Quantitative Precipitation Forecast Discussion...Corrected NWS Weather Prediction Center College Park MD 532 PM EDT Tue Apr 10 2018 Corrected for typo in second paragraph of Day 2/3 discussion. Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 11/0000 UTC thru Apr 14/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern California---coastal Pacific Northwest... The upper trof over the northeast Pacific will be amplifying day 1---helping to propel a strong frontal boundary toward the west coast Tuesday night/early Wednesday. Strengthening south southwesterly low level flow and increasing pw values ahead of this front will support organized frontal/pre-frontal precipitation pushing inland Wednesday across the Pacific Northwest from the coast ranges into the Cascades---southward into northern California. The best upslope component to the south southwest low level flow will be over northwest California...far southwest Oregon and into the Olympic Range of Washington State. Across these areas heavy precip totals in the 1-2" range likely...with lighter totals elsewhere from the Cascades west to the coast ranges where the low level flow will not have as significant upslope component. Overall---very good model agreement with precip amounts and distributions--leading to a high confidence forecast this period. ...Northern Rockies into the Northern Plains... Ahead of the strong front approaching the west coast day 1---a lead area of height falls will be pushing through the Northern Rockies and into the Northern Plains. Above average pw values will support moderate to locally heavy precip totals over the Northern Rockies from northwest WY into far eastern ID---central to northern ID...northeast Oregon into southwest MT. To the east of the Rockies---isentropic lift will strengthen along and north of the northwest to southeast oriented frontal boundary across the Northern Plains. This will support increasing overrunning precip Wednesday from central Montana---eastward across ND and northern portions of SD---with areal average .10-.25"+ amounts possible. ...Upper Great Lakes into the eastern Great Lakes... Northern stream shortwave pushing quickly southeast from Saskatchewan into the Upper Great Lakes tonight and toward the eastern Great Lakes by Wednesday evening will push an associated front southeast across these areas. The fast movement and lack of moisture will keep any precip on the light side. ...Florida... The stationary front across north Florida this afternoon will be pushing southward tonight into early Wednesday across central to south Florida. The model consensus is for lessening organization of convection ahead of this front as the current favorable right entrance region jet dynamics in the vicinity of this front weaken. This weakening dynamics and expected progression should keep any precip on the light to moderate side. Days 2/3 ...West Coast...Intermountain West to the Upper Midwest... A potent shortwave trough will swing around a Gulf of Alaska low, digging an upper trough down the West Coast through 00Z/12. This trough will close into a low over the central Rockies near 00Z/13 and be a notable weather maker this weekend into next week. While a NAM/GFS/ECMWF blend was used for QPF across the West, timing differences between the faster NAM/GFS and slower ECMWF/UKMET/CMC grow as the trough axis reaches the Plains and cyclogenesis occurs across the central Plains. Similar to reasoning described below for the southern Plains and as stated in the PMDHMD, WPC stayed closer to the slower ECMWF/UKMET timing. Ahead of the associated Pacific cold front along the California coast at 00Z/12, integrated moisture transport should continue to weaken with a quick shot of QPF to the Sierra Nevada but less so for southern California as flow becomes more parallel to the coast along with lowering precipitable water values with the cold front. The focus then shifts inland across the Wasatch and Rockies and into eastern Montana with the development of a deformation zone. Downstream should be the development of a 850-700 mb low over the central High Plains. This low is expected to track from the Nebraska Panhandle to eastern Nebraska between 12Z/13 and 00Z/14. Farther east, frontogenetic forcing is expected to help align precipitation axes from west to east through the upper Mississippi valley once the atmosphere moistens up at the nose of a low level jet, with the GFS quickest to do so. Increasing upper level diffluence and divergence ahead of a possible coupled upper level jet signature should help to enhance vertical motion and associated precipitation from Nebraska, South Dakota into Minnesota, north of the strongly defined frontal zone at the surface. Slow movement of the 850-700 mb low is reflected with areal average liquid equivalent values of 1 to 2.5 inches over western South Dakota for the 24 hour period ending 00Z/14. ...Upper Mississippi valley through Great Lakes and New England... The tail end of a mid-upper level shortwave near the West Coast 00Z/11 will move east into a region of ridging and emerge into the Plains during the day on the 11th. A surface low will develop along a strong frontal boundary and likely track into central Lake Michigan for 12Z/12 and near Down East Maine for 12Z/13. Minor timing differences existed in the latest 12Z model guidance, so a general blend was used for QPF, led by the 00/12Z ECMWF and 12Z GFS. Warm advection driven precipitation should be ongoing at the start of the Day 2 period (00Z/12) across the upper Mississippi valley, at the nose of a strengthening 50-60 kt 850 mb jet moving across the IA/MO/IL border. Low level frontogenesis seemed to be maximized slightly farther south than indicated in the model QPF maxima, so adjustments were made over Wisconsin and Lower Michigan to reflect this notion prior to 12Z/12. The storm system should quickly move off toward the east in a zonal flow pattern aloft, with the moderately strong southwesterly 850 mb flow reaching the Northeast by 00Z/13. The quick moving nature of the shortwave and low track near the U.S/Canadian border is expected to keep precipitation totals rather light across the Northeast between 12Z/12 - 12Z/13. ...Southern Plains... Timing differences still exist with a large closed low forecast to set up over the central Rockies through 12Z/13 and eject out into the Plains through 00Z/14. The NAM/GFS remain quicker here and at the leading edge of the latest ECMWF/GEFS/CMC ensemble members, partially why WPC leaned toward the slower ECMWF/UKMET. Rapid moisture return into Texas and Oklahoma, ahead of a dryline and eventual cold front, should help set the stage for heavy rainfall, once it does develop. Following the timing of the ECMWF/UKMET, precipitable water values of 1.5 to 1.75+ inches in the vicinity of the ArkLaTex along with CAPE > 1000 J/kg between 18Z/13 and 00Z/14 along with increasingly diffluent flow aloft are expected to be present. The cold front should be the trigger to develop convection with unidirectional flow potentially allowing for training across a region which has seen above average rainfall over the past 2-4 weeks. This will need to be given consideration for future Excessive Rainfall Outlooks, beyond 12Z/13. Oravec/Otto Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml