Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Fri Dec 15 2023 Valid 12Z Mon Dec 18 2023 - 12Z Fri Dec 22 2023 ...Coastal storm to lift Heavy Rain/Wind and Maritime Threats from the Mid-Atlantic to New England early next week... ...Emerging Heavy Precipitation Threat to focus into California and the Southwest next week... ...Overview... A main and deepened coastal storm is forecast to continue a track up along the East Coast from the coastal Mid-Atlantic to off New England early next week with widespread heavy wrapping rainfall/far interior snows along with high wind and maritime threats. Some uncertainty remains in the amounts and placement of the heavy rain due to model differences in the upper levels and complex surface low track. After this system pushes offshore, a drier pattern will develop over the eastern two-thirds of the nation as an upper low moves south along the West Coast toward the Southwest, with periods of increasingly organized precipitation with track and slow inland approach. ...Guidance/Predictability Assessment... The models generally agree that there will be a leading southern stream low swinging up the East Coast and a second northern stream low diving south from the Upper Midwest towards the Mid-Atlantic that will interact along the East Coast early-mid next week. However, there are still some model differences with how they handle the interactions between upper level lows. 12/18Z guidance forecast spread now seems to portend near average confidence in the sensible weather forecast details like QPF given lingering uncertainties, but the overall pattern and threat potential remains more universally noted, so a composite guidance solution was used to mitigate smaller variances. Agreement among models remains much better in the West with ridging over the interior West early next week and an upper level trough/low moving south along the West Coast mid-next week prior to working along with organized precipitation gradually to the Southwest later next week. There are expected differences in the position and timing of the system, and a general middle of the road solution was used for this forecast. The WPC forecast was based on the 12/18Z guidance that was available at forecast production time. A blend of the 12Z ECMWF/UKMET/CMC and 18Z GFS deterministic guidance along with the GEFS and ECMWF ensemble means was used along with the 01Z NBM and WPC continuity. A composite of the latest 00z guidance overall maintains good product continuity. ...Weather/Hazards Highlights... A deepened coastal low and deep troughing aloft will support heavy rainfall that could be widespread into early next week. There are some lingering differences in the placement and amounts of rainfall, but forecast spread is decreasing. By Monday, the low is forecast to lift quickly northward and rain (and some plowable snow on the backside of the system) will spread/focus into the Northeast. A Day 4/Monday Marginal Risk of Excessive Rainfall is in place for the potentially heavy rainfall and plan an upgrade to a Slight Risk for northern New England given the wet antecedent conditions and given model and ensemble probabilities for enhanced wrap-back axis amounts. Precipitation should pull away on Tuesday, but lingering coastal showers could continue. The deepened system should also favor enhanced winds. Precipitation chances will be increasing next week for California as shortwave energy supporting a possible atmospheric river approaches. Rain amounts and rates into Monday could be sufficient to cause some flooding concerns, especially given some instability could reach coastal areas. Marginal Risks of Excessive Rainfall are in place for Monday and Tuesday where flooding may be a concern. Precipitation is forecast to also pick up in the Pacific Northwest Monday into Tuesday as a an upper low moves down the coast. Some higher elevation snow will also be possible, though confidence in the details remains somewhat low. Away from the coasts, the Central U.S. will see mainly dry conditions next week. Areas from the West into much of the Plains will see warmer than normal temperatures next week underneath upper ridging, with temperatures 10-20F above average periodically affecting the northern/central High Plains into the Upper Midwest. Meanwhile, the northeastern U.S. should see above normal temperatures early in the week but moderating back toward normal by Tuesday behind the low pressure system. The Southeast including Florida looks to be cooler than average next week behind multiple frontal passages. Schichtel 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