Extended Forecast Discussion NWS Weather Prediction Center College Park MD 315 PM EST Fri Jan 24 2020 Valid 12Z Mon Jan 27 2020 - 12Z Fri Jan 31 2020 ...Overview and Guidance/Predictability Assessment... Model and ensemble guidance still depicts progressive and fairly low-amplitude flow aloft across the lower 48 through about the first half of next week. Then most solutions show the pattern trending toward larger scale mean troughing oriented from the Upper Mississippi Valley/Great Lakes into northwest Mexico as a strengthening eastern Pacific ridge builds into the West. Within this consensus evolution there are still significant detail uncertainties due to medium/small-scale shortwaves whose specifics and interactions are not typically well resolved several days out in time. Expect the best focus/highest confidence for heaviest precipitation during the period to remain over the Pacific Northwest into extreme northwest California with lesser amounts and lower confidence in details from the Rockies through the southern tier of the central/eastern states. Pacific origin of most flow (aside from shortwaves brushing New England) should maintain near to above normal temperatures over a majority of the contiguous U.S. next week. A blend incorporating the most common ideas of the 06Z GFS and 00Z ECMWF/UKMET/CMC provided the best representation of latest guidance during the first half of the period. A transition toward a model/mean blend (emphasizing the 06Z GFS/GEFS mean and 00Z ECMWF/ECMWF mean) accounted for large scale pattern agreement later in the period while downplaying increasingly uncertain specifics. Through today's 06Z cycle the guidance had furthered recent trends toward weaker and more suppressed low pressure after the system reaches the southern Plains Tue. The 00Z ECMWF joined the suppressed cluster after lagging the GFS/GEFS mean and some other guidance in this trend. However some new 12Z guidance (GFS/CMC) has reversed course somewhat, highlighting the ongoing uncertainty. The new 12Z ECMWF maintains the very suppressed scenario though. Farther upstream the models/ensembles have generally gravitated toward yesterday's 06Z GFS/GEFS idea of more pronounced amplification of shortwave energy nearing the Pacific Northwest early Tue and reaching the Four Corners then the southern Rockies/northwest Mexico by Wed-Thu. An embedded upper low may close off for at least a brief period of time. 12Z guidance thus far is providing decent continuity for this evolution. Meanwhile flow entering western Canada should feed into mean troughing over the northern U.S. after midweek. The 12Z GFS leans to the slow side with this developing northern trough. With typical detail differences there is decent agreement in principle for the individual fast-moving northeastern Pacific systems/fronts that will affect the Northwest during the period. ...Weather/Hazard Highlights... Models/ensembles continue to highlight the extended period of significant rain/mountain snow for the Pacific Northwest and far northwest California in association with the passage of multiple shortwaves aloft/frontal systems. Guidance is consistent in showing potential for five-day liquid totals in the 5-10 inch range over favored terrain along the Cascades and coastal areas. The best signal for most intense activity exists over western Washington and perhaps into parts of Oregon around next Wed-Thu when GEFS/ECMWF ensemble means briefly show precipitable water vales reaching at least 1.5-2.0 standard deviations above normal with operational runs depicting even higher anomalies. Strengthening flow between the persistent Northeast Pacific/Alaska upper trough and building eastern Pacific/western U.S. upper ridge should also enhance the moisture focus. Precipitation extending into the northern Rockies should yield lower totals than in the Pacific Northwest but still be significant. Expect separate shortwaves aloft crossing the West on Mon and Wed-Thu to produce lighter activity over parts of the Great Basin and central-southern Rockies. A weak wave over the eastern Gulf of Mexico on Mon should produce a brief period of mostly light-moderate rain over primarily the southern half of the Florida Peninsula. Guidance has still not arrived at a robust consensus for coverage/intensity though. The system reaching the southern Plains Mon-Tue should produce some precipitation over the southern half of the Plains into the Lower Mississippi Valley during the first half of the week. There may be a few pockets of at least moderate rainfall while any wintry weather will likely be confined to the central Plains in the northern fringe of the moisture shield. Confidence in precip coverage/intensity rapidly decreases from the Mississippi Valley eastward mid-late week. Upstream trough amplification aloft should promote surface low pressure reaching the Gulf of Mexico by late next week with a front forming to its northeast. This evolution should lead to an expanding area of rain between the southern Plains and Southeast Thu-Fri. Again confidence is below average in the precise details of coverage/intensity/timing. An area of light and scattered rain south/snow north may cross the East on Mon, followed by one or more northern stream impulses that may produce mostly light snow from the northern Plains eastward. The forecast is consistent in showing the greatest temperature anomalies from the northern High Plains into Upper Great Lakes with highs 10-20F above normal and morning lows 15-25F above normal most days. By next Thu-Fri most locations to the west of the Rockies should trend warmer, with increasing coverage of plus 10-20F anomalies, as upper ridging builds over the region. Expect below average temperatures to remain limited next week. The central-southern Rockies may see modestly below normal highs under shortwaves aloft while Florida may also see one or more days with highs up to a few degrees below average. Passage of one or more eastern Canada fronts may bring readings close to or slightly below normal over northern New England. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of Northern California, the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Mon-Fri, Jan 27-Jan 31. - Heavy snow across portions of the Central Rockies, the Central Great Basin, the Northern Rockies, and the Northern Great Basin, Tue-Wed, Jan 28-Jan 29. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Mon-Thu, Jan 27-Jan 30. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Northern Plains, the Tennessee Valley, the Great Lakes, the Middle Mississippi Valley, the Pacific Northwest, and the Ohio Valley. - High winds across portions of the Central Rockies, the Central Plains, the Central Great Basin, the Northern Plains, and the Northern Rockies, Thu-Fri, Jan 30-Jan 31 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