Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 433 PM EDT Sat Apr 28 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 29/0000 UTC thru May 02/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northeast... The mid to upper level trof moving through the Great Lakes region this afternoon will continue to strengthen into a well defined closed low moving from the eastern Great Lakes across the northern Mid-Atlantic tonight and into eastern NY/western New England during the day on Sunday. Large scale lift will strengthen to the north of this amplifying closed low---supporting an increasingly well defined comma head/deformation precipitation region affecting northern NY state into northwestern New England. Model consensus is for moderate to locally heavy precip totals across these regions. The lowering heights associated with the deepening mid to upper low will support rains changing over to snow across the Adirondacks late tonight into Sunday with generally light snowfall accumulations expected, Precipitation along the trailing cold front forecast to sweep east off the Mid-Atlantic coast tonight and through New England Sunday will be more progressive than the comma head/deformation area---limiting precip totals to the light to moderate side. ...Pacific Northwest---northern Great Basin into the Northern Rockies... The closed low along the Oregon coast this afternoon will be moving inland tonight through the Pacific Northwest and into the northern Great Basin-Northern Rockies region Sunday. Model consensus for the greatest precip totals is to the 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. 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 at the beginning of the day 1 period over the Central to Southern Rockies and then again Sunday afternoon farther east into the Southern High Plains along the dry line/lee trof position. Light to locally moderate precip totals possible through the Central to Southern Rockies. Hi-res models are more emphatic about locally moderate to heavy precip totals along the dry line/lee trof Sunday afternoon--although confidence is low with placement. 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 Day 3 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 Sunday night with height falls spreading east across the northern Plains. This was well said before and it remains true, model guidance still likes the idea of a plethora of small scale vort centers ejecting out of the base of the trough through the period and downwind across the central/northern Plains and the upper Midwest ahead of the main trough axis...which is an inherently difficult pattern to accurately time individual shortwaves. Frontogenetically enhanced precipitation is expected by late Sunday and Monday across the far northern High Plains around the back side of developing low pressure east of the northern Rockies. Farther off to the east, there will be a well-defined cold front beginning to push east across the Dakotas, and this will interact with strengthening southerly low level jet which will enhance warm advection and moisture return. Precipitable water values will initially be very modest, but will increase to near or over 1 inch by Sunday night and 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. A non-GFS general model blend was used for QPF across this area. The 12Z GFS suffered from convective feedback issues and was not used due to decent agreement among the 12Z ECMWF/NAM/UKMET. ...Central/Southern Rockies to the High Plains... Anticyclonic flow over the central CONUS ahead of the upstream upper low crossing the northwest will allow increasing moisture across central Colorado to New Mexico as weak shortwave energy embedded within difluent flow arrives out ahead of approaching height falls. The result will be some diurnally enhanced and orographically focused convection over the central and southern High Plains region. 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. Rain, moderate at times will wrap into the system from the Atlantic with wet snow (particularly at higher elevations) on the west side over NY state under the cold core upper low. Good agreement among models allowed a general model blend for QPF with the 12Z ECMWF/NAM given the greatest weight. Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml