Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 406 AM EDT Mon May 07 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 07/1200 UTC thru May 10/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central to Southern Appalachians... The mean mid to upper level eastern trof will remain centered across the central to southern Appalachians day 1. Increasing instability underneath this trof---mu cape values 500+ j/kg-- will support scattered diurnal showers through the central to southern Appalachians Monday afternoon into Monday evening. Light to moderate precip totals in the .10-.25" range depicted from western NC/northeast TN---Southwest VA---central to eastern WV into far western MD and far southwest PA. ...Northern Rockies into the Northern Plains... Height falls that moved into the Northern Rockies early Monday morning will be pressing eastward across the Northern Plains day 1. Well defined upper difluence ahead of these height falls will support continued fairly organized precipitation over the Northern Rockies in the vicinity of northern ID into northwest MT Monday morning with .25-.50"+ totals possible. As this well defined upper difluence pushes eastward Monday...scattered convection should increase through the Northern Plains in a region of strengthening low level southerly flow and increasing pw values--up to 2 to 2.5 standard deviations above the mean. While models do show a lot of spread with the qpf details---there is general agreement on potential for .25-.50"+ precip totals across the Northern Plains from northeast MT into large portions of ND and SD. Days 2/3... ...Northern Plains to Great Lakes... Two shortwave troughs move east across the northern Plains Tuesday before merging over MN Wednesday. This shortwave will interact with a plume of deepening moisture, pooling on the nose of a strengthening low level jet reaching into the Dakotas. By early Tue, the GFS show PW of one inch (1.5-2.5 standard deviations above normal) extending across much of the Dakotas - supporting the potential for moderate to locally heavy amounts across the region. Late Tuesday into Wednesday, models show the initial wave continuing to move east into the upper Mississippi valley as it phases with an upstream wave sliding southeast off of the ridge out of western Canada. Follow a compromise between the 00Z GFS and 00Z ECMWF, with generally light to moderate amounts moving east ahead of a deepening surface wave tracking across WI/MI on Wednesday. ...Central Plains to the southern Great Lakes... Shortwave energy moving east from the central Rockies Monday night will amplify the upper trough across the central Plains into the Mid Mississippi valley Tuesday night to Wednesday morning. This lift, along with 1.25 inch PW (1.5 standard deviations) on 40kt southwesterly flow and 500-1000 j/kg of MLCAPE is expected to spur multi-cell development that may organize somewhat as it shifts east across northern MO/southern IA. There remains some model signal for slow-moving and heavy back-building storms - which may result in local runoff concerns Tue evening. Therefore, the 'Marginal' risk, centered south of the IA/MO was maintained for the Day 2 Excessive Rainfall Outlook. Here also, WPC QPF largely reflects a compromise between the 00Z GFS and 00Z ECMWF. The 00Z NAM is thought to be too aggressive with activity and too far north (it is well into IA). Shear increases out of the west Wednesday aiding more progression to activity that develops over the Midwest/southern Great Lakes. Therefore, so excessive rainfall risk was added for Day 3 (12Z Wed-12Z Thu). ...Pacific Northwest... A closed upper low will transition to negatively-tilted open wave as it approaches Vancouver Island Tuesday night with further shortwave activity swinging into BC through Wednesday night. Southwesterly inflow ahead of the trough will bring one inch PW air (2-3 standard deviations above normal) across northern Oregon into Washington Tuesday afternoon and evening, fueling the potential for moderate rain across the Olympics and northern Cascades. PW diminishes to around 0.75 inches as subtropical moisture is shunted south/out to sea. Mainly light rain is expected on Day 3 (12Z Wed-12Z Thu). WPC QPF is mainly a blend of the 00Z GFS and ECMWF through the period. Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml