Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Tue May 05 2020 Valid 12Z Fri May 08 2020 - 12Z Tue May 12 2020 ...Record cold possible over the East especially this weekend... ...Western U.S. warmth/heat to subside early next week... ...Overview... An amplified evolution aloft featuring a narrow western North America ridge and downstream trough will lead to a pronounced temperature contrast over the lower 48, especially through the weekend. The pattern will look and feel like March over the eastern half of the country while much above normal temperatures prevail over the West, with daily records on both sides of the temperature spectrum being approached or exceeded. Consensus shows a gradual erosion/eastward drift of the western upper ridge by early next week as an upper trough (with embedded low) approaches the West Coast. This will serve to bring temperatures closer to normal over the West while moderation farther east will be more modest and most noticeable over southern areas. ...Guidance/Predictability Assessment... For specifics of the forecast, an operational model blend with greatest emphasis on the 06Z GFS/00Z ECMWF represented consensus well through mid-period. Those two models and their ensemble means maintained good enough agreement to allow for 60-65 percent operational input through day 7 Tue. The resulting blend yielded good continuity in principle with the primary adjustment being a faster trend for the Northern Plains through Northeast wave and its trailing front Sat-Mon. Slower trend in the new 12Z ECMWF is returning the guidance average to slower progression though. Guidance continues to show a compact and deep upper low tracking just northeast of the Upper Great Lakes and possibly through northern New England Fri-Sat, with associated heights at least 3-4 standard deviations below normal over some areas. This feature will promote significant deepening of a wave forecast to track over the Mid-Atlantic region and New England coast into the Canadian Maritimes. Specifics of this evolution and resulting effects on New England will depend on small-scale details that will likely take into the short range to be resolved sufficiently well to increase confidence. One notable trend of the past day or so involves the GFS/GEFS mean adjusting quicker/northward for development like the ECMWF/ECMWF mean which have been very consistent in principle thus far. There are still other solutions such as the 00Z UKMET that deepen the storm a little later and northward versus the current majority cluster. Across lower latitudes, greater definition in the latest ECMWF runs versus earlier versions has strengthened the overall guidance signal for potential waviness across parts of the southern Gulf of Mexico and Florida Peninsula during the weekend/early next week, with an associated heavy rain threat. Specifics remain fairly uncertain as upper dynamics appear to originate from northern Mexico where mean ridging prevails during the short range, with the shortwave energy becoming more enhanced over the western Gulf. Individual solutions also vary considerably for strength and timing. At the moment there is better than average agreement/continuity for the upper trough and embedded low (with associated surface system) that approach the West Coast during the first half of next week. More noticeable differences exist with the shape of the upper ridge to the north/northeast so some adjustments may still be in store for the eastern Pacific feature. Latest operational runs are remarkably similar with the idea that energy originating from northern Canada will close off a compact upper low that approaches the Upper Great Lakes by next Mon-Tue. This feature may bring another front into the northern tier states. Small scale of this feature and occasional guidance tendency to have difficulty with flow from high latitudes may temper confidence with this system for a while. ...Weather/Hazard Highlights... Over the West expect mostly dry conditions with well above normal temperatures from late this week through the weekend. Anomalies should be in the plus 10-20F range over the West Coast states and into the interior, with some scattered daily records for highs/warm lows. The eastern Pacific system nearing the coast during the first half of next week will push cooler temperatures into the West and areas of rainfall into northern California and Pacific Northwest. By next Tue above normal readings should be confined to the Great Basin and vicinity. To the east of the Rockies the pattern will seem more like late winter, especially over the Great Lakes and Northeast. The most extreme temperature anomalies should be on Sat when locations from the eastern Great Lakes through the central Appalachians may see highs 20-25F below normal. Temperatures on Sat and/or Sun may approach or break daily records for lows/cold highs within the broader area of minus 10F or greater anomalies covering much of the eastern half of the lower 48. Also over some areas this cold surge would lead to a late frost/freeze where the growing season has already started. By next Mon-Tue parts of the South may still see lows reaching 5-10F or so below normal but highs may recover to near-normal levels. However min/max temperatures will tend to remain 10-20F below normal from the northern half of the Plains into the Mid-Atlantic/Northeast. The Mid-Atlantic into Canadian Maritimes system and trailing front will bring one area of precipitation to locations between the western Gulf coast and the Northeast late this week into the weekend. Some snow may fall from near the central Appalachians into the higher elevations of New York/New England and even down to higher valley floors, depending on the evolution of the system. Confidence in precise intensity/duration of precipitation is still moderate at best given ongoing detail uncertainty. Snow in May is an infrequent but not unprecedented occurrence in the Northeast. Behind the system, lake-effect/enhanced rain/snow is also possible and a period of brisk to strong winds will add to the late-winter feel of the chilly air over the Northeast. Western/southern Gulf of Mexico waviness that may develop along the tail end of the late-week/weekend system's front may bring a period of heavy rainfall to southern Texas and then to parts of the Florida Peninsula between Fri night and early next week. Especially over Florida the confidence in specifics is fairly low thus far. The precipitation shield with the Northern Plains to Northeast wave Sat onward should be fairly compact and confined near the surface low, with light to isolated moderate amounts. By next Mon/Tue the Central Plains and vicinity may see some rainfall as moisture and shortwave energy interact with the stationary front draped over central High Plains/Southern Plains. 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