Extended Forecast Discussion NWS Weather Prediction Center College Park MD 415 PM EDT Thu Apr 02 2020 Valid 12Z Sun Apr 05 2020 - 12Z Thu Apr 09 2020 ...Significant precipitation potential over parts of California Sunday into midweek... ...Overview and Guidance/Predictability Assessment... One dominant feature of the forecast will be a deep upper low expected to drop southward near the California coast and reach the Southwest U.S. by next Thu. This system will likely bring multiple days of precipitation to California with exact amounts/coverage sensitive to precise track and timing of the upper low. Farther east there will be mean ridge anchored over/near the Gulf of Mexico and extending into the eastern U.S. during the first half of next week. Leading shortwave energy flowing around the northern periphery of this ridge and underneath a strong upper high to the north/northeast of Hudson Bay will bring a front into the East to start the week. This front will stall and lift back north as a warm front. Stronger energy originating from the Bering Sea and Alaska will push southeastward and ultimately lead to an amplifying upper trough that reaches the eastern U.S. by next Thu with a possible closed low over eastern Canada. The leading cold front will suppress the well above normal temperatures forecast over the central/eastern states during the first half of the week. Guidance clustering has improved over the past day for the upper low tracking near California, though by the latter half of the period there is still some timing divergence. Recent trends suggest the GFS may still be a bit on the fast side (06Z GFS looking somewhat better than the 00Z version) while the GEFS mean becomes one of the slower solutions. By midweek the ECMWF has nudged a bit faster than it was 24 hours ago, favoring maintenance of an intermediate solution. Around Wed-Thu the persistent spread for Northeast Pacific flow reaching western North America still has some lingering influence on the forecast in the medium range, with more amplified troughing in the GFS likely contributing to its faster timing for the upper low to the south. Uncertainty in the northern stream flow becomes much greater issue immediately after day 7 Thu. As for the amplifying upper trough reaching the East by next Thu, operational GFS/ECMWF runs have been fairly steady in principle over the past 24-36 hours of runs. In this same time span their respective means have been trending deeper with the trough and now show the closed or nearly closed upper low over eastern Canada by day 7 Thu, adding some confidence to the GFS/ECMWF ideas. Ahead of the late-period upper trough, the uncertainty has greatly increased for the surface pattern across the Great Lakes/New England around Tue-Wed. In particular the 00Z ECMWF has become quite strong and fast with a wave that crosses this region. The ECMWF mean shows a broad area of low pressure which at least favors backing off from the operational run but supports more/faster waviness than seen in prior consensus. Confidence over this area and southeastern Canada is not very high due to complexities in flow between the upper high north of Hudson Bay and ridge over the remainder of the eastern U.S. Preference sided with a compromise approach based on data available through the 06Z cycle. The new 12Z guidance is still very conflicted as UKMET/CMC runs side more with the 00Z ECMWF in principle while the 12Z ECMWF has reverted back to a surface wave even slower and weaker than the GFS/GEFS. Forecast considerations led to a starting mass field blend consisting of the 06Z GFS/00Z ECMWF and lesser weight of the 00Z UKMET/CMC day 3 Sun into early day 5 Tue, followed by a trend to the GFS/ECMWF with their respective means and modest input from continuity. ...Weather/Hazard Highlights... The upper low tracking near the California coast will likely bring areas of significant precipitation into California and possibly Nevada/Great Basin with lesser amounts reaching parts of the Rockies. Greatest confidence in heavy rain/higher elevation snow will be over the Sierra Nevada, where model and ensemble signals are the most agreeable. Expect the coastal ranges to see meaningful precipitation as well. There is a wider range for potential precip totals over the southern ranges (with some high elevation snow possible) due to current spread for exact track/timing of the upper low. Slower half of the envelope could support five-day totals at least as high as those expected over the Sierra Nevada while the faster half would correspond to less extreme values. Current preference is for an intermediate solution. Most areas over the eastern half of the U.S. will see one or more episodes of rainfall with varying intensity. A brief period of low level flow from the Gulf will contribute to some of this activity, while Plains through Northeast wave(s) and associated fronts as well as a trailing cold front reaching the South/East by next Thu will provide some focus for rainfall. Guidance has been fairly consistent in suggesting that some locally moderate/heavy rainfall could be possible from Texas into parts of Tennessee/Mississippi/Alabama but without a great commitment to specifics. Expect multiple days of chilly daytime highs over California and southwestern half of Nevada during the first part of next week. Upper low path should shift the core of coolest readings versus normal into southern California and the Southwest by Wed-Thu. Minus 5-15F anomalies should be common for highs over this area. Meanwhile much of the central/eastern U.S. will see well above normal temperatures with plus 10-20F anomalies, and possibly greater for morning lows at some locations. The best potential for daily record highs/warm lows Tue-Thu will be over the South/Gulf Coast region. Exceptions to this warmth may be over the far northern Plains and New England. The cold front pushing southeast into the Plains and beyond next Wed-Thu will bring cooler Canadian air but with anomalies retreating only toward climatology. Southern tier states will likely stay warm into 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 Hazards: - Heavy rain across portions of California, Sun-Tue, Apr 5-Apr 7. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Sun-Tue, Apr 5-Apr 7. - Flooding possible across portions of the Great Lakes, the Upper/Middle Mississippi Valley, the Southern Plains, and the Northern Plains. - Flooding occurring or imminent across portions of the Mississippi Valley, the Northern Plains, the Tennessee Valley, the Southern Plains, and the Ohio Valley. - Flooding likely across portions of the Upper/Middle Mississippi Valley, the Great Lakes, and the Northern Plains. 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