Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 508 AM EDT Fri Jun 01 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Jun 01/1200 UTC thru Jun 04/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr ...Central-Northern Plains... A well-defined shortwave trough will continue to translate east, assuming a negative tilt as it pivots across the northern Rockies and into the central-northern Plains. Favorable upper level forcing (robust upper difluence and resultant deep-layer Qs vector forcing) will result in highly-anomalous 850-700 mb moisture flux into the central and especially northern Plains on Friday. PW values (1.5+ inches) will be equally as impressive, averaging about 2-2.5 standard deviations above normal. By afternoon, 700-500 mb lapse rates between 8-8.5 c/km will foster vigorous deep-layer instability (MUCAPES 2000-3000+ j/kg). The 00Z guidance, especially the high-res CAMs, continue to show a signal for heavy (potentially excessive) rainfall across ND (especially central ND) late this afternoon into the evening as the southerly low-level inflow increases to 50+ kts. The high-res simulated reflectivity in fact shows the convection growing upscale (QLCS) while the eastward progression begins to slow down a bit during the evening (corresponding to the strengthening LLJ), which owing to the strength of the upwind propagation, would lead to a greater potential for cell training. As a result, WPC will maintain a Slight risk in the day 1 ERO, which is supported by the 00Z HREF probabilities of 3 hourly QPF exceeding FFG (50-70+ percent across western-central ND between 21-03Z). Farther south -- the guidance also depicts a heavy rainfall footprint across portions of the central Plains into the lower MO Valley -- especially from eastern NE-northeast KS into northwest MO. This area of convection is expected to be triggered by the compact upper jet streak/associated vort lobe that will traverse the 4 Corners area this morning and lift into the central high Plains this evening. In this region, also expect rather robust deep-layer instability (again largely due to the steep mid-level lapse rates) as over this area MUCAPEs would be more likely to exceed 3000 j/kg per the models. The models show the southerly LLJ in this region not as strong compared to the northern Plains, while also denoting considerably more deep layer shear (both directional and speed shear). As a result expect more forward/downwind propagation than otherwise as the convection grows upscale during the peak late-day heating -- however given the strength of the thermodynamic parameters (instability and anomalous deep-layer moisture), the risk for 1-3 hour QPF>FFG exceedance will continue to warrant a Slight risk in the day 1 ERO. ...Eastern U.S.... Slow-moving, positively-tilted upper trough will nudge eastward toward the Atlantic coast by Saturday. Until then, a broad area of modest dynamical forcing east of the trough axis and surface front (weak upper ageostrophic divergence and deep layer Qs vector convergence) will favor scattered-widespread convection once again today, especially during the peak heating hours from late-afternoon into early evening, though also with an early onset (by early afternoon) given the absence of a capping inversion and thus relatively low convective temps. The relatively low 0-6 km bulk shear values again would support mostly limited upscale/organizational growth outside of pulse-type clusters with limited persistence. One exception would be over portions of the Deep South (northern AL-into northwest GA), where the high-res CAMs simulate more w-e oriented convection later this afternoon and evening, on the southern periphery of the mid-level vort lobe where steeper 700-500 mb lapse rates are expected to lie (between 7-8 c/km). The latest high-res projections show an uptick in the wsw low-level wind (20-25 kts at 850 mb) across this region, which given the wind profiles aloft support some upwind propagation/backbuilding and thus a heightened risk of cell training. WPC noted a Slight risk area across this particular area, which includes BHM, HSV, and RMG. Elsewhere to the n-ne, WPC included another more focused slight risk area across eastern portions of the OH Valley into the central Appalachians, Shenandoah Valley, Blue Ridge Mtns, and into parts of the Piedmont where 1/3/6 hourly FFG values remain quite low. Given the continued favorable deep subtropical moisture along with modest instability (tall/skinny CAPE distribution with equilibrium layers aoa 45 kft), the highly-efficient short-term rainfall rates, even less than 1 hour, could certainly eclipse these low 1 hourly FFG values despite the pulse nature of the convection. Days 2/3 ...Upper to Middle MS Valley... A mid-level trough over the Dakotas Saturday morning becomes negatively tilted as it shifts east to the Great Lakes through Saturday night. Strong upper dynamics ahead of this through in an axis of PW 2 standard deviations above the mean, confidence is fairly high for widespread moderate to heavy precipitation moving into the Upper to Middle MS Valley Saturday. Notable differences in the placement and strength of the upper trough persist in the 00Z model suite, including the number of and location for MCS activity. Instability decreases north while dynamics increase north. Maintained two Slight Risks of excessive rainfall for Day 2 (12Z Sat-12Z Sun) with a northwest shift in location per the 00Z GFS/NAM/ECMWF. ...Southern Plains to Gulf Coast... Low level forcing/a cold front pushes south through the Southern Plains and toward the Lower MS Valley Saturday. Strong instability (MUCAPE peaking around 00Z Sunday in the 3000-4000 j/kg range) takes advantage of Gulf moisture. Expect organized convective activity to shift from OK/AR Saturday to the Gulf Coast Sunday. Better upper level forcing north should keep the greater excessive rain risk north of this area both days. ...Southern to Central Appalachians---Mid-Atlantic... Slow moving negatively tilted trough crosses the eastern CONUS Saturday night through Sunday night. Slow motion of the mid level height falls and much above average pw values will persist both days from the Central Appalachians into the Mid-Atlantic---with additional moderate to heavy precip likely across these areas. A Slight excessive rainfall risk is maintained for the central Mid-Atlantic, particularly Saturday when instability will be greatest along and ahead of the southward moving west to east oriented front moving across these regions. Increasingly stable surface conditions day 3 across these areas as an overrunning regime sets up as low pressure stalls on the southern Mid-Atlantic coast. A Marginal Risk was raised for much of this region for Day 3 (12Z Sun-12Z Mon). ...Central to Southern Rockies... A cutoff upper low drifts northeast from Yuma Saturday. Desert inflow dominates with little to no QPF on Day 2. However, by Sunday the opening low crosses the four corners and begins to draw Gulf moisture toward CO. Modest instability with difluence aloft allows convective thunderstorms over the terrain. Northern NM looks preferred at this time with a Marginal Risk of excessive rainfall introduced for Day 3 (12Z Sun-12Z Mon). ...Pacific Northwest into the Northern Rockies... A shortwave trough swings around the base of a closed low off BC and comes ashore with 0.75 inch PW over WA late Sunday. Light to moderate rainfall is expected on Day 3 for the northern Cascades into northern ID per 00Z model consensus. Hurley/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml