Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 631 AM EDT Sun May 27 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 27/1200 UTC thru May 30/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Southeast / Alberto... The forecast track for Alberto trended east and a little quicker overnight. The core of the system will track up through the eastern Gulf today. Generally difluent upper flow and deep layer ascent will support generous rainfall coverage over much of Florida / drying out a bit over southwest Florida. The conveyor belt out to the east of the parent synoptic trough may set up convergence right over southeast Florida. Much of the guidance is not especially well focused in the QPF fields, but the pattern is a little concerning as cells dropping tropical style rainfall could train over the urban areas / Miami. Some models are just offshore to the east with the heaviest...but based on the NAM mass fields we painted healthy areal average rainfall over Miami and more generally the southeast coast of Florida to the tune of 2-3 inches. Expect embedded heavier amounts. Meanwhile the difluent region farther north and east will migrate up through southeast Georgia and into the Carolinas with time. Expect again heavy areal average rainfall and embedded totals exceeding 3 inches through Monday morning. WPC QPF was initially derived from the UKMET, WRF-ARW2, and continuity...and later trended toward the developing consensus between the global models, including GFS and ECMWF. ...Mid Atlantic... A shortwave trough had ignited a well of instability pooled along a warm front in New Jersey, resulting in flash flooding early this morning. Other scattered activity was ongoing in the moist plume over Virginia to southern Maryland. In the wake of this wave, convergent 850-700-mb flow is forecast to set up during peak heating from central Virginia to southern Maryland. The hi-res models, on average, light up this axis with organized afternoon thunderstorms and locally heavy rainfall. The global models, however - ECMWF and GEFS ensemble 6-hour probabilities - have shown a slight trend to the north which could place more heavy rain into the Washington D.C. vicinity. WPC QPF was centered toward the HREF mean and ECMWF. ...Western/Northern U.S... There will be another round of widely scattered diurnal convection to the rear of the upper trough where low level moisture and mid level lapse rates are sufficient. Farther east height falls and upper difluence will overspread the high plains, and despite warm mid level temperatures it does appear the low level moisture will improve to the extent needed to support clusters of thunderstorms from MT/WY down through western KS and into the OK/TX panhandles. An early morning MCS will also drift downstream through ND/MN...with additional afternoon activity expected to form there along a front. All these things in the west and north were best described by the WRF-ARW2 and HREF Mean, blended with WPC continuity. Days 2/3... ...Southeast and Florida Peninsula... *** MODERATE RISK of Excessive Rainfall on Monday and Monday Night for southeast Alabama, the Florida Panhandle, and far southwest Georgia *** Subtropical Storm Alberto will spread moderate to heavy rainfall into most of the Southeast region during the Day 2-3 forecast period. Overall, WPC QPF maintains fairly good continuity with previous forecasts. The largest adjustments were to shift the heaviest QPF axis near the center of Alberto to the east, matching a shift in the NHC official forecast track to the east. The QPF generally was closest to a blend of the 00Z GFS, ECMWF and NAM, and the 00Z HWRF was also referenced at times. These models showed reasonably good agreement in the track now, and therefore some of the QPF gradients were sharpened to narrow down the axis of heaviest rainfall. Hi-res models show some of the challenges, with very localized swaths of heavy rainfall -- even close to the center of Alberto. This will likely be due to the subtropical nature of the storm, with a prominent dry slot expected to wrap around the south and east side of the circulation (and perhaps even the north side by Monday). Therefore, a bubble of higher PWATs and deep tropical moisture will likely become cutoff near Alberto's center, and some of the slightly drier air may get ingested into the circulation. This may lead to more convective rainfall, even near the center of the circulation, and this is likely what the hi-res models are suggesting. Therefore, the localized maxima will likely be in excess of what is depicted in the WPC QPF, with some locales (even in the center of the track) likely receiving far less. The maximum values in WPC QPF were closer to the NAM and GFS, and our internal bias-corrected pseudo-ensemble. Other than along Alberto's track, the most likely areas for heavy rainfall on Days 2-3 will be in the southern Blue Ridge Mountains near the confluence of the NC-SC-GA borders as moist low-level flow impinges on the terrain; in central and eastern North Carolina given a favorable coupled jet structure and a position near the nose of the warm conveyor belt; and in south Florida which should continue to be situated in a deep tropical moisture plume with the potential for several days of convective rainfall. With the shift in the track and QPF, the Moderate Risk on the Day 2 Excessive Rainfall Outlook was also shifted east. In addition to the heavy rainfall generally expected near the center of Alberto as it moves inland, convective bands may produce prolific rain rates. Several hi-res models, such as the 3km NAM and HWRF nest, show narrow swaths of 3+ inches of rain in 6 hours only about 10-15 miles in width. Some of these actually form in the wake of Alberto, in an area of low-level confluent flow off the Gulf of Mexico into the Florida Panhandle. For the Day 3 Excessive Rainfall Outlook, a Slight Risk was extended along the track up into the Lower Ohio River Valley, and maintained across portions of the Southeast where convective rain bands will remain possible. Please refer to the National Hurricane Center forecasts for the latest information on Alberto. ...Northern Rockies, and Central and Northern Plains... No major changes were made to the QPF in this region, with continued tweaks toward a combination of the NBM and our internal multi-model ensemble. The GFS and NAM did show heavier concentrations of QPF south of 38N in the Plains, but preferred to trend toward the ECMWF which kept most of the QPF north of that latitude due to a combination of possible capping and stronger DCVA to the north. Isolated thunderstorms further south may produce localized swaths of heavy rainfall, but there was low confidence in pinpointing any one location. Marginal Risks were included on both Day 2 and 3 given the expected abundance of convection in a moderate to strongly unstable environment -- any storm clusters should be able to maintain some organization and could conceivably produce isolated flash flooding. Burke/Lamers Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml