Extended Forecast Discussion NWS Weather Prediction Center College Park MD 107 AM EST Sun Mar 08 2020 Valid 12Z Wed Mar 11 2020 - 12Z Sun Mar 15 2020 ...Heavy Precipitation Threat next week for Southern California/Southwest... ...Guidance/Predictability Assessment... An interesting and active flow regime is 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 Southeast/Southern 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 by next weekend, with the GFS continuing to show a much more amplified wave than much of the other guidance. These differences play out in dramatically different solutions at the surface between the GFS and ECMWF next weekend, with the ECMWF showing a more suppressed surface low track from the Southern Plains to the Tennessee Valley, and a formidable round of cold polar air farther north from the Midwest to the Northeast. Meanwhile, the GFS depicts more phasing and quickly deepens a low pressure system as it moves from the Midwest into Ontario. Based on an assessment of the current ensemble spread, something closer to the ECMWF 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. Some differences remain among the guidance with respect to the character of this feature (deep closed low in the ECMWF or amplified positively-tilted shortwave in the GFS). The WPC forecast initially based heavily on the 12Z ECMWF during days 3-4 (Wed-Thu). The ECMWF was preferred for its perceived better handling of the southwestern upper low later this week, and general agreement with the overall model/ensemble consensus elsewhere. During days 5-7 (Fri-Sun), weight was gradually increased on the ensemble means, with some continued use of the ECMWF. The ECENS mean was weighted more heavily than the GEFS mean. Thus, by next weekend the forecast was based approximately 3/4 on the combination of the ECENS/ECMWF. ...Weather/Hazard Highlights... Portions of the Southwest will see a period of heavy precipitation potential Wed-Thu as the upper-level low moves across inland. The heaviest rains are forecast across the Mogollon Rim in Arizona, where 1-2 inches, with locally higher amounts, will be possible Wed-Thu. Farther east, moist southerly flow should focus showers and thunderstorms along a stationary front from the Southern Plains to the Ohio/Tennessee Valleys Wed-Thu. 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 the main system and overrunning a northern stream frontal drape. There remains much uncertainty with the main axis of any heaviest amounts and there could be sufficient cold air to the north of the drape 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 associated frontal system affect the region with increasing vigor. Rain/snow associated with this system are forecast to spread inland across the Great Basin and northern Rockies by Sat. Temperatures are forecast to be above average across much of the CONUS Wed-Fri. Highs are forecast to range from 5 to as much as 20 deg F above average from the Plains to the Eastern Seaboard. Cooler temperatures are forecast across much of these areas (with the exception of the South) by next weekend as a polar air mass spreads southward across the central/eastern states (reinforcing a strong surface front across the South. Ryan 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