Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sun Feb 24 2019 Valid 12Z Wed Feb 27 2019 - 12Z Sun Mar 03 2019 ...Pattern Overview... The persistent Rex block pattern over the east-central Pacific--and in particular the ridge extending through northwestern North America into the Arctic--will continue to provide some challenges for determining forecast specifics. However in spite of the ongoing uncertainty for some aspects of the forecast, there are the following common themes: Significant precipitation from California eastward into the Rockies due to systems early and late in the period, a continuation of very cold temperatures over and near the northern half of the Plains with reinforcement from a deep upper low that may track near the Canadian border for a time, and potential for a late week/weekend storm system to affect the eastern U.S. with various weather hazards. ...Guidance/Predictability Assessment and Preferences... Two dominant features of interest during the period will be an upper low initially off the Pacific Northwest coast and an upper low over southern Canada. The combined evolution of these features will determine the character of low pressure development for which guidance is showing a stronger signal over the eastern states into southeastern Canada by Fri into the weekend. Through the 18Z model cycle, guidance still showed some spread for how the Pacific Northwest upper low energy will eject inland but with some apparent trends toward a common solution. GFS runs have been on the fast side of the spectrum--but with the 12Z UKMET adding some support to the 12-18Z GFS which were not quite as fast/amplified with ejecting energy as some earlier runs. On the other hand ECMWF-based guidance still wants to keep the energy farthest west but not to the extent seen 24 hours ago. Meanwhile guidance suggests that strong jet energy dropping down the east side of the upper ridge may pull the initial southern Canada upper low center a bit to the west/southwest mid-late week with a subsequent track very close to the U.S.-Canadian border. GFS runs had been on the southern side of the solution envelope for this feature but there is some increasing model support for a fairly far southward track. Trends with the Canadian upper low on their own are raising the probability of surface development reaching the Great Lakes region by day 6 Sat. Strength of this development appears more uncertain due to lower confidence in the timing/strength of ejecting Pacific Northwest energy and how it may interact with the Canadian upper low. Through the 18Z GFS run the model had been backing off the depth of its Great Lakes system due to its slower trend for the Pacific Northwest energy, but the 00Z run has reverted back to a deep scenario more similar from 24 hours ago. 12Z ECMWF and past couple CMC runs offer a less extreme evolution. A compromise among the daytime model runs and ensemble means should provide a reasonable starting point to reflect increasing likelihood of system existence but uncertainty for depth. For the system approach California during the latter half of the period, there are ongoing timing differences that have yet to be worked out. The ECMWF/ECMWF mean have adjusted back somewhat from earlier runs that had been on the faster side of the envelope. In contrast the 18Z GFS and the new 00Z run are on the slow side. An intermediate solution still appears best until there is a pronounced shift toward either side of the spread. Early in the period over the eastern half of the continent, there are still discrepancies over small-medium scale shortwave details over southern Canada/northern U.S. with corresponding surface differences. As with the previous couple days the GFS is strongest and ECMWF weakest while the CMC/UKMET generally offer a compromise. A blend offers the most reasonable starting point given low predictability due to relatively small scale, and incoming 00Z guidance seems to support this idea. Based on above considerations the updated forecast started with an operational model blend (12-18Z GFS and 12Z ECMWF/UKMET/CMC) days 3-4 Wed-Thu and then gradually trended toward almost even weight among operational models and 18Z GEFS/12Z ECMWF means. ...Weather Highlights/Hazards... Over the West expect areas of enhanced precipitation (rain and higher elevation snow) to extend from California through the Great Basin and into the north-central/central Rockies. There will be two main systems of note, one arriving around the start of the period and bringing rain/snow during Wed-Thu and the next most likely spreading moisture across the region during the weekend. The second system's moisture axis should be somewhat southward of the first. Highest totals during the period should be over the Sierra Nevada range. Around midweek modest shortwave energy/surface waviness may bring an area of mostly light snow across the northeastern quadrant of the Lower 48. Another small area of rain/snow may cross parts of the Ohio Valley/Mid-Atlantic later in the week. Then guidance is starting to show more similar ideas for potentially strong surface development with low pressure reaching the Great Lakes around Sat and continuing northeastward thereafter. Highest snow potential would extend across parts of the Midwest into the northwestern Great Lakes with meaningful snowfall also possible over New England. Expect rain farther south and some pockets of locally moderate-heavy activity may be possible. Slightly better potential for highest totals will be over the Gulf Coast states but relative maxima may be possible over other parts of the East as well. Continue to expect very cold temperatures centered over and near the northern half of the Plains. Wed-Fri readings will be extreme enough at 10-30F below normal but a stronger surge of cold air Fri into the weekend could bring some areas to 30-40F below normal while less extreme cold anomalies spread farther south and east. Meanwhile Pacific flow will keep min temperatures in particular above normal over the southern 2/3 of the West. The southern tier states should see above normal temperatures on average but with a cooling trend next weekend. Rausch WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities are found 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