Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Sun Feb 17 2019 Valid 12Z Wed Feb 20 2019 - 12Z Sun Feb 24 2019 ...Multi-day heavy rain threat for Lower Mississippi/Tennessee Valleys to the Southern/Central Appalachians and snow/ice for the Mid-Atlantic to Southern New England... ...Cold weather persists in the western to central U.S., with unsettled weather focusing into the Southwest... ...Overview... Persistent western U.S. mid-upper level troughing will be reinforced during the beginning of the medium range period by shots of renewed energy. The trough will eventually move eastward into the central U.S. by next weekend. This feature plus an unyielding Bahamas upper ridge creates a favorable pattern for a multi-day heavy rain threat this week from the lower MS/TN Valleys to the Southern/Central Appalachians and a snow/ice threat for parts of the Mid-Atlantic and the Northeast Wed-Thu. ...Guidance/Predictability Assessment and Preferences... Guidance is in general agreement for days 3-4 of the medium range period regarding the pattern of troughing across the western U.S. and a leading trough moving across the north central U.S. on Wed and into the Northeast on Thu, overtop the southeastern ridge. A multi-model blend of deterministic models was used for this time frame. By days 4-5, while model agreement across the CONUS is reasonably good, the 00Z deterministic ECMWF develops an upper low in the Gulf of Alaska/northern Pacific that is much stronger than other models and its mean, due to its handling of energy upstream in the northern Pacific. This cutoff low (where other models have a trough) progresses southward through the rest of the period and affects the western U.S. Thus leaned away from the 00Z deterministic ECMWF for this feature. By the end of the forecast period, a surface low is expected to develop in the central/eastern U.S. as the main trough in the western U.S. ejects out into the central U.S. The most recent GFS runs seemed too suppressed with this surface low, considering the strength of the upper vort max and the reasonable forecast confidence. A blend of the means was used by the end of the period, along with the deterministic CMC and the parallel FV3 GFS to maintain details in strength. ...Weather Highlights/Hazards... An prolonged excessive rainfall threat from the lower MS/TN Valleys to the Srn/Central Appalachians will be one main story this week. Deep layered and highly anomalously moist southwest flow channeled from the Southern Plains to the East will act to sustain a Southeast upper ridge and fuel multi-day threats of heavy rain/training. On the northern edge, a cold surface high and cold air damming in the Mid-Atlantic region is expected, leading to frozen precipitation. Potentially heavy snow and ice is a possibility especially in the northern Mid-Atlantic into southern New England Wed-Thu. With anomalously low heights in the western U.S., much below average temperatures will prevail for the western and central U.S., while ridging in the Southeast will lead to above average temperatures. A main low is progged to dig from the Pacific Northwest to California Wed/Thu to the Southwest by Fri along with enhanced precipitation, including higher elevation heavy snow. The main system is expected to eject northeastward across the central and east-central U.S. next weekend. Tate/Schichtel WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities are found 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