Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Wed Oct 03 2018 Valid 12Z Sat Oct 06 2018 - 12Z Wed Oct 10 2018 ...Stormy Pattern for the West/Rockies and a Central U.S. Excessive Rainfall Threat... ...Pattern Overview and Guidance/Predictability Assessment... A highly amplified synoptic regime will remain a fixture during the medium range period. Positive height anomalies with mid-level heights often exceeding 591-dm should be commonplace over the Eastern Seaboard. Meanwhile, the blocking regime will afford a persistence of troughing across the southwestern U.S. as an upper low pinches off over the region. This axis of lower heights should gradually drift northeastward next week with the potential for a reinforcement of this pattern as a deep upper low descends from British Columbia. Relative to climatology, many of these mentioned features are in the 2 to 3 sigma departure range, particularly with the upper lows. The squeeze play between this pair of persistent features will lead to tremendous poleward transport of Gulf moisture into the central U.S. Only modest changes in the pattern are anticipated given the stable positive height anomalies inhabiting the eastern U.S. and eastern Pacific, respectively. Shearing height falls within broad mid-level southwesterly flow will carry a dampening wave of low pressure across the Great Lakes and into southern Quebec over the weekend. Model agreement is much stronger with this guidance package relative to yesterday. Shifting focus to western North America, multi-day ensembles continue to favor an upper low dropping down from the Central Great Basin while closing off over the Desert Southwest and Four Corners region. The 00Z ECMWF sits a bit to the west of the consensus by Day 5/Monday although it does sit within the ensemble cluster. Where models diverge is with the strength and timing of a wave of low pressure possibly lifting into the north-central U.S. on Monday. The past several runs of the GFS as well as the 06Z FV3-GFS remain adamant about a deeper solution although this camp sits on its own. The 00Z CMC is about a day later with this evolution while the previous couple ECMWF cycles depict a more innocuous pattern. Further issues arise across the higher latitudes with the amount of polar energy able to drive southward across western Canada and into the Pacific Northwest. It appears the complexities lie with a band of shortwave energy peeling off a vortex currently sitting over Nunavut. A compact upper low sitting underneath a closed high across the northern Northwest Territories appears to be the culprit. This particular scenario is not shown by recent runs of the GFS which allows shortwave energy to freely drop down through British Columbia and into the northwestern states. In contrast, the 00Z/12Z ECMWF form that high-latitude block which delays the arrival of lower heights into the Pacific Northwest by roughly a day. Given the uncertainty that looms beyond Day 4/Sunday, only kept substantial amounts of operational guidance in the mix through the weekend. This was compromised primarily of the 00Z GFS/ECMWF with minor contributions from the 00Z CMC/UKMET. By Day 5/Monday onward, added more significant chunks of the 00Z GEFS/ECMWF ensemble means into the picture. Decided against using the 00Z NAEFS mean as it is biased toward the Gulf development depicted by the CMC ensembles. ...Weather Highlights/Threats... With the level of amplification anticipated during the period, it is not surprising that a heavy rainfall threat will ensue over the middle of the country with impressive temperature gradients bounding the meandering frontal zone. Models depict precipitable water values exceeding the 95th percentile over the central U.S. with deep layered southerly flow allowing for the potential for training convective elements. The multi-day nature of the rainfall will likely augment flash flood concerns, particularly along the Red River northward to the Central Plains where models are most robust. Several inches of rain are likely during the forecast period with the threat gradually lifting north-northeastward into the Middle Mississippi Valley and Upper Midwest. Farther west within the dome of cold air, wintry precipitation is anticipated over large sections of Montana and Wyoming extending into the western Dakotas. Regarding temperatures, well below average numbers are likely from the Upper Midwest back to the West Coast. The bowl of coldest air will reside across the Northern/Central Rockies into the adjacent High Plains with departures from climatology on the order of 20 to 30 degrees in some cases. Some higher elevation locations will struggle to get out of the 30s for highs while overnight lows plunge into the 20s. On the other side of the boundary, it will feel more like the end of Summer with highs around 10 to 15 degrees above average, especially around the Great Lakes and Ohio Valley. Even some daily records may be broken given the vast overnight warmth over the region. Rubin-Oster 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