Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 602 PM EDT Fri Apr 27 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 28/0000 UTC thru May 01/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...South Florida... The convective activity enhancing across south Florida Friday afternoon will likely be weakening by the beginning of the upcoming day 1 time period...0000 UTC Saturday...and pressing to the south and east of south Florida. Model consensus is for the axis of the heaviest precip to be to the south of south Florida and the Keys---with the northern edge of the precip shield bringing moderate precip potential to south Florida. ...Eastern New England... The surface wave along the northern Mid-Atlantic coast this afternoon will continue to move quickly northeastward this evening along the New England coast and into southeast Canada. The area of moderate to heavy rains ahead of this wave will primarily affect coastal to eastern Maine early in the upcoming day 1 period---supporting .25-.50" areal average amounts. ...Great Lakes into northern NY state... In the wake of the wave affecting eastern New England---the upstream large scale trof will continue to amplify as it pushes southeast across the Great Lakes tonight into Saturday. Two distinct surface waves expected to rotate through this amplifying trof---helping to concentrate moderate precip totals ahead of each wave. A lead wave will move northeast from the eastern Great Lakes into northern NY state and southern QB...followed by a second wave pushing southeast from the lower Great Lakes toward the Central Appalachians. A stripe of moderate precip totals in the .10-.25"+ range possible from the lower to eastern Great Lakes region into northern NY state associated with each wave. ...Northern California...Pacific Northwest---northern Great Basin into the Northern Rockies... The latest model guidance is showing good agreement with the east northeast push of the well defined closed low off the California/Oregon coast this afternoon into western Oregon during Saturday. A broad region of showery conditions likely to the east and northeast of this closed low from northern California---through the Pacific Northwest---northern Great Basin and Northern Rockies region. Model consensus for max totals expected over northern California where the onshore southwest flow will have the best upslope component and over Washington from the Cascades to the coast range where a west to east oriented precip band to the north of the closed low is expected to pivot during Saturday. In both regions---.25-50"+ areal average amounts depicted with heavier localized totals likely in the favored terrain regions. ...Northeast Mexico into far South Texas Southern stream shortwave energy expected to push across far northeast Mexico and far south TX early Saturday. Shower activity ahead of these height falls expected to primarily affect far northeast TX where the axis of much above average pw values will persist day 1. The northern edge of this precip may affect far south TX with light to moderate totals---with the better chance of any heavy rain amounts to the south of the U.S. border. Days 2/3... ...Northeast... Shortwave energy already focusing up across the northeastern CONUS will be absorbed by the arrival of a stronger northern stream trough arriving from the Great Lakes. The result will be a well-defined closed low that will slow down and meander over the Northeast through Sunday and Monday. A combination of relatively steep mid level lapse rates, DPVA, Atlantic moisture, and local orographics will foster numerous showers across the interior of the Northeast and especially northern New England where some locally heavy banded rainfall is possible. Some locally heavy amounts cannot be ruled out across the Adirondacks and the Green and White mountains as well. Multi-day totals of 1 to 2 inches will be possible locally. A multi-model consensus weighted toward the GFS and ECMWF was used for QPF with adjustments for topography. ...Northwest to Northern Plains and Upper Midwest... An upper low will drift eastward across the Northwest U.S. on Saturday and then begin to open up into a more progressive trough axis through Saturday night and Sunday with the height falls spreading east across the northern Rockies and well out across the northern Plains. The model guidance again 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. Widespread showers are expected across the Northwest and over the northern Rockies in association with the arrival of the upper low owing to moist mid levels, steep lapse rates and upslope flow. More 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. The convection could tend to be locally organized given the expectation that some of the upstream vort energy rounding the base of the trough will also be lifting northeast up across this region, but for now, the rainfall amounts are not expected to be particularly heavy. A multi global model consensus was used for QPF with perhaps a bit more weight toward the GFS. ...Central/Southern Rockies to the High Plains... Anticyclonic flow over the central CONUS ahead of the upstream upper low entering the West 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 high terrain of the central and southern Rockies and out across the central and southern High Plains region. Rainfall amounts should be generally light given rather low precipitable water values. Oravec/Orrison Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml