Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Thu Apr 23 2020 Valid 12Z Sun Apr 26 2020 - 12Z Thu Apr 30 2020 ...Extended Period of Much Above Normal Temperatures Across the West... ...Cool With Two Storm Systems Affecting Eastern States... ...Overview... Most guidance is stable in showing a persistent western ridge/eastern trough pattern aloft through next Thu, and likely beyond given the Climate Predication Center 8-14 day forecast. The western ridge will promote much above normal temperatures with the strongest upper ridging and greatest temperature anomalies likely to be around Tue-Thu. Shortwave energy rounding the northern periphery of the upper ridge should bring periods of precipitation to parts of the Pacific Northwest and extreme northern Rockies. At the same time the eastern U.S. will see below normal temperatures due to the mean trough aloft. There will be two primary storm systems of note. One will track northeast from the Mid-Atlantic early Sun onward. Upstream northwestern U.S. energy will amplify and generate another system that should intensify as it tracks from the Plains into or near the Great Lakes Tue-Wed, continuing eastward thereafter. Both systems should produce meaningful rainfall over some areas with locally moderate/heavy activity possible. ...Guidance/Predictability Assessment... As of the 12Z/18Z runs the GFS has continued to be on the fast/northern side of the spread for the system tracking northeast from the Mid-Atlantic and vicinity early in the period. The 18Z GEFS mean adjusted somewhat in the operational run's direction while the 12Z run was closer to the majority consensus/continuity. A blend emphasizing the 12Z ECMWF along with some CMC/UKMET input along with more 12Z GEFS versus operational GFS maintained a consistent solution with only modest detail adjustments. Most new 00Z runs maintain the same theme with the GFS/GEFS northward of other guidance during Sun but the ECMWF has adjusted to a compromise between the southern cluster and the GFS. Guidance continues to show some spread and variability for the next system to be supported by energy that reaches the Northwest by next Mon, begins to amplify over the Plains on Tue, and continues through the East thereafter. 12Z/18Z GFS runs strayed well ahead of nearly all other guidance, including the GEFS mean, from about early Tue onward. The past two 12Z ECMWF runs (not so much the 00Z/22 run) compared rather well to the remarkably agreeable ensemble means while the 12Z CMC eventually trended a little slow/south with the surface low. New 00Z guidance shows the same relationship with the GFS racing ahead of other solutions including the GEFS mean. Over the West the GFS/GEFS mean continued to be a bit stronger than consensus with leading weak shortwave energy crossing the region on Sun. Then around Wed the GEFS mean agreed with other non-GFS guidance that showed a somewhat stronger upper ridge than the 12Z/18Z GFS. After early Wed uncertainty increases regarding the progression and amplitude of ejecting eastern Pacific shortwave energy. The 12Z ECMWF/CMC shared similar ideas and the prior ECMWF was close, but otherwise there has not been enough agreement or continuity to provide any confidence in those specifics. Thus the preference was to lean increasingly toward the ensemble means for the time being. Not surprisingly the new 00Z guidance offers added detail possibilities. The aforementioned blend for the leading eastern system reflected preferences elsewhere through the first half of the period. By days 6-7 the initial blend rapidly increased total GEFS/ECMWF ensemble input to 60-80 percent with remaining operational input coming mostly from the 12Z ECMWF. ...Weather/Hazard Highlights... During Sun-Mon expect plus 10-15F anomalies for highs to cover a large area across the southern half to two-thirds of the West and into the High Plains, while clouds and precipitation keep temperatures closer to normal over northern areas. The forecast is consistent in showing intensification of warmth/heat over the West by Tue-Wed as the upper ridge strengthens, with above normal temperatures also expanding northward. Highs that are 15-25F above normal should be common across much of the West aside from northern areas and guidance continues to highlight the Great Basin for best potential to see plus 20F or greater anomalies especially on Wed. The upper ridge may weaken just a bit on Thu leading to slightly fewer plus 20F anomalies and greater eastward extent of warmth into the High Plains. Some daily records for highs/warm lows are possible through the period and likely to be most numerous Tue-Thu. On the other hand expect generally below normal highs over the East under mean troughing aloft. Coolest temperatures versus normal are likely to be on Sun-Mon from the Upper Ohio Valley/Appalachians into the Northeast where highs may be 10-15F or so below normal. Single-digit negative anomalies should prevail Tue-Thu. The system tracking northeastward from the northern Mid-Atlantic or vicinity Sun-Mon will bring precipitation of varying intensity to areas north and west of the surface low track. Higher elevations in parts of the Northeast may see some snow from this system. There is still enough detail/track uncertainty to temper confidence in locating the axis and magnitude of highest precip totals from early Sun onward. Shortwave energy will support rain and possibly some higher elevation snow over the Pacific Northwest and into the extreme northern Rockies from the weekend into early next week. Expect highest totals along favored terrain in western Washington. Amplification of this energy will bring a developing storm system and associated fronts from the Plains into the eastern U.S. Tue onward. The fronts and anchoring low pressure should provide focus for multiple areas of rain and thunderstorms with some activity possibly heavy. A fairly broad area has at least some potential for significant rainfall, including parts of the Southern Plains along with much of the Mississippi Valley and Great Lakes/Ohio Valley into the Northeast. One concern may be for any regions that also saw significant rainfall from the first system (and/or the system affecting the East during the latter half of this week). Consult Storm Prediction Center outlooks for latest information on severe potential as important details become more clear over the coming days. At some point in the mid-late week time frame the Northwest may see another episode of precipitation as additional shortwave energy ejects from the eastern Pacific, but with low confidence in specifics. 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