Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 540 AM EDT Mon Apr 30 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 30/1200 UTC thru May 03/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northeast... The closed mid-upper low along the New England coast early Monday morning will make slow eastward progress during the day 1 period...passing just south of Nova Scotia by 12Z Tue. Precipitation is expected to remain fairly well defined in the comma head/deformation axis n-nw-w of the closed low, though will become less organized today as drier air continues to wrap into the mid-upper circulation. With continued consensus among the latest models, the day 1 QPF in this region was fairly straightforward and essentially a blend of the high-res ensembles (including the HREF, NBM, and dynamically-downscaled 5km WPC ensemble QPF). Highest additional QPF (12Z Mon-12Z Tue) will average around 0.50+ inch across far northern portions of ME, with pockets of 0.25-0.50" over the remainder of northern ME as well as over elevated terrain in eastern NY (Adirondacks and Catskills) and across the Green-White Mtns in VT-NH. ...Northern Rockies - Uinta Mountains - Wasatch Range... A broad mean upper trof will persist day 1 across the western U.S. from the Pacific Northwest through the Great Basin and into the Rockies. A lead shortwave will be rotating through the base of this trough across UT-WY and into the northern High Plains by 12Z Tuesday. This feature will be responsible for moderate-heavy pcpn from the Wasatch/Uinta Mountains into the WY ranges, while convective clusters develop out ahead of this wave Tuesday night across portions of the northern Plains (including north-central NE), owing to some enhanced deep layer lift/low level frontogenesis via the right entrance region of an upper level jet streak. The strong deep-layer shear environment (0-6km bulk shear values of 50+ kts) will allow for swift moving cell motions and thus mitigate an excessive rainfall threat. ...Southern High Plains... Broadly difluent upper flow downstream of the mean western trof will support scattered convection during the peak heating hours this afternoon and evening along the dry line situated across the Southern High Plains. Once again, confidence remains low with regard to the precip details on any precip details in this scattered convective pattern -- as some of the high-res CAMs (ARW and ARW2 in particular) are a bit more prolific with the QPF totals. Given the degree of spread with the amounts and placement, continued with the light to locally moderate areal-average totals. Days 2/3... ...Western United States... A series of three shortwave troughs will track over CA along the positively-tilted mean western CONUS trough as it amplifies to southern CA through Tuesday night. Continental air with this interior trough will limit moisture under the forcing from the shortwave passages over the Sierra Nevada and southern CA. QPF is limited each day to around a quarter inch for preferred higher terrain. As the trough amplifies over the western CONUS will wrap Pacific-sourced moisture across the Intermountain West to the Rockies including along a stalled surface front over NV/UT/CO that sets up from Tuesday into Wednesday. Precipitable water values around half an inch shown by the NCEP guidance 0.5 inch PW is two standard deviations above normal for the Great Basin which enables moderate precip over preferred terrain with areal average QPF max over the Wasatch of Utah. Model guidance from the mass fields were not too disimilar from each other, but their respective QPF varied quite a bit. The manual WPC QPF was largely based on a GFS/NAM/ECMWF blend...with the ECMWF getting least weighting until the end of the day 3 period. The ECMWF seemed to be showing its dry bias. By the end of day 3, the GFS appeared to be too fast. Confidence in the details of the forecast was low. ...Central/Southern Plains to Upper Midwest... An amplifying trough over the western CONUS will maintain and eventually enhance southerly moisture advection. A stationary front will set up over the central plains Tuesday night as the positively-tilted trough ejects shortwave energy over the central CONUS. This provides a focus for moisture convergence and instability, both of which are above normal. Maintained a slight and marginal risk for excessive rainfall mainly over NE/IA with a slight shift northwest per 12Z guidance. An areal average of up to two inches is forecast in the Day 2 (12Z Tue-12Z Wed) time frame. Introduced a marginal risk on day 3 across the Southern Plains southward towards the international border east of the dry line. The models all tend to develop convection here with the NAM being most bullish in terms of QPF. We tended to lean towards the wetter guidance given the above-mentioned idea that southerly flow in the low levels will be transporting plenty of moisture northward from the Gulf. Hurley/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml