Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Fri Nov 30 2018 Valid 12Z Mon Dec 03 2018 - 12Z Fri Dec 07 2018 ...Cold pattern more typical of mid-winter to grip much of the contiguous U.S. next week... ...Overview... A cold pattern is in store for much of the CONUS during the medium range period as mean upper-level troughing persists from the northern Plains/Upper Midwest to New England. Upstream support for this pattern shows no signs of waning, with a highly amplified and multi-center blocking pattern extending from Siberia across Asia to the North Pacific, which will maintain anomalous ridging from western British Columbia into Alaska. The flow resulting from this pattern will favor continued southward transport of polar/arctic air through Canada and into the CONUS. An active northern stream flow regime will result in a dominant storm track from central Canada southward into the central/eastern U.S., with the potential for cyclogenesis off the Eastern Seaboard. With the flow across the North Atlantic much less blocked than that farther west, systems should become more progressive through time as they move east, with limited threat for significant coastal storms. Meanwhile a fairly deep upper low over the eastern Pacific early in the week will steadily move toward California through midweek, likely opening up as the system progresses into the Southwest U.S./northwestern Mexico late in the week. This system will bring significant precipitation to portions of California/Arizona, and potentially the southern Plains toward the end of the week. ...Guidance Evaluation/Preferences... For about the first half of the period the forecast emphasized the 06Z GFS/00Z FV3 GFS and 00Z ECMWF along with modest inclusion of the 00Z UKMET. The CMC differed from consensus for both the upper low tracking toward California and the shortwave crossing the Northeast during the first half of the period. The blend introduced 00Z GEFS/ECMWF mean input by mid-period with the means reaching greater than half of the total weight late in the period. The late part of the forecast retained some aspects of the FV3 GFS/ECMWF as operational GFS runs were quicker than consensus for progression of Pacific flow upstream from the system reaching the Southwest and had lower confidence details within the eastern North America trough. There is still lower than desired predictability in exact details of how rapidly progressing northern stream shortwaves will interact with southern stream flow. One aspect of the forecast that has been in focus over recent days has been the evolution of shortwave energy emerging from the Rockies/Plains with very uncertain effects on the eastern U.S. surface pattern around day 4 Tue. Some spread persists but there is a notable trend toward keeping the energy aloft more sheared and any Southeast U.S. frontal waviness more suppressed until it reaches the western Atlantic. Compared to yesterday the latest ensemble surface low plots valid 12Z Tue also display a significant reduction of members showing any waviness to the northwest of the southeastern coast. Farther westward, the consensus/majority cluster is holding up fairly well for the upper low approaching California. CMC runs have been consistently straying from the majority cluster in some fashion, most commonly on the slow side. The 00Z run is initially fast and then becomes slower than consensus. By mid-late period the ECMWF/ECMWF have trended somewhat slower over the past 1-2 days to yield fairly good timing agreement in today's forecast. The 00Z GEFS run was preferred over the 06Z version as the former maintained a better defined and slower southwestern system/upstream ridge. Toward day 7 Fri the CMC ensemble mean becomes questionably more progressive over the eastern Pacific. 00Z/06Z solutions agree in general principle with the forecast of a strong shortwave dropping through western Canada toward the northern Plains Tue-Wed. Most guidance picked up on this feature in the 12Z/29 cycle with prior runs much flatter/more sheared. This feature is providing a stronger push of cold air into the northern Rockies and southward through the Plains after early Wed. By day 7 Fri there are pronounced differences regarding how far south the front reaches. The best clustering consists of the 00Z FV3 GFS/GEFS mean/ECMWF/ECMWF mean while operational GFS runs hold the cold air much farther north. Frontal position will likely prove to be very important in determining precipitation type as moisture from the California/Southwest system reaches the southern Plains. ...Weather Highlights/Hazards... As described above, the flow pattern will favor southward transport of polar/arctic air into the CONUS through much of the medium range. In a relatively rare occurrence, nearly the entire CONUS will see below average temperatures by late next week. For the five-day period expect the most consistently low anomalies to be over/just north of the Great Basin and over the northern-central High Plains. High temperatures across those areas and at times nearby locations will tend to range from 10 to at least 20 deg F below average on multiple days. After a brief warm up along/ahead of a frontal boundary, below average temps (5-10 deg F) will return to the Eastern Seaboard by Tue-Wed, with temps dropping further by later week as additional rounds of cold air arrive. In terms of precipitation, this will be a relatively dry pattern for much of the northern tier as low pressure systems traversing the northwesterly flow remain relatively moisture-starved. Periods of lake effect snow are possible though. One or more low pressure waves along the front across the Southeast on Mon-Tue should focus more widespread rains for portions of Florida and the coastal Southeast before quickly moving out into the Atlantic Ocean. The slow-moving Pacific system will increase rain and mountain snows along the West Coast by Tue/Wed before spreading precipitation into the Southwest and portions of the southern Great Basin by Thu and the southern Plains by Fri. Some areas of heavy rainfall may be possible as the moisture shield spreads across the southern Plains and possibly into the lower Mississippi Valley. Wintry weather may also be a threat at some locations depending on precise location of the front expected to drop southward through the Plains mid-late week. Rausch/Ryan WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes 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 https://www.wpc.ncep.noaa.gov/heat_index.shtml