Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 PM EST Thu Dec 16 2021 Valid 12Z Sun Dec 19 2021 - 12Z Thu Dec 23 2021 ...Pattern Overview and Guidance/Predictability Assessment... The period begins Friday with a vigorous shortwave moving through the Northeast U.S. with surface low pressure exiting the coast. Meanwhile, a southern stream shortwave will move through the Southern Plains this weekend, eventually into the Southeast, with a potentially well organized coastal low developing in the Gulf of Mexico and sliding across Florida on Tuesday and into the western Atlantic by Wednesday. Elsewhere, weaker shortwaves move through the flow across the far northern tier of the U.S. with upper level troughing becoming established off/over the West Coast early next week. Overall, the guidance is in good agreement with the synoptic pattern through much of the period. The UKMET seemed a bit faster with systems than the rest of the guidance, so it was not included in todays blend. Otherwise, a general model blend between the 06z GFS and the 00z ECMWF and CMC worked well for days 3-5 within a pattern with above average forecast confidence. By days 6-7, some bigger questions begin to emerge regarding details of an organized surface low near the Southeast early next week and its proximity to the coast. There are also differences out west with evolution of a system in the eastern Pacific and interactions with the West. Generally, these are typical differences for that late in the period and so a roughly 50/50 blend of the ensemble means with the deterministics seemed to offer a good starting point consistent with the previous WPC forecast. ...Weather/Hazards Highlights... Northern stream upper trough passage and an exiting frontal low may support some lingering snows Sun over northern New England and off the lower Great Lakes. Locally enhanced rain should focus with waves along a trailing/stalling front sunk through the Gulf Coast and Southeast into early next week. The heaviest rains should stay off the Gulf Coast on Monday, but a period of moderate to heavy rain will likely impact portions of Florida by Tuesday. Depending on proximity of the surface low to the coast, locally heavy rainfall may clip portions of the Southeast coast as the low moves out into the western Atlantic. An upper trough and surface system passage will support lingering terrain enhanced precipitation over the Northwest on Sunday. Details remain less certain upstream over the eastern Pacific, but there remains a good guidance signal that an amplified upper trough digging early next week would offer potential to focus ingredients for moderate to heavy precipitation for north-central CA considering slow system approach, a developing moisture plume, and favored southern facing terrain enhancement. Some heavier model qpf solutions would favor local runoff/flooding issues. The main precipitation focus, including heavy Shasta and Sierra snows, is set to spread down across central-southern CA into midweek, with additional activity spreading across the Great Basin terrain to the Rockies with inland shortwave system progression. There is also then an emerging guidance signal for upper trough energy to dig from Alaska to renew precipitation into the Pacific Northwest by next Wed/Thu. Upper ridging across the nations midsection should spread above normal temperatures (though not nearly as extreme as current values) from the northern/central Plains southward towards the Gulf Coast with daytime highs potentially 15 to 20F above normal for the southern High Plains by next Thursday. Elsewhere, temperatures through the medium range should be near seasonal averages for much of the East, with slightly below normal temps associated with weak shortwaves through the northern tier and also the shortwave through the Southeast early in the period. Santorelli/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, 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