Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 551 AM EDT Sun Apr 29 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 29/1200 UTC thru May 02/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northeast... The closed mid-upper low will slowly pivot across the eastern Great Lakes and Northeast Sunday-Sunday night, with the large-scale forcing strengthening as a result of the increased mid-layer deformation/frontogenesis. This will support widespread mod-heavy precipitation along a well-defined comma head within a deepening trowal zone from northern NY state into northwestern New England. Models have trended wetter across these regions, and are in reasonable agreement with respect to the location of heaviest precipitation (widespread areal-average 24 hour totals between 0.50-1.25+ inches). Meanwhile, the low thicknesses and favorable column cooling due to the pcpn intensity will support rains changing over to snow across the Adirondacks today. Please see the latest QPFHSD for further details regarding the snowfall potential. ...Pacific Northwest / Central-Northern Great Basin / Northern Rockies... The closed mid level low across the PAC NW early Sunday will move slowly across the northern-central Great Basin through Sunday night, becoming an open trough in the process. Broad deep-layer Qs vector convergence with the difluent upper flow will allow for widespread light-moderate orographically-aided pcpn totals, with the model consensus for the highest totals north of this closed low where comma head precip may pivot from just north of the U.S./Canadian borer across southern B.C. into southwest Alberta (also along the axis of above-average PW values). To the south of this, showery conditions will persist through the Pacific Northwest---northern Great Basin into the Northern Rockies. ...Central to Southern Rockies into the Southern High Plains... A broadly upper difluent flow pattern will spread downstream of the inland moving closed low across the northwest U.S. through the Central to Southern Rockies and into the Southern High Plains. This will support widely scattered showers Sunday afternoon and evening across the central and southern High Plains along the dry line/lee trof position. High-res models are more emphatic about locally moderate to heavy precip totals along the dry line/lee trof Sunday afternoon, although confidence remains low with placement. Moreover, areal-average totals will be mitigated somewhat by the limited coverage. Days 2/3... ...West Coast... A shortwave low will shift south off the PAC NW coast around the mean trough Sunday night through Monday. Half inch PW ahead of the low will aid light to locally moderate rain for the Coast Ranges of WA/OR on Day 2. Flow remains parallel or offshore for late Day 2 and during the first half of the Day 3 forecast which limits light precip to the Sierra Nevada and southern CA coast ranges. ...Northern Rockies and Upper Midwest... A newly opened upper trough will shift east from the northern Intermountain West before the start of the Day 2 forecast on Monday morning. Height falls associated with this wave will be spreading east across the northern Plains on Monday Model guidance still wants to eject a plethora of small shortwaves across the central and northern plains during the period ahead of the main trough axis. As a result, the models continue to struggle since it is an inherently difficult pattern in which to accurately time individual shortwaves. One area of precipitation...with some enhancement tied to low/mid level Frontogenesis is expected across the far northern High Plains around the back side of developing low pressure east of the northern Rockies in upslope/northeast flow. Farther off to the east, a well-defined cold front will be taking shape and then beginning to push east across the Dakotas. The front will interact with strengthening southerly low level jet which will enhance warm advection and moisture return. Precipitable water values will initially be very modest, which explains why the WPC QPF was on the low end on day 2. But the precipitable water values increase to near or over 1.25 inch by Monday across the northern Plains and Midwest. This will allow for some convection to likely develop along or just behind the cold front given a more favorable thermodynamics environment. Despite some differences in the mass fields, there was some agreement on the location of surface moisture flux convergence maxima and the magnitude of moisture transport vectors over the southeast corner of Nebraska and southwest Iowa which WPC used to place the rainfall max on day 3. Given the amount of instability involved, think that there could some intense rainfall rates capable of producing flooding over this area. As a result, hoisted a slight risk of excessive rainfall on day 3 with a marginal risk generally focused in the unstable airmass along and ahead of the front. ...Central/Southern Rockies to the High Plains... Rainfall amounts should be generally light given higher precipitable water values will be sequestered farther east with low level southwesterly flow developing as the shortwave pushes down the CA coast Monday night. ...Northeast... An occluded low will slowly weaken as it drifts east across New England Sunday night through Monday. Generally light amounts of precipitation will be lingering across the area...although spotty moderate amounts are possible...until the system pulls far enough out to sea. Hurley/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml