Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sat Feb 11 2023 Valid 12Z Tue Feb 14 2023 - 12Z Sat Feb 18 2023 ...Stormy flow with a series of upper lows in split southern stream flow from the Western U.S. northeastward... ...Overview... Latest guidance continues to advertise an active weather pattern, highlighted by an opening upper low tracking from the central High Plains through Great Lakes Tuesday-Wednesday and a stronger system digging into the West Tuesday-Wednesday and then continuing through the Plains/Great Lakes later in the week. The combination of these systems, and likely to a greater extent the latter, should produce a broad area of significant weather including heavy rain, snow, wind, and severe weather across portions of the lower 48. Meanwhile an upper trough and possible embedded low nearing the West Coast toward the end of the week may bring some precipitation to that region but with lower confidence in the specifics at this time. ...Guidance/Predictability Assessment... The early part of the forecast has held up quite well in terms of guidance clustering and continuity for the two features of interest through day 4 Wednesday, the High Plains to Upper Midwest/Great Lakes system and the upper trough digging into the West yielding a closed low nearing the Four Corners as of early Wednesday. A consensus approach represents these systems well in that time frame. Then the guidance shows more spread and run-to-run variability by days 5-7 Thursday-Saturday. For the western system emerging into the Plains by day 5 Thursday and continuing northeastward thereafter, there is continued uncertainty over exactly how the upper low itself will evolve and how/when northern stream shortwave energy will interact with it. Through the 12Z/18Z model cycles (serving as the basis for the updated forecast), recent ECMWF/CMC runs had been the most consistent and best clustered with the ensemble means. GFS runs have been more erratic, with some earlier runs straying faster than consensus, the 12Z GFS fairly close to consensus, and then the 18Z GFS trending slower/westward. In the new 00Z cycle, the CMC has adjusted somewhat slower and the GFS maintains its slower timing from the 18Z run. The 00Z ECMWF is holding onto its timing near the means though. Over the eastern Pacific/West Coast, the models and ensembles have continued to be split regarding if/where an upper low could pull off. Recent GFS runs through 18Z (and joined by the 12Z UKMET) have been on the west side of the spread for an eastern Pacific upper low while through day 6 Friday the ECMWF/CMC have been closer to the ensemble means that are somewhat more open but with an implied upper low by Friday. Then by day 7 Saturday the ECMWF has been pulling off a closed low farther southwest than where the CMC and ensemble means have been suggesting, though the new 00Z CMC has trended somewhat southwest as well. At the very least, the resulting D+8 multi-day means in the 12Z/18Z GFS runs yield significantly higher heights aloft over the West versus the eventual troughing suggested by teleconnections relative to the strong core of positive height anomalies south of Alaska. This favored removing the GFS from the forecast blend late in the period. The new 00Z GFS is a still a bit extreme from Friday onward but is much closer to remaining guidance through and beyond the late part of the medium range period. The combination of early consensus and subsequent forecast preferences led to starting with a 12Z operational model composite for days 3-4 Tuesday-Wednesday followed by a little increase of the CMC weight at the expense of the GFS day 5 Thursday. Days 6-7 Friday-Saturday emphasized the CMC/ECMWF (lower ECMWF weight by day 7) on the operational side while incorporating 30-50 percent total weight of the 18Z GEFS/12Z ECens means. ...Weather/Hazard Highlights... The system tracking from the central High Plains through the Upper Midwest/Upper Great Lakes Tuesday into Wednesday will spread an area of precipitation across the eastern Plains through east-central U.S. This should be a fairly warm system with the chance for some light to moderate snow confined to the northern Plains/Upper Midwest. Rainfall may be locally moderate to heavy over the Plains/Mississippi Valley, though guidance has shifted enough on some details for totals along/ahead of a briefly stalling front over the Mississippi Valley to suggest a somewhat lower threat of any isolated flash flooding than depicted earlier. The system digging into the West as of Tuesday should bring mostly light precipitation to the central West Coast but more significant snowfall and low elevation rain are likely over the favored terrain of Arizona/Utah/Colorado as an upper low passes over the region. Expected system emergence over the south-central Plains and then a track northeastward into eastern Canada mid-late week will likely renew a heavy precipitation pattern over the eastern half of the country. More meaningful snow is possible in the cold sector of this system relative to its predecessor, with the most probable axis for significant accumulations currently extending from the central High Plains into the central/upper Great Lakes. Latest guidance has been waffling on timing, which will be important for determining maximum snowfall totals within the favored axis. Meanwhile leading Gulf inflow ahead of the system's trailing front should promote areas of heavy rainfall for the Southeast/southern Appalachians and up the East Coast. Areas of severe weather are also possible from the Lower Mississippi Valley into the Southeast given strong low-level and deep-layer shear and sufficiently buoyant air spreading northward from the Gulf. Check the latest Storm Prediction Center products for more information on severe threats. Guidance is signaling this storm could become deep enough to produce fairly strong winds. High pressure will settle in over the Interior West as this system continues to the east of the Rockies/Plains. The upper trough nearing the West Coast should bring an increase of precipitation to that region late in the week. Amounts should be mostly light to moderate, while uncertainty regarding the exact character of this trough and possible embedded low keeps confidence on the lower side for the specifics of coverage and amounts. The upper trough/closed low digging over the West will bring highs down to 10-25F below normal over the southern two-thirds of the region during Tuesday-Thursday, with some of this cold air reaching the Plains by Thursday as well. Cool but more moderate temperatures should persist over the West through the end of the week. Warm flow ahead of the two primary systems next week will bring well above normal temperatures to parts of the central U.S. into midweek and the eastern U.S. through at least Thursday and possibly Friday. Some readings may reach at least 20-25F above normal for both high and low temperatures. Widespread record warm lows are possible especially Wednesday-Friday (if they hold through the end of the calendar day), while some record highs could be possible around Thursday over New England and the Florida Peninsula. New England could also see some record highs on Friday if the approaching cold front is slow enough. 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, experimental excessive rainfall outlook, 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/#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