Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 449 AM EDT Tue May 29 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 29/1200 UTC thru Jun 01/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Alberto / Southeast U.S. / Ohio-Mississippi Confluence... The Alberto track forecast was generally doing well, and not changing in any dramatic way. The inherited QPF had this system handled pretty well - we merely enhanced amounts just a bit near the center as it tracks toward western Tennessee, but we also did trend a little westward with the QPF swath there. The hi-res models, in particular, want to deflect the more intense rainfall west of the circulation center. This seems a little counter-intuitive, although with southerly shear the stronger echoes can advect north of the center and get drawn back to the W or SW. We handled the trend by using 40 percent continuity and 20 percent each of the HREF blended mean, 00z NAM, and WRF-NMMB. The NMMB has done well this spring, and represented a less stark contrast to the continuity forecast. Elsewhere there is concern that the confluent moist inflow south of the circulation will overlap with areas in Alabama which experienced the core rains on Monday, leading to particularly high 2-day totals which may warrant maintenance of a Moderate Risk of excessive rainfall there. Moderate Risk will likely be included up along the center track - pending collaboration with the Field, and also maintained over through the upslope areas in the Carolinas. The QPF in the higher terrain did not change much, but we did split the precipitation over central and eastern North Carolina as the conveyor belt is shed from the cyclone, and model QPFs trended downward. ...Central U.S... The MCS in northwest Oklahoma will have a hard time making eastward progress into an area of large inhibition this morning. Expect the next round to have more success Tuesday afternoon as the western U.S. trough punches out into a negative tilt and acts to cool the mid levels north of I-40. WPC expects an MCS track somewhere across central to southeast KS and northern OK Tue night, leaning on the GFS, ECMWF, and WRF-ARW. We spread generous coverage of convective QPF northward along the instability axis to Minnesota, and also within the secondary frontal zone that hangs back over the High Plains and Dakotas. Overall the QPF in this region leaned toward the GFS, WRF-ARW, and WRF-NMMB. The Canadian GEM Regional also gave support. Days 2/3... ...Great Lakes, Ohio Valley, and Mid Atlantic... Whatever surface low that remains from Alberto will advance from the Ohio Valley into the Great Lakes on Wednesday, eventually getting absorbed in broader pressure falls associated with a trough advancing east from the Northern Plains. This should allow the QPF pattern to transition from a more focused corridor of heavier rainfall during the day on Wednesday, to broader moderate amounts by Wednesday Night before the low continues pushing northeast into Canada on Thursday. QPF generally followed a broad model blend with Alberto and its eventual remnants, with greatest weight placed on the GFS and ECMWF. Away from the track of the center of Alberto, rain should be far more convective in nature. With a lack of a strong focusing mechanism, but a broad area of tropical moisture, there should be scattered convection over most of the region. The focus was putting in a broad areal average of about 0.25 to 0.50 inches of rain over most of the region, and a broad model blend generally delivered those amounts with some minor adjustments. With a lack of a strong low-level focusing mechanism (other than Alberto's center), the Excessive Rainfall Outlook for Day 2 shows a broad Slight Risk for most of the region. Although rain may be focused more along the track of the low, flash flood guidance is higher in those areas suggesting that antecedent conditions may prevent more widespread flooding. There is concern about flash flood potential in the broad warm sector to the east (OH, WV, VA) where convective instability should build through the day on Wednesday with anomalously high precipitable water values around or above the 99th percentile for late May and early June. Such environments typically support very heavy rain rates with convection, and the potential for more significant episodes of flash flooding. However, given the lack of distinct focusing mechanisms, pinpointing the highest threat areas is somewhat uncertain at the moment. Therefore, a Slight Risk is maintained across this region with the possibility of an upgrade at a later time if a more concentrated area of convection seems likely. Please refer to the National Hurricane Center forecasts for the latest information on Alberto. ...Southeast and Florida Peninsula... The portion of the Southeast to the east of 86-87W longitude should remain within a plume of deeper tropical moisture with PWATS generally above 1.8 inches. These are less anomalous relative to climatology than points further north, but should still be supportive of heavy rain rates and localized flash flooding. A Marginal Risk was extended down into parts of the region for the Day 2 Excessive Rainfall Outlook. Rainfall amounts are generally close to the NBM, but were enhanced in the 18-06Z time frame down the spine of the Florida Peninsula and up into south Georgia where converging sea breeze boundaries in the very moist environment may support a greater concentration of convection. ...Central Plains into the Ozarks... Models show an expanding low-mid level thermal ridge into the southern Plains over the next several days, with sporadic QPF along the periphery of the elevated mixed layer from eastern Colorado, through Kansas, and into the Ozarks and Mid Mississippi Valley regions. From a pattern recognition standpoint, this seems reasonable and also provides greater than normal uncertainty in the QPF. Models can struggle with the timing of MCSs as they roll in a generally easterly direction around the periphery of a ridge, and also struggle with how much latitude they can lose as they mature and propagate further into the higher thetae air mass. Generally followed a compromise of the ECMWF and GFS in terms of timing and position of QPF maxima in this region, with a marked increase in nocturnal QPF relative to the NBM. ...Northern Rockies... An amplified trough pushing into the Northwest and eventually the northern Rockies should lead to increasingly widespread convection, especially by the Day 3 period from Idaho into Montana. Slightly different timing produces different results in the QPF with convection initiating on terrain features and low-level boundaries at different times. Generally followed a blend of the GFS and ECMWF in this region, and weighted the QPF maxima toward the GFS in magnitude given the ECMWF typical dry bias. Given the very wet antecedent conditions across the region, flash flooding will be a threat, and a Slight Risk was introduced on Day 3 across Montana. Burke/Lamers Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml