Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 445 PM EDT Thu Apr 05 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 06/0000 UTC thru Apr 09/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Pacific Northwest/California... ...Central/Northern Rockies and adjacent High Plains... The models bring an area of low pressure toward the West Coast over the next 24 to 36 hours in association with a gradually amplifying upper trough. The trough will be interacting with a substantial pool of subtropical moisture and this will set the stage for a strong atmospheric river to impact the West Coast tonight through Friday, with enhanced moisture transport and strong forcing helping to focus heavy rainfall across the coastal ranges of central and northern California, and extending up across foothills and upslope areas of the northern Sierra-Nevada. The 850/700 moisture flux anomalies are forecast to increase to over 5 standard deviations above normal along the coast which will be aided by the arrival of a 40 kt low level jet. Additionally, the precipitable water values are forecast to rise to between 1.5 and 1.75 inches across coastal areas of central California and extending offshore which in itself would be over 5 standard deviations above normal. A combination of increasing onshore flow and enhanced rainfall efficiency given the depth of subtropical moisture will result in heavy rainfall. Expect as much 2 to 4 inches of rain locally through late Friday, with some areas possibly exceeding 5 inches involving the central/northern California coastal ranges and northern Sierra-Nevada. Runoff problems and flooding concerns are expected given the amounts. The influx of Pacific moisture will extend well inland and all the way through the central and northern Rockies where some lead shortwave energy coupled with the moisture will be interacting with an Arctic cold front draped up against the eastern slopes of the high terrain. A combination of warm air advection and associated isentropic lift along with relatively deep frontogenetic forcing should support a stripe of light to moderate precipitation, including accumulating snowfall, out across the High Plains. WPC leaned toward a multi-model consensus with a bit more weight toward the ARW and ARW2 hires solutions. Please consult the latest QPFHSD for more details on the expected winter weather impacts. ...Southern Plains/Lower Mississippi Valley... Increasingly moist southerly flow ahead of a developing area of low pressure over the southern Plains will set the stage for heavy convective rainfall beginning late tonight in an elevated fashion late tonight over far eastern Oklahoma and western Arkansas, and then expanding east across much of the lower Mississippi Valley on Friday with focused impacts on central and southern Arkansas and into central and northern Mississippi. The convection will be enhanced by the arrival of a strong cold front from the northwest and the development of a separate west/east oriented stationary front ahead of the surface low. These boundaries will focus precipitable water values of up over 1.5 inches and foster stronger lower level convergence which will take advantage of the pooling of instability for heavier rainfall rates and convective organization. WPC generally leaned toward the wetter hires CAM guidance with a focus on the ARW2 in particular. The environment is conducive for back-building and training convective cells, and thus expect heavy to excessive totals. As much as 2 to 4 inches of rain through Friday evening is expected with locally heavier amounts possibly approaching 5 inches. This will drive concerns for runoff problems and flash flooding. ...Great Lakes/Northeast... Low pressure and an associated shortwave trough will traverse the Great Lakes region before lifting northeast across southeast Ontario through late Friday. Warm advection and moisture transport extending downwind of the low center will promote an expansive area of light to moderate snowfall and some rain across the Great Lakes and interior of the Northeast. Generally a blend of the global models was favored for the QPF. ...Florida... Some lingering showers are possible along the southeast coast of the Florida peninsula this period with some low level convergence and pooling of modest instability along a weakening front. The amounts should be generally light. Days 2/3 Pacific Northwest, California eastward through the Northern and Central Rockies: The strong atmospheric river event that sets up during Day 1 is expected to continue over northern and central California through much of the Day 2 period. A deepening cyclone moving through the eastern Pacific toward the northern Washington coast will advect anomalously high PWs in the 1.5-1.75 inch range (upwards of 5-6 standard deviations above normal) into the coast ahead of a strong cold front. The combination of the deep moisture along with strong frontal dynamics and height falls will support heavy precipitation across northern and