Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 538 AM EDT Sat Apr 14 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 14/1200 UTC thru Apr 17/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central to Northern Plains---Upper MS Valley---Great Lakes into northern NY state and northern New England... A surface low near the NE/MO/IA border will track east across IL tonight. To the north and northwest of this low will see continued wintry precipitation across SD/NE, pushing east into MN/IA/WI through the day. Strong 850 mb moisture transport ahead of the low will transport convection up and over the warm front into MI tonight, although rainfall intensity north of the warm front should be tempered by the lack of instability. Meanwhile, a lead wave will push precipitation across the Great Lakes this morning and into portions of upstate NY and northern New England this afternoon into tonight. In general, models are in pretty good agreement. Have noted some south/north fluctuations with the deformation axis of wintry precipitation. The 0z model guidance appears to be focusing in with this axis...forecasting it to stretch across southern MN into central and southern WI. This axis keep very good continuity from our previous forecast...although the new WPC is higher with the magnitude of amounts across this axis...and is more in line with the 0z HREF mean. Across upstate NY and northern New England have noted a northward shift in most of the new guidance...and thus WPC trended in this direction as well. ...Lower MS Valley into TN and OH Valley and the Southeast... As of 08z this morning a convective line stretches from IN south into the Lower MS Valley and into east TX...with the heaviest amounts focusing across southeast AR into northwest MS where some training will continue into the early morning hours. Indications are that this convective line will become more progressive by 12z as it continues to push eastward. In general the line is forecast to remain progressive through the day 1 period...likely limiting the potential for any extreme rainfall amounts. With that said, still think some slowing of the line is likely by this afternoon, resulting in a corridor of enhanced flash flood potential. As the shortwave energy over the Plains digs into the MS Valley this afternoon the entire trough will take on more of a negative tilt. This should act to slow the forward progression of the line across portions of southeast LA into southern MS and much of AL. The expectation is that the digging wave and negatively tilted trough will aid in upstream convective development that will expand along the already present eastward moving convective line. Thus, even though the line will maintain some movement east, the slowing of it and upstream development will allow for some southwest to northeast training of the cells embedded within the line. Moisture parameters will remain impressive...with strong 850 mb moisture transport and PWATs approaching climatological max values. Thus any training would present a flash flood threat. WPC QPF for the warm sector of this system generally blended the HRW ARW, ARW2 and the HREF Mean. The HRRR was also heavily used early in the period, as it was the best match of the high res guidance to the current radar. The ARW was also a good match, although feel like it may become a bit too progressive by later today. Thus incorporated some of the slower ARW2 and HREF mean to account for this. Given that organized convection is ongoing and will maintain through the day...generally stuck with the high res guidance for this warm sector QPF...as it is situations such as this when they typically are much more useful than the global guidance. Overall the new WPC QPF is a bit more progressive than our previous forecast...which fits in line with current observational trends and the high res guidance. However, overall the axis of highest amounts remains very similar to our previous forecast stretching from southeast LA into southern MS and into AL. Did shrink the areal extent of the higher amounts to better fit the model consensus. Amounts were decreased some from northern AL into the OH Valley. Instability will be less here and the orientation of convection further south may help cut back on totals over this area as well. Again, think this is a case where the high res guidance is much more useful than the global models. Did however keep amounts slightly higher than some of the CAMs show...as sometimes they do under do QPF some on the northern edge of convective lines. ...Pacific Northwest into the Northern Rockies... An additional 1-2" of QPF expected across the coastal ranges of WA/OR and the WA Cascades. WPC QPF follows close to the HREF mean. Days 2/3 ...West Coast... As a closed low drops south from the Gulf of Alaska on Sunday, embedded shortwave energy along with a steady stream of Pacific Moisture will be directed onshore the California/Pacific Northeast coast. PW values will generally be 0.75" early in the periods before tapering off once heights begin to fall. Enhanced orographic lift will result in the favored upslope regions having higher qpf. such areas include the Cascades, were snowfall amounts will likely exceed 4 inches. the area of precipitation will expand into the Northern Rockies as the trough moves further onshore. Heavy snow will be possible for the Sierras and the higher terrain of the Intermountain West Sunday night through Monday. The WPC forecast is a blend of the prior forecast with ECMWF/GFS/UKMET with a nudge toward the hires ARW. ...Upper Midwest across the Great Lakes to the Northeast... A robust closed low and associated trough will swing through the north-central U.S. and become negatively tilted. Tropical moisture (TPWs up to 1.75" in Mid-MS valley at the start of the period) will continue to lift north into the TROWAL/left branch of the warm conveyor belt producing a strong comma head producing notable late season wintry conditions (please refer to QPFHSD for additional details). The latest suite of deterministic models and ensemble means persist in keeping this feature negatively tilted, which further supports a slower progression to the east. The GFS continues to be the faster of the solutions, albeit qpf amounts remain fairly close to the favorable ECWMF. The exact placement of the local maximas with the axis of precipitation still differ slightly, but the NAM and UKMET were also fairly clustered. A general model blend with less weighting on the 00Z GFS was preferred. ...Southeast to the Ohio Valley to the Mid-Atlantic and eastern Great Lakes... Widespread to numerous showers and thunderstorms develop along and ahead of a front passing through the Gulf states, Tennessee Valley, Southeast, Appalachians and the Mid-Atlantic. A broad axis of 1.5 inch PWATS narrowing to a 1.75 to 2.0+ inch axis, or roughly 2 to 3 standard deviations above normal, will be feeding into the closed low over the north-central U.S. ahead of the approaching cold front. The environment will be favorable for slow moving storms that produce heavy rainfall-- training can be expected to continue east of the MS river Saturday into early Sunday. Additional backing low level flow along the Gulf Coast may further enhance convection to pose an increased thunderstorm activity with efficient rainfall production. A broad areal coverage of 1.5-3+" QPF is forecast from Florida to the the Great Lakes by Monday morning. WPC maintained the Day 2 Excessive Rainfall Outlook (12z Sat-12z Sun) of a Moderate Risk; both the Moderate and Slight Risks were adjusted slightly further east to account for some of the faster guidance Into Day 3, as the upper low elongates...a second focus/center will focus moisture flux just west of the spine of the Appalachians leading to response across the Sargasso Sea...with return moisture (TPWs to 1.25" reaching the Carolinas by early Sunday morning) combining with deep moisture feed with the main warm conveyor leading to strong flux convergence along the southern Appalachians of NC/SA/N GA before slowly sliding northward into SW VA/W VA/W MD/Central PA by Monday leading to increased uptick in convective vigor and rainfall efficiency. Deep southerly flow ahead of the cold front will present ideal South to North training of thunderstorms before being orographically enhanced. As such areal QPF forecasts of 2-3" with some isolated higher totals up to 4-5" possible leading to an expansion of the Day 3 Excessive Rainfall Outlook (12z Sun-12z Mon) Slight Risk to stretch from Northern GA to Central PA. A backdoor cold front/stationary front will be laid out across the northern Mid-Atlantic to Southern New England and will likely add additional focus to the south to north streaming convection leading to a secondary maxima of precipitation Monday across this region. Chenard/Campbell Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml