Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 441 PM EDT Tue Mar 20 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 21/0000 UTC thru Mar 24/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...eastern Ohio Valley/Central Appalachians/Mid Atlantic/Northeast... All the ingredients were coming into place for a heavy snowfall from Washington, DC to Philadelphia and New York City. Refer to the WPC Heavy Snowfall graphics and discussion for winter weather amounts and probabilities. The storm will also produce heavy rainfall over the coastal mid Atlantic, with rain changing to snow in coastal areas of the northern mid Atlantic. The models have come in better agreement with cyclogenesis in the southern mid Atlantic with the low intensifying over the coastal waters early Wed. Snow redevelops along the existing low level frontal zone in southern PA and expands across NJ into the metro New York City area. The strong lift, followed by enhanced moisture fluxes on easterly low level flow off the ocean, leads to 1.5 to 2 inches liquid equivalent precipitation in NJ, with over an inch to Long Island and into southeast PA and northern MD and DE. The WPC QPF followed the model consensus, and blended the high resolutions windows with the 12z NAM Conus Nest, and 12z Canadian regional model. The 12z ECMWF trended south with its QPF axis in the mid Atlantic in response to its axis of low pressure shifting south, so no the northern edge amounts remain uncertain in northern PA/southwest NY/New England. The high res windows and ensembles have concurred with the 12z nam to produce one secondary maxima in the Ohio Valley centered on IN and western Ohio as 850-700 mb convergence maxima develop west of the circulation in the Oh Valley. There is potential for banded snow in this area. Another area of heavier precip is where the circulation produces 700 mb convergence in southern WV to adjacent VA with lift aided by upslope flow. Given good multi-model agreement, a blend of the models/ensembles was used. ...California... The models continue to advertise widespread rain in California and heavy rain in portions of southern CA, particularly the coastal ranges. Secondary maxima develop in the interior mountains. Please see the risks of excessive rainfall / flash flooding graphics and discussion. The models show an atmospheric river with moisture transport tonight increasing into California, with the 12z nam showing precipitable water values increasing to 1.25-1.5 inches in portions of coastal southern CA. A well defined 870-700 mb convergence maxima moving onshore between Santa Maria and Los Angeles, This results in heavy rain developing Wed near Santa Barbara and nearby ranges. As the upper jet across north into northern CA, widespread rain develops in northern CA. Lift from well defined upper divergence maxima in central CA continue overnight and drift north into northern CA in tandem with the upper jet maxima on Wed. Due to good large scale agreement, a consensus approach to the QPF was used, including the high resolution ensemble forecast mean for the day 1 forecast. This approach usually does well along the West Coast. Rainfall rates tonight gradually ramp up to half to three fourths of an inch in six hours, increasing to 1-1.25 inches in 6 hours during the day on Wed. Day 2 California There continues to be a strong model signal on day 2...Wednesday evening through Thursday evening...for widespread heavy precipitation across large portions of California from the Transverse range/southern California coastal range--through the Sierra and into the northwest California coast range. The atmospheric river event which commences during the day 1 period will continue into the day 2 period as a mid to upper level trof off the California coast continues to press eastward day 2. This will sustain anomalously strong and moist onshore flow into much of California with the emphasis for the strongest moisture flux focused into the southern California coast range/Transverse range and into large portions of the Sierra. Precipitable water anomalies of 4 to 6+ standard deviations above the climatological mean...and moisture flux anomalies of 5+ standard deviations above the climatological mean are expected during day 2. Model consensus is for very heavy precipitation potential across these areas with a high likelihood of runoff issues over the southern California coast range/Transverse range and heavy snows into the central to southern Sierra. With respect to the runoff issues over the southern California coast range/Transverse range---a high risk area was maintained with few changes from the overnight issuance...namely from near Morro Pt---southeastward through the Transverse range to cover the Canyon--Sherpa---Whittier---Rey and Thomas burn areas. The expected magnitude of the rainfall across these burn areas---upwards to .25-50" per hour will likely produce life-threatening debris flow/mudslide potential. Pacific Northwest Anomalously high precipitable water values will also be drawn northeastward into the Pacific Northwest ahead of the strong closed low amplifying and dropping southward to a position off the B.C. coast. While pw values will rise to 1 to 1.5 standard deviations above the mean into the Pacific Northwest---the low level flow will have a more terrain parallel trajectory---supporting much lower precip totals than areas farther to the south. Areal average precipitation amounts in the range of .25-.50+ inches were depicted from the Washington-Oregon Cascades to the Washington-Oregon coast ranges. Heavier totals in the 1-2"+ range likely from far southwest Oregon into coastal northwest California where the low level south southwesterly flow will have a greater upslope component. Inland into the Great Basin---widespread moderate precip totals likely day 2 as the upstream anomalous pw axis pushes inland. There remains a lot of detail differences among the latest guidance leading to lower confidence than other areas in the anomalous pw plume. Mid Atlantic into New England There should still be low pressure lingering off the coast of southern New England by the time the day 2 forecast period begins at 22/00Z. Between the flow around the surface feature and the cyclonic circulation aloft, there will still be a band of precipitation along the coastline extending from southern New England back towards the Mid Atlantic region in the first 6 to 12 hours of the forecast period. Gradually, the system will ease far enough out to sea where any of the dynamically forced precipitation will come to an end. Northwest flow behind the system will draw colder air across the eastern Great Lakes resulting in some lake enhanced precipitation before the wind direction changes. Mesoscale snow bands likely which will support potential for enhanced snow totals depending upon where the bands set up. See the latest QPFHSD for additional winter weather information. Southern to central Appalachians Accumulating snows also likely from the central to southern Appalachians early on day 2 where persistent northwesterly to north northwesterly upslope flow is expected in the wake of the deepening coastal low. Day 3 California The axis of much above average pw and moisture flux values impacting southern California during day 2 will begin to shift south of the area during the latter part of day 3 as mid level heights associated with the main troughs pushes inland. The moderate to heavy rainfall occurring in and near the Tranverse range and coastal ranges to the south will gradually push south and eventually taper off. Additional heavy precipitation and heavy snows likely through the central to southern Sierra on day 3 as persistent west southwesterly upslope flow continues, although this should also gradually taper off during the day on Friday. Pacific Northwest The low level flow that had a significant terrain parallel component day 2 over the Pacific Northwest will veer to a more upslope direction day 3 as height falls from the strong closed low off the Pacific Northwest/southwest Canada coast swing inland. This will increase precipitation potential across the Pacific Northwest from the Cascades to the coast ranges. As the core of the upper trough makes its way eastward late on day 3, there should be a general decrease in areal coverage and precipitation rates late in the day 3 period. Northern Rockies into the Great Basin The inland push of height falls through the northern Rockies and Great Basin will raise pw values to much above average levels day 3. This will support additional widespread moderate to heavy precip totals across the Great Basin into the northern Rockies. Model consensus is for best 24-hour total precipitation amounts to straddle the day 2/day 3 timeframes. There should be a general decrease in areal coverage and amounts by the end of day 3. Northern plains The inland moving height falls across the northern Rockies will spread isentropic lift into the northern Plains early Friday morning. Strengthening southeasterly low level flow will support increasing precipitation potential through broad isentropic lift/warm advection pattern from far northeast MT into western ND and western SD. Precipitation amounts should generally be in the 0.10 inch to 0.25 inch range, although a few areas in the 0.25 inches to 0.50 inch range should be embedded within that area. Petersen/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml