Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 427 AM EDT Thu Mar 15 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 15/1200 UTC thru Mar 18/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Western U.S... Flow was generally onshore into California, with steep lapse rates supporting showers farther north along the Pacific Northwest coast. Other rain and snow showers were sweeping gradually through the intermountain region as broad cyclonic flow had taken up residence in the West. The trough had carved out enough territory that the sub-tropical jet was suppressed well south, so precipitation was not especially heavy, but will remain fairly widespread through Thursday. One stripe of some of the more hydrologically significant precipitation will lift from Wyoming up into Montana early in the day, associated with a well defined shortwave. Heavier, upslope-enhanced precipitation will occur through the daytime hours before winds switch to downslope as the feature lifts north of the Canadian border and is caught wheeling around the gyre / upper low over the Pacific Northwest coast. The frontal zone trailing the Montana shortwave and marking the leading edge of the larger scale trough as it undercuts Northern Plains ridging, will promote orographically enhanced precipitation generally under a half inch liquid, but spiking above a half inch in some of the highest terrain in Utah / Colorado / northern New Mexico. Meanwhile, back over California the upper low coming ashore will bring stronger gradients and some sense of a new surface frontal structure. The combination of difluence east through north of the low and enhanced lower level onshore flow should yield some healthy precipitation totals, both rain and snow. Precipitable water values were fairly low, under a half inch as of the 00z soundings. PW is forecast to increase above a half inch, but given the the cold air aloft / steep lapse rate environment, this event could be rather showery with shallow convection, leading to more variability in local precipitation amounts. This might perhaps argue for lesser areal average QPF. Still, the high terrain and westward facing terrain should be favored for steadier precipitation, and guidance was in such strong agreement that our blend resulted in the same or slightly heavier QPF in most areas. Throughout the West, WPC QPF on Day 1 was a 40/30/30 blend of the HREF blended mean / 00z GFS / latest in-house ensemble pre-bias correction. ...Central Plains... With a blocky pattern downstream toward the Atlantic, the trough and Pacific front cresting over the Rockies will have a limited playing field, resulting in a very concentrated Central Plains storm beginning in the Day 1 period. Model agreement on the large scale is pretty strong, and the QPF agreement is good aside from the ECMWF which is locked more onto the terrain with its higher amounts west of Cheyenne, WY. It would seem more likely that the heaviest precipitation will fall along and inverted trough axis that pushes out onto the Plains in tandem with the developing surface low. The majority of guidance, especially the NCEP models and hi-res suite place the bulls-eye over southeast WY / northwest NE / southwest SD through Thursday night, with strong low level convergence and moist easterly inflow to the north of the developing cyclone. Upglide / warm advection should cause rain to expand east and southeast through eastern NE/KS and much of Missouri as well, and a jet axis with embedded shortwave farther south will likely yield some spotty rainfall across Texas, and more concentrated rain breaking out early Friday morning over the lower MS Valley. Precip type will be an issue across the Central Plains, but some ice and locally heavy snow are possible - see WPC Heavy Snowfall Discussion for details. Here, as was the case in the West, we used a 40/30/30 blend of the HREF blended mean / 00z GFS / latest in-house ensemble pre-bias correction. ...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 on Thursday. Generally, the trough that spawned the unsettled weather is spent and is no longer a threat for heavy precipitation, but daytime heating and steep low level lapse rates will support rain and snow showers all the way to the coast and as far south as PA/NJ. We used a number of hi-res models to derive the forecast, but areal average amounts should be minimal. Days 2/3... WPC QPF was based on a blend of the 00Z GFS/ECMWF with some consideration of the 00Z NAM/UKMET. ...West Coast to the Rockies... The center of a deep upper-level low spanning the entire west, will drift east along the CA/OR border Friday into Saturday, and slowly fill as the system occludes. Circulation around the upper-level low with a series of vort lobes rounding the main low will stream half inch PW in from the Pacific across central CA Friday with 1.5 inch QPF maxima in the Sierra. A secondary maxima around a half inch QPF will be farther south in the Santa Barbara/Ventura area as a low level trough sets up. Rain rates do not look excessive in southern CA. Low snow elevations under the low will keep much of the precip snow in the Sierra Nevada. A baroclinic zone associated with the surface low will allow moderate precip across eastern NV into eastern ID Friday night into Saturday with half inch average QPF before the zone weakens. ...Central/Northern High Plains to the Ohio/Tennessee Valleys and Mid-Atlantic... Short wave energy ejected will shift east over southern CO Thursday night and allow surface low development over KS to be in a mature phase by Friday morning. This low will shift east into the ridge over the Midwest and weaken through Saturday as it tracks east to the OH Valley. 0.75 inch PW will wrap around the low on a low-level flow of 45 to 50 knots Friday, weakening through Friday night as the low weakens. Frontogenesis along the north side of the low/baroclinic zone will enable locally heavy precip with the heaviest swath of up to one inch stretching from western IA to the SD/NE border per 00Z guidance consensus. The GFS remains the heaviest with precip east of the central Appalachians Saturday due to a stronger low. The 00Z ECMWF came in with a similar coverage of precip in the central Mid-Atlantic, but at a much lower magnitude. A compromise between these two was drawn for Saturday. ...Gulf Coast... Weakening low pressure pushing east across the OH Valley Saturday with an associated front stalling near the Gulf Coast. 1.25 inch PW along the Gulf Coast along with strengthening bulk shear and MLCAPE over 1000 j/kg will allow any activity that develops to be sustained. However, a general lack of forcing should limit coverage with areal averages around a quarter inch. Burke/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml