Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 448 AM EDT Tue May 01 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 01/1200 UTC thru May 04/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Lower Missouri Valley into the Upper Midwest... Shortwave energy over the eastern Pacific will pivot southeastward toward the base of the longwave western U.S. trough, and in doing will further amplify the trough as height falls push southward across Baja and northwest Mexico while a mid-upper low closes off across the desert SW. While there continues to be consensus among the models with respect to the longwave pattern, spatial and timing differences continue with respect to the convectively-aided shortwaves ejecting northeast of the trough and along the northern periphery of the downstream upper ridge. Meanwhile, as separate northern stream trough across western Canada begins to impinge the the aforementioned upper ridge late in the period (Tue night) across the upper MS Valley, the increased confluence aloft will generate a 90-100+ kt upper level jet streak across far northern portions of the Upper Midwest into southern Canada. All of the models are picking up on these favorable dynamical signals for heavy rainfall ahead of the MCVs and into the favorable right entrance region of the aforementioned jet streak; however, the bi-modal distribution of QPF maxima from some of the models, along with the typical warm-season latitudinal differences among the operational guidance (especially global vs. high res) is once again making for a low-confidence QPF in terms of the timing/location details. Believe the bi-modal distribution per much of the guidance is indicative of the MCVs not entirely coupled within the most favorable right entrance region upper jet forcing. Thus a rather broad upper divergence signature depicted, with two distinct areas of stronger deep-layer ascent noted -- one farther south across the lower MO Valley associated on the southern periphery of the MCV and resultant outflow boundaries displaced farther s-se of the main surface front, with the other one farther north over portions of the upper Midwest/MS Valley where the favorable upper jet forcing will more of a factor. WPC noted the heaviest QPF along the former (southern) axis -- at the apex of a 50+ kt low-level jet where greater (untapped) deep-layer instability will be present. ...Western U.S.... The amplifying upper trough will generate a broad area of light-moderate QPF -- which given the degree of forcing (broad area of favorable upper level difluence) -- will not be linked exclusively to the elevated terrain. Highest areal-average totals (0.50+ inch) were noted across the southern NV mountains into the Wasatch and Uintas, as well as along the San Juan and Laramie Mtns. Days 2/3... ...Western United States... An upper trough that was amplifying over the western U.S. on Day 1 should generally be located over the southern Great Basin at the start of the Day 2 period. The precipitation shield will be widespread from Utah and the southern half of Wyoming southward to the Mogollon Rim in Arizona eastward into New Mexico at the start of the forecast period. A closed low should begin to fill as the system makes its way across the Rockies and the Front Range. Even though the best dynamics will be emerging over the Western High Plains, precipitation will be slow to taper off given the amount of low level upslope flow north of the surface/H85 low that develops. The model QPF in this part of the country varied quite a bit from model to model and from run to run of each model. The WPC QPF tended to put more weight on the higher resolution runs of the GFS and ECMWF as well as the NAM...taking cues from the WPC pseudo bias corrected ensemble QPF as well for placement. Confidence in the details of the forecast was low, especially as the system crosses the Rockies. ...Central Plains to southern Great Lakes... The amplified trough mentioned above will be making its way from the Western High Plains to the Central Plains on Wednesday or early Thursday. As it does so, it will interact with a stationary front. This provides a focus for moisture convergence and instability (1.25 PW is 2 standard deviations above normal) near the borders of NE/IA/MO. Maintained a slight and marginal risk for excessive rainfall mainly over parts of KS/NE/IA and extended it as far east as the southern Great Lakes region on Day 2 (12Z Wednesday to 12Z Thursday). The WPC and model QPF were approaching 2 inches in places...and there are areas of lower flash flood guidance in the Great Lakes region. QPF was based on a blend of the 01/00Z GFS/ECMWF with some inclusion of the NAM for both days and hi-res models like the ARW2 for Tuesday night. ...Southern Plains... Continued a marginal risk on day 2 for west TX from the Rio Grande to the Red River. An ejecting upper low over the desert SW will allow surface low pressure to persist over northeastern NM/the TX panhandle with a dry line shifting east across west TX. The precipitable water value of 1.5 inch streaming northward from the Gulf is about 2 standard deviations above normal for west TX which along with ample instability allows 1 to 2 inch areal average QPF in thunderstorms particularly Wednesday evening. ...Great Lakes to Northeast... A plume of Gulf moisture shifts east from the central plains on southwest flow ahead of the system making its way out of the Southwest. Precipitable water values of 1.25 inch will be widespread across the Great Lakes and into the Northeast. This is some 2 to 2.5 standard deviations above climatology with moderate to locally rainfall expected. The forecast problem in this part of the country was how much interaction will occur between the synoptic scale system moving through the Great Lakes and a shortwave trough approaching from the northwest out of Canada. The 30/12Z ECMWF allowed for far more interaction between energy in the two streams while the 30/18Z ECMWF was flatter. There was some convergence of the two solution between the 00Z run of the GFS and ECMWF while the NAM was much flatter with an axis displaced north. WPC aimed to be between the 00Z GFS/ECMWF without much weight given to the NAM/CMC or UKMET. In the new Day 3 Excessive Rainfall Outlook, valid 12Z Thursday through 12Z Friday, we essentially extended the marginal risk eastward from Day 2. The global models had signals for 1 to 2 inches of rain in the period and enough instability to suggest that rainfall rates will be capable of short term flooding problems. In fact, the 01/00Z run of the GFS increased its 24 hour QPF to between 2.0 to 2.5 inches. While the WPC deterministic rainfall amounts were not as high as this, some amounts this high are certainly possible especially from the most intense convection. Uncertainly about where the axis of heaviest rainfall sets up and fairly high 1- and 3-hour flash flood guidance were the primary reasons for remaining with a marginal risk at this point. Hurley/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml