Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 532 PM EDT Fri May 25 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 26/0000 UTC thru May 29/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ..Northern California---Great Basin into the Northern Rockies... There continues to be good model agreement with the inland push of the strong closed low off the north central California coast this morning---inland across northern to central California tonight and into the Great Basin Saturday. An active precip pattern likely to the north and northeast of this closed low in a region of well defined upper difluence from northern California---southern to eastern Oregon---central to northern NV--much of ID into northwest MT. PW values across these regions will remain 1-2 standard deviations above the mean---supporting potential for widespread moderate to locally heavy precip totals. WPC qpf did not deviate from the model consensus of the heaviest precip potential from northeast California into eastern Oregon---far northwest NV and western ID. A broad marginal risk area was maintained in this max qpf area in the Excessive Rainfall Outlook. ...South Florida.. Showers likely to re-develop tonight across south Florida as tropical moisture continues to be drawn northward on the eastern side of Alberto. This will be the beginning of a multi-day heavy precip event for South Florida---expanding farther to the north after day 1 into central to north Florida. With pw values rising to 2-3 standard deviations above the mean---confidence is high on additional heavy precip totals this period from the Keys into South Florida. See the latest NHC advisories for additional updates on this system. ...MS Valley eastward to the east coast... A broad elongated mid to upper level trof will become established thru the MS Valley this period. With pw values along and to the east of this trof expected to remain above average this period---widespread scattered convection likely. Confidence is rather low with the details of qpf across most areas from the MS Valley eastward thru the Great Lakes and south across the eastern third of the nation. the href and in house hi res means used primarily to mitigate the range of qpf solutions offered in the latest guidance. Areal average moderate precip totals depicted with the likelihood of locally heavy totals where convection does organize. ...Southern Ontario---southern QB---northern NY state into northern New England... A stationary front is expected to remain draped west to east from north of the Upper Lakes---eastward across southern QB and into northern New England. Showers should remain active along this boundary day 1 where pw values will pool to much above average levels. Areas near this boundary from northern NY state into northern New England will possibly see multiple bands of precip---with moderate to locally heavy precip values likely across these areas Days 2/3... ...Southeast and southern Mid-Atlantic... Overall, forecast remains much the same with widespread moderate to heavy rainfall expected to spread across the region as Subtropical Storm Alberto tracks north through the eastern Gulf of Mexico this weekend. Forecast rainfall uncertainty again remains largely tied to the track of Alberto, with the models continuing to show a fair amount of spread as the system approaches the Gulf Coast late Sun-Mon. WPC QPF reflects a solution close to center of the model envelope, similar to a GFS/ECMWF compromise, with heavy amounts approaching the coast in the vicinity of Mobile Bay on Monday. Regardless of the track specifics, models are in generally good agreement showing moderate to heavy amounts spreading north, well to the northeast of the center and along an axis of very moist southerly flow. Models show PWs in excess of 2 inches spreading north from the Florida peninsula Sunday morning into Georgia and South Carolina later in day, surging into eastern North Carolina by early Monday. This is expected to support periods of moderate to heavy rains across the region, with the models offering a good signal for heavy amounts along the Georgia-Carolina coasts as this moisture interacts with a weak wave and area of enhanced upper divergence moving north along the coast on Monday. Here too WPC QPF generally followed a blend of the GFS and ECMWF. WPC Excessive Rainfall Outlooks continue to show a northward expanding Slight Risk from South Florida on Day 2, into portions of Georgia, the Carolinas and Mississippi on Day 3. Refer to NHC outlooks for further guidance regarding Alberto. ...Northeast/Mid-Atlantic... A weak wave moving east along a lingering boundary and corresponding axis of deep moisture is expected to provide the focus for moderate to heavy amounts early in the period. A weak wave is expected to support heavy amounts as it propagates east from the northern Mid-Atlantic region into the western Atlantic south of Long Island and southern New England on Sunday. ...Intermountain West, Northern Rockies, and Northern Plains... A closed low centered over the Great Basin Sunday morning is expected to gradually lift to the northeast into the northern Rockies on Monday. Deformation aloft along with low level frontogenesis/convergence is expected to support widespread light to locally heavier amounts from the High Plains back into the northern Rockies and Great Basin on Sunday. As the low lifts further to the northeast on Monday, amplifying low level easterly flow is expected to tap deeper moisture, drawing it back across the northern Plains back into the northern Rockies. This moist, upslope flow interacting with favorable forcing is expected to generate an increasing potential for heavy amounts from southeast Montana and northeast Wyoming into the Dakotas. Given the wet antecedent conditions, a Slight Risk was introduced into the Day 3 Outlook across portions of south-central to southeast Montana and far northern Wyoming. WPC QPF generally followed a blend of the GFS and ECMWF through late Sunday, with less weight given to the GFS thereafter. The GFS develops heavier amounts further to the south across across Wyoming. These may be the product of feedback issues and at this point has little support from the other models, so more weight was given to the ECMWF for the Day 3 forecast. Oravec/Pereira Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml