Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 354 AM EDT Fri Mar 23 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 23/1200 UTC thru Mar 26/1200 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 farther south (toward the northern periphery of the forecast MUCAPE axis). Model progged 700-500 mb theta-e lapse rates in fact show zero to slightly negative lapse rates lifting into southeast NE and southern IA. ...Western U.S... A deep trough/closed low digging south offshore the Pacific Northwest will begin to gradually shift inland today across the West Coast and maintain deep, moist cyclonic flow across the coastal ranges of WA, OR and northern CA and into the Cascades. This will sustain numerous shower activity, which will be aided by slowly steepening mid level lapse rates and favorable onshore/upslope flow over the terrain. Expecting 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. WPC leaned toward a multi-model consensus for QPF, with generally a blend of the GFS, ECMWF and hires CAM guidance. Days 2/3 ...Pacific Northwest/northern California into the Intermountain West and northern Rockies... A broad area of lower heights across the western U.S. will feature multiple embedded shortwaves which will focus periods of organized precipitation over the region. More specifically, one such disturbance is forecast to lift out of far western Montana into southern Alberta/Saskatchewan early Sunday. To the south, another shortwave will dig through northern California/Nevada, albeit with a rather limited reservoir of moisture. Strong mid-level lift combined with upslope augmentation will bring locally heavy precipitation to the Shasta, Siskiyou, and Sierra Nevada ranges through early Sunday morning. Such amounts were quite comparable among the 00Z NAM/GFS/ECMWF output which bolstered confidence in the forecast. General lighter amounts were painted downstream as the upper trough becomes more stretched out across the central Great Basin. While much of the West Coast eastward into the northern Rockies should be on the wet side, amounts are quite modest. ...Central U.S./MS River Valley into the OH/TN Valleys and southeastern states... On Saturday morning, modest mid-level height falls ejecting from the middle Mississippi Valley toward the lower Ohio Valley will be in the process of becoming more stretched out in time. The attendant surface cyclone tracking from Missouri toward the upper Tennessee Valley should continue to weaken given the shearing nature of the pattern aloft. In spite of this, expect a healthy swath of precipitation spreading across southeastern Kentucky/northeastern Tennessee/far western North Carolina and Virginia. Models show sufficient 850-mb flow in excess of 40 to 45 knots advecting 1 to 1.25 inch precipitable water values toward the southern Appalachians. This would allow for additional upslope forcing to the QPF amounts with areal averages on the order of 1 to 1.50 inches, primarily through early Sunday. Much of the action should shift toward the southeastern U.S. by the end of the weekend although the activity may become more elevated in nature as surface ridging extends down into the eastern Gulf of Mexico. The overall model trend on Day 2 has been to stay the course or shift southward compared to recent days. As such, followed a consensus of the 00Z GFS and 12Z ECMWF/CMC/UKMET with a primary focus on the higher resolution output of the GFS/ECMWF solutions. By Day 3, reverted back to the 18Z GFS given the most recent run showed spurious amounts over the southeastern U.S. Its output looked unreasonable as mid-level heights gradually rise and the better surface convergence sits well upstream. Back over the center of the country, a few areas of focus are noted in the guidance. Initially, a light swath of precipitation across the northern-central U.S. will give way to a brief drying period. With broad southwesterly flow stretching over much of the western half of the country, weak to moderate perturbations will help increase the coverage of precipitation over the Northern/Central Plains. Models are quite abrupt in return flow advecting moisture back up the Plains with 850-mb winds shifting to at least 30 to 35 knots by Sunday morning. This will occur in response to increasing warm advection with areas of lower pressure shifting up the higher terrain of the Northern Rockies. With the better forcing lurking well upstream, amounts remain modest at best. Across the Southern/Central Plains into the Ozarks, convection should spread over the region primarily on Day 3. The primary drivers appear to be a multitude of weak impulses pushing out of northern Mexico and the Four Corners region. The better focus would take place along a poleward lifting warm front although an additional area of interest would occur back along the dryline. Among the latest guidance, the 00Z GFS is the wettest while the 00Z/12Z ECMWF are a bit more disorganized with the activity. Given some potential capping issues, the latter solutions may be a more accurate portrayal of the situation. The manual QPF did take a blend of these two solutions but did downplay the amounts somewhat. ...New England... Within a region of deep cyclonic flow, a pronounced shortwave will dig through the northeastern U.S. early in the weekend. With the best moisture well offshore, much of the activity will be driven by instability forced showers underneath the cooling mid-level temperatures. A consensus approach was utilized which kept general amounts under 0.15 inches for the most part during the period. Hurley/Rubin-Oster Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml