Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 PM EST Fri Dec 15 2023 Valid 12Z Mon Dec 18 2023 - 12Z Fri Dec 22 2023 ...Coastal storm to bring 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 deep coastal storm is forecast to track up along the East Coast early next week with widespread heavy rainfall, high winds, and maritime threats. There may also be some snow on the far northwestern/back side of the storm. Some uncertainty remains in the amounts and placement of the heavy rain due to model differences in the upper levels and a 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. This feature will bring periods of increasingly organized precipitation as it slowly moves inland into the Southwest. ...Guidance/Predictability Assessment... Model agreement concerning the East Coast system has improved over the past couple days. The southern stream low will quickly swing up the East Coast while a northern stream low moves from the Upper Midwest towards the Mid-Atlantic. Most recent model runs have been trending faster with the southern stream low, which would allow at least a portion of the upper level energy of the northern stream low to continue to push into the Atlantic. The most uncertainty in the forecast is surrounding this split in upper level energy mid-next week. Even so, the overall pattern and threat potential seems 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. This feature will gradually push into the Southwest with organized precipitation 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 00/06Z guidance that was available at forecast production time. A blend of the 00Z ECMWF/UKMET/CMC and 06Z GFS deterministic guidance along with the GEFS and ECMWF ensemble means was used over the period with gradually increasing weight on ensemble guidance over time. ...Weather/Hazards Highlights... A deep coastal low and deep troughing aloft will support widespread heavy rainfall and strong coastal winds 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 with an embedded Slight Risk area for northern New England given the wet antecedent conditions and model/ensemble probabilities for enhanced wrap-back axis amounts. Precipitation should pull away on Tuesday, but lingering coastal showers could continue. Precipitation chances will be increasing next week for California as shortwave energy supporting a possible atmospheric river approaches. Rain amounts and rates on 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. Moderate 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. The slow approach of the closed system toward southern California mid-later next week would act to spread precipitation gradually inland, with organized lead activity working into the Southwest later next week. There may be an emerging wet pattern later next week in the south-central U.S. with some downstream energy flow and return Gulf inflow. Temperatures will be well above average in the Northeast Monday into Tuesday in the warm sector of the East Coast storm system, but temperatures will trend downwards through the rest of the week. Above average temperatures are also expected to persist across the West and spread into the Central U.S. for the second half of next week. Temperatures in the Southeast will likely remain below average Monday through Wednesday as a cold front passes through and high pressure builds into the region, and frost and freeze conditions may extend into parts of North Florida. Dolan/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