Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Fri Dec 08 2023 Valid 12Z Mon Dec 11 2023 - 12Z Fri Dec 15 2023 ...Overview... Deepening surface low pressure crossing New England on Monday will bring the potential for heavy rain and snow to the region before the system exits by Tuesday. After a bout of light to moderate precipitation in the Northwest on Monday as well, much of the lower 48 will trend drier by Tuesday. By midweek and beyond though, return flow from the Gulf of Mexico streaming into the southern High Plains ahead of a Four Corners upper low should lead to increasing precipitation chances there. Moist easterly flow could spread convection into the Florida Peninsula with a tendency toward higher rainfall totals late in the week. ...Guidance Evaluation/Predictability Assessment... The models and ensembles agree with the larger scale aspects of the pattern. A leading amplified trough should lift away from the East while a more shallow trough centered over south-central Canada as of Monday ultimately crosses the northeastern quadrant of the lower 48 and eastern Canada. Meanwhile consensus is maintaining the idea of incoming Pacific energy consolidating into a closed low over the Great Basin/Southwest/Four Corners area by midweek while an upper ridge builds into the Northwest and then the central U.S. around the upper low. There is also good agreement with eastern Pacific trough amplification mid-late week with a ridge rebuilding near the West Coast. However, there is considerable spread on how a shortwave nearing the Pacific Northwest around Wednesday may split as it nears the coast, with downstream impacts on the western upper low. Upper level/surface specifics across the Gulf of Mexico and Florida also become a question mark by late next week, with implications for details of the general wetter trend signaled in the guidance. Focusing first on the spread that develops over the western half of the country after midweek, there is still a relative majority of guidance that is not as strong as recent ECMWF runs with incoming midweek energy which that model (at least in the 00Z run) uses to form a separate closed low and eject the leading midweek western upper low with flatter trough to the south. The 00Z CMC fell into the ECMWF camp as well but the new 12Z run has trended more sheared with its energy. Not surprisingly, a meaningful number of ECMWF ensembles reflect some aspects of the operational run but the resulting mean does tone down the more extreme aspects of the operational run. The majority of GEFS/CMC ensembles side with recent GFS runs in showing minimal influence from the incoming energy, holding onto a strong western ridge by Friday while a well-defined trough with at least an implied upper low reaches the High Plains. Over the past couple days the 00Z runs of ECMWF-initialized machine learning models have strongly favored some variation of the GFS/GEFS/CMCens cluster. Given this majority scenario and the multi-day mean perspective favoring mean ridging over the Northwest (which tends toward low predictability for whatever energy may try to embed within it), preference maintains continuity by using a late-period model/ensemble mean blend that minimizes influence of the operational ECMWF. Meanwhile, southern tier/Gulf of Mexico detail differences become noticeable late in the period as well. In varying ways the latest GFS/ECMWF runs are somewhat stronger with approaching shortwave energy by Friday, leading to more pronounced surface waviness over the southeastern Gulf of Mexico and heavier rainfall extending into Florida. Thus far the CMC and GEFS/ECens/CMCens means keep the surface pattern suppressed over and near the Gulf, so for the time being will await a more pronounced clustering before adjusting toward the ECMWF/GFS details. The blend favored for the late-period western pattern reflects this preferences as well. A composite of 00Z/06Z models (more GFS/ECMWF weight relative to the UKMET/CMC) provided a good starting point for the first half of the period, with good clustering for the New England system on Monday and other meaningful Monday-Wednesday features. ...Weather/Hazards Highlights... Precipitation will be ongoing across the Northeast on Monday. Much warmer than average temperatures are possible across Maine in particular early in the day, which along with a potential band of heavy rain (within an axis of precipitable water values at least 2-3 standard deviations above normal) could cause quickly melting snow for possible flooding there. The Day 4/Monday Excessive Rainfall Outlook maintains a Marginal Risk for this potential, and river flooding is also a threat. On the backside of the low, probabilities for notable snow are still gradually increasing especially in the higher elevations of the interior Northeast on Monday. Some brisk to strong winds are also possible from the south ahead of the cold front and from the west behind it. Precipitation should clear out by Monday night, perhaps aside from lingering light snow to the lee of Lake Ontario and favored terrain near the Canadian border. Some light to locally moderate precipitation is likely across the northern Rockies on Monday with potential for some lingering enhanced snow totals. After that, precipitation will be at a minimum across the lower 48 on Tuesday, but chances for precipitation should increase on Wednesday-Thursday across the southern High Plains as an upper low settles over the Southwest/Four Corners and moisture interacts with a front. Some of this precipitation may be snow, with typical elevation/coverage uncertainty. Increasing spread for specifics of western-central U.S. flow aloft by the end of the week lowers confidence in how precipitation may progress eastward over the Plains on Friday. A weakening front may bring some light precipitation to the Pacific Northwest around midweek or so, with only light/scattered activity extending farther inland if at all. Expect Florida to see more rainfall with time as easterly flow sets up to the south of strong eastern U.S. high pressure and deep moisture increases. There are some signals for heavier rainfall by the end of the week but details will depend on low-predictability impulses in southern stream flow and whether they are strong enough to support a surface feature. As noted above, temperatures will be well above average in the Northeast on Monday. Morning lows could be near or above record high minimum temperatures for the date but colder air moving in later in the day could make it difficult to hold such values for the calendar day. Coolest anomalies over the East early in the week will tend to be south of the Ohio River, with the Gulf Coast/Florida seeing a day or two of temperatures 10F or so below normal. Otherwise the East should see generally near normal temperatures, perhaps with moderately above normal lows over the Northeast most days and above normal highs over all but the South by Friday. Farther west, the most persistent above average temperatures will be across the north-central U.S. (other than a brief cooler spell on Tuesday) as upper ridging builds. The greatest anomalies look to be around Thursday-Friday for the Dakotas into the Upper Midwest with readings up to 15-20F above normal. Most of the West will tend to see near to somewhat above normal temperatures. Clouds/precipitation over and near the southern High Plains mid-late week will support a fairly narrow temperature range (above normal lows, below normal daytime highs). Rausch/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