Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon May 06 2019 Valid 12Z Thu May 09 2019 - 12Z Mon May 13 2019 ...Active springtime pattern for parts of the Lower 48... ...Overview... Significant precipitation will be in the forecast across portions of the southern and eastern U.S. during the period. A storm system reaching the Midwest by the start of the period early Thu and tracking northeast with time will bring one area of enhanced rainfall across the Great Lakes/Northeast late this week, along with a threat of strong to severe convection in the warm sector. Meanwhile there is still a pronounced signal for heavy rainfall potential from eastern Texas through the Lower Mississippi Valley and to some degree extending farther northeastward. This threat corresponds to guidance continuing to show the Midwest storm's trailing front stalling near and northeast from the western Gulf Coast, with some degree of low level Gulf inflow providing moisture while impulses progress along in southwesterly flow aloft. At the same time upper troughing over the West will bring multiple days of precipitation to locations from central-southern California into the southern half of the Rockies with highest totals over areas with favorable terrain enhancement. ...Guidance Evaluation/Predictability Assessment... The majority of guidance shows a similar theme for the evolution of mean flow aloft, an eastern Pacific ridge building into the Northwest states while split flow prevails to the southeast--a mean trough settling over east-central Canada into the Upper Midwest/Great Lakes and a developing upper low tracking near the western U.S.-Mexico border. The relative agreement in mean flow conceals significant differences/uncertainty in some day-to-day details. Greatest differences over the past couple days have involved northern steam flow, both within the mean trough over southern Canada and northern U.S. as well as North Pacific flow heading toward/into North America. The most noticeable multi-day trend has been for stronger ridging to build over the Northwest and to persist for a longer time. Uncertain details of North Pacific flow rounding the ridge continue to cause difficulty in resolving the timing and amplitude of a potential surface low/frontal system dropping from Canada into the northern U.S. late week onward. Based on the full array of guidance GFS runs may be somewhat strong/fast with the system but the 12Z ECMWF/ECMWF mean are slowest/weakest. The old 00Z ECMWF was somewhat closer to other guidance. Across this expanse of northern latitudes a blend/compromise approach provides reasonable continuity while reflecting latest trends for the Northwest ridge aloft. Guidance still shows a complex evolution taking place along and to the south of the California coast, as Interior West energy pushes southwest to interact with an offshore upper low moving eastward along 30N latitude. There will be various ways that this evolution could ultimately stray from consensus but so far the majority/intermediate solution has been consistent in showing the center of the upper low reaching near the southern Arizona border by Sun. Given the ridge building over the Northwest, suspect the ECMWF/ECMWF mean may be too quick to open up/eject the feature after Sun. The new 00Z GFS/GEFS mean maintain support for slower timing versus ECMWF-based solutions by Mon. For the storm tracking northeast from the Midwest, UKMET runs are notable for being on the strong/northwest fringe of the model spread. Otherwise consensus has trended a bit weaker from prior cycle but to a degree that is generally within the noise range. Most solutions continue to show a triple point wave off New England by early Sat. Along the trailing front that will become the focus for heavy rainfall over the South, some uncertainty in waviness/frontal position arises toward Sun-Mon in response to details of the upper trough to the north. For the full forecast domain a general model blend early in the period trending toward a blend among the 18Z GFS/GEFS mean and 12Z ECMWF/ECMWF mean provided a reasonable starting point for representing the desired consensus or intermediate solution for features of interest. This approach provided fairly good continuity while awaiting any possible adjustments over areas of greatest uncertainty. ...Weather Highlights/Threats... The storm tracking northeast from the Midwest Thu-Fri will produce an area of rainfall that may be heavy at times over the Great Lakes/Northeast as well as thunderstorms (some strong to severe) in the warm sector from the southeastern Plains/Lower Mississippi Valley into the Ohio Valley/Lower Great Lakes, and possibly extending farther eastward with time. Consult Storm Prediction Center products for latest info on the severe weather forecast. In the northern part of the precip shield there may be localized areas of snow at times, with best potential (though with fairly ow probability in absolute terms) over the western UP of Michigan and extreme northern Wisconsin. The favorable setup of a stalling front, low level Gulf inflow, and impulses aloft carried by southwesterly mean flow, are maintaining the threat for heavy rain across parts of the South. The best potential for highest 5-day totals still exists over eastern Texas through Louisiana with significant totals extending farther northeastward. Guidance continues to show the possibility for at least 5-10" accumulations at some locations. The front may drift a little southeastward late in the period to tone down the heavy rainfall threat. Within the area of terrain-enhanced precipitation from central-southern California eastward into the Rockies, the best potential for higher elevation snow will be centered over the Rockies of Colorado Thu into Fri before heights aloft rise over that area. There is still uncertainty over details of the upper low that may track along the southwestern U.S./northern Mexico border but the favored average of guidance during the weekend into early next week yields a fairly consistent forecast for the time being. Rainfall may increase in coverage/intensity over the southern High Plains early next week as the feature approaches. The upper ridge building into the Northwest will bring multiple days of well above normal temperatures--generally plus 10-20F anomalies and maybe locally higher. Some daily records for highs/warm lows will be possible. On the other hand much below normal highs (minus 15-25F anomalies) will push down through the central/southern High Plains late this week and slowly moderate thereafter. To a less extreme extent the Southwest will see cool daytime highs as well. The East will be warm late this week ahead of the Midwest system, followed by a trend toward more normal readings. 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