Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 656 PM EDT Fri Mar 23 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 24/0000 UTC thru Mar 27/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Upper Midwest/middle Mississippi Valley to lower Ohio Valley/Tennessee Valley and Southeast... A vigorous short wave trough will emerge from the West into the Upper Midwest this evening into Sat. This southern stream feature, which impacted central/southern CA earlier this week with torrential rains, will help induce a surface low across KS, as a northern Polar boundary drapes over the circulation. A considerable amount of moisture return from the Gulf of Mexico with broad large scale warm advection regime will allow precip to flourish from the mid MS Valley to the OH Valley. The guidance captures this scenario, as the upper disturbance tracking south and east or downstream within the upper-level confluent flow. This will result in a weakening storm system over time with a very sharp edge or cutoff to the precip shield, while a surface ridge builds over the nern quad of the country. WPC favored a four way blend of the 12z gfs, 00z/23 ecmwf, 12z nam nest and 12z href mean for a streak of 1-2 areal avg qpf amounts with locally higher values. The heaviest qpf appears likely to set up from IA through south/central IL into southern IN and a large portion of KY. The southern extent of the qpf will be moderate to heavy rain, while potential heavy snow impacts the northern extent. For more on the extent of the frozen precip and heavy snowfall, please refer to the latest qpfhsd. ...Western U.S... A very active and wet regime will persist across the West on Sat. A large 500mb closed low and deep upper trough offshore of the Pac NW will sweep into the region. The main mid to upper circulation will dig south and east toward southern OR/northern CA, while the main thermal zone pushes into the inter-mountain West. Overall the onshore flow will remain intact but maybe elevated in one location. A dynamic upper jet/jet streak will focus strong Pac moisture transport into swrn OR and northern CA for possible 1-2 inch areal avg qpf amounts, especially from the coast to terrain. Overall WPC followed a blend of the gfs and ecmwf with the hi-res suite for qpf amounts. Days 2/3 QPF was based on the following blend with minor consideration of the previous forecast: Day 2 12Z ECMWF 50% 12Z GFS 40% 12Z NAM 10% Day 3 12Z ECMWF 60% 12Z UKMET 20% 12Z GFS 10% 12Z NAM 10%. No excessive rainfall outlook is in effect. ...The West... A long wave trough will shift onshore Saturday evening with a closed 500mb low center near the OR/CA border. This low will quickly open as the trough moves inland Saturday night. Associated onshore flow over CA will have PW less than 0.5 inches and allow locally moderate precip for northern CA coast ranges, the CA Cascades, and Sierra Nevada for Day 2 (00Z Sun-00Z Mon). A low level trough ahead of the upper trough axis will focus precip across the northern Great Basin to WY Rockies. A shortwave trough rounding the long wave will amplify the trough axis down to the CA/Mexico border Sunday night though little-if-any precip is expected in Days 2/3 from this over the desert southwest or southern CA coast. Strong onshore flow south of a broad trough south of Alaska Monday will send 0.75 inch PW into western WA where locally moderate precip can be expected with moderate snow elevations. ...Central and Northern Great Plains... A shortwave trough axis over northwestern MT Saturday evening shifts northeast across southern Saskatchewan through Sunday. The surface low will remain in Canada, but the associated cold front will allow convergence of Gulf-sourced moisture on a 50kt southerly low level jet over eastern ND to far northwestern MN. This is expected to be all snow. Surface low development in the lee of the southern CO Rockies is expected Sunday ahead of the long wave trough axis moving through the Intermountain West. Despite surface low development over the TX and OK panhandles (an area particularly parched), moisture will come from the southeast and be directed up the middle portions of the plains, not the southern high plains before wrapping around the developing low and into WY. A swath of one inch PW will begin to shift north Sunday from the southern Great Plains and reach across the Mid-South Sunday night (2 standard deviations above normal there) and into MO/IL Monday (2 to 3 standard deviations above normal there). A warm front in this vicinity will allow moderate to locally heavy rainfall that spreads north into IA. Farther north across the Dakotas and MN will be convergence with a surface high extending south from the Canadian Prairies which will result in a baroclinic zone and a swath of wintry precip. ...Southern Appalachians and Southeast... A remnant mid-level vort lobe and surface trough will shift ESE across the southern Appalachians and Carolinas Saturday night before sagging across the Deep South through Monday. Guidance continues to waiver a bit with the location of the max precip Saturday night, but it will generally be from SW VA to northern NC with a 0.5 inch to 0.75 inch swath expected. Abundant gulf moisture will be around 1.5 standard deviations above normal. With little forcing and weakening jet dynamics, expect only localized heavy rainfall across the south Sunday through Monday. ...New England... A closed northern stream low will push south across NY state/NJ Saturday night through Sunday. Little moisture is associated with this low, though on onshore flow will aide precip along the southern New England coast where the blend kept average QPF around a tenth of an inch. Musher/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml