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WPC Medium Range Archive
Medium Range Products Valid On:
12/09/2023
(Day 7 PMDEPD: Valid 12/05/2023 to 12/09/2023)
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD 200 PM EST Sat Dec 02 2023 Valid 12Z Tue Dec 05 2023 - 12Z Sat Dec 09 2023 ...Heavy rain expected for the Pacific Northwest with an atmospheric river event continuing into Tuesday and Wednesday... ...Pattern Overview... A primary concern into next week will be a strong Pacific jet stream that will be accompanied by an atmospheric river event, with heavy rainfall pattern expected to continue for the Pacific Northwest. A building upper ridge axis is expected across the Intermountain West and then across the Plains going into the middle of the week with well above average temperatures expected. Meanwhile, a broad upper trough is forecast to be in place from the Great Lakes to the East Coast, with a departing storm system followed by a clipper type system crossing the Upper Midwest and reaching the Ohio Valley through Wednesday with some snow showers accompanying it. A more zonal flow pattern aloft likely resumes after this system exits the East Coast, followed by a potentially organized storm system over the Southwest to central U.S. into next weekend. ...Guidance Evaluation/Predictability Assessment... Model and ensemble forecast spread has improved early-mid next week in a pattern with seemingly above normal predictability. Accordingly, the WPC medium range product suite was primarily derived from a composite of the 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian models along with the compatible 13 UTC National Blend of Models (NBM) and WPC continuity for best detail Tuesday into Thursday. Newer 12 UTC guidance remains mainly in line and multi-center ensemble support remains very good in this period. Forecast spread increases later next week into next weekend. A composite blend of best clustered guidance from the 06 UTC GFS/GEFS mean and 00 UTC ECMWF/ECMWF ensemble mean was preferred along with the NBM and WPC continuity to attempt to mitigate growing embedded system differences over time. The most significant guidance variance in this blend aloft was that the 06 UTC GFS and 00 UTC ECMWF models similarly showed a less progressive and more amplified southern stream upper trough out through the Southwest than the GEFS/ECENS means, albeit with a subset of model matching individual ensemble members. The newer 12 UTC GFS/ECMWF models have now trended more progressive/less amplified, more in line with the ensemble means. ...Weather/Hazards Highlights... Out West, precipitation will be ongoing on Tuesday, with a series of atmospheric rivers into the Northwest lasting into the middle of the week with earnest, with rising creeks and river levels expected. Given enhanced rainfall during the weekend and on Monday, the soils will be more vulnerable and thus the potential for greater run-off and flooding. Snow levels are initially expected to be rather high, and therefore western facing slopes of the Cascades may be subject to an additional risk of flooding due to snow melt. Therefore, a Slight Risk area remains valid for western slopes of the Washington Cascades on Tuesday, and also for the Oregon Coastal Ranges on Tuesday (Day 4). With the atmospheric river focus most likely weakening some and shifting a bit more south into Oregon and northwest California on Wednesday, a Marginal Risk is valid for the Day 5 outlook for far western Oregon into extreme northwest California. Farther inland across northern portions of the Intermountain West and Rockies, some moderately heavy snow is expected into next midweek with the passage of the next organized system. Some upslope snow showers are also likely for the higher elevations of the central Appalachians and vicinity going into midweek as the next low pressure system passes by. In terms of temperatures, much of the central and northern Plains will enjoy late season warmth with highs possibly 20-30 degrees above early December averages from northeast Colorado to the Canadian border on Wednesday and into Thursday, with some record highs within the realm of possibility. The same is expected to hold true with overnight lows as well across much of the central and western U.S., with the greatest positive anomalies across Montana and the Dakotas. Conditions should be close to seasonal averages for the eastern U.S. for much of the week with no major arctic air intrusions expected. Hamrick/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
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