Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 535 PM EDT Sat Mar 31 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 01/0000 UTC thru Apr 04/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 Update Minor adjustments, focusing mainly on the overnight to early morning periods, were made based on recent runs of the HRRR and ensemble guidance; but overall, no significant changes were made to the preliminary Day 1 package. Pereira Day 1 ...Southern Plains into the Lower MS Valley and TN/OH Valley... Showers and embedded thunderstorms will continue to build south along the cold front into much of central and northern AR into eastern OK and far northeast TX. Would expect to see coverage increase by 0z as low level convergence along the front increases. Based on the 12z set of high res models, WPC did trend higher with QPF amounts from far northeast TX northeast into western KY. The consensus is for a broad swath of 0.5"+, with amounts locally as high as 1-2". From northern AR into Ky instability is pretty meager...thus while the duration of showers here may be a bit longer than further south...rates should generally not be high enough to promote much of a flash flood threat. Further to the southwest...from far northeast TX into western AR...instability is higher...but even here it is progged to remain under 1000 J/KG. With the slightly better instability...rainfall rates should be locally higher here...although in general storms are expected to remain pretty progressive...limiting any flooding threat. This is supported by the 12Z HREF...which shows high probabilities of exceeding 1"...but only very low chances of isolated 2" amounts. The fact that some of these areas have been anomalously wet as of late suggests the flash flooding threat is non zero in any more susceptible locations...however think the threat remains below 5% and thus will continue with no marginal risk area. WPC QPF across this area leaned towards the 12z high res runs...with more weighting given to the ARW and ARW2 which appeared to show a more realistic evolution of convection. ...Lee of the Central Rockies into the Central Plains... A wave ejecting east out of the Rockies will support increasing isentropic lift ahead of it...with decent mid level frontogenesis also noted. Thus should see a band of locally heavy snow from southeast WY southeast across NE. Overall models are in pretty good agreement (better agreement than the band early this morning over the NP/Upper MS Valley)...although some minor north/south axis differences remain. WPC QPF ends up closest to the 12z HREF blended mean...which seemed like a good middle ground solution. ...Northwest... Showers will return to the Pacific Northwest during the day Sunday as a good amount of mid/upper level energy drops southeast into the area. For the day 1 period models have trended wetter on recent runs...and thus WPC went in this direction as well. Given the relatively good agreement seen...a multi model blend was used. Days 2 & 3 ...Pacific Northwest... A potent shortwave trough over the Salish Sea Sunday evening shifts southeast to WY by Monday night. Continued onshore flow both ahead and behind this wave will bring quarter to half inch PW into the Washington/Oregon coast with locally moderate Coast Range and Cascade precip Sunday night, tapering off Monday. Good agreement among the models warranted reusing the 12Z GFS/GEFS/ECMWF/UKMET blend for the QPF that was used elsewhere. A plume of tropically sourced moisture approaches the PAC NW coast late Tuesday. Rain onset is somewhat uncertain, but is by Tuesday evening according to the 12Z GFS/UKMET which are more progressive than the 12Z ECMWF. ...Northern and Central Rockies Eastward Across the Plains to the Great Lakes... A potent shortwave trough pushes southeast from Washington Sunday night to WY by Monday night, turning east and reaching the upper Midwest Tuesday. Broad surface low development over the central high plains with zonal jet support will bring a broad swath of precip from the High Plains adjacent to the Central and Northern Rockies eastward to Michigan through Monday night. The surface low center then tracks northeast across the Midwest to Michigan Tuesday. Many details remain fuzzy for likely significant winter storm in the Day 3 time frame (00Z Tue-00Z Wed) across the Midwest. The 12Z ECMWF remains the slowest to eject the shortwave trough axis across the plains Monday night/Tuesday while the timing of the 12Z GFS and UKMET was closest. Therefore more weighting went to the 12Z GFS/GEFS and UKMET. The 12Z NAM is only slightly more progressive with the upper shortwave trough axis, but is more amplified and potent making its QPF much different with two separate swaths both ahead and behind the developing low as opposed to a more defined deformation zone apparent in the 12Z GFS/UKMET/ECMWF. The 12Z GEFS was included in the blend given the uncertainty and to damp the greater QPF in the operational GFS. ...Mid-Mississippi Valley/TN and OH Valleys/Mid-Atlantic/Northeast... The track of the weak shortwave trough/surface low pivoted a bit more to the northeast in the 12Z guidance which brings more QPF to NYC and southern New England (and less to Richmond). The 12Z NAM was disregarded for it having the heaviest and farthest north QPF from the major model suite. A difference in the swath axis of 0.5 inch QPF was noted between the 12Z GFS (south of the Mason-Dixon line) and the 12Z ECMWF (north). A compromise blend of the 12Z GFS/GEFS/ECMWF/UKMET was used. Given the weak mid-level forcing further changes can be anticipated over the coming day. A second round of precip is expected to sweep across and north of this area late Monday night through Tuesday on the warm front ahead of the Midwest low. One inch PW sweeping across this area in 50+kt SW flow is 2 to 2.5 standard deviations above the standardized anomaly. As noted in the section above, there is uncertainty with this low, so no excessive rainfall was entered as of this time. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml