Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 516 PM EDT Fri May 11 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 12/0000 UTC thru May 15/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Upper Mississippi Valley/Great Lakes/Northeast... Rainfall is expected overnight and again on Saturday in a zone of ascent associated with confluent mid level flow to loosely located to the rear of an upper jet over southeast Canada. Low level moisture transport will increase around the northern periphery of anomalous ridging centered over the southeast United States, with 850 mb convergence setting up from the Midwest states toward the northern Mid-Atlantic states. Capping and generally dry boundary layer conditions should greatly limit or suppress convection in the warm sector / over the plains and lower Mississippi Valley such that most precipitation tonight should focus along the gradient between anticipated convergent 850 mb flow and stronger deep layer lift beginning up at 700 mb farther to the north. Another organizing influence is an upstream MCV leftover from yesterday's Nebraska-based MCS. Rain and embedded thunder should increase ahead of this feature this evening from near Chicago-land through lower Michigan. The events today and tonight will not be strongly forced, but sufficient moisture transport into the region will support modest CAPE values and persistent ascent within the elevated frontal zone. Even still, PW values are not particularly anomalous, and greatest deep layer lift is offset from the CAPE axis such that excessive rainfall should occur only on an isolated basis. The most persistent overlap of parameters appears to be over lower Michigan, especially during the evening and overnight hours, when multiple rounds of convectively enhanced precipitation could lead to quick accumulations greater than one inch and perhaps approaching Flash Flood Guidance values. Farther downstream the synoptic wave along with convectively enhanced mid level gradients will likely come into phase with the diurnal cycle over the mid Atlantic, especially Pennsylvania, during the day Saturday. We leaned a little heavier with QPF here compared to previous forecasts, but areal coverage and placement remained nearly the same. This forecast is most consistent with the 12Z NAM CONUS Nest, 12Z WRF-NMM, and the ECMWF. ...Great Basin/Northern Rockies/High Plains... The trough in the West will continue to amplify this period, with a closed low dropping into the Great Basin. Some of the heaviest amounts of the period are expected to center across Wyoming and northeast Utah where anomalous moisture and sufficient instability, supported by low level easterly inflow, is expected to interact with larger scale forcing along the left-exit region of the upper jet. Heavy snows are expected for portions of the Big Horns and western Wyoming ranges, with moderate to locally heavy rainfall totals across the lower elevations. Farther south, some moderate to locally heavy accumulations (including heavy snows in the Uintas) are possible along a low-mid level frontal boundary dropping south into the region. Here we followed trends by using a blend of the NMMB, ECMWF, and previous WPC. One of the more difficult aspects of the forecast was the extent to which measurable amounts can spill out into the Dakotas. Some models like the NAM and WRF-ARW indicate locally heavy downpours while the ECWMF, GFS are much more conservative and/or confined farther west. WPC favored confining any hydrologically significant amounts to the western High Plains given the extent to which dry high pressure has asserted itself southward into Nebraska and the fact that deep layer ascent will be limited with the upper low being located so far west into the mountains. ...South Florida... Deep layer moisture will just begin to increase, with associated rainfall reaching the Keys and far south Florida, especially by Saturday afternoon. Greater coverage and intensity is expected to expand northward on Days 2 and 3. Days 2/3... ...Great Basin/Rockies... A closed mid level low over NV on Day 2 tracks slowly northeast into Day 3, then weakens. Ahead of the mid level system, a low level flow of increasing moisture is expected to result in locally heavy QPF across the higher terrain, especially during Day 2. For the most part, there was good model agreement concerning the synoptic scale features, so the WPC QPF was based on a blend of the 12Z ECMWF/GFS. Further west, closer to the weakening mid level system, there should be sufficient instability to feed developing convection, especially over the higher terrain of northern and eastern NV. Moisture will be the limiting factor for higher QPF amounts, so for now local 0.50 inch QPF amounts were placed in these locations. ...Central and Southern Plains... Moisture and instability surging along a quasi stationary front extending from the western OK Panhandle into west TX from late Day 2 into Day 3 will feed convection that produces locally heavy rainfall during the second half of the period. The WPC QPF was based more closely on a multi model blend with least weight given to the GFS. Synoptic scale ascent will be focused over a quasi-stationary frontal boundary from western OK into west TX wi. Ahead of the front, a 30 to 40 knot low level south southwest flow is expected to transport 1.25 inch precipitable water air, mainly after 14/00z with moderate to locally strong instability shown by the forecast models ahead of the front. This should result in some locally heavy rainfall amounts, but the forecast values are under the 3-hourly Flash Flood Guidance. As a result, no organized flash flooding is expected at this time. ...Upper MS Valley/Ohio Valley/Northeast/Mid Atlantic... Impulses riding along in the quasi zonal mid level flow extending from the Upper MS across the OH Valley into the Northeast states during Days 2/3 will tap moisture and instability focused on a slowly southward-sagging frontal zone. The most active convection are expected to heavy to locally excessive rainfall amounts along the front each day. The atmosphere ahead of the front over the Upper Mississippi Valley and Upper Great Lakes should be characterized by a precipitable water value ranging from 1.25 inches to 1.50 inches along with low level west to southwesterly flow at 20 to 30 kts. Given the instability and the progged cell motion, the WPC QPF was largely maintained without change and saw little reason to alter the previous excessive rainfall outlook. The 12Z GFS continued to be a bit further south with its maximum QPF amounts when compared with other global guidance, and this southerly placement seemed to better fit the positioning of the best instability. Because of this, the WPC QPF was based on a blend of the 12Z ECMWF/GFS, then dropped a bit south to better fit the expected instability placement. ...FL... Deep moisture and instability become focused on an inverted trough over the east coast of FL during Days 2/3. Convection along the trough is expected to produce heavy to locally excessive rainfall over portion of the east coast of Fl, especially during Day 3. There was convergence in the models in terms of placement of the heaviest QPF axis...although some spread remained in terms of precipitation amounts based on the depth of the surface feature. We aimed for a consensus value which nudges us back, but not all the way, to the GFS/ECMWF solution. WPC maintained the marginal risk on days 2 and 3 without more than some subtle adjustments. Burke/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml