Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 429 PM EDT Tue Apr 24 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 25/0000 UTC thru Apr 28/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Eastern U.S... An active closed low steadily/gradually ejects from TN/KY to over the s-central Mid-Atlantic today into Wednesday before shifting offshore under increasing influence from nrn stream trough energy digging into the Great Lakes. GOES-16 and blended TPW loop imagery leads well into a model guidance composite that continues to depict a plume of deep layered moisture with low latitude connection feeding into the associated surface low/system with lead thetae advection especially into terrain and coastal areas along the warm front and inverted coastal trough. Expect widespread enhanced rains/convection to spread from the Ohio Valley/Appalachians and srn Mid-Atlantic northward across the nrn Mid-Atlantic and Northeast with system progression. The linearity of the forecast mass field evolution lends a WPC day1 qpf forecast derivation mainly from a blend heavy on continuity but incorporating latest model and radar rainfall/convective trends that seem well represented from a WPC in-house bias corrected qpf, HRRR and National Blend of Models. A Marginal Risk of excessive rainfall remains centered over the central Appalachians/Mid-Atlantic. ...Central U.S... The next in a series of fairly vigorous mid level lows will remain well defined as it drops south to southeastward through the Plains today to the lee of an amplifying wrn u.s. mid-upper level ridge. Given relatively scant moisture in the wake of the lead system, expect most precipitation will be slightly post-frontal and rooted in strong mid level ascent from South Dakota down through Kansas, and eventually parts of TX/OK and mid-MS Valley. There may be quite a bit of mesoscale influence to the precipitation pattern, given the steep lapse rates, marginal low level moisture, and interplay between the synoptics and terrain influences. Model guidance remains in decent overall agreement for such an environment and WPC took a consensus/continuity approach, albeit with modest areal coverage. Days 2/3... ...Pacific Northwest... Low pressure off the OR/CA border shifts east to that border Friday which wraps Pacific moisture across far northern CA, up the Willamette Valley, and off the western WA shore. The GFS continues to have higher PW (around one inch) than the ECMWF (around 0.75 inch) which results in more QPF for the GFS. A general model blend gave a Day 3 (00Z Fri-00Z Sat) areal average QPF around 0.75 inch. ...Central and Southern Great Plains to the Southeast... A pair of shortwave troughs will swing east from the central plains and southeast Wednesday night through Friday. An area of one inch PW ahead of the first trough will bring an areal average of an inch from roughly Memphis to Atlanta on Day 2 (00Z Thu-00Z Fri). Uncertainty remains which the track of the lead trough which is discussed in the section below. PW is a little less for the second trough (around 0.75 inch) as it is associated with more continental air than Gulf moisture. QPF for Day 3 (00Z Fri-00Z Sat) is generally a quarter to half inch from KS/OK...the Ozarks...to southern AL. WPC QPF generally followed a general model blend including the 12Z GEFS through both days. ...Mid-Atlantic to the Northeast... Low pressure shifts across the northeastern CONUS Wednesday night through Thursday. Gulf stream moisture flowing into this system brings 1 inch PW to the New England coast which is 2 standard deviations above normal. A pivot from an east direction from the eastern Great Lakes Wednesday evening to northeast across interior Maine Wednesday night allows a slightly longer period of heavy rainfall for coastal Maine. A marginal risk for excessive rainfall for the Day 2 ERO (12Z Wed-12Z Thu) was maintained which a slight draw northeast per 12Z guidance consensus. Great uncertainty persists with the Day 3 QPF forecast (00Z Fri-00Z Sat) with NCEP guidance (NAM/GFS) much farther inland with the shortwave trough lifting from the southeast while non-NCEP (ECMWF/CMC/UKMET) are much farther east along the Mid-Atlantic coast. Some narrowing of the difference was noted in the 12Z guidance, so a general model blend with usage of the 12Z GEFS was taken for the Day 3 QPF. Schichtel/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml