Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri Mar 29 2019 Valid 12Z Mon Apr 01 2019 - 12Z Fri Apr 05 2019 ...Overview... A ridge over mainland Alaska as of early Mon will likely split with a part collapsing to the southeast and the rest rebuilding northwest toward the Bering Strait and eastern Russia. Even with this transition there is some support for maintaining cyclonic mean flow over Canada into the northern Lower 48. Meanwhile periodic Pacific shortwaves will bring precipitation to areas across the western U.S. as well as parts of the central/eastern states. ...Guidance Assessment/Preferences... Within the fairly agreeable mean pattern there are some significant issues with specifics. Differences in handling the initial Alaska ridge play at least a partial role in the divergence of solutions for flow across southern Canada and northern U.S. Through the 12-18Z cycles there are two general clusters, one including the ECMWF/ECMWF mean and CMC/UKMET which show more troughing in western Canada and higher heights over eastern Canada, versus the GFS/GEFS which keep troughing away from southwest Canada and bring a more amplified trough across the Upper Mississippi Valley/Great Lakes into New England Tue-Wed. The 00Z UKMET has adjusted somewhat closer to the GFS up to the point of the trough reaching the Great Lakes--after which point the UKMET trends less amplified than GFS runs. At the same time guidance suggests a shortwave reaching the southern Plains by Mon will continue onward and generate low pressure along or offshore the southeastern coast by Tue. The ECMWF is in the slower part of the spread and GFS runs in the middle to faster side of the envelope. By mid-late week, what happens with southern Canada/northern U.S. flow will affect the forecast of the southern stream system. For now GFS runs and some GEFS members are in the minority with their more phased scenario that would bring the system rapidly northward into/through the Canadian Maritimes instead of spending a longer time over the western Atlantic--albeit with very uncertain duration of effects along the East Coast. Based on data through the 18Z cycle, forecast preferences encompassing these features end up leaning 2/3 away from the GFS for southern Canada/northern U.S. flow and an intermediate solution for the southern stream system. However an early look at the new 00Z ECMWF shows a shift toward the GFS scenario with both features so needless to say there is high sensitivity in this pattern. Looking farther westward, most solutions expect energy offshore British Columbia to combine with a separate system tracking toward the West Coast and produce an upper trough that progresses through the West around midweek and the Plains/Mississippi Valley Thu-Fri. There is still potential these features may not merge though. At the surface low pressure will likely track into the Northwest early in the week and then a wavy frontal system should reach the central/east-central states Wed-Fri. Not surprisingly the operational models develop significant differences with some specifics at the surface and aloft by the latter half of the period--favoring greater emphasis on the means at that time. In the wake of this system expect a modest ridge to build into the West Coast/Interior West while more trough energy pushes into the eastern Pacific. Thus far GFS runs have tended to be a bit on the aggressive side of the envelope with bringing the energy into the ridge though the 12Z ECMWF starts to become questionably amplified with its trough by late Fri. The ensemble means are fairly close (joined in principle by the 18Z FV3 GFS) and merit the greatest weight in the forecast by late in the period. ...Weather Highlights/Hazards Expect rain/higher elevation snow to spread through the northern half to two-thirds of the West in the Mon-Wed period, followed by additional precipitation reaching the West Coast late in the week. Some moisture may have a connection to lower latitudes. Highest precipitation totals are most likely over northern California/Sierra Nevada with enhanced activity reaching into the northern/central Rockies. Locations across the South and East Coast regions may see a period of rain (and possibly some snow over higher elevations) with the system forecast to track into the Atlantic. Wide guidance spread for track and timing keeps confidence low with respect to precipitation coverage and intensity. Pockets of locally moderate/heavy rainfall may be possible. The system emerging from the West may spread precipitation along a wavy frontal system into the Plains/Mississippi Valley region during the latter half of the week. Thus far signals for significant amounts are not agreeable enough to provide meaningful confidence in specifics. Temperatures will vary in accordance with system progression. The South and East will be on the chilly side early in the week with some areas of minus 10-20F anomalies. Then above normal readings (mostly plus 5-15F anomalies) should progress from the West and into the central/eastern states over the course of the week. 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