Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 556 AM EDT Wed May 30 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 30/1200 UTC thru Jun 02/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Overview... Guidance was all generally well clustered with QPF output for large scale synoptic systems in the Midwest (absorbing Tropical Depression Alberto) and in the Great Basin / northern Rockies. Somewhat more tedious convective forecasts were seen over the southern Plains into the Southeast, including Florida. WPC QPF leaned toward details in the WRF-NMMB, which has performed better this year than in any previous year, and seems a better choice than the other hi-res options much of the last few months. We also used the HREF probability matched mean - which boosts the signal for embedded heavier amounts. Overall this approach led to an increase of QPF along the past of T.D. Alberto, and also in association with the next MCS expected to top the thermal ridge over Oklahoma/Kansas Wednesday evening. Please see also WPC Tropical Cyclone Advisories and Storm Summaries for Alberto. Days 2/3... ...Eastern Seaboard from the Mid Atlantic south to Florida... The Day 2 period (Thursday) will begin with abundant moisture in place for most of the East Coast extending west to the Appalachians. Precipitable water values should be above 1.6 to 1.7 inches in most places, with some values approaching 2 inches likely. An upper level trough over the Great Lakes on Thursday will steadily advance to the east, and perhaps dig slightly, through the Day 2-3 period with an associated low-level cold front also pushing into northern portions of the region. The anomalously high precipitable water values along the entire East Coast and general upper level support for vertical motion should contribute to scattered convection through much of the region. WPC QPF was based on a blend of the 00Z ECMWF, UKMET, and GFS, with the greatest weight placed on the ECMWF. The GFS appeared to be advancing the front too quickly, and this affects its distribution of rainfall, particularly in the Mid Atlantic. The differences between the NBM and WPC QPF are not too significant overall, but may be larger in areas where the NAM is indicating heavy rainfall. The NAM was generally excluded from WPC QPF due to potential issues with convectively induced vorticity maxima over the Plains propagating downstream. A Slight Risk was continued on Day 2 over the Mid Atlantic, and introduced on Day 3 for similar areas. Several rounds of scattered convection may not produce significant areal averaged rainfall totals, but the convective nature of the rain should lead to more substantial localized totals. With PWATs around the 99th percentile on Thursday (slightly lower but still above average on Friday), the environment would be supportive of rain rates as high as 2 in/hr. Many parts of the area also have above normal streamflow given a wet pattern in recent months, so some isolated to scattered episodes of flash flooding appear possible both days. ...Ozarks into the Tennessee Valley... This is the region with the greatest forecast uncertainty. The influence of the trough should be restricted to the north, and as such the dominant forcing for convection may be convective processes (outflow, cold pools) as well as low-level boundaries and the low-level jet. Thus, the precipitation forecast is highly likely to be influenced by convective evolution in previous days, and this leads to uncertainty in placing the swath of heaviest rainfall. It also appears likely that one or more MCSs may evolve given that the region will be situated on the N/NE periphery of a low-level thermal ridge and elevated mixed layer. One MCS may be ongoing at the beginning of the period in the Ozarks, with another one potentially initiating Thursday evening where low-level inflow intercepts a residual outflow boundary. For now, a blend of the ECMWF and GFS was generally followed, but several changes were made: (1) to nudge the QPF axis south, as MCS propagation tends to bend a bit more toward the higher thetae values than models indicate; (2) show more of a natural progression, as the models tend to show the higher QPF moving slowly or sitting in similar locations for consecutive 6-hr periods; and (3) to increase the QPF maxima above what a simple blend would yield. Slight Risks were included on the Day 2 and 3 Excessive Rainfall Outlooks to account for the potential for organized convective clusters/lines to produce flash flooding. ...Northern Rockies into Northern Plains... A potent shortwave will focus DCVA and mid-upper level height falls across the region, spreading from the Northern Rockies on Thursday, into the Northern Plains on Friday. Given the strong forcing and above average PWATs, it is unsurprising that the models also show a clear signal for heavy rain. This is particularly true on Friday (Day 3), as the height falls arrive in the Plains and reach a corridor of significant convective instability. Models develop widespread convection during the afternoon and then spread it east, likely in the form of one or more convective lines. Followed a general blend of the ECMWF, GFS, and UKMET, but QPF maxima were increased to match our internal bias-corrected ensemble. Global models can tend to underestimate QPF in cases of widespread convection or significant convective lines. The GFS and ECMWF show a large QPF maximum in Nebraska, and this makes sense. On the southern periphery of the advancing trough, flow and storm motions will be weaker. Any convective clusters or lines may propagate more slowly than further north, and this may also allow the convection to take on an increasing west-east orientation with time (which would favor heavier rain). A Slight Risk was maintained in Montana on Day 2, and introduced in the Plains on Day 3. Burke/Lamers Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml