Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Thu Mar 21 2019 Valid 12Z Sun Mar 24 2019 - 12Z Thu Mar 28 2019 ...Overview and Guidance Assessment/Preferences... Today's guidance continues the recent general theme aloft of strong mean ridging over western Canada and/or Alaska, periodic downstream troughing, and features progressing from the eastern Pacific into/across the Lower 48. Recent model/ensemble runs have displayed some significant differences/trending with individual western-central U.S. features that continue eastward and affect the forecast over the eastern half of the country. Guidance has been a little better behaved for a fairly deep eastern Pacific trough/embedded upper low that should reach the West by Wed-Thu but typical amplitude/timing differences still exist. The forecast over the eastern half of the Lower 48 depends on specifics of a compact upper low over the north-central Plains and Interior West energy as of early day 3 Sun, and perhaps to some degree Canadian troughing that dips into the Great Lakes/Northeast. Over the past day there have been some trends toward more emphasis on the Plains feature and less on the western energy. This has led the ECMWF/CMC and their means in particular to trend faster with associated low pressure crossing the mid-lower latitudes of the central/eastern states during Sun-Tue. The adjustment brings the guidance cluster closer to the prior GFS/GEFS scenario that was fairly progressive. However by early Tue the 00Z ECMWF/ECMWF mean may have overshot the trend just a bit, favoring partial inclusion of the 12Z/20 run as part of a consensus blend that includes the 06Z GFS and 00Z UKMET/CMC into Tue. The 00Z GFS was slightly faster. It remains a close call as to whether the Canadian/Northeast U.S. trough may influence the southern stream system. Recent trends for the amplitude of this trough are mixed. A blend/consensus approach looks reasonable for depicting the evolution of the amplified trough/embedded upper low settling over the eastern Pacific during the first half of next week and then opening up with progression inland. The overall upper low contains multiple small shortwaves whose scale results in low predictability for those details. By Wed-Thu the models and ensembles have varied somewhat with amplitude/timing as trough energy progresses into the West. Recent ECMWF runs and their ensembles have tended to be on the faster side of the overall spread in the past day and the 00Z ECMWF looks particularly fast in bringing height falls into the northern Plains by Thu. The ECMWF mean has trended slower over the past 24 hours and the 06Z GFS is a bit slower than the 00Z run so preferences for this feature late in the period tilt more toward the latest ECMWF mean/06Z GEFS mean/06Z GFS and 12Z/20 ECMWF. Downstream from an upper high expected to close off over eastern Alaska and the Yukon, there is decent agreement that an amplifying trough over central Canada will push a cold front into the northern Plains around day 7 Thu. There have been enough spread and variability for the character of the upper ridge to suggest some future guidance adjustments are possible over the northern U.S. and southern Canada during the latter half of the week. ...Weather Highlights and Hazards... One or more waves progressing across the eastern half of the country will produce areas of mostly rain Sun-Tue, with thunderstorms possible from the southern half of the Plains into the Southeast. Some rainfall could be locally moderate/heavy but recent faster trends suggest that any intense activity should be fairly brief. At the same time the faster progression of moisture may limit any snow in the northern part of the moisture shield to farther north latitudes than previously anticipated--due to a higher proportion of moisture crossing the East ahead of the colder air pushing southeast from Canada. Mostly during Sun-Sun night an initial western U.S. shortwave will generate mountain snow/valley rain over the Great Basin and northern-central Rockies. The system settling into the eastern Pacific during the first half of next week will likely bring multiple surges of moisture into the West Coast and extending into favored terrain farther inland over northern/central parts of the West. The first round of enhanced precipitation should commence by Sun night-Mon. By Thu there is some uncertainty over the exact character of upper flow along the West Coast, lowering confidence in how much precipitation may persist over the West Coast states at that time. Guidance is persistent in showing best potential for highest precip totals from northern California/extreme southwest Oregon into the Sierra Nevada. The combination of events will provide less extreme but still significant totals over the northern/central Rockies. System progression will lead to variable temperatures over many areas during the period. Expect warmest anomalies to be for morning lows from the Rockies into Plains/Mississippi Valley Sun-Mon and then for max/min readings from the Interior West/Rockies into the Plains/Upper Mississippi Valley Tue-Thu. There may be fairly broad coverage of plus 10-20F anomalies over the central U.S. by next Thu. Parts of the east should see temperatures at least 10F below normal for a brief time around Tue-Wed. 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