Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 514 PM EDT Sat May 26 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 27/0000 UTC thru May 30/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Florida into the Southeast... No significant changes to the forecast thinking with the heavy to excessive rainfall potential day 1 associated with the northward movement of Alberto across the eastern portions of the Gulf of Mexico. Heavy rains will continue on the eastern side of Alberto in the persistent axis of much above average pw values---2.5-3+ standard deviations above the mean---across much of FL. Models do show potential for a developing dry slot pushing northeastward off the eastern Gulf and across portions of FL tonight into Sunday. The highest pw values may focus along the east coast of FL where a few of the hi res runs are indicating potential bands setting up parallel to the coast late Saturday night into Sunday--with a lessening heavy rain threat for the western portion of the peninsula. Farther to the northeast---the precipitation threat will be increasing across the Southeast across southern GA into coastal SC as the higher pw values on the east side of Alberto continue to stream northward. For the day 1 time period---the potential very heavy rains near the center of Alberto will likely remain offshore given the current official NHC track---with this core heavy rains a day 2 and beyond threat as the low continues to push north. ...Mid-Atlantic into the Central Appalachians... Much above average pw values expected to remain in a east to west axis across portions of the Mid-Atlantic---extending into the Central Appalachians. Initially this is well to the south of the frontal boundary over the northeast. With time this front will be pressing south into the Mid-Atlantic and may become a focus for heavy to excessive rainfall amounts Sunday. A predecessor rainfall band well to the northeast of Alberto is possible along this front where favorable upper difluence in the axis of much above average pw values will support heavy to locally excessive precip totals Sunday. ...Great Basin into the Northern Rockies The strong Great Basin closed low will be moving only very slowly eastward day 1. Well defined upper difluence ahead of this closed low will support fairly well organized comma head deformation precipitation areas to the north and northeast of this strong closed low. This will support widespread moderate to locally heavy precip totals from the Great Basin into the Northern Rockies. The href mean and in house hi res ensemble used primarily to mitigate any run to run and model to model qpf detail differences. Days 2/3... ...Southeast and Florida Peninsula... *** MODERATE RISK of Excessive Rainfall on Monday and Monday Night for extreme southeast Mississippi, southern Alabama, and the western Florida Panhandle *** The rain associated with the center of Subtropical Storm Alberto will begin spreading into the Southeast region in earnest Sunday, with the center approaching the central Gulf Coast Monday per the latest NHC forecast. The plume of tropical moisture east of the storm center will create a mainly separate area of heavy rainfall north from Florida to the coastal Carolinas through the Day 2-3 period. A mid-level dry slot is expected in between, as depicted in WPC QPF through the Day 2-3 period. However, due to the potential for repeating cells/bands in the dry slot, the Slight Risk for excessive rainfall that spans that southeast was maintained for both Days 2 and 3 per consultation with some Florida WFOs. Also, the northern extent of the Slight and Marginal Risks were shifted north a bit per 12Z guidance. WPC QPF was based on an even blend of the 12Z GFS/NAM/ECMWF which are in good agreement on Day 2 (when 3km NAM was used). The 12Z ECMWF shifted the axis of heaviest rainfall east to Alabama from Mississippi on Day 3 which is in line with the 12Z GFS (QPF max axis placement in the 00Z ECMWF had been quite similar to the 12Z NAM), so the final QPF also included a shift east. The area of heavy rain around the center of Alberto may be relatively small as it approaches the coast, per 12Z guidance consensus of a small core circulation and limited diameter of heavy QPF. As such, even small deviations in the track could significantly shift the expected heavy rain and flash flooding impacts near the center. However, there is relatively high confidence in that flash flood threat existing near the track, particularly given the expected slow forward motion (around 7-8 knots) hence the Moderate Risk for excessive rainfall on Day 3 (12Z Mon-12Z Tue). Please refer to the National Hurricane Center forecasts further information on Alberto. ...Mid-Atlantic... A swath of heavy rain is expected to redevelop across the Mid-Atlantic Sunday afternoon. The combination of a lingering low-level front and theta-e gradient, along with the right entrance region of a jet streak, should provide enough focus for over areal average QPF of 2 to 2.5 inches from northern VA across DC to central MD (including Baltimore City) and east across the Delmarva Peninsula. Given the generally uni-directional flow roughly parallel to the low-level boundary, and model consensus on increased rainfall, a Day 2 (12Z Sun-12Z Mon) Slight Risk for excessive rainfall was raised across this area. The maximum guidance for QPF in Day 2 is the 12Z GEM-regional which averages around 5 inches across this swath in what appears to be a predecessor rainfall event on the north side of the tropical airmass. This area will continue to be monitored, particularly given the wet antecedent conditions over the past week as well as ongoing activity tonight. ...Intermountain West, Northern Rockies, and Northern Plains... An upper level low across the Intermountain West and Great Basin should maintain a chance for scattered convection through Sunday/Day 2, likely concentrated near terrain features, before the low devolves into a trough and kicks into the Plains by Day 3. QPF was based on the 12Z ECMWF/GFS/NAM. The Slight Risk for excessive rainfall for Day 2 (12Z Sun-12Z Mon) was expanded a bit over Wyoming and South Dakota. Strong ENE low-level inflow should create both convergence and possibly some orographic ascent, and models generally agree on PWATs over 1 inch being advected into the area in this band of low-level inflow. Additionally, upper level divergence should be enhanced by coupled jet structure just northeast of the upper level low. Given the strong synoptic scale forcing and PWATs around the 95th percentile for late May, there seems to be sufficient threat of flash flooding to maintain the Slight Risk. Over an inch of QPF is forecast over southeast Montana and the Black Hills on Day 2. This area shifts east to the northern plains for Day 3 (12Z Mon-12Z Tue) and the Marginal Risk for excessive rainfall was maintained. Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml