Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Thu Mar 14 2019 Valid 12Z Sun Mar 17 2019 - 12Z Thu Mar 21 2019 ...Overview... Model and ensemble guidance maintains good agreement/continuity in showing the mean ridge-trough pattern over North America drifting slowly eastward with time. As has been the case in recent days, there is still some question as to the specifics of smaller scale shortwaves within/around the western mean ridge and exact character of the eastern trough. Meanwhile there is steadily increasing potential that an upper trough approaching the West Coast during the first half of next week will close off an upper low that reaches California around late Wed-Thu. The overall pattern will be fairly dry across the Lower 48 aside from the possibility of heavy rainfall over the Florida Peninsula next week due to a wavy front near the Keys/southern stream shortwave energy, and moisture moving into the West with the upper low. ...Guidance Assessment/Preferences... An operational model blend (06Z GFS/00Z ECMWF and some aspects of the 00Z UKMET/CMC) provides a reasonable starting early in the period. Then increasing 06Z GEFS/00Z ECMWF ensemble mean weight toward half along with the GFS/ECMWF (and CMC near the West Coast) helps to smooth out the increasingly uncertain shortwave details and yields a forecast close to continuity. Model differences and run to run variability persist for the evolution of energy initially coming through/around the western North America ridge and then feeding into the eastern mean trough. Thus far whichever solutions have deposited the most energy over/near the northern Rockies/Interior West by early next week have not held up in successive runs, which in this cycle favors leaning away from the 00Z GFS/06Z FV3 GFS. Then beyond this issue there is still a fair degree of spread for the shortwave energy over the central/eastern states. The 00Z UKMET/CMC are in the western part of the model/ensemble envelope, with some similarity to a couple earlier ECMWF runs. On the other hand the 00Z ECMWF eventually strays to the faster side of the spread. At the moment GFS runs are closest to the ensemble means. The spread/variability favor maintaining an intermediate solution for the deterministic manual forecast, but with the understanding that the potential still exists for a slightly farther westward trough and more sensible weather effects over the East. Farther south there is a reasonable signal that southern stream energy help generate a frontal wave near the Florida Keys with a track continuing northeastward but guidance is still attempting to resolve precise path and timing. A compromise appears best for this feature as well. Over the past day the ensemble means have trended deeper and more closed with the upper trough energy heading toward California by the middle of next week. This represents a trend toward recent GFS/ECMWF runs which were earlier to depict a closed system. A northward adjustment in the ECMWF over the past 1-2 days could be a tad overdone by late day 7 Thu but otherwise brings it into reasonable agreement with the majority cluster. The 06Z FV3 GFS becomes faster than consensus late in the period. GEFS means become slowest late in the period--plausible in light of multi-day trends but possibly a little slow based on overall progression of southern stream flow. ...Weather Highlights and Hazards... The Florida Peninsula--more likely the southern half--will have the best potential to see the highest rainfall totals during the period. A wavy front stalled near/south of the Keys and southern stream energy aloft will promote a period of unsettled weather from the weekend into next week, with the best rainfall enhancement occurring as a more defined frontal wave develops after Mon. Moisture extending to the northeast from Florida should remain offshore but it will be close enough to the southeastern coast that only a slight change in guidance would lead to more rainfall over that region. Over the rest of the U.S. east of the Rockies there may be one or more areas of precipitation with shortwave energy aloft and associated surface reflections. Any such precip is most likely to be light/scattered and confidence remains low in specifics. Meanwhile the system drifting toward California should start to bring moisture into the state Tue night-Wed with precipitation spreading farther into the West through Thu. There may be localized areas of focused activity but location of highest precip totals will be sensitive to exact track/evolution of the system. The large scale pattern evolution will tend to favor an expanding area of plus 5-15F temperature anomalies over the West during the period, though max temps should cool down over/near California with arrival of the Wed-Thu system. Upper troughing over the East will keep that part of the country below normal by up to 5-10F. Expect chilly readings up to 10-20F below normal over parts of Texas and the extreme southern Rockies Sun-Mon with gradual moderation toward normal thereafter. The northern Plains should also see a warming trend during the period. 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