Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Thu Dec 14 2023 Valid 12Z Sun Dec 17 2023 - 12Z Thu Dec 21 2023 ...A wet pattern is likely for the East Coast next week... ...Overview... A strong surface low is forecast to track along the East Coast early next week with widespread heavy rainfall and high winds near the coastline. Some uncertainty remains in the amounts and placement of the heavy rain due to model differences in the upper levels and the surface low track. After this system pushes offshore, a drier pattern will develop over the eastern two-thirds of the nation while an upper low moves south along the West Coast, producing periods of precipitation. ...Guidance/Predictability Assessment... Model guidance continues to show a westward trend in the track of the East Coast surface low, and there are still significant differences between the models with how they handle the interactions between upper level lows. The models all 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. The 00Z guidance suite (with the exception of the ECMWF) was showing more of a consensus towards a faster progression of the lows, but the 12Z guidance suite (with the exception of the GFS) is starting to trend towards a slower solution where the second low lingers longer near the Northeast United States. There is lower than average confidence in the sensible weather forecast details like QPF due to the uncertainties in the upper levels. Agreement among models is much higher in the West with ridging over the interior West early next week and an upper level low moving south along the West Coast mid-next week. There are expected differences in the position and timing of the southward moving low, 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. More specifically, a blend of the 00Z ECMWF/UKMET/CMC and 06Z GFS deterministic guidance along with a bit of the GEFS and EC ensemble means was used early in the period. Later in the period, the weight on ensemble means was increased to up to 50% by days 6 and 7. ...Weather/Hazards Highlights... The surface low and deep troughing aloft will support heavy rainfall that could be widespread on Sunday and Monday. There are significant differences in the placement and amounts of rainfall. This forecast continues the trend toward more rainfall farther west in the eastern U.S. on Sunday. There is a Slight Risk of Excessive Rainfall for most of the Carolinas and Virginia, southern Maryland, and Washington DC where many models have multiple inches of rain. Instability will likely be maximized near the coast for some higher rain rates, but there is enough dynamical support for lift that heavy rainfall causing instances of flooding is possible inland as well. By Monday, the low is forecast to lift quickly northward and rain (and some snow on the backside of the system) should spread into the Northeast. A Marginal Risk of Excessive Rainfall is in place for the potentially heavy rainfall. An upgrade to a Slight Risk may eventually be needed for Maine given the wet antecedent conditions and sensitivity to heavy rain there. 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 Sunday and 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. Dolan/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 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