Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Mon Jan 25 2021 Valid 12Z Thu Jan 28 2021 - 12Z Mon Feb 01 2021 ...Heavy precipitation likely for California into late week followed by another event focusing along the central West Coast from the weekend into early next week... ...Weather Pattern Overview and Guidance/Predictability Assessment... The large scale pattern will continue to feature eastern Pacific mean troughing aloft, supporting episodes of significant precipitation along portions of the West Coast and California in particular. Ejecting shortwaves will maintain the theme from the short range, generating varying degrees of low pressure and spreading precipitation of various types eastward across portions of the lower 48. A leading vigorous shortwave should reach near the East Coast at the start of the forecast early Thu with associated low pressure deepening rapidly as it tracks over the western Atlantic. Guidance has been oscillating over recent days for the exact timing and when an embedded closed upper low may form. The latest adjustments in most guidance are toward faster progression and less potential for a closed low before the feature crosses the East Coast--reducing precipitation amounts with this system. The 12Z CMC was a notable slow solution for this system. To the northwest, most guidance is following through with yesterday's 00Z cycle initial trends toward a southern Canada upper low tracking southeastward across New England late this week. The blocky pattern from which this upper low emerges has caused difficulty with the forecast as solutions have varied between having the low lift northward (the majority scenario before the 00Z/24 cycle) or tracking southeastward (before that cycle, generally represented by only a stray CMC or UKMET run). With the exception of the fast CMC, guidance is well clustered for the upper trough initially near the West Coast and then progressing inland. Even among non-CMC solutions there has been some spread for the trough's acceleration in response to upstream flow. Latest convergence has been more from faster GFS trends than the ECMWF adjusting slower. There is ongoing uncertainty over the shortwave specifics in the approaching Pacific flow and how this energy as well as the leading ejecting shortwave evolve within the overall consensus trough expected to reach the central/eastern U.S. by days 6-7 Sun-Mon. This tempers confidence somewhat for the specifics of the surface pattern. Over the past 24-36 hours guidance has trended more suppressed with the eastern U.S. system late in the period. This is due to the change in handling of the upper low now forecast to cross New England late this week along with residual cyclonic flow in its wake, plus stronger surface high pressure expected over eastern Canada. The next upper trough taking shape off the West Coast during the weekend/early next week should reach near 130W longitude like its predecessor, but in contrast be somewhat more rounded and less amplified. Toward the end of the period GFS runs (especially the 12Z run) became sharper/deeper than consensus. This was likely due to its upstream evolution that built a stronger upper ridge into southwestern Alaska than other guidance that at least had lower heights if not troughing. The new 00Z GFS has nudged slightly away from its prior runs. Guidance comparisons among 12Z/18Z guidance favored a forecast blend emphasizing the 18Z GFS/12Z ECMWF along with modest 12Z UKMET input for about the first half of the period followed by a trend to a model/mean mix consisting of slightly more ECMWF than GFS and even weighting of their respective means. ...Weather/Threats Highlights... Expect heavy precipitation over California to extend from the short range period into late this week. Highest totals of rain/mountain snow should be during Thursday-Thursday night over the central Coastal Ranges and Sierra Nevada followed by locally heavy activity but somewhat less extreme totals over the southern ranges by Friday. The supporting upper trough will spread areas of generally moderate precipitation over the remainder of the West into the Rockies late this week with highest totals over favored terrain. Upstream Pacific flow that evolves into another upper trough offshore will likely bring an episode of rain and higher elevation snow to areas from the Pacific Northwest into northern California during the weekend into early next week. Guidance is not yet fully committed to the specifics but the overall signal for highest totals has been fairly stable, over southwestern Oregon and northern California. Some of this West Coast moisture should extend into the northern Rockies. Farther east, the deepening storm reaching the western Atlantic by early Thursday should be far enough offshore to yield a dry day along the East Coast. However the tight gradient between this low and the surface high building toward the eastern states will support a period of brisk to strong winds. Meanwhile some lake effect snow (mostly on the lighter side of the spectrum) will be possible late this week, especially downstream from the northern and eastern Great Lakes. The next system ejecting from the West will bring a broad area of precipitation across the eastern half of the country from the weekend into the start of next week. Recent guidance changes for the pattern over the Great Lakes/Northeast and eastern Canada ahead of this system have led to a somewhat broader area of winter weather potential, from the Midwest across the Great Lakes/parts of the Ohio Valley and into the Mid-Atlantic/Northeast. Rainfall in the warm sector could be locally moderate to heavy at some locations. Behind the deepening storm tracking through the western Atlantic, eastern U.S. temperatures will tend to be below normal from late this week into the start of the weekend. Best potential for highs 10-20F below normal will be over the Northeast around the end of the week as a compact upper low tracks overhead. The southwestern states will also be on the cool side for most of the period with some locations up to 5-10F or so below normal for highs. Readings may reach closer to normal by early next week. As the initial upper trough near the West Coast starts to eject eastward expect a warming trend over the Plains late this week followed by eastward progression of above normal anomalies into the east-central U.S. during the weekend. Plus 10-20F anomalies will be common for morning lows while there will be less coverage of plus 10F or greater anomalies for highs. Cooler temperatures will be possible where snow cover exits. By early next week low temperatures over the East should reach 5-15F above normal but highs may not exceed normal by more than a few degrees. 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, 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