Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 PM EDT Sun Mar 08 2020 Valid 12Z Wed Mar 11 2020 - 12Z Sun Mar 15 2020 ...Southwest heavy rain threat into mid-late week to emerge over the south-central U.S... ...Guidance/Predictability Assessment... It remains the case that an active flow regime remains on tap for the lower 48 and vicinity during the medium range as a relatively progressive northern stream and less progressive/separated southern stream are forecast. Through the first half of the week, models/ensembles showed relatively good consensus, but that degrades quite a bit toward the end of the forecast period. An area of low pressure is forecast to move away from the Northeast on Wed, with the cold front trailing back into the south-central Plains. Farther west, across the east Pacific, a deep cutoff upper-level low is forecast by all guidance to approach Southern California by Wed-Thu, eventually reaching the central U.S. Models/ensembles over the past 24 hours have shown a trend toward a slower progression of this system, although that trend seems to be ending with the latest arriving solutions appearing more stable in terms of timing. The GFS has largely lagged the ECMWF on this trend. This slower progression makes for less phasing/interaction with northern stream energy than was shown in previous days, as southern system now completely misses the leading northern stream energy in terms of timing. Thus, latest solutions show a continued separate southern stream wave emerging into the Plains by Fri night. At this time model/ensemble spread has reduced somewhat around this southern stream feature, but a larger difference was noted with another northern stream wave forecast to cross the Great Lakes into next weekend. Differences play out in dramatically different solutions at the surface between recent runs of the GFS and ECMWF next weekend, with the ECMWF/ECMWF ensembles showing more suppressed surface low tracks from the Southern Plains through the South, and more pronounced round of cold polar air farther north from the Midwest to the Northeast. Meanwhile, recent GFS and GEFS have been showing more phasing/low pressure system development and tracks that tend to lift farther northeastward than the ECMWF/ECMWF ensembles into the cold dome. Based on an assessment of the current ensemble spread, something closer to the ECMWF ensemble mean was preferred. Farther west, models continue to depict an amplified shortwave trough/upper low reaching the Pacific Northwest next Fri-Sat. This feature is present across multiple runs/models and recent run-to-run continuity has improved despite lingering uncertainty with respect to guiding upstream ridge specifics. The WPC forecast was based heavily on the compatable 00 UTC ECMWF/ECMWF ensemble mean during days 3-5 (Wed-Fri). For next weekend, the WPC product suite was mainly derived from the ECMWF ensemble mean amid growing model spread and uncertainty and with an eye on continuity. ...Weather/Hazard Highlights... Portions of the Southwest will see a period of heavy precipitation potential mainly Wed-Thu as the upper-level low moves across inland. Farther east, moist southerly flow should focus showers and thunderstorms along a slow moving/wavy front stretching eastward from the south-central U.S. As the Pacific shortwave energy (former upper low) reaches the central U.S. by Fri/Sat, showers and thunderstorms, some strong, are expected to become more widespread from the Southern Plains northeastward to the Appalachians ahead and wrapping into the main system. There remains much uncertainty with the main axis of any heaviest amounts and associated runoff risks. There should also be sufficient cold air to the north to support some snow/ice on the northern periphery of the expanding precipitation shield. Meanwhile, the Northwest should see an increase in rain/mountain snow heading into next weekend as the aforementioned trough and frontal system affect the region with increasing vigor. Rain/snow with this system are forecast to spread inland across the West through next weekend. Ryan/Schichtel 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