Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Tue Dec 05 2023 Valid 12Z Fri Dec 08 2023 - 12Z Tue Dec 12 2023 ...Pattern Overview... Guidance shows broadly cyclonic mean flow setting up over the lower 48 by the weekend into next week. Initially setting up this pattern will be a vigorous shortwave emerging from the West late this week, supporting a deepening surface low forecast to track from the southern Plains through the Great Lakes. This system and connecting fronts will likely produce significant precipitation over the eastern third of the country plus a period of brisk to strong winds from parts of the Rockies eastward. Upstream Pacific energy feeding into North America, followed by an amplifying eastern Pacific ridge, should help to reinforce the overall mean trough early next week but with a more positive tilt that should lead to more benign weather. Moisture ahead of the Pacific energy and associated surface system will bring the potential for a brief period of heavy precipitation over the Pacific Northwest around Saturday. ...Guidance Evaluation/Predictability Assessment... Models/ensembles agree in concept on the idea of the leading trough emerging from the West helping to deepen Plains through Great Lakes/Canada low pressure from the end of this week into next Monday. However there is still considerable spread for the surface low track from the Mississippi Valley onward. Recent GFS runs and the 00Z UKMET represent the northern or northwestern side of the spread (12Z UKMET adjusted closer to the middle) while the 12Z CMC now represents the southeastern side of the envelope. Some ECMWF runs had been on the southern side of the spread but the 00Z version and the fairly consistent ECens mean are closer to the middle of the spread. GEFS means lean a bit north in deference to the operational runs but are a bit less extreme at some forecast hours. Given the lack of pronounced clustering so far, prefer to maintain an intermediate solution that provides reasonable continuity. Behind this system the guidance has made more noticeable trends over the past day or so, toward a more amplified eastern Pacific upper ridge by next Tuesday and more downstream troughing. Also the latest consensus shows Pacific-origin low pressure reaching the vicinity of the Great Lakes at that time. The latest guidance through the 00Z/06Z cycles favored an operational model composite for the first half of the period and then a transition to include some 06Z GEFS/00Z ECens means by days 6-7 Monday-Tuesday. ...Weather/Hazards Highlights... After the ongoing atmospheric river event ends in the Pacific Northwest, there will be a brief break in heavy precipitation on Friday before another surge of moisture reaches the coast on Saturday. The region will be sensitive to additional rainfall after the first event, so Day 5 Excessive Rainfall Outlook introduced a Marginal Risk of Excessive Rainfall for western Washington and Oregon into far northwestern California with an embedded Slight Risk area along the coast. The updated ERO issuance made only modest adjustment to these areas. In addition to heavy rainfall, widespread moderate to locally heavy snow will be possible in the higher elevations of the Cascades and northern Rockies as moisture pushes inland. Precipitation should quickly trend lighter after Saturday but with some uncertainty over specifics. The surface low that will deepen as it tracks from the southern Plains into the Great Lakes late Friday into the weekend will likely produce widespread rainfall from the Gulf Coast up through the mid-Mississippi Valley and into the Ohio Valley and southern Great Lakes region. Conditions will be favorable for thunderstorm development on Saturday from the Gulf Coast to the mid-Mississippi Valley, and the Storm Prediction Center has highlighted this area for a severe thunderstorm threat. Meanwhile the Day 5 ERO has introduced a Marginal Risk of excessive rainfall from parts of the Lower Ohio Valley south to the Gulf Coast. Moisture anomalies and instability allow for some enhancement of rainfall, and although frontal progression may keep totals from becoming too extreme, rain rates could still be intense for a time to yield the localized totals that some models are producing. Specifics within this area will still take time to be resolved though. As the low pushes northeast on Sunday, precipitation will spread to the East Coast and up into New England while thunderstorm chances will decrease. The northern Mid-Atlantic into Northeast will see the best potential for locally heavy rainfall on Sunday. Some snow will be possible in the northwestern part of the precipitation shield, but details are currently uncertain due to lack of guidance agreement for surface low track. Probabilities for a quarter inch of liquid in the form of snow are low on an absolute basis, but what probabilities do exist cover parts of the Midwest into the Great Lakes. Also some snow may fall after system passage, mainly over the Great Lakes into Appalachians. Finally, the gradient around this developing storm will produce a broad area of brisk to strong winds. The greatest wind threats currently appear to be behind the storm over portions of the Rockies, but other areas as far east as the East Coast (including in the warm sector) should also monitor forecasts for wind speeds. The eastern U.S. will trend drier after Sunday as high pressure builds into the South and Mid-Atlantic. Expect well above normal temperatures in the warm sector ahead of the developing Plains-Great Lakes system. From Friday into the weekend, highs should be 10-20F above normal from the Plains into the East with some morning lows even warmer versus average. Frontal progression will bring temperatures closer to normal (or even a little below normal over the southern Plains) over the central U.S. by the weekend. Above normal readings should be confined to the Northeast by next Monday. The Intermountain West and central-southern Rockies will tend to see moderately below normal highs through the period. Rausch/Dolan 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 WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw