Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 530 PM EST Fri Mar 09 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 10/0000 UTC thru Mar 13/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...MS valley into the TN and OH Valleys... A compact, vigorous shortwave traversing the northern Intermountain West early this afternoon will track down the east side of the longwave upper ridge and reach the Central Plains by the end of the Day 1 period (00Z Sun). Out ahead and south of this feature will be a fairly elongated upper jet streak, with peak 250 mb winds of 110-130kts, which will lead to a quick increase in left exit region forcing (increased low level southerly inflow/moisture transport from the western-central Gulf and deepening waa/moist isentropic upglide). Model QPF spread associated with this feature remains higher than normal (for a Day 1 forecast), owing to multiple (yet subtle) areas of frontogenetical forcing as the eastern upper jet streak pushes from the mid MS Valley-TN Valley-lower OH Valley to off the mid Atlantic coast. WPC opted for a blend of the 12Z ECMWF, NAM, RGEM, ARW, ARW2, and HREF mean with the bi-modal distribution -- one axis of 0.50-1.0" amounts draped w-e across KY (associated along the elevated front), another across a portion of the lower MS Valley from LA and southeast AR east-northeast into northern MS and northwest AL (closer to the surface front). The more southerly, heavier QPF axis will have more instability to work with, especially closer to the MS River, where MUCAPES will average between 400-1000 J/kg. Thus expect more variability from areal average amounts over these areas, as suggested per the high-res CAMs (isolated amounts exceeding 1.5"). ...Central-Southern CA... An upper jet streak and associated shortwave energy along the subtropical jet will approach central-southern CA on Saturday -- and with it an axis of deep moisture as PW values of 1.25" reach the southern CA coast by toward the end of the period. These PW values will average between 4-4.5 standard deviations above normal per the latest SREF and GEFS, with similar positive anomalies with regard to the 850-700 mb moisture flux. The gradual uptick in left exit region upper forcing (favorable deep-layer waa/moist isentropic lift) within the warm conveyor belt (WCB) will result in modest rainfall totals later Saturday across the southern coastal ranges into the San Gabriel and San Bernardino Mtns during the Day 1 period (through 00Z Sun), with more expected thereafter Sat night into early Sunday. Through 00Z Sun, a consensus of the 12Z GFS, ECMWF, WRF-ARW, and HREF mean would support areal average totals between 0.25-0.50" over these aforementioned orographically-favored locations, with locally higher totals expected as per the high-res guidance. Days 2/3... ...California/Southwest... Sharp shortwave enters southern California at the start of Day 2 (just after 00Z Sun) along with a weak sfc wave and then slides eastward through the deserts into Arizona into New Mexico by the start of day 3 (00Z Mon). Precipitable water values around 1.00-1.25 inches (+4 to +5 standard deviations which is near monthly max values) supports at least a brief window of locally moderate to heavier rains and highest elevation snow as a 110kt jet streak noses into SoCal between SBA-SAN. Though the mid-level vort max shears out east of the coast through AZ, favorable lower level winds into the Transverse Ranges should be able to squeeze out more than an inch QPF (local max at least double that) with much lesser amounts east of the mountains. Guidance has wobbly trended a bit higher with amounts but the hi-res ARW/NMMb were split higher/lower, respectively, at the end of their runs (12Z Sun). Model blend offered values near the 00Z GEFS reforecast which was reasonable for now. Maintained a marginal risk for flash flooding though burn scar areas will be much more sensitive. Into Nevada, flatter flow shunted the precip more into N AZ rather than central/northern NV, with even lesser amounts into New Mexico but enough upper level support should wring out a couple tenths of an inch liquid for at least the mountains. ...Central/Southern Plains and Mid/Lower MS Valley into South/Southeast and Mid-Atlantic... Upper flow evolution over eastern Canada/CONUS continues to be a challenge, with myriad solutions as the northern/southern stream converge east of the Mississippi. 12Z deterministic guidance, outside the UKMET and NAM, converged a bit as the system moves out of the southeast and off the coast, but the ensemble spread (in all systems) remains rather large for only 60-84+ hrs out considering the last couple of runs. Please consult the PMDHMD for more information. Forged the middle ground until a clearer picture emerges, hopefully with the next cycle or two. Wary of the suppressed solutions given the tendency of recent storms to develop robustly, though every storm is unique. For snowfall information, consult the latest QPFHSD. Before low pressure exits the coast, moderate to locally heavy rain is forecast over parts of MS/AL as the 850mb moisture plume intersects the sfc boundary and developing low pressure beneath the 110-125 kt jet streak. Model QPF has varied N-S over the past several runs but consensus position minus some latitude offered a good position mostly in line with continuity. Upper support will transfer to the coast by late Sunday with expands the precip shield northeastward at the expense of the heavier QPF in AL/GA. By the end of day 3 (00Z/Tue) kept the QPF axis through NC into the Atlantic but again ensemble spread was large N-S and E-W by then. Held the precip just south of New England for now despite the multi-center ensemble mean of a tenth of an inch to the Mass Pike and nearly a quarter inch just south of Long Island. The Day 2 (12Z Sat-12Z Sun) Slight Risk for excessive rainfall over the Deep South was maintained with little modification given the good signal. Farther north, models have yet to resolve another feature on the heels of the lead shortwave that will be responsible for the sfc low development on the east coast -- what is initially strung out vorticity through ND/SD/MN on Sunday that the NAM/GFS develop much more than the ECMWF/UKMET. Blended solution was favored given the uncertainty. ...Northern New England... A blocking ridge stretching from Newfoundland to Labrador keeps low pressure over Maine/the Canadian Maritimes Saturday with wrap-around snows over northern New England. Kept some light QPF in the forecast, especially in the higher elevations, as these events can linger longer than what the global models think. Fracasso/Hurley Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml