Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Tue Mar 12 2019 Valid 12Z Fri Mar 15 2019 - 12Z Tue Mar 19 2019 ...Overview and Guidance Assessment/Preferences... Expect broad troughing aloft to prevail over eastern North America downstream from a ridge that builds over the Canadian Rockies and Interior West. Over the eastern Pacific a leading weak shortwave will likely pass into/around the building ridge while a trailing trough should amplify and sharpen as it approaches the West Coast. There are essentially two larger-scale guidance disagreements. The first involves energy initially moving into the Southwest and eventual possible interaction with northern stream shortwave energy amplifying into the East. Even compared to its ensembles the 00Z CMC is rather extreme with its combined strength and fast progression of the Southwest shortwave. Then a fairly strong depiction of an amplifying Midwest shortwave by Sun-Mon in the CMC (similar to the 00Z UKMET through the end of its run late Sun) allows for a phased trough which by early Mon leads to a surface low tracking closer to the East Coast than most other guidance. Relative to the full array of ensemble members the 00Z CMC is on the fringe of the envelope but not extreme enough to be considered an outlier. The CMC scenario has sufficiently low probability not to include in a most-likely single deterministic forecast. However the potential for evolution to change from the current consensus represented by the ECMWF/GFS and its ensemble means will have to be monitored. The second issue involves the weakening Pacific shortwave energy that reaches western North America by around Sun. In varying ways the latest GFS/GEFS runs (and FV3 GFS) are more enthusiastic about the shortwave's strength passing through/around the mean ridge and then westward/southwestward elongation of this energy into the first half of next week. By then this evolution leads to a flatter upper trough over the East versus most other guidance. The 06Z GFS/GEFS mean are more pronounced with the westward/southwestward elongation than their 00Z counterparts. Teleconnections relative the strong core of positive height anomalies over the western half of Canada seen in the D+8 multi-day means suggest an eastern U.S. trough that is rounded enough to allow for some 00Z GFS/GEFS input along with the more amplified ECMWF/ECMWF mean solution. On the other hand the telecons support lower heights over the East than depicted in the 06Z GFS/GEFS mean and last couple FV3 GFS runs. Based on majority clustering the forecast incorporated the 00Z GFS/ECMWF and lesser weight of the UKMET from early day 3 Fri into day 5 Sun. The forecast started to include 00Z GEFS/ECMWF means late Sun and increased their total influence to 60 percent total by day 7 Tue, with lingering operational input from the 00Z GFS/ECMWF. ...Weather Highlights and Hazards... The cold front crossing much of the East Coast late this week will likely settle near the Florida Keys as a wavy stationary front. The combination of this front and one or more southern stream shortwaves aloft may produce periods of locally moderate or heavy rainfall over the Florida Peninsula. Extreme southern Texas may see some enhanced rainfall with the energetic flow aloft as well. The rest of the East will see a drier trend as frontal rainfall passes off the East Coast after Fri. Cool cyclonic flow will produce some areas of lake effect snow primarily to the lee of open waters (mostly Lake Michigan and Lake Ontario). During the weekend light precipitation could brush parts of the Pacific Northwest and some light/scattered activity may occur over the southern Rockies. Otherwise the West should be mostly dry until moisture reaches the West Coast next Tue. There may be one or more areas of precipitation between the northern half of the Plains and Great Lakes during the period, in association with features progressing into the Lower 48 from Canada, but with low confidence in timing and coverage. Expect below normal temperatures initially over the West to moderate with time while cool air moving into the East behind Friday's cold front should rebound closer to normal after Sat. The north-central and southern Rockies/High Plains may see the most persistent cold anomalies with some locations at least 15-20F below normal for highs through the weekend. The warming trend over/near the West Coast states should bring readings 5-10F above normal by Sun-Tue. 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