Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Thu Dec 05 2019 Valid 12Z Sun Dec 08 2019 - 12Z Thu Dec 12 2019 ...Rain/snow spreading through the West this weekend will taper off early next week... ...Precipitation to increase over the East early next week but with uncertain specifics... ...Much below normal temperatures settle into the northern Plains/Upper Midwest early-mid week... ...Pattern Overview and Guidance/Predictability Assessment... Latest guidance has been showing fairly good agreement with the large scale flow but with some medium to smaller scale discrepancies that have significant sensible weather implications. For the most part a blend approach (more operational model input early and more ensemble means late) continues to offer the best way to navigate forecast uncertainty. From day 3 Sun through the first half of the upcoming week guidance continues to show strong upper trough amplification over central-eastern North America in response to a sharp but very amplified ridge extending from the eastern Pacific/West Coast north through northwestern North America and into the Arctic. As the upper trough amplifies, expect a strong cold front dropping south from the northern Plains to interact with southern U.S. flow/central Plains low pressure, with strengthening low pressure likely to track through the Great Lakes into eastern Canada Mon-Tue while a trailing cold front reaches the eastern states. By days 6-7 Wed-Thu the eastern trough aloft should become more shallow as eastern Pacific trough energy filters through the longer term mean ridge near the West Coast (rebuilding by Thu). One ongoing uncertainty in the forecast is the ultimate evolution of trough energy near the West Coast as of early Sun, with individual models and ensemble members continuing to exhibit wide spread. Through the 12Z/18Z cycles CMC runs had been on the fast side while ECMWF runs have tended to be slowest--with the 12Z ECMWF mean starting to hint at a slower/more separated feature. GFS runs have been moderately progressive with the GEFS mean somewhat slower--a trend adopted by the 00Z GFS. The major revelation in the new 00Z cycle thus far is a complete about-face by the CMC to the slow scenario, but to add more confusion the new 00Z ECMWF has adjusted faster. At the same time there are timing/amplitude differences with northern stream flow. These two issues put together lead to significant discrepancies in precipitation coverage and intensity/duration over the eastern U.S., with low confidence in a specific solution. Later in the period the issues with the initial western energy still impact the forecast, as exhibited by the the 12Z ECMWF's Gulf Coast system versus nearly all other guidance that has no such system. However there are enough 12Z ECMWF members that suggest at least a more subdued feature (whether from the initial energy or the second bundle of energy that passes through the West Coast ridge later in the period) to suggest some southern tier precipitation potential. One other question mark late in the period is the orientation of northern tier U.S./southern Canada flow and influence on temperatures over/near the northern Plains. Teleconnections relative to the most prominent positive height anomaly centers in latest D+8 multi-day means (near the West Coast and over the North Atlantic) favor a pattern more like the ECMWF/CMC and their means that would lead to a warming trend versus the GFS/GEFS mean that keep cold air entrenched over the northern Plains. For now went with an intermediate approach given the distant time frame and the occasional tendency for some guidance to displace cold air too quickly. The early to middle part of the period used various components of the 18Z GFS/GEFS mean, 12Z and 00Z/04 ECMWF, and 12Z UKMET. The low-confidence Gulf Coast low in the 12Z ECMWF required phasing it out of the blend late in the period. By day 7 Thu the forecast leaned about 2/3 to the GEFS/ECMWF means with a very slight tilt in the ECMWF/ECMWF mean direction. ...Weather Highlights/Threats... Expect areas of rain and higher elevation snow over much of the West on Sun though with a drying trend already in progress over parts of the Pacific Northwest. Best focus will be over favored terrain but with amounts beginning to trend lighter than what are expected during the late short-range period. Much of the West will see drier conditions early next week. Shortwave energy reaching the West Coast by Tue-Wed and then flow around the periphery of a rebuilding Pacific ridge should tend to focus rain and high elevation snow over the Pacific Northwest and possibly the northern Rockies by mid-late week. The forecast for precipitation specifics over the eastern U.S. has become even more uncertain than in the past couple days. There are still signals for heavy rainfall potential over some locations within a broad area from the South northeastward into the Mid-Atlantic and southern/eastern New England. However latest guidance has been too diverse and variable with separate important features aloft to have much confidence in determining where highest precipitation totals will be and their magnitude. Best potential for snow remains over the Upper Midwest/Upper Great Lakes which could see a combination of synoptic snow and lake effect/enhancement, with lesser probabilities extending into New England. Lake effect snow should spread across the remainder of the Great Lakes Tue onward as colder air moves into the region. During the first half of the period the warm sector ahead of the front advancing toward the East will lead to a day or so of well above normal temperatures. Some areas may see plus 15-25F anomalies for morning lows while plus 5-15F anomalies for highs should be common. Behind the front expect cold air feeding into the northern tier to bring readings down to as much as 15-25F below normal to parts of the northern Plains/Upper Midwest from Mon onward. In modified form this cold air will continue southward/eastward with the East seeing temperatures generally 5-15F below normal by Wed-Thu. 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