Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 PM EST Thu Jan 23 2020 Valid 12Z Sun Jan 26 2020 - 12Z Thu Jan 30 2020 ...Overview... Progressive Pacific flow aloft will split as it reaches North America with separate streams across the contiguous U.S. and north-central Canada, between general mean ridging or at least above average heights across southern Canada. Some of the northeastern Canada flow may extend into New England by Wed-Thu. Over the course of the period there will be a trend toward greater mean troughing over the lower 48. By next Wed-Thu an upper ridge building from the Pacific into the West Coast and vicinity should encourage the downstream trough to become larger in scale. The forecast pattern will promote an extended period of precipitation focused over the Pacific Northwest and northwestern California. Meanwhile significant uncertainties regarding important shortwave details keep confidence low in determining surface evolution and precipitation coverage/amounts over the eastern two-thirds of the country. ...Guidance/Predictability Assessment... For about the first half of the period a blend of latest model guidance reflects consensus/average ideas for meaningful systems. The upper low and surface system affecting the Northeast early in the period continue to show some modest adjustments in consecutive runs. Latest trends include previously fast GFS runs returning back to the west while the full model/ensemble cluster has adjusted northeastward for the surface low Sun-Mon. Some 12Z guidance again shows some wobbling slower than established consensus at some valid times. Farther upstream a modest shortwave near the West Coast as of early Sun should amplify somewhat as it heads toward the Plains while multiple weak trailing impulses stream across the eastern Pacific/western U.S.. Guidance is still having a challenge resolving details of this flow with the primary trend of note over the past 1-2 days being a faster/more open depiction of the leading shortwave that reaches the Plains by Tue. This results in a weaker trend for corresponding surface low pressure. In spite of the shortwave uncertainties, the model blend provided good continuity for the sequence of features affecting the West Coast. By day 5 Wed the 06Z GFS begins to stray from the consensus and continuity near the West Coast, favoring a transition to include some of the 00Z run for GFS input. Meanwhile there is still good consensus in principle regarding a wave tracking across the Gulf of Mexico Sun-Mon but with typical spread for specifics. Guidance displays increasing divergence and continuity changes during the latter half of the period. One prominent issue involves the ultimate evolution of the energy reaching the Plains by early Tue and upstream shortwaves. There has been a general trend toward more progression and less phasing of shortwaves, leading to weaker and more suppressed surface low pressure beyond the southern Plains. The operational 00Z ECMWF is a lingering holdout in depicting well-defined and more northward low pressure over the East by Wed-Thu. Remaining models and ensemble means are quite suppressed by next Thu. The new 12Z ECMWF does reflect some of the consensus trends. Farther west the 06Z/12Z GFS and GEFS mean runs are notably westward/sharper than most other guidance with energy that comes into the western states. By the end of the period the 00Z GFS/ECMWF/ECMWF mean and CMC offer the best clustering though CMC ensembles suggest potential for some southwestward elongation of shortwave energy. Overall preference was to account for the suppressed trends at the surface over the East but to a somewhat lesser extent than the current means, while giving minimal weight to the 06Z GFS/GEFS idea over the West. This was accomplished by leaning more to the 00Z GFS/CMC/ECMWF mean while including 20-35 percent WPC continuity. ...Weather/Hazard Highlights... The most confident aspect of the forecast is the multi-day period of significant rainfall/mountain snow expected over the Pacific Northwest and northern California. Passage of multiple shortwaves aloft and waves/fronts at the surface will produce some north-south oscillation of strongest focus during the period. Teleconnections relative to the strong core of negative height anomalies over the Gulf of Alaska and mainland Alaska continue to highlight the central/northern West Coast for best precipitation potential, while the upper ridge that builds into the West Coast late in the period may lead to a northward shift of strongest moisture focus into Washington by around next Thu. Guidance consensus is still advertising potential for five-day totals of 5-10 inches liquid at some locations along favored coastal and Cascades terrain. Expect lighter activity farther inland as moisture spreads across the West, with highest totals over the northern Rockies and most of the southern quarter of the West staying fairly dry. There will be one or more areas of precipitation across the central and eastern U.S. with the progression of shortwaves aloft and associated surface features but confidence is below average in determining coverage and amounts especially by Tue-Thu. There is general agreement that a southern stream feature will bring some light to locally moderate rainfall to the southern tier and Florida Peninsula Sun-Mon. Heaviest rain should be confined to the Gulf of Mexico. The system reaching the southern Plains by Tue should produce some rain, with wintry weather possible in the northern part of its moisture shield. After Tue the coverage and intensity of precipitation become increasingly uncertain. Recent trends toward a faster and more suppressed surface system suggest decreasing potential for significant amounts compared to forecasts from previous days. Trailing energy could produce some precipitation over the southern Plains and/or Gulf Coast region by next Thu. The system over/near New England as of early Sun and surrounding flow will initially produce some snow from the Great Lakes into the Appalachians and western-northern New England. The forecast is consistent in showing near to above average temperatures over most of the lower 48 during the period. Norther tier states will see the greatest anomalies with highs 10-20F above normal and morning lows 15-25F above normal. The Northwest/Great Basin and vicinity will see warm lows during much of the period with readings on Sun possibly challenging daily records at some locations. Clouds/precipitation should lead to modestly below normal highs over the South including Florida Sun-Mon with cool anomalies persisting over the Florida Peninsula into Tue. Parts of the central Rockies may see highs a few degrees below normal around Tue-Thu. 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 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