Extended Forecast Discussion NWS Weather Prediction Center College Park MD 308 PM EDT Fri Mar 15 2019 Valid 12Z Mon Mar 18 2019 - 12Z Fri Mar 22 2019 ...Overview... A blocking mid/upper-level ridge initially centered from the Pacific Northwest to western Canada early next week should very slowly shift eastward through the forecast period. An upstream trough reaching the northeastern Pacific by the latter half of the week may help to reinforce/rebuild the western Canada ridge by Fri. The western North America ridge will promote a mean trough over the eastern part of the continent. Multiple shortwaves embedded within the overall trough--less predictable due to their smaller scale--will make it difficult to resolve some forecast details. Strong westerly flow across the North Central Pacific will split as it encounters the ridge, with some shortwave energy directed north into the higher latitudes of Canada and the rest separating into the southern stream. By late next week the persistence of ridging over western Canada may begin to direct stronger height falls from the North Pacific closer to the Pacific Northwest. ...Guidance Assessment/Preferences... Confidence in specifics of the system forecast to approach/reach California around midweek has decreased over the past day due to a significant divergence of solutions for the feature itself and upstream flow that may influence it. GFS runs (the 00Z version in particular) have trended toward much stronger upstream energy reaching the Pacific Northwest--leading to a faster and more open upper system reaching California. On the opposite side of the spectrum, the 00Z CMC becomes slower than consensus with flow farther upstream over the Pacific--resulting in an upper low that becomes an extreme to outlier southwestward solution by Thu-Fri. Overall prefer the 00Z ECMWF/ECMWF mean/CMC mean and 06Z FV3 GFS which represent the most consistent forecast relative to prior runs. Modestly slower adjustment in the 06Z GFS provides a little added confidence in the favored scenario. Note that by late next Fri the 06Z FV3 GFS may become a little too aggressive with its shortwave nearing California. At the same time the models and individual ensemble members continue to be quite diverse for the evolution for shortwave energy reaching the northern Plains and vicinity by Tue and continuing into the East thereafter. UKMET/CMC runs have been persistent in depicting a slow and amplified/closed feature while latest GFS/ECMWF runs are quite open and progressive. Some ECMWF runs from a couple days ago were slower though. Teleconnections relative to the positive height anomaly center seen in multi-day means over western Canada leave the door open to some trough elongation over the Midwest/Plains but perhaps with greater progression than the UKMET/CMC scenario. Would recommend a blended approach for the purposes of a single deterministic forecast. Later in the week there are typical differences in central/eastern Canada flow that may amplify into the Northeast. A model/ensemble mean blend looks reasonable for this part of the forecast, aside from the 06Z FV3 GFS that becomes sharper and more amplified than most other guidance. Based on preferences over the West in particular, the forecast places greatest emphasis on the 00Z ECMWF during the first half of the period with minority input from the 00Z UKMET/CMC and 06Z FV3 GFS. After midweek the forecast reduces CMC/FV3 input while increasing weight of the 00Z ECMWF mean and NAEFS mean (which includes the CMC ensembles). ...Weather Highlights and Hazards... A lingering frontal boundary south of the Florida Peninsula and one or more embedded waves will support widespread and potentially heavy rains over the southern half of the Florida Peninsula during the first half of the week. Farther west, the preferred forecast for the Pacific system tracking into California should spread rain and high elevation snow into the Sierra Nevada, Great Basin, and central/southern Rockies by next Wed-Fri. Some localized enhancement of precipitation over favored terrain will be possible. Note that a less likely scenario exists that would bring more moisture into northern California. Moisture from upstream flow may reach the Pacific Northwest by next Fri. One or more areas of mostly light rain/snow may progress across the Great Lakes/Northeast within cyclonic flow aloft, with some activity possibly extending farther southwestward along a front. The most prominent temperature trend next week will be the development of above normal readings (especially for morning lows) over the West and progressing/extending into the north-central Plains. There should be decent coverage of plus 10-20F anomalies for min temps over the Interior West by Wed-Thu and parts of the High Plains by Fri. Warmest highs (plus 10F or so anomalies) over the West will likely be near the West Coast early in the week while highs from California into the Four Corners states trend gradually cooler with the arrival of clouds/precipitation. Cool temperatures over the South/East (10-15F below normal over southwestern Texas early in the week, generally 5-10F below normal elsewhere) early in the week should moderate closer to normal by Thu-Fri. Rausch/Ryan 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