Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 528 AM EDT Thu May 31 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 31/1200 UTC thru Jun 03/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr ...Lower-Mid Mississippi Valley into the lower Ohio and Tennessee Valleys... Ongoing MCS over the southern Plains early this morning (and ensuing MCV) will certainly have implications on the new day 1 QPF downstream into the lower-mid MS Valley-lower OH Valley, and TN Valley. While still showing a a good heavy rainfall footprint, the spread among the 00Z high-res CAM guidance (notably N-S) was less than desirable for a day 1 period. The WPC QPF initialized with continuity and a blend with the NBM and HREF mean, however evolved toward a NSSL-WRF/WRF-ARW2 combination late Thu and Thu night. Like the idea from these models of 2 main axes of higher totals, the first one farther south (across much of Arkansas) where the instability pool in the warm sector should be more than sufficient (while mid level capping not an inhibition) in supporting a developing QLCS south of the MCV. The other QPF max was noted across the mid MS Valley, including se MO, southern IL-IN, and western KY given the bolstered 850-700 mb moisture flux/theta-e transport out ahead of the MCV track. ...Lower Great Lakes / Upper Ohio Valley / Central Appalachians / Mid Atlantic... As the trough over the upper Midwest translates eastward this period, and the upper ridge axis pushes off the Atlantic coast, a northwest-southeast oriented boundary and the leading edge of a very moist airmass extending from the Mid-Atlantic from the Great Lakes is expected to move north this period. Guidance shows deep moisture surging north along the central Appalachians and Mid-Atlantic as the low level flow backs ahead of the approaching trough. This moisture along with afternoon heating and the larger scale forcing afforded by the approaching trough (broad-scale upper difluence) is expected to support and increasing threat for moderate to locally heavy amounts focused along the higher terrain and ahead of a cold front pushing across the lower Great Lakes. WPC QPF gave significant weighting to the HREF Mean through most of the period. ...Northern Intermountain West / Northern Rockies and High Plains... As an upper level trough continues to amplify across the Northwest, a developing upper jet streak across southern Alberta-Manitoba along with ample (anomalous) moisture along and east of the low-level front will support mid-heavy rainfall along the eastern slopes of the northern Rockies and spreading into the northern Plains. WPC QPF reflects a consensus of the hi-res models here as well. The shear profiles support more forward (downwind) propagation of convection than otherwise; however, given the antecedent wet soils (relatively low FFG across much of the region), areas that receive the heaviest rainfall may experience isolated short-term runoff issues, especially across much of central and western MT. Days 2/3... ...Plains/Midwest... The guidance suggests that a wet pattern is in store from the Dakotas/Nebraska eastward into the upper and middle Mississippi Valley in a very diffluent pattern. The highest amounts are likely to be within the southern portion of splayed height/thickness pattern near the NE/KS border eastward in the Mid-Mississippi Valley due to convective complexes hemmed in to the south by very warm 700 hPa temperatures/a mid-level capping inversion. To the north across the upper Mississippi Valley and Midwest, some frontogenesis in the low- to mid-levels interacts with progressive anomalous moisture across MN/IA to yield heavy precipitation. On Friday/Friday night, the pattern best resembles a compromise of the 00z GFS/12z & 00z ECMWF solutions. For Saturday/Saturday night, the pattern is a compromise of the 00z GFS and a southward-adjusted in-house bias corrected ensemble QPF. For MN/WI, heavy amounts appeared uncertain as, despite a highly favorable divergent flow pattern aloft ahead of a negatively tilted shortwave, low-level inflow appears to be fading -- and in southern WI diverging -- as the system butts up against a small pseudo Rex Block with the southern member near the Mid-Atlantic States causing issues in the low-levels. Kept amounts modest there, for now. Excessive rainfall areas have been depicted in this area both days, with a broad marginal risk to account for forecast uncertainty. ....TN Valley/Central Appalachians/Mid-Atlantic States... Confluent flow at 850 hPa associated with high precipitable water values and a slowly progressive frontal boundary are expected to yield heavy rains from the OH/WV/PA border junction on Friday afternoon and evening to MD/northern VA/the Eastern Shore Saturday afternoon/evening. A deep layer low in the Mid-Atlantic States should generate highly efficient rainfall. Used a rough compromise of the 12z & 00z ECMWF/00z GFS here but amped up the volume of the heavy rains to fit 00z GFS mass fields towards the in-house bias-corrected ensemble QPF. Excessive rainfall areas have been depicted in this area both days. ...Florida... A shortwave moving across the eastern Gulf into FL is expected to lead to heavy rains across portions of the southern and central FL peninsula Friday afternoon/evening as it interacts with sea breeze fronts merging in the western peninsula and 2"+ precipitable water values. Amounts should lower Saturday afternoon/evening within a slightly less moist airmass across central and southern sections of the peninsula near and southeast of the upper trough. Went for broader coverage Saturday most similar to the 00z UKMET as cooler air aloft should allow for convection up into the central peninsula, unlike the more suppressed solutions of the 00z GFS/12z ECMWF. Hurley/Roth Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml