Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Thu Mar 15 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 16/0000 UTC thru Mar 19/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Western U.S... An upper low digging southward along the northwest U.S. coastline will eject a couple shortwave into the West through the Day 1 period. Low-level onshore flow of 20 kt will provide the moisture and some (relatively modest) orographic lift. Some enhancement of precipitation rates is likely, however, due to fairly steep height falls advancing into CA during the period, and cold temperatures aloft even resulting in some areas of modest convective instability. The best combination of these three factors appears to be across northern/central CA, and this is where the heaviest precip is forecast on Day 1. Liquid equivalent amounts exceeding a half inch are expected over the northern CA coastal ranges, while the heaviest amounts (1-2 inch liquid) are expected across the northern/central Sierras. Much of this precip will fall as snow. Please refer to products issued by the Winter Weather Desk for further details on the winter weather threat. Farther inland, relatively lighter amounts are expected given relatively poor moisture flux ahead of this system, but some convective enhancement appears possible across the Great Basin as well. Areal averaged QPF amounts of a quarter to perhaps locally reaching a half inch are expected. WPC QPF was based on a multi-model blend including the WRF-ARW/NMM, ECMWF/GFS, and in-house bias corrected ensemble guidance. ...Central Rockies/Central Plains... Mid/upper-level shortwave crossing the Rockies this evening is expected to eject into the central High Plains tonight. Models are in good agreement that this feature will quickly intensify as the vortex stretches, with a surface low expected to rapidly spin up near the CO/KS border late this evening into the overnight hours. The upper wave will initially produce generally light precip across portions of the central Rockies tonight, before tapering off by Fri morning as the best energy shifts into the plains. As the upper wave amplifies and the developing surface low quickly deepens, a dynamically forced band of precipitation is expected to develop on the north side of the system, from eastern WY east across the plains. The warm conveyor belt should begin to quickly ramp up tonight as strengthening low-level flow develops across the southern/central plains ahead of the developing system. A 50-60 kt low-level jet shown by the GFS by 06-12Z Fri would result in a fairly impressive area of moisture flux convergence in a west-east oriented band centered roughly along the SD/NE border, with even some modest elevated instability making it into the region. The Day 1 QPF shows a max across this area, with the heaviest QPF amounts exceeding 1.25" possible. Another max is expected across southeastern WY where low-level upslope enhancement will likely occur. Snow and freezing rain are expected across portions of this area. Please refer to products issued by the Winter Weather Desk for further details on the winter weather threat. WPC QPF was based on a multi-model blend including the WRF-ARW/NMM, ECMWF/GFS, and in-house bias corrected ensemble guidance. ...Gulf Coast to lower/mid MS Valley... Farther southeast, increasing moisture and instability will support areas of convection focused along the cold front from the mid-Mississippi valley to the Gulf Coast. Pwat values in the 1-1.25 inch range along with expected decent forward movement (owing to 40-50 kt mid-level flow) should keep overall amounts in check. The heaviest amounts are expected across portions of central/southern MS, where areal averaged amounts exceeding a half inch are expected, with localized amounts potentially nearing 1 inch in the heaviest areas. WPC QPF was based on a multi-model blend including the WRF-ARW/NMM, ECMWF/GFS, and in-house bias corrected ensemble guidance. ...Northeast... Northwesterly flow will favor certain terrain and lake-enhanced areas of New York and New England for additional 0.10 inch or greater liquid equivalent tonight into Friday morning. Rising upper heights and decreasing moisture after 12Z Fri should begin to suppress shower activity. Days 2/3... ...West Coast to the Rockies and northern high Plains... The models indicate a deep layer trough drifts onshore into CA and the Pacific northwest. An initial upper jet max across central CA moves south with time. Upper divergence maxima coupled with low level convergence and upslope flow provide a day 2 maxima in QPF in the central to southern CA Sierra before the couplet moves further inland in tandem with the jet. Pre-frontal low level convergence leads to valley rain and mountain snows in central to northeast NV and adjacent ID and UT. On Sat night-Sun the low level front and associated upper trough progress across UT into CO. The NAM and GFS have precip spreading to the CO front range by Sun evening as a mid level deformation axis sets up northwest of the lee low in eastern CO. Separate maxima related to the 700 mb circulation and bndry are forecast to occur from the OR Blue Mountains across to the ranges of southern ID Fri night-Sat. As the circulation drifts slowly east northeast out of ID across western and central MT Sat night-Sun, precipitation is focused with highest amounts near the ID/MT border, including the Bitterroots. The models are in good agreement on the timing of the 700 mb low, thus a consensus approach was taken. Precip breaks out on the northern high Plains Sun afternoon as low level southerly flow advects moisture northern and the 850-700 mb boundary acts to focus lift to produce precipitation. The axis of the gfs and nam QPF and averaged amounts was used for QPF. ...Mid MS Valley to the Ohio/Tennessee Valleys, Mid-Atlantic and Southeast... The models show a defined initial 700 mb circulation moving east from NE across areas near the IA/MO border and gradually becoming an open wave as it moves across the Oh Valley. Frontogenesis along the north side of the low/baroclinic zone will enable locally heavy precip with the heaviest swath of up to one half inch stretching from northern IL and IN, with more weighting given to the NAM and GFS as their QPF lines up nicely with 850-700 mb convergence and theta-e advection, with good overlap along the frontogenesis zone. Further south, precipitation extends across KY and the central Appalachians along the trajectory with bndry layer moisture convergence. The wave weakens as it comes into confluent flow across the mid Atlantic, with most models producing one maxima into the central Appalachians and then further southeast towards southern VA and North Carolina. A band of showers briefly extends along the trailing front south across the TN Valley and southeast. The short duration of favorable bndry layer moisture convergence leads to light amounts. WPC QPF was based on a blend of the 12Z GFS/ECMWF/NAM. ...Eastern OK and northeast TX to AR, LA, and MS... The models indicate a southern stream upper jet maxima develops in the southern Plains as an upper trough crosses the southern Rockies. The combination of 850-700 mb warm/moist advection and convergence developing underneath the upper divergence maxima that increases in the ARKLATEX sun afternoon leads to an increase in showers/storms. The models agree on the increase In amounts/coverage but not the axis/orientation yet. Manual progs used a centroid approach until better agreement develops. The 12z NAM shows precipitable water values increase to near 1.5 inches by 00z Mon so this supports the general model increase in heavier amounts in the period ending 00z Mon. Ryan/Petersen Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml