortwaves, leading to a wide range of possible outcomes for precipitation totals and southward extent of moisture. Above normal temperatures focused over and near California and the Southwest late this week should expand across most of the southern 2/3 or 3/4 of the West by Sat-Mon as ridging aloft builds over the region. Within this broadening area of warmth expect some highs to reach 10-15F above normal and possibly locally higher. Readings may be the warmest of the season so far and could approach daily records at a few locations. The Desert Southwest will likely see highs near or slightly over 100F. The eastern U.S. will tend to be on the near to below average side of the spectrum, with the system crossing the East late this week bringing clouds/rain followed by another push of cool air behind potential eastern U.S./western Atlantic surface development. Best potential for some highs 10-15F below normal will be on Sun-Mon. 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