Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM EST Mon Mar 4 2019 Valid 12Z Thu Mar 7 2019 - 12Z Mon Mar 11 2019 Weather Pattern Overview ~~~~~~~~~~~~~~~~~~~~~~~~ The amplified flow pattern that exists during the short range forecast period is expected to transition to a more quasi-zonal pattern across the continental U.S. by the beginning of the medium range period on Thursday. The anomalous upper level ridge over the eastern U.S. lifts out while an upper level trough builds in across the West Coast region by Friday, and it will be this trough axis that induces surface low cyclogenesis across the Plains by this weekend. A second and weaker disturbance is likely to affect the West Coast region by the end of the forecast period next Monday with a more amplified pattern in place. Model Guidance Evaluation and Preferences ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The models and ensemble means indicate reasonable agreement on the synoptic scale for the end of the week, but not quite as good as previous days when confidence was above average. One of the main differences on Thursday was the CMC having a flatter 500mb height configuration over the West Coast region compared to more troughing evident in all of the other guidance. Both the 18Z and 00Z GFS indicate a more developed lead surface low crossing the Ohio Valley region on Friday, whereas the model consensus and FV3 support more of an elongated surface trough. The strong surface low that is forecast to develop across the Plains and then track northeastward is indicating very good agreement on Saturday morning across eastern Colorado as lee cyclogenesis gets underway. Model differences grow significantly by Sunday regarding the track of the surface low across the Midwest states and then the Great Lakes region. In terms of trends, both the EC mean and GEFS mean have trended stronger with this storm system, and the Canadian mean more suppressed to the south and not as strong. The FV3 is faster than the model consensus and is not preferred beyond Saturday. There is better agreement among the EC and GEFS ensemble members compared to the CMC members, and the EC mean provided a reasonable idea on low placement. Taking all of these factors into account, the forecast was primarily derived from the 12Z ECMWF/18Z FV3/12Z UKMET for the first half of the forecast period, and then some of the ECMWF and EC/NAEFS mean for the last two days. Sensible Weather and Potential Hazards ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Heavy snow will likely be ongoing across the higher elevations of the Sierra Nevada and much of the Intermountain West and Rockies through Friday as the strong Pacific disturbance crosses the region. Snowfall amounts on the order of 1 to 2 feet are likely across parts of the central and northern Rockies, mainly on Friday. Temperatures are expected to slowly moderate across the north-central U.S. compared to early this week while still remaining below average. With a large storm system likely developing across the central U.S. next weekend, widespread rain is expected to develop from the Gulf Coast to the Ohio Valley and southern Appalachians as a rich supply of Gulf moisture surges northward ahead of a strong cold front. The heaviest rainfall is likely across the Deep South and Tennessee River Valley, where a general 1 to 3 inches of rain is likely that may lead to some flooding issues based on the current forecast. Depending on where the surface low develops, a corridor of blizzard conditions will be possible across parts of the northern plains, Upper Midwest, and western Great Lakes region. It is still too early to mention precise locations given current model differences. Another facet to this low pressure system will be the potential for strong to severe thunderstorms to develop in the warm sector, and this is something that the Storm Prediction Center is currently monitoring. D. Hamrick 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