Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 702 PM EDT Thu Mar 22 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 23/0000 UTC thru Mar 26/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Western U.S... A deep trough/closed low digging south offshore the Pacific Northwest will begin to gradually shift inland tonight through Friday across the West Coast which will maintain deep and moist cyclonic flow across the coastal ranges of WA, OR and northern CA and into the Cascades. This will maintain numerous shower activity which will be aided by slowly steepening mid level lapse rates and favorable onshore/upslope flow over the terrain. Expecting locally an additional 1 to 3 inches of rain for the coastal ranges and especially far northwest CA and southwest OR. Lesser amounts are expected over the Cascades. Meanwhile, the lead shortwave currently coming into CA out ahead of the aforementioned trough/closed low offshore the Pacific Northwest will eject rapidly inland across the Intermountain region tonight and then across the Rockies on Friday. This will take the remnants of a well-defined atmospheric river in across the region which is characterized by 850/700 mb moisture flux anomalies of over 5 standard deviations above normal, and precipitable water values of locally over 4 standard deviations above normal. This will result in a highly efficient precipitation processes over the higher terrain with west and southwest facing slope of the Wasatch, Tetons and high country of western CO seeing moderate to heavy precipitation. Water equivalents will locally be over 1 inch through late Friday as the energy traverses the region. WPC leaned toward a multi-model consensus for QPF with generally a blend of the GFS, ECMWF and hires CAM guidance used. ...Northern/Central Plains southeast to the Middle Mississippi Valley... The aforementioned shortwave crossing through the Intermountain region through tonight will ultimate encroach on the High Plains by late Friday. The strong influx of Pacific moisture within the deep layer warm conveyor belt will be encountering a much colder, drier air mass out across the northern Plains in particular. As low pressure develops in the lee of the Rockies, expect an axis of frontogenetically enhanced precipitation to break out over northeast MT and the Dakotas late tonight and early Friday which will ultimate begin to streak southeastward toward the middle MS Valley by Friday evening. The enhanced Pacific moisture transport will likely result in moderate to heavy precipitation, including heavy snowfall across portions of North and South Dakota. Some relatively strong frontogenetical banding of snowfall will be likely which will enhanced the snowfall rates. Please consult the latest QPFHSD for more details on the expected winter weather impacts. In general, WPC leaned toward a consensus of the GFS, ECMWF and the hires models including the ARW, ARW2 and NMMB. Days 2/3 WPC QPF was based on a blend of the 12Z ECMWF/GFS (which had good agreement) with a little 12Z NAM included. No excessive rainfall is expected in this period. ...Pacific Northwest to the Northern Rockies... Occluded and weakening low pressure comes ashore near the OR/CA border late Saturday reaching the Great Basin as a weakened open wave Sunday. Short wave energy ahead of the low taps Pacific moisture (though only half inch PW) to produce locally heavy QPF over higher terrain of coastal OR/CA Friday night into Saturday. Local areal averages around one inch can be expected in the CA coast ranges and Cascades for Day 2 (00Z Sat-00Z Sun) and the Sierra Nevada for Day 3 (00Z Sun-00Z Mon). Associated height falls allow mainly higher terrain QPF of half inch or less across the northwestern CONUS. ...Northern Plains/Upper MS Valley/OH Valley/Carolinas... Short wave energy tracking over the CO Rockies Friday evening with a surface low tracking east across the central Great Plains Friday night. In doing so the wave runs into the central CONUS upper level ridge and weakens. Before the low can weaken too much Friday night a baroclinic zone develops north/east of the low with increasing southeasterly low level flow. One inch PW from the western Gulf spreads north ahead of the low and into the Great Plains where a swath of rain to the southwest and snow to the northeast sets up. Moderate intensity can be expected in this frontal zone over MN/IA/IL/MO and into southern IN/KY. The low weakens to a remnant trough which drifts across the Carolinas Saturday night. Rates will decrease as it moves on, though locally enhanced precip can be expected over the southern Appalachians/Smokies late Saturday. 12Z guidance took a southward jog with precip over southern VA and over NC Saturday night. Considered continuity a little bit with this southward shift to slightly limit the southward movement of the QPF. Orrison/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml