Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Sat Apr 14 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 15/0000 UTC thru Apr 18/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Lower MS Valley into TN and OH Valley and the Southeast... Multi-cellular linear convection remains well organized (and progressive) early this afternoon across the lower MS Valley and western TN Valley ahead of the cold front. Deep-layer, surface-based instability downwind remains favorable for a continued progressive squall line, with MUCAPES between 1000-2000 j/kg along with modest downdraft CAPE (400-800 j/kg). Continue to expect some slowing in terms of the eastward progression of the convection this evening, owing to the arrival of shortwave energy and resultant upper height falls across the western Gulf region, thereby allowing for a more negative tilt in the longwave trough orientation. In addition, the nocturnal uptick in the above-surface low-level flow this evening and overnight (50-55 kts across the eastern TN Valley by 04Z) will also aid in decelerating the forward