Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 453 AM EDT Fri Apr 13 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 13/1200 UTC thru Apr 16/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern and Central Plains east into the Mid/Upper MS Valley and Great Lakes... A large and energetic system will bring a swath of moderate to locally heavy QPF amounts from portions of the Central and Northern Plains east across the Mid/Upper MS into the Great Lakes. While models are in fairly good agreement...some important differences remain. Fist off, it would appear that precipitation early Friday will focus a bit further west than previously forecast...with the trough ejecting east a bit stronger and more neutral to negatively tilted as it moves into the Plains. This is supported by the 0z HREF components and recent HRRR runs. How the system evolves through the day is a bit less clear...and thus a general multi model blend seemed fine. A concern will be the northern gradient...with a strong high to north supplying low level dry air. Thus likely to be a pretty tight gradient. In general have noted a southward shift over the past several cycles of the global models...with the 0z HREF components also a bit south. Overall using a blend of the global and high res models seemed to produce a good compromise for this northern gradient...with the new WPC forecast pretty similar to our previous forecast with this aspect. The 0z HREF mean and UKMET are the closest individual solutions to our QPF preference over this region. Another area of interest will be near and just north of the strong warm front forecast to stretch from IA east into IL and southern MI. The combination of favorable mid/upper level dynamics, strong 850 mb moisture transport into and over the boundary, and decent elevated instability....should all support a favorable environment for convective development by Friday afternoon into the overnight. Initiation should be over IA with activity moving northeastward into portions of southern MN/WI/MI. The threat for the heaviest rainfall rates will be closest to the front...with the activity becoming increasingly elevated as you head north of the front. Did nudge our QPF a bit closer to the ECMWF/UKMET for this activity...which have been more consistent showing some locally heavier totals. The 0z HREF mean seemed a bit light across portions of IA into northern IL and southern WI. ...Eastern portions of the Southern Plains toward the Lower MS Valley and TN Valley... Convection will initiate Friday afternoon in a line from northeast TX into northwest LA into western AR. Moisture parameters will be high enough to support heavy rainfall...with PWATs approaching the climatological 95th percentile to go along with strong 850 mb moisture transport. Thus heavy rates are likely...and so the determining factor for flash flooding concerns will be the duration. Convection will become more organized as we head into Friday evening and the better forcing moves into the area. Overall the system as a whole is progressive in nature...although there may be a period where training is possible. Would appear that storms will initially form out ahead of the surface cold front in a region of confluent 850mb flow and enhanced moisture transport. As the wave to the west digs southeast into the area, storms may tend to form further southwest closer to the surface front...with this activity then merging into the prefrontal storms. If this were to occur then we could end up with a corridor of heavier rainfall totals and a higher flash flood risk. Thus in general looks like a broad region where some flash flood concerns could arise given the favorable environment for high rates...and a smaller region where potential training could cause a more significant threat. Some model differences remain with the axis and magnitude of rainfall totals. While good to start, thought the GFS probably becomes a bit too progressive by the end of the period. Thus preferred a blend of the 0z HREF, UKMET and 12z ECWMF...which seemed to produce a good consensus solution. This results in a broad 1-3" area from northeast TX into western TN. The consensus region for the best training threat is from northern LA into southern AR and western MS...where a swath of 3-5" (locally higher) is expected. This is slightly north of our previous forecast...with a cap likely defining the southern edge of the threat through day 1. The new WPC forecast is also slightly more progressive than our previous forecast to take into account the ARW and ARW2. ...Pacific Northwest... Onshore west southwest low level flow will be re-strengthening into the Pacific Northwest tonight into Friday to the southeast of a developing closed low just to the south of the Gulf of AK. This will support moderate to heavy precip values from the Washington-Oregon Cascades---west to the northern Oregon coast range into the Olympic Range. Days 2/3 ...West Coast... A closed low over the Gulf of Alaska Saturday morning will eject south and swing into the west coast Monday. A series of smaller scale mid-level shortwaves are expected to crash into the Northwest coast south of the low center forcing precip particularly along a narrow plume of 0.75 inch PW pushing in from the coast also south of the low center. Precip will make its way to central CA Sunday night with precip enhanced on western slopes of the coast ranges and Cascades through both days. ...Upper Midwest across the Great Lakes to the Northeast... A potent closed low over the central Great Plains Saturday morning will lift northeast to MI through Sunday night as the amplifying mid-level trough axis south of the upper low center acquires a negative tilt. Tropical moisture will continue to lift from the Gulf of Mexico and around the comma head of the occluding low making for a notable late season winter storm (see QPFHSD for further winter information). The northward extent of the precip will be determined by the presence of a strengthening surface high over northern Manitoba/Ontario. Dry air from this high will make for a sharp cutoff on the northern side which still wavers from run-to-run. WPC stayed close to continuity despite some of these shifts, with this axis continuing to lie near the middle of the model spread. WPC QPF was based on a blend of the 00Z ECMWF/GFS/NAM/UKMET. ...Southeast to the Ohio Valley to the Mid-Atlantic and eastern Great Lakes... 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, is expected to pivot over the southeast from LA to KY (and even IN/OH) as the mid-level trough swings into a neutral tilt Saturday. Heavy rainfall with unidirectional flow promoting periods of training can be expected to continue east of the MS river Saturday. The Slight Risk for excessive rainfall for Day 2 (12Z Sat-12Z Sun) was maintained. The anomalous warm air advection through the OH Valley to the eastern Great Lakes warrants a northward expansion of the slight risk to roughly Cincinnati and a marginal risk for excessive rain where rates could exceed half inch per hour all the way to Detroit and east to Buffalo. As the mid-level trough axis swings into a negative tilt over the southeastern CONUS Sunday, progression of the leading convective line is expected to increase. This increase in forward speed is expected to occur of AL which lowers QPF east from there. Backing low level flow and the system lifting northeast will focus enhanced precipitation on the eastern slopes of the central Appalachians. Despite generally less than an inch of rainfall over this area in the past two weeks, the anomalous moisture and strong low level jet with topographic enhancement warrants a slight risk for portions of the central Appalachians and adjoining Piedmont for the Day 3 excessive rainfall outlook (12Z Sun-12Z Mon). Heavy rain is expected along the front farther south, but progression is expected to be great enough to limit areal averaged rainfall to two inches or less, so the marginal risk was limited to north of this area. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml