Extended Forecast Discussion NWS Weather Prediction Center College Park MD 318 PM EST Fri Mar 01 2019 Valid 12Z Mon Mar 04 2019 - 12Z Fri Mar 08 2019 1500 UTC Update... Overall model/ensemble consensus remains above average through much of the medium range, with general agreement at the large scale even through day 7. Clustering of solutions was sufficient both with respect to the low pressure system off New England on day 3 (Mon) and a low pressure system approaching California on day 5 (Wed) that a multi-model deterministic blend (including the ECMWF/UKMET/CMC/GFS/FV3) served as a good forecast starting point. Differences emerge after day 5 with respect to how this West Coast system evolves as it moves inland, with the GFS (and CMC) keeping a much more amplified shortwave across the Rockies and central U.S. days 6-7 (Thu-Fri) relative to the ECMWF/FV3 which quickly weaken the energy as it moves into broad anticyclonic flow. At this time ensemble means all support the existence of this broad anticyclonic flow over the Rockies by that time, so support for the GFS solutions seems limited. Thus, chose to with a solution for days 6-7 based on a blend of the ECMWF and majority ensemble means (ECENS and GEFS). For the most part, adjustments to continuity were relatively small. The previous discussion from overnight follows below. Ryan Previous Discussion (issued at 0643 UTC)... ...Overview... A return to a more amplified upper level flow pattern is expected across the continental U.S. for next week. The passage of a strong cold front across the eastern U.S. by Monday morning will herald a much colder airmass across the central and eastern U.S., accompanied by a pronounced upper level trough and sprawling surface high from Canada. The presence of the eastern U.S./Canada trough supports a broad upper ridge across the Rockies and High Plains by midweek which gradually flattens out towards the end of the forecast period. An unsettled weather pattern is expected to return across the West Coast region as multiple Pacific disturbances move inland. ...Model Guidance Evaluation and Preferences... With the transition to a more amplified pattern over the continental U.S. compared to the quasi-zonal pattern in the short range period, there has been an improvement in model agreement over the past few model cycles regarding the medium range forecast period next week. This particularly pertains to the large scale upper level trough anchored over the eastern half of the nation through Thursday, although the 12Z ECMWF indicates a faster shortwave progression across the eastern U.S. on Tuesday compared to the other guidance. The main forecast challenge resides over the West Coast region beginning on Wednesday with the approach of a strong Pacific storm system towards California. The 12Z ECMWF and the EC mean are suggesting a more northern track with the surface low, and the GFS and CMC are indicating a southerly track, with the GEFS mean close to the model consensus. There are indications in the deterministic guidance and many ensemble members that a Rex block may try to develop over the northeast Pacific and western Canada as an upper ridge tries to build north of the West Coast upper trough. This is where the greatest model differences currently exist during the second half of the forecast period, and the ensemble means provide a more reasonable idea on placement of main weather features. The forecast was primarily derived with a blend of the latest deterministic solutions (ECMWF/GFS/CMC/UKMET) as a starting point for the Monday through Wednesday time period. For Thursday and Friday, the GEFS and EC means along with smaller percentages of the aforementioned deterministic model were incorporated into the forecast process. No major changes from previous WPC continuity were warranted. ...Sensible Weather and Potential Hazards... The winter storm across the East Coast Monday morning will be intensifying as it continues tracking towards the northeast and exiting New England later that day. Heavy snow is likely to be ongoing across interior portions of New England and rain for coastal areas, with most of the precipitation ending prior to 12Z Monday. Behind this nor'easter will be a strong cold front that will usher in an anomalously cold air mass by early March standards. A huge arctic surface high will encompass all of the central and eastern U.S. through the middle of the week, and reaching well into Mexico. The projected departures from early March climatology are astounding, with high temperatures on Monday on the order of 35 to 40 degrees below normal across much of the central and northern plains, and extending eastward across the Upper Midwest! This equates to subzero lows from Montana to Lake Michigan, and highs in the single digits and low teens. There will be a high likelihood for numerous daily record lows to be established with this arctic airmass. Temperatures only slowly moderate through Tuesday and Wednesday with anomalies of 20 to 30 degrees below normal, and this will extend eastward towards the Appalachians. Cold weather will continue through the remainder of the week across these areas. Heavy rain and mountain snow are expected to make a return to the western U.S. by Tuesday night as an atmospheric river event accompanies a strong Pacific low headed towards California. The Sierra Nevada will likely get hammered by very heavy snow, thus adding to its extensive snow pack. Significant snow is forecast for the central and northern Rockies by Wednesday and Thursday as the moisture plume advects inland, albeit not to the same degree as California. Heavy rainfall across the lower elevations of California may lead to some flooding problems, and future excessive rainfall outlooks will have additional information as this event enters the short range forecast period. D. Hamrick 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