Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 PM EST Wed Jan 08 2020 Valid 12Z Sat Jan 11 2020 - 12Z Wed Jan 15 2020 ...Eastern U.S. storm this weekend to bring a heavy rain threat to lower half of the Mississippi Valley and areas to the east/northeast, severe weather potential to parts of the South, and wintry precipitation to the Midwest through Great Lakes/northern New England... ...Active pattern over the Northwest... ...Overview/Guidance Assessment... Into the first part of next week guidance is consistent and agreeable in showing persistent and amplified western U.S. mean troughing between strong ridges off the Southeast U.S. and over the east-central Pacific. This pattern will support at least a couple significant precipitation events over parts of the eastern half of the lower 48. The first will be a widespread event associated with low pressure tracking from the Mid-Lower Mississippi Valley through the Northeast this weekend (straddling the short-medium range overlap). Additional events next week have a less-defined focus at this time and thus have lower confidence in specifics, with best signals for highest rainfall totals over the interior Southeast. Multiple systems reaching the Northwest will produce significant precipitation over favored terrain in terms of five-day totals. There will be a pronounced temperature contrast between much above normal temperatures over the East and much below normal temperatures overspreading the northern Plains and to a lesser extent the northern/central portions of the West. Guidance is still in the process of resolving the evolution of the shortwave/surface low tracking northeastward from the Plains/Mississippi Valley through the Northeast this weekend. The 00Z ECMWF/ECMWF mean/CMC/UKMET were generally slower and depicted an occluded (or nearly so) surface system by early day 4 Sun with timing slower than the sheared 00Z/06Z GFS/GEFS mean. Shift in the ensemble consensus a bit quicker than 12-24 hrs ago suggested the quicker GFS was not to be discounted. Retained a 30% weighting of the GFS solutions vs 70s% ECMWF-led solutions through day 5 Mon. Upstream flow, with another shortwave progressing into the West, shows reasonable agreement into day 4 Sun. However after that time the models/ensembles begin to go astray regarding the timing and amplitude of this western energy once it continues beyond the Rockies as well as for upstream impulses. Thus there is very poor agreement over specifics of one or more surface waves that should emerge over the Plains and continue to the east/northeast. Then around Tue-Wed larger scale model discrepancies develop with respect to how North Pacific energy could begin to dampen the east-central Pacific upper ridge. Downstream effects may reach the western U.S. by day 7 Wed. Over recent days the guidance has varied with details of western Canadian flow aloft that should dip into the northern parts of the West/Plains and support the forecast trend toward colder temperatures over this area. Consensus approach served as a best start with increased weight toward the ensemble means by next Wed, given the quite varied solutions (and low confidence) upstream just south of Alaska by then. ...Weather Highlights/Threats... Heavy rainfall expected to be in progress over/near the Mid-Lower Mississippi Valley at the start of the period early Sat should continue to the east/northeast during the weekend as low pressure tracks from the Mississippi Valley through the Northeast. In addition to the heavy rainfall, expect this system to produce a severe weather threat across portions of the South. Check latest Storm Prediction Center products for updated information regarding the severe weather forecast. Meanwhile winter weather will be likely on the cold side of the system with a wintry mix in between. The current winter weather outlook highlights an axis from near Missouri through the central Great Lakes and northern New England for the best potential of meaningful snowfall and narrower swaths of sleet/ice. Additional waves emerging from the Plains may promote the stalling/northward return of the front trailing from the above weekend system. However wave specifics are very unclear at this time. As a result confidence is moderate to low regarding the next possible heavy rain event. Currently the best signal extends from the Lower Mississippi Valley into the far southern Mid-Atlantic with highest totals over the interior Southeast. Lighter precipitation will be possible farther to the north/northeast--snow over northern latitudes and mostly rain farther south. Over the Northwest continue to expect repeated episodes of rain and mountain snow with highest five-day totals focused along favored terrain of the Cascades and Pacific Northwest coast/northwest California. The northern Rockies will see a secondary maximum of snow while areas from the Sierra Nevada into the central Rockies will see somewhat lower totals. By the middle of next week forecast specifics become more uncertain as eastern Pacific flow differences range between persistence of the pattern with another system reaching the Pacific Northwest (currently preferred based on the ensemble means) or a farther offshore system that would allow for a break in the precipitation. With the expected southward movement of the cold air from Canada, snow levels may lower toward the lower elevations south of the U.S./Canadian border. This was reflected on the Winter Weather Outlook probabilities nearing sea-level next Tue-Wed. Gradually lowering heights aloft along the central/western U.S.-Canadian border will lead to much below normal temperatures spreading over an increasing portion of the northern Plains and West. The core of coldest air should be over/near Montana with some readings 30-40F below normal by early next week. Northern parts of the West may see highs 10-20F below normal by early-mid week. Eastern U.S. warmth should be the most extreme this weekend ahead of the Mississippi Valley through Northeast system, with widespread plus 20-35F anomalies that may reach/exceed daily records for highs and/or mild/warm lows. Even after the system's passage temperatures should remain 10-20F above normal for highs with some higher anomalies for morning lows in the vicinity of the frontal boundary. Fracasso/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