Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri Mar 23 2018 Prelim Day 1 QPF Discussion Valid Mar 23/1200 UTC thru Mar 24/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern Plains southeast to the middle Mississippi Valley and western Ohio Valley... A very dynamic shortwave over southern CA at 0600Z will undergo a bit of shear over the next 24 or so hours as it encounters the broad central U.S. ridge. At the same time however, the upper jet streak maintaining strength as it progresses eastward into the Central Plains by Friday night will continue to support a vigorous (albeit more compact) vort max on the northern flank. Narrow left-exit region forcing (upper difluence/DPVA and associated ribbon of deep-layer WAA/isentropic lift) ahead of this shortwave will push east across the northern Plains toward 00Z, while pushing into the mid and (southern) portions of the Upper MS Valley overnight Fri into early Sat. Meanwhile, as the NW-SE oriented upper jet streak in the lee of the upper ridge pushes east across the upper Midwest-Upper Great Lakes and Ohio Valley, the ensuing right entrance region forcing will augment the upper divergence/large-scale ascent (steepen the S-N isentropic lift) while also enhancing the low-mid layer frontogenesis. The models continue to depict this "coupled" dynamical setup in between the approaching (southern) and receding (northern) upper jet streaks -- though differ in terms of location of best deep-layer lift, low-level frontogenesis, and thus the axis of heaviest QPF. In terms of the 00Z model guidance, the NAM remains the farthest northeast with the QPF axis, with the ECMWF on the SW flank of the the guidance spread. The latest WPCQPF comprised a blend of the GFS and UKMET -- both solutions serving as a middle ground between the NAM and ECMWF, while getting support from both the 00Z HREF mean and latest NBM (which were also utilized in the multi-model blend). Am a bit concerned that we could see a s-sw trend with the heaviest QPF given the weaker static stability profile (i.e. into eastern NE, northern MO). ...Western U.S... A deep trough/closed low digging south offshore the Pacific Northwest will begin to gradually shift inland tonight through Friday across the West Coast which will maintain deep and moist cyclonic flow across the coastal ranges of WA, OR and northern CA and into the Cascades. This will maintain numerous shower activity which will be aided by slowly steepening mid level lapse rates and favorable onshore/upslope flow over the terrain. Expecting locally an additional 1 to 3 inches of rain for the coastal ranges and especially far northwest CA and southwest OR. Lesser amounts are expected over the Cascades. Meanwhile, the lead shortwave currently coming into CA out ahead of the aforementioned trough/closed low offshore the Pacific Northwest will eject rapidly inland across the Intermountain region tonight and then across the Rockies on Friday. This will take the remnants of a well-defined atmospheric river in across the region which is characterized by 850/700 mb moisture flux anomalies of over 5 standard deviations above normal, and precipitable water values of locally over 4 standard deviations above normal. This will result in a highly efficient precipitation processes over the higher terrain with west and southwest facing slope of the Wasatch, Tetons and high country of western CO seeing moderate to heavy precipitation. Water equivalents will locally be over 1 inch through late Friday as the energy traverses the region. WPC leaned toward a multi-model consensus for QPF with generally a blend of the GFS, ECMWF and hires CAM guidance used. Hurley