Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Nov 27 2021 Valid 12Z Tue Nov 30 2021 - 12Z Sat Dec 04 2021 ...Overview... Persistent moist inflow into the Pacific Northwest should cause additional precipitation there, with the heaviest likely Tuesday into Wednesday. Multiple shortwaves will pivot through the mean trough that will be in place over the central and eastern portions of the CONUS during the extended periods; which will help deamplify the pattern somewhat as next week progresses. Rounds of light precipitation are possible for the Great Lakes region and interior Northeast through the week. Persistent moist inflow into the Pacific Northwest should cause additional precipitation there, with the heaviest likely on Wednesday. ...Guidance/Predictability Assessment... The 12Z/18Z guidance has a fairly decent handle on the large scale pattern evolution with the troughiness over the East and ridging over the West. However, uncertainties remain with energy coming into the trough in the East that will affect positioning and depth of a surface low off the Northeast coast. Ensemble guidance continues to show spread with the central pressure as well as its position. The 18Z run of the GFS has trended somewhat slower and less amplified than the 12Z run with the shortwaves traversing central and eastern Canada/United States. By Thursday evening, it reverses and the pattern becomes more progressive across the Northeast U.S. There is fairly good agreement for potent shortwave energy to move across the Great Lakes through Wednesday; however with potentially more shortwave energy passes through the flow over the Great Lakes region by Thursday, the spread in possible tracks increase as it tracks northeastward. The WPC once again attempted to take a middle ground solution of the GFS and ECMWF deterministic and ensemble mean positions for the low. For the West, at least initially, model guidance is clustered fairly well with the ridging but this changes beyond Wednesday. These differences may impact the forecast not only in regards to how much moisture is drawn onshore but also the amounts of precipitation reaching western Washington from British Columbia as well as precipitation type (the ridgier GFS suite has less precipitation farther south and staying all rain, with the EC/CMC wetter and lower snow levels). The model spread increases further by the mid-to-late periods which decreases the predictability and forecast confidence. The EC mean and the GEFS means end up showing a mean trough across the central U.S. on Friday, which the 00Z deterministic ECMWF and CMC were closer to compared to the 00Z and 06Z GFS runs that kept low amplitude troughing over the Northwest and Northeast instead. Uncertainty is rather high by that point given the small magnitude of the impactful shortwaves and the energy arising from high latitudes and over the Pacific. The WPC forecast was based on a combination of the deterministic 00Z ECMWF, OOZ CMC,00Z and 06Z GFS initially. As the period progressed, weighted the GEFS and ECWMF means more to minimize individual model differences, as well as lessening GFS influence as GFS runs were not aligned with the means by later in the week. ...Weather/Hazards Highlights... Portions of the northern Cascades and the Olympic Peninsula will begin to have precipitation ramping back up becoming moderate to heavy through Wednesday for the western portions of Washington. Some of this will likely fall as snow near the highest terrain but there is uncertainty in snow levels. Portions of the Northern Rockies are also expected to have light precipitation with this flow. Periodic shortwave energy across the north-central to northeastern U.S. will cause rounds of light precipitation chances through those areas. Favored lake enhancement areas will see the best chance for snowfall becoming moderate, especially downwind of Lake Ontario, as northwesterly flow passes across the open lakes. Additionally, the low offshore of the Northeast could cause light wraparound rain or snowfall through Monday, with amounts depending on the low track. Other than these two highlighted regions, a majority of the country will be generally dry other than increasing moisture for Texas midweek onward and spreading into the Mid-South by Thursday/Friday. Temperatures are expected to mostly be slightly below normal across the Eastern U.S. Monday as the mean trough becomes reinforced, with moderation back towards normal Wednesday and Thursday. Much of the western and central states will maintain mild temperatures through the extended forecast thanks to the ridging over the region. A few locations may reach or exceed daily maximum and minimum temperature records, where some places could register 20 to 30 degrees above seasonal average. As the flow flattens during the late period, above normal temperatures should also begin expanding into the Midwest and South. Campbell/Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Tue-Wed, Nov 30-Dec 1. - Heavy rain across portions of the Pacific Northwest, Tue-Wed, Nov 30-Dec 1. - Flooding occurring or imminent across portions of the Pacific Northwest. - Flooding likely across portions of the Pacific Northwest and the Northern Great Basin. 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