Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 655 PM EDT Sun May 27 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 28/0000 UTC thru May 31/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Southeast / Alberto... The NHC forecast track for Alberto trended east and a little slower at 15Z compared to 09Z, though in terms of the track, the east shift that was observed in the past few model cycles has ended. The core of the system will track up through the eastern Gulf today, then is forecasted per the latest NHC advisory near Panama City by 12Z Monday. 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 multi-model ensemble, and continuity, then later trended toward the consensus of the more recent HRRR/experimental HRRR runs through the Sunday night period. ...Mid Atlantic... A shortwave trough had ignited a well of instability pooled along a warm front in New Jersey overnight, resulting in flash flooding early this morning. In the wake of this wave and along/ahead of a backdoor surface front that is getting a southerly boost from convective outflows this afternoon, focused sfc-700 mb convergence timed with the peak diurnal heating is setting the stage for additional widespread convection with w-e cell training late in the afternoon/early evening from northeast WV-northern VA eastward into northern-eastern MD. WPC had updated the earlier periods (00-06Z Mon especially) based on the latest convective trends. ...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/Monday Night for central and southeast Alabama, the Florida Panhandle, and far southwest Georgia *** Subtropical Storm Alberto will spread moderate to heavy rainfall over a swath of the Southeast region during the Days 2-3. A bit of a shift back west was made to WPC QPF since the QPF is based on a blend of the 12Z ECMWF/GFS/NAM of which the GFS is farther west than the ECMWF/NAM and agrees with the light shift west in the 15Z NHC official forecast track. 12Z runs of the ARW/ARW2/3km NAM are in good agreement for both placement and magnitude for the excessive rain portion of Day 2 (12Z Mon-12Z Tue). The max QPF is along and east of the center of circulation where tropical moisture is streaming in with a sharp cut off west of the low center. This is 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. Given this good agreement, the magnitude was raised through the Day 2 period to around 6 inches. As with any tropical activity, localized higher amounts are likely. The Day 2 Moderate Risk Excessive Rainfall Outlook was shifted a bit north. A warm conveyor belt of tropical moisture will bring a second area of risk east of the dry slot east of the low center. This is across eastern NC. 12Z guidance consensus brings this into SE VA where the Slight Risk was expanded. Furthermore, the moisture plume associated with the low center will lift over the southern Appalachians near the NC-SC-GA borders. Also, south Florida will continue to be situated in a deep tropical moisture plume with the potential for several days of convective rainfall. A swath of Marginal Risk continues for central and northern FL, coastal GA and into SC for the dry slot. The track of Alberto will reach the western OH valley on Day 3. The Slight Risk for the Day 3 Excessive Rainfall Outlook was maintained. Please refer to the National Hurricane Center forecasts for the latest information on Alberto. ...Northern Rockies and Central Plains... QPF was based on the 12Z GFS/NAM/ECMWF which raised the magnitude a bit, particularly over the central plains of NE/KS for both Days 2 and 3. Gulf moisture is available to this activity via deep layer southerly flow which may allow maintenance/training of thunderstorms. 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 produce isolated flash flooding. ...North Dakota... A marginal risk of excessive rain was raised for northwest ND on Day 3 due to convergent flow of Gulf-sourced moisture on the north side of a surface low over the ND/SD border. FFG is low in this area due to rainfall in the past week which warrants the risk despite average QPF only around an inch. Hurley/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml