Extended Forecast Discussion NWS Weather Prediction Center College Park MD 212 AM EST Mon Dec 9 2024 Valid 12Z Thu Dec 12 2024 - 12Z Mon Dec 16 2024 ...Overview... Behind the deep eastern North American upper trough that supports deep low pressure lifting through eastern Canada as of early Thursday, most guidance continues to show a fairly progressive mean pattern across the lower 48. However the models have been challenged in resolving the details of individual embedded shortwaves and their reflection at the surface along with corresponding sensible weather effects. The overall pattern will favor episodes of rain and higher elevation snow over portions of the West and then development of precipitation over the Plains/Mississippi Valley as shortwaves reach the central U.S. Aside from a brief period of cold temperatures from the Upper Midwest through most of the East late this week, much of the lower 48 should see near to above temperatures through the period with warmest anomalies centered over the Plains from the weekend into early next week. ...Guidance/Predictability Assessment... Predictability in the advertised low amplitude mean pattern (from a multi-day mean perspective) has been below average over recent days, with operational runs showing varying ideas for individual shortwaves and what surface pattern they will produce. There have also been discrepancies between some general themes of machine learning (ML) guidance and dynamical solutions. On the positive side, latest guidance seems to be starting to cluster a little better for individual features, and incorporating a combination of ideas from earlier dynamical and ML solutions. With consensus remaining good for the upper trough lifting out of the East early, the next feature of interest is a shortwave reaching near the West Coast by early Thursday. There has been considerable spread for how energy will be distributed within the shortwave as it continues eastward, while ML models earlier on had been showing a more defined surface system reaching into the eastern half of the country by the weekend versus a majority of dynamical runs. Latest dynamical model/ensemble trends seem to be toward a somewhat better defined central-eastern U.S. shortwave with more precipitation and some degree of low pressure. Multi-day ML trends have been a little slower to bring low pressure eastward, ultimately leading to a compromise between prior ML and dynamical themes. Toward next Sunday-Monday the guidance is hinting at the potential for some surface waviness to develop near the East Coast but this will be very sensitive to exact details of shortwave energy aloft. This will likely take a while to resolve, given how long it has taken to figure out the details farther back over the east-central U.S. (where spread still exists, with ML runs leaning somewhat more toward GFS/UKMET runs versus the slower ECMWF). Farther west, guidance shows what looks like a challenging-to- forecast but potentially impactful eastern Pacific system late this week. Supporting dynamics arise from zonal North Pacific flow as of Wednesday into a fairly sharp trough with embedded upper low by Friday-Saturday, with guidance developing quite a bit of spread for strength and track/timing. For forecasts valid 12Z Saturday, the ensemble means have been fairly consistent with a track offshore Washington/Vancouver Island. Some runs like the 12Z GFS/UKMET have been faster (as are a couple of the 12Z ML models) while CMC runs have been slow to the extent that they appear questionable (and have a more elongated surface evolution). The 12Z ECMWF was somewhat south while the new 00Z run has adjusted northward somewhat. Strength is another issue, with considerable spread there as well but with a fair signal from operational runs and ensemble members for central pressure that could reach as low as the 970s or 980s mb. The manual forecast based on 12Z/18Z data was more conservative given the spread, but the new 00Z GFS/UKMET/ECMWF runs would support a deeper trend going forward. For track, preference aligned with a compromise between the 18Z GFS and the ensemble means at 12Z Saturday. After early Saturday, recent ECMWF/CMC runs have been straying slower with the overall upper trough and in turn the trailing upper ridge versus what the ML models have been recommending (closer to an average of GFS runs and the GEFS mean). The new 00Z ECMWF has made a favorable partial trend toward the ML model and GFS/GEFS cluster. The progressive nature of the pattern seems to favor leaning away from the slow side of the envelope. This feature should reach the central U.S. by next Monday while flow ahead of another Pacific system should come into the picture near the West Coast at that time. Guidance considerations led to updating the forecast with the 12Z/18Z GFS and 12Z ECMWF/UKMET early in the period. Then the blend tilted more toward GFS/GEFS guidance mid-late period (more 18Z GFS versus 12Z Saturday-Sunday) with some ECMWF mean as a slight compromise. The ensemble means comprised 50-60 percent of the forecast late in the period. ...Weather/Hazards Highlights... Cold cyclonic flow behind deep eastern Canada low pressure will support snow to the lee of the Great Lakes continuing into Thursday. Snow should taper off thereafter as the storm and supporting upper trough rapidly lift away. A shortwave reaching the West Coast by early Thursday will continue eastward with time. This feature should produce some rain and higher elevation snow but with generally light to locally moderate amounts. The shortwave and associated surface wave/frontal system may spread precipitation across the east-central U.S. by Friday-Saturday and continuing east thereafter but with considerable uncertainty for magnitude and location of highest totals. Currently expect sufficient progression to keep totals below excessive thresholds through Friday, so the Day 5 Excessive Rainfall Outlook depicts no risk area for this activity. The northern part of the precipitation shield may contain some wintry weather at times. The storm likely nearing the Pacific Northwest by Saturday should produce heavier precipitation and possibly a period of brisk to strong winds. However guidance has shown sufficient spread for strength/track/timing to lower confidence enough in rainfall specifics not to merit a risk area in the Day 5/Friday ERO yet. Improvement in clustering may lead to introduction of a risk area in the near future depending on system details. After early Saturday expect this system's moisture to continue across the West, with the shortwave/surface system likely producing another area of precipitation over the east-central states by around the start of next week. Within the cold pattern over the East late this week, the most extreme anomalies will extend from the Upper Midwest into the central Appalachians on Thursday with readings 15-25F below normal. Areas closer to the East Coast will generally see temperatures 5-15F below normal Thursday-Friday. Then the East will trend closer to normal while above normal temperatures expand from the Interior West and Rockies into the Plains/Mississippi Valley. Some areas over the central U.S. may reach greater than 10F above normal from the weekend into Monday. Rausch 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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