north-central California Friday night...with the bulk of the QPF shifting southward into central California early in the day and gradually diminishing as the main energy pushes inland. Orographic lift will enhance amounts along west and southwest facing slopes...with rainfall amounts of 2-4 inches expected across the coastal ranges in northern California, the Shastas, and into the northern and central Sierras. Overall...did not make any notable changes to the previous forecast given good run-to-run continuity in the guidance. So will maintain the slight risk area for this region on Day 2, with the greatest flooding potential remaining over burn scar areas. Farther north through the Pacific Northwest...widespread precipitation is expected through much of the Day 2 into Day 3 period, initially from the height falls and frontal passage Friday night to early Saturday, then with persistent and strong onshore flow south of the cyclone as it pushes into Vancouver Island and weakens. Areal average amounts over the two-day period will range in the 2-3 inch range, with locally higher amounts expected over favored terrain in the Cascades and Olympics. While the heaviest QPF will focus over California and from the Cascades westward through Washington/Oregon, a significant amount of Pacific moisture will continue to stream inland through the Northern and Central Great Basins/Rockies. As short wave energy rounds the base of the eastern Pacific closed low and ejects eastward...expect widespread precipitation to break out on Saturday, with the heaviest amounts across northern Washington and northern/central Idaho in response to enhanced lift from a combination of coupled jet dynamics and orographic lift. Drier conditions are likely on Sunday, though as the eastern Pacific closed low weakens and shears inland, it will act on remaining available moisture to support some light valley rain/mountain snow over the northern Intermountain region. Southern Plains, Tennessee Valley, Southeast and Mid-Atlantic region: Forecasting the interaction between northern and southern stream energy moving through the mean eastern U.S. trough has proven very challenging. The main trend in all of the guidance from the previous overnight runs has been to shift southward the Day 2 QPF from the Tennessee/lower Ohio Valleys across the mid-Atlantic states. This appears to be a result of a weaker southern stream feature and less phasing with the northern stream, which is leading to a flatter and more suppressed wave ejecting along the arctic front. Overall, favored a blend of the NAM/ECWMF/GFS over this region with the cool sector precipitation, with the heaviest totals (around .75-1.5 inches) from central Tennessee eastward across Virginia/North Carolina. Right entrance region jet dynamics may enhance the QPF across eastern sections of Virginia/North Carolina, where local amounts of up to 2 inches are possible. This system will move off the coast Saturday evening, with the trailing cold front potentially spawning convection Sunday over the central and southern Florida peninsula. Opted for a blend of GFS/ECMWF solutions over this region. Across the Southern Plains and lower Mississippi Valley, strong low level transport of Gulf moisture into the surface front will support a continuation of the organized convection from Day 1. Models show some notable differences with how quickly the convection advances into the more unstable air toward the Gulf, with the high resolution ARW runs showing a cold-pool dominant solution and quickly advancing a squall line southward. The majority of global and regional guidance is slower, maintaining the convection along the front. Leaning at this point toward the more progressive solutions, but with below average confidence. Overall, expecting around 1-3 inches across this region, with locally higher amounts in any training convection. Since we are favoring faster progression, have opted to shift the Day 2 slight risk area southward. Northern/Central Plains, Upper and Mid Mississippi Valleys: The system that impacts the Western states on Day 2 will move out into the Plains during Day 3. The short wave pattern is rather complex, but there's generally good agreement that a low will spin up on the arctic front over Montana Saturday evening and advance into the Upper Mississippi Valley by late Sunday. This will support a band of overrunning snowfall along and north of the low track, with liquid equivalent amounts ranging from about .25-.65". Farther south...trailing short wave energy will induce lee-side cyclogenesis early Sunday over the south-central high Plains. Moisture overrunning a front developing east of this low should result in a second band of light precipitation across the south-central Plains into the mid-Mississippi Valley during the day Sunday. There are some timing differences among the models, so generally favored a compromise that blends the ECMWF with the National Blend. Orrison/Klein Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml