Extended Forecast Discussion NWS Weather Prediction Center College Park MD 111 PM EDT Fri Apr 17 2020 Valid 12Z Mon Apr 20 2020 - 12Z Fri Apr 24 2020 ...Another Excessive Rainfall/Severe Weather Threat over the South Mid-Late Week... 17Z Update: There is good overall deterministic model agreement among the GFS, ECMWF, and UKMET in terms of mass fields across the continental U.S. for the first half of the forecast period. The 00/12Z CMC is much stronger with the upper level ridge over the Pacific Northwest and also differed on the erosion of the ridge along the West Coast with the arrival of the southern stream trough, so it was not used in the forecast process. By Thursday and Friday of next week, recent runs of the GFS were more amplified across the interior western U.S. when compared to the ensemble spread, so a slightly greater weighting towards the ECMWF/ECENS was used for Thursday and Friday, which also maintains previous continuity. With the strong storm system crossing the southern Plains and Deep South during the Wednesday to Friday time period, the ECENS members have been more amplified with this shortwave compared to the CMC and GEFS members, and this also has support from the stronger 12Z UKMET, which is now the strongest deterministic model across Oklahoma and Texas. Periods of heavy rain with strong/severe thunderstorms remain likely across portions of the Gulf Coast region as this disturbance moves through the region. The previous discussion from last night is appended below for reference. /Hamrick ...Overview... Latest guidance agrees upon a fairly progressive pattern with an active southern stream that will carry one system away from the East Coast at the start of the period Mon and bring a trailing system across southern parts of the West and eventually toward the East Coast. This second system has the potential to generate another episode of heavy rainfall and severe weather over portions of the southeast quadrant of the lower 48, after the prior event expected to affect the region into at least Sun night (now in the short range time frame). Meanwhile a deep upper trough initially extending from Canada into the northeastern quarter of the U.S. will relax after midweek, though some degree of cyclonic mean flow is likely to persist across most of Canada and northern fringe of the lower 48 through late week. By Thu-Fri Pacific energy moving into the West and an upstream ridge should lead to at least a modest trough over the western half of the country but with uncertain depth/timing. ...Guidance/Predictability Assessment... The updated forecast emphasized an operational model blend Mon into early Wed and then trended toward an even model/mean blend with slightly more emphasis on the 12Z ECMWF/ECMWF mean relative to the 18Z GEFS/GEFS mean. The overall consensus of guidance has nudged a bit faster with the system departing from the East early in the week and the trailing system forecast to traverse the country during the period. This faster adjustment was accepted given the progressive nature of the large scale flow. However there is some degree of stream separation that provides a lingering potential for a return to slightly slower timing. As it is the current solution lies between the slightly slower continuity and faster 12Z CMC/CMC mean. System evolution over the eastern half of the country after Wed will become increasingly influenced by uncertain interaction with southern Canada/northern tier U.S. flow--adding support for a conservative model/mean approach. 00Z CMC switching to a slow solution and new GFS/ECMWF runs that are similar to or a bit faster than previous cycle highlight the current uncertainty. Also late in the week, recent model runs and ensembles have been diverse and inconsistent for the specifics of the shortwave energy that comes into the West. This is due in part to differences in how upstream energy moves into the eastern Pacific upper ridge. The 18Z GEFS mean was on the slower/amplified side of the full range of guidance for the late week western trough but within typical error range for a day 7 forecast. The new 00Z GEFS mean has adjusted toward the 12Z ECMWF mean. Other 00Z guidance shows varying timing/amplitude but at least leans away from the slow scenario. Overall clustering appears better than average for the strong dynamics crossing the Great Lakes/New England Mon-Wed and associated surface low pressure/frontal system. ...Weather/Hazard Highlights... The front trailing from the system affecting the South/East through Sun night may still support a band of rainfall extending into the extreme Southeast/northern Florida during the day Mon. The upstream system reaching the Southwest early in the week should produce mostly light to locally moderate precipitation over parts of the southern half of the West. Somewhat heavier activity is possible over central/south-central parts of the Plains by Tue-Tue night. Then the continued progression of upper dynamics/surface low pressure and corresponding rebound of Gulf moisture should lead to a period of heavy rainfall/severe weather potential from the Lower Mississippi Valley into the Southeast into at least Wed-Thu. There continues to be a good signal from the guidance for existence of this system though timing and system evolution specifics remain uncertain. Some areas will be very sensitive to any additional rainfall given what is expected with the preceding system through Sun night and significant totals already observed in the past week. Shortwave energy and moisture reaching the West during the latter half of the week will bring areas of terrain-enhanced rain and higher elevation snow across the Pacific Northwest through the northern Rockies. Light to locally moderate precipitation will accompany the low pressure/frontal system affecting the Great Lakes/Northeast during the first half of the week. Upper troughing over the northeast quarter of the country will support below normal temperatures over the northern half of the East Mon into Wed, with readings 5-15F below normal most common across the Great Lakes/Northeast. On the other hand a weakening upper ridge over the Northwest U.S. and western Canada as of Mon will bring similar anomalies on the warm side of normal between the Northwest and Northern Plains early-mid week. Elsewhere temperatures will likely be within 10F on either side of normal and exhibit some variation with system progression. 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