Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 453 PM EDT Thu May 10 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 11/0000 UTC thru May 14/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Eastern U.S... A trough encountering seasonably steep lapse rates and sufficient moisture will yield scattered strong but quick moving thunderstorms this afternoon across the Carolinas / Mid-Atlantic States / interior New England. Predictability of the individual thunderstorm swaths is not high given this particular convective mode, so we generally followed observational trends and hi-res ensemble means, yielding no more than a half to one inch areal average precipitation (heaviest swaths in the mid Atlantic). ...Central Plains to the Great Lakes... A Great Plains style precipitation event is expected to evolve tonight, with quasi-surface-based convection growing upscale and heading east across Nebraska and western Iowa. Models are strongly suggestive of a second precip maximum to occur overnight near the IA/MN border in a zone of upglide focused to the north of a frontal zone that had dipped into the area. There is some potential for these events to overlap somewhat, and to boost the event totals over parts of IA/MN, if the convection coming out of Nebraska can be maintained to some extent by the low level jet. Moisture availability is not optimum, with PW values climbing just above 1.00 inch in the pre convective environment. The thermodynamic environment may hamper rainfall efficiency and promote forward motion through strong cold pools. Deep layer forcing with the low amplitude upper wave is also expected to remain farther north, just off the instability axis. So there are some negative factors regarding any truly focused potential for excessive rainfall. Still, the model QPFs actually converged a bit toward a better consensus, and they do favor areal average 1 to 2 inch amounts from near Sioux Falls eastward overnight, with locally higher amounts possible, although extreme amounts are unlikely given the transient nature of the forcing. The previous WPC forecast had seemingly done a good job handling wide model spread. The 12Z cycle saw models trend toward the WPC QPF placement, but with somewhat lesser amounts. Using the HREF Mean and WRF-NMM (which has been confusingly good at QPF this spring) we trended toward more confined maxima and just a slight reduction to QPF magnitude, while keeping placement nearly the same as before. These thoughts were coordinated with local offices and NCRFC. ...Rockies / northern Intermountain West... A well-defined shortwave will continue to dig toward the Great Basin, with a closed mid level center forming near Idaho during Day 1. A surface high settling toward the international border out on the plains will help to focus easterly low level flow into the foothills and mountains. As a difluent upper flow pattern develops east / northeast of the low center, there should be some pockets of enhanced vertical motion and precipitation. One such event is expected tonight in south-central Montana, and another is expected Friday in southeast Wyoming. This is evident on somewhat larger scales, while there may be other local mountain enhancement scattered around this system as well. WPC started with a blend that included our in-house ensemble as influenced by the hi-res models, along with the GFS and NAM CONUS Nest. Continuity was also considered. Although not very anomalous in the height field, this system should produce widespread quarter inch and greater precipitation, with the more favored areas receiving 0.50 to 1.25 inches liquid through Friday afternoon. This may compound spring snowmelt flooding in the long run, and wherever pockets of instability develop there may be some convectively enhanced precipitation leading to at least a low percentage chance of some flash flooding. Days 2/3... ...Intermountain West, Northern/Central Rockies, and High Plains... A closed upper low will persist over the Great Basin through Saturday afternoon/evening before weakening Friday through Saturday night before it begins to weaken. The positive tilt to this low and surrounding trough will wrap both Pacific and Gulf moisture into the Intermountain West. This, combined with favorable upper jet dynamics and upslope flow, will support moderate to heavy precipitation with this system. A couple of small marginal risk areas where hoisted where the models were generous with QPF. The risk areas were confined to elevations below snow levels. Overall, though, the fundamental thinking of the overnight shift remained that same...ongoing snow melt will raise flood concerns for this area and WFOs in the area have already been addressing the concern with hydrologic outlooks. ...North-central Great Plains to the Middle/Upper Mississippi Valley and Northeast... Broad warm advection ahead of the amplifying western U.S. trough/closed low will bring an increasing risk for rainfall from the north-central Great Plains to the Middle/Upper Mississippi Valley and Northeast late this week...with potential for moderate to heavy rainfall amounts in areas that are hydrologically sensitive. Two main threats for excessive rainfall will be with weak shortwave energy streaking out of the western U.S. and across a frontal boundary over the Midwest late Friday and Saturday. The Friday activity will mainly be on the cold side of the front with little instability limiting convective potential. Therefore the marginal risk for Day 2 (12Z Fri-12Z Sat) was maintained. A slight risk for excessive rainfall was maintained largely unchanged for Day 3 (12Z Sat-12Z Sun). The 12Z run of the GFS largely maintained continuity in its mass field and its precipitation forecasts. The WPC QPF was blending of ideas shown by the ECMWF...GFS and UKMET. The choice of those models in the first 12 hours of the Day 2 forecast had very good support from the ARW/NMMB in terms of the location of the precipitation axis. ...Southern Florida... The majority of model guidance is in agreement that an axis of highly anomalous moisture and a weakening frontal boundary should focus periods of heavy rain just south of the Florida peninsula on Friday, but then there is a growing model consensus for the heavy rain to spread northward into the greater Miami area Saturday as a weakness aloft progresses through the Gulf of Mexico and across Florida. Localized particularly heavy rainfall is possible, so a marginal risk for excessive rainfall was maintained for Day 3 (12Z Sat-12Z Sun). Burke/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml