Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Sun May 09 2021 Valid 12Z Wed May 12 2021 - 12Z Sun May 16 2021 ...Overview... Within the moderately progressive regime across most of the lower 48 and vicinity, guidance is gradually trending toward better agreement for a more muted surface reflection associated with Plains shortwave energy that tracks into the East behind a departing Northeast trough. On the other hand the arrival of yesterday's 00Z ECMWF has ultimately heralded what continues to be a more chaotic trend for how eastern Pacific shortwave energy may evolve as it approaches the western U.S. Some of these differences may ultimately have an impact on the eastern half of the country later in the period. Between these features an upper ridge will move into the West and then the Plains, bringing well above normal temperatures to parts of the West in particular. Southern tier states will see a drying trend after midweek as a rainfall-focusing front sags southward from the Gulf Coast region. Unseasonably chilly temperatures north of this front as of midweek will moderate with time. ...Guidance Evaluation and Preferences... Latest ensemble means and GFS runs have been trending toward the theme of recent ECMWF/CMC runs that shortwave energy emerging from the Rockies/Plains will not be sufficiently strong to produce a defined surface wave near the central/northern East Coast after midweek. Current consensus is noticeably more sheared with the shortwave while it is over the Plains though the feature still becomes somewhat better organized farther east. Frontal waviness now remains confined rather far south, takes longer to organize, and then tracks much farther offshore once it gains latitude around Friday-Saturday. There is still some concern that the upper trough could produce a little more of a dent in the surface high pressure near the East Coast but it is hard to argue against the trends from the past couple days. In spite of fairly good agreement for a strong upper low/storm system forecast to reach the Aleutians and Bering Sea during the latter half of the week, models and ensembles have become more diverse in handling multiple pieces of eastern Pacific energy that had recently been forecast to amplify into the West Coast as a phased trough. Taken as a whole, the past day of guidance suggests a significantly increased potential for some degree of flow separation to take place. The question is where/when this separation occurs. The last two ECMWF runs (three counting the new 00Z run) pull off an upper low farther west than other models while the GFS has tended to be the most progressive (and steadily trending slower), and farthest east with separation (if it occurs). Latest CMC/UKMET runs have been between the two extremes. The elongated nature of the ECMWF mean reflects the variance in possible separation. GEFS/CMC means have remained more phased for the time being but reflect a slower and more diffuse trend for the trough. Teleconnections relative to the positive height anomaly between Alaska/Hawai'i and negative anomaly just north of Hudson Bay, the most common features in the D+8 multi-day mean charts, provide conflicting guidance--which could explain some of the difficulty guidance is having within this area. The exact nature of shortwave energy will eventually have a significant effect on wave/frontal details to the east of the Rockies. The updated forecast started with a 12Z operational model blend early in the period and then transitioned to about two-thirds weight of the 18Z GEFS/12Z ECMWF means with only minority inclusion of the GFS/ECMWF/CMC by next weekend. This solution reflected latest trends over the eastern U.S./western Atlantic while leading to a conservative adjustment amid the considerable spread over the eastern Pacific and western U.S. ...Weather Highlights/Hazards... Some moderate to perhaps locally heavy rainfall near the Gulf Coast may extend into Wednesday while lighter rain should continue over parts of the Southeast into Thursday. Then the southern tier will see a drier period as the focusing surface front gradually pushes southward over the Gulf and the Florida Peninsula, with the latter to see varying intensity of rainfall persist the longest during the period. Scattered areas of rainfall may extend from the northern Rockies through parts of the Plains/Mississippi Valley ahead of a wavy front that reaches the Plains. Some of this activity could eventually extend farther eastward but with low confidence at this time. Areas from the central/southern Plains through the Southeast will see unseasonably cool highs on Wednesday with broad coverage of minus 10-20F anomalies. A few locations could challenge daily records for cool highs. The Southeast and to a lesser extent farther back into Texas may still see highs at least 10-15F below normal on Thursday. Other portions of the East will likely be below normal as well but with less extreme anomalies. A moderating trend over the South and East should yield temperatures within a few degrees on either side of normal by next Sunday. The upper ridge moving into the West and then the Plains will favor an area of temperatures 10-20F above normal from California and Oregon into the Great Basin with plus 5-10F or so anomalies extending into the central Rockies/Plains. An approaching upper trough should gradually erode this area of warmth but recent slower/weaker trends for the trough in general plus uncertainty over the character of the trough suggest a slower and less pronounced cooling trend. 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