Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 417 AM EDT Sat May 19 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 19/1200 UTC thru May 22/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Eastern U.S... Will be another wet day across much of the east, although the pattern will finally begin to progress today. The broad mid/upper level trough will finally begin pushing off to the northeast, which will push increased moisture into the northeast and push the Mid Atlantic front north as well. Thus anticipate showers will overspread the northeast with time, and generally become less focused over the Mid Atlantic. Given weak to no instability, not anticipating rainfall rates will be all that high today from the northern Mid Atlantic into the Northeast. Will need to continue to monitor portions of eastern SC and NC through the day for the potential of an axis of heavier rainfall. An axis of near 2" PWATs is expected to focus across this area for an extended period of time, with at least weakly convergent flow in the lower levels. Also appears like one or more weak shortwaves will push north in the southerly flow, locally enhancing lift and low level moisture transport. Not expecting convection to be too organized, and instability is not all that great. However, the persistent convergence axis, higher PWATs and mean flow does support the potential for a period of south to north moving repeat cells capable of producing briefly heavy rates. The 0z HREF shows moderate to high probabilities of exceeding 3" across portions of the area, and so think the setup suggests at least a localized flash flood threat. The shortwave that has only slowly been progressing east across the MS Valley into the OH valley over the last few days will pick up a bit more speed off to the northeast today. Anticipate another day of ample instability and moisture ahead of this wave and trough axis for more scattered to widespread diurnally driven convection. Main focus will be across the OH Valley into the western Mid Atlantic. Mean flow is greater today, thus anticipate storms will be more progressive than we have seen the past few days. Thus in general would expect rainfall magnitudes to be lower, although with continued high PWATs, briefly intense rates are still likely. Normally this setup would probably not be too much of a flash flood threat, however where recent rainfall is saturating soils and lowering FFG, this rain may cause additional flash flood concerns. Biggest threat appears to be across portions of southeast WV south into northwest NC, where some signal this convection today could briefly hang up and/or repeat near the terrain resulting in localized heavier amounts. Given antecedent rainfall here, and the possibility of intense rates with this activity, additional flash flooding is possible. Another wet day across Florida as well. PWATs and 850 mb moisture transport should both be higher today than previous days, potentially resulting in a wetter overall day. Activity should initially focus along the west coast sea breeze boundary, before potentially shifting to the east coast overnight. Most of the 0z high res guidance is quiet wet over eastern FL with this overnight activity. The potential is certainly there for these heavier totals given the increased southeasterly flow, strong convergence signature and near climatological record PWATs. Appears like the high res models are hinting at a subtle wave riding north, interacting with these favorable parameters and producing a period of south to north training. Tough to say for sure if this will materialize, but certainly something to watch, as given wet antecedent conditions, a greater flood threat could exist. ...Central U.S... Another active convective day expected for portions of the Central and Southern Plains into the Mid MS Valley. By this afternoon the surface setup should feature a cold front extending from the Southern Plains into the Mid and Upper MS Valley, and a dryline across western TX. The mid/upper levels will remain broadly divergent and generally uncapped. Thus by afternoon will see an uptick in convection along both these boundaries. PWAT values will remain high enough to support heavy rains. Thus any flash flood threat will come down to if and where convection is able to train and or repeat for a period. The general consensus is that the first area to convect will be along the front across OK/KS this afternoon. This activity should move northeast along the front for a period, but then should advect eastward into the very unstable airmass forecast over MO. The orientation of propagation vectors relative to the instability gradient does support the potential for a period of west to east training of convection across this area. Also, should see storms form further northeast along the front from southeast NE into southern IA. Some training along this front is also possible. Seems most likely that one of these two areas will see the larger convective cluster and better training potential, probably not both. Uncertainty on exactly which one becomes most dominant, with the ARW/NMMB favoring the further north area over IA, and the NSSl WRF and Experimental HRRR more over MO. Confidence is low, although did lean a bit more towards the solutions over MO, with ongoing convection this morning possibly acting to displace the effective boundary a bit further southeast. Higher confidence in what evolves over TX/OK...with widespread development appearing likely along/east of the dryline. Would appear like there is a window of potential training near the intersection of the cold front and dryline from north central TX into southern OK. This is depicted by a good amount of the 0z high res guidance, and given the orientation of the fronts at play and increasing 850 mb moisture transport, it seems plausible. Eventually would anticipate any complex to become more progressive off to the south into the instability pool. Generally preferred leaning towards the more aggressive QPF solutions here, with the 0z Experimental HRRR seeming pretty good (although it likely does not develop convection far enough south along the dryline). Chenard Days 2/3... ...Mid and Upper MS Valley to the OH Valley/Central Appalachians and east of the mountains in PA/MD on Mon... A low-mid level wave over IA on Sunday moves east northeast towards the Great Lakes, with the deformation zone focus over eastern IA/northern IL/northern IN. The Marginal risk for excessive rainfall continues with slow moving cells possible near the elongated circulation. More progressive showers/storms develop in an axis of the low level jet with well defined 700 mb convergence near the jet maxima crossing central IL and IN on Sun, supported by upper divergence maxima. This continues downstream on Monday as bands of low level moisture convergence stream across much of Ohio and then PA. The mid level westerly flow provides an upslope component of flow so moisture is lifted by the mountains with showery conditions expected in the terrain of WV. Manual progs used a blend of the 12-00z ECMWF/00z NAM/00z UKMET with the least weight on the GFS, which depicted a weaker wave across the Great Lakes. ...FL/eastern Gulf coast/southeast... Deep moisture within confluent flow with corresponding lift provided by the passage on an inverted 850 mb trough will support bands of showers over FL on day 2 and 3. The NAM hinted a closed low developing which led to strong low level wind fields and resultant convergence and lift, and thus showed heavier totals than other models. Manual progs blended continuity with the gfs/ecmwf/ukmet/Canadian global models. Towards the end of day 2 into day 3 the nam/gfs/ukmet closed a closed 850 mb low with the gfs on the eastern edge of the suite of solutions. Consequently, the gfs was given less weight than the ukmet/nam/ecmwf. Marginal risks for excessive rainfall over the southern FL peninsula were maintained for both days 2 and 3. The inflow off the gulf advects the high moisture into the FL panhandle across into southern Alabama and Georgia, so combined with diurnal instability and sea breeze initiated lift, the higher amounts are shown in these areas for Mon. ...Great Basin/Northern Rockies... A long wave trough will slowly amplify this weekend as it crosses OR and CA before closing into a low over southern CA Monday. The focus of rainfall develops with upper divergence maxima just downstream from the trough as it crosses OR and adjacent northern CA and NV. The higher amounts over eastern OR Sun correspond to low-mid level frontal convergence plus lift aided by terrain in the area, like the Blue Mountains. On Monday the focus shifts a little further east into the ranges of ID and western MT in tandem with both the low level front and upper trough. The maxima in NV continues as the closed low and low level front are slow to move across the state. Good agreement in the 00Z models supported a general model blend for QPF. ...West Texas/eastern NM... East of the developing western US trough is confluent flow over Eastern NM and Texas with southeast flow bringing gulf moisture into NM. A Marginal Risk for excessive rainfall was raised for north of the Big Bed to the NM border for 1.5 inch PW Sunday night which is 2 standard deviations above normal with high CAPE and low shear which should keep storm motion slow. The continuation on day 3 promises to bring another round of showers/storms with a couple of the models showing locally 2-3 inches of rain. The risk was raised to slight on day 3 given both trends towards continued showery conditions plus potential for locally heavy amounts. Chenard/Petersen Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml