Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 457 PM EDT Sat May 19 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 20/0000 UTC thru May 23/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Eastern Carolinas... Will need to continue to monitor portions of eastern SC and NC through tonight 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 12z HREF shows moderate to high probabilities of exceeding 3" across portions of the area, especially into the evening, and so think the setup suggests at least a localized flash flood threat. Will maintain continuity and have a Slight risk to account for this risk. ...Florida... Another wet forecast across Florida as well. PWATs and 850 mb moisture transport should both be higher tonight into Sun than previous days, potentially resulting in a wetter overall period. Activity should initially focus along the west coast sea breeze boundary, before potentially shifting to the east coast overnight. 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. With the above environmental parameters in place, 0z HREF probabilities of locally exceeding 5" in the moderate to high category, and wet antecedent conditions, will go with a Slight risk across the southeast Florida coast. Lower confidence with this risk area, but the conditional risk appears high enough to warrant it at this time and will monitor trends through the evening/overnight. ...Southern-Central Plains into the Mid MS Valley... A cold front extends from the Southern Plains into the Mid and Upper MS Valley, with a dryline across western TX. The mid/upper levels will remain broadly divergent and generally uncapped -- as the bulk of the vort energy early traverses the central Plains and eventually the mid MS Valley. Vigorous deep-layer instability (surface-based CAPES 2000-3000+ j/kg), buoyed by steep mid-level lapse rates (7.5-8.0 c/km between 700-500 mb) along/ahead of the front and dry line across north and west TX will spawn new convection by late afternoon, with the activity expected to grow upscale later in the evening along an axis of increased 850 mb moisture transport as the southerly low-level inflow increases to 40-45 kts. Given the degree of directional shear progged, per the high-res CAM simulated reflectivity expect the segments to be more progressive/forward propagating than otherwise. Still, there continues to be a multi-model signal for hefty areal-average totals across portions of north TX across the Red River into southwest OK -- maintaining the higher QPF confidence over this region. The WPCQPF comprised a blend of the previous forecast (continuity) with the HREF probability-matched mean, NBM, and WRF-ARW -- which noted a slight timing difference (a little more progressive) from the previous WPC forecast. Farther north -- another concentrated area of heavier rainfall is still anticipated across the lower MO into the mid MS Valley, based on the additional destabilization later this afternoon underneath a thinning canopy of last night's mid/upper level convective debris, while also ahead of the main vort lobe that across KS-NE around midday. Areal-average totals up to around one inch are anticipated given the multi-model consensus; however, per the high-res CAMs, localized amounts of 3-5+ inches will be possible within the axis of anomalous PW (deep TROWAL zone). This especially given the relative weak downwind propagation given the low-mid layer wind fields. Hurley/Chenard Days 2/3... ...Mid and Upper Mississippi Valley to the Ohio Valley/Central Appalachians and east of the mountains in Pennsylvania/Maryland on Monday... A low-mid level wave over eastern Iowa on Sunday evening moves towards the Great Lakes, with the deformation zone focus over eastern Iowa/northern Illinois/northern Indiana. 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 Illinois and Indiana late Sunday, supported by upper divergence maxima. This continues downstream on Monday and Monday evening as bands of low level moisture convergence stream across much of Ohio and then Pennsylvania. 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 West Virginia and Tennessee. Manual progs used a blend of the latest available ECMWF/12Z NAM/12Z UKMET. ...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 with less weight given to the UKMET and Canadian global models. Marginal risks for excessive rainfall over the southern Florida peninsula were maintained for both Day 2 and Day 3 excessive rainfall outlook. The inflow off the gulf advects the high moisture into the Florida 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 Monday. ...Great Basin/Northern Rockies... A long wave trough will slowly amplify this weekend as it crosses Oregon and California before closing into a low over southern California Monday. The focus of rainfall develops with upper divergence maxima just downstream from the trough as it crosses OR and adjacent northern California and Nevada. Highest QPF remained over portions of Oregon, northern Nevada which corresponds 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 farther east into the ranges of Idaho, western Montana and western/northern Wyoming in tandem with both the low level front and upper trough. Good agreement in the 12Z models supported a general model blend for QPF. ...West Texas/eastern NM... East of the developing western U.S. trough, southeasterly confluent low level flow over Eastern NM and across much of Texas will draw moisture off the Gulf of Mexico inland. A Marginal Risk for excessive rainfall was maintained for north of the Big Bend to the New Mexico border with only minor adjustments. The airmass will be characterized by a precipitable water value around 1.5 inches by Monday...which is 2 standard deviations above normal with high CAPE and low shear which should keep storm motion slow. There large scale picture does not change much on day 3, which will lead to another round of showers/storms with a couple of the models showing locally 2-3 inches of rain. The excessive rainfall outlook was maintained at the slight risk category. Hurley/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml