Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 453 AM EDT Sat Apr 28 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 28/1200 UTC thru May 01/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northeast... Shortwave energy pivoting across the Ohio Valley early Saturday will further amplify the longwave trough, as the vort lobe ultimately ends up rotating around the the eventual closed mid-upper low. With the main warm conveyor belt (wcb) off the Atlantic coast, a secondary wcb will develop ahead of the amplifying trough -- though with moisture anomalies (PW, low-mid layer moisture flux) negligible this time around. The slowly-evolving setup will nevertheless prolong the shower and embedded thunderstorm activity, resulting in modest 24 hour QPF totals per a multi-model blend (0.25-0.50+ inch) straddling the Hudson Valley. ...Northern California...Pacific Northwest...Central-Northern Rockies... The latest model guidance continues to denote good agreement with the gradual east-northeast progression of the closed upper low currently along the northern California/southern Oregon coast early this morning. By the end of the period (12z Sunday), the circulation will broaden a bit as it moves across OR-WA. A broad region of showery conditions will continue in the difluent upper flow east and northeast of this closed low, particularly across orographically-favored terrain. areas where the flow is more orthogonal to the elevated terrain. Highest areal-average totals (0.50-1.0+ inch) were noted across northwest CA and the Olympics-northern Cascades in WA, with a broader area of 0.25-0.50" over the remainder of the coastal ranges and western slopes of the Cascades. Days 2/3... ...Northeast... Shortwave energy already focusing up across the northeastern CONUS will be absorbed by the arrival of a second and stronger northern stream trough arriving from the Great Lakes. The result will be a well-defined closed low that will slow down and meander over the Northeast through Monday. A combination of relatively steep mid level lapse rates, DPVA, Atlantic moisture, and local orographics will foster numerous showers across the interior of the Northeast and especially northern New England where some locally heavy banded precipitation is possible. A multi-model consensus weighted toward the GFS and ECMWF was used for QPF with adjustments for topography. ...Northwest to Northern Plains and Upper Midwest... An upper low making its way eastward across the Northwest U.S. by the start of the Day 2 forecast will be in the process of opening up into a more progressive trough axis Sunday. Height falls will be spreading east across the northern Rockies and well out across the northern Plains. The model guidance still likes the idea of a plethora of small scale vort centers ejecting out of the base of the trough through the period and downwind across the central/northern Plains and the upper Midwest ahead of the main trough axis...which is an inherently difficult pattern to accurately time individual shortwaves. Widespread showers are expected across the Northwest and over the northern Rockies in association with the arrival of the upper low owing to moist mid levels, steep lapse rates and upslope flow. More frontogenetically enhanced precipitation is expected by late Sunday and Monday across the far northern High Plains around the back side of developing low pressure east of the northern Rockies. Farther off to the east, there will be a well-defined cold front beginning to push east across the Dakotas, and this will interact with strengthening southerly low level jet which will enhance warm advection and moisture return. Precipitable water values will initially be very modest, but will increase to near or over 1 inch by Sunday night and Monday across the northern Plains and Midwest. This will allow for some convection to likely develop along or just behind the cold front given a more favorable thermodynamics environment. A multi global model consensus was used for QPF with perhaps a bit more weight toward the GFS. ...Central/Southern Rockies to the High Plains... Anticyclonic flow over the central CONUS ahead of the upstream upper low entering the West will allow increasing moisture across central Colorado to New Mexico as weak shortwave energy embedded within difluent flow arrives out ahead of approaching height falls. The result will be some diurnally enhanced and orographically focused convection over the high terrain of the central and southern Rockies and out across the central and southern High Plains region. Rainfall amounts should be generally light given rather low precipitable water values. Hurley/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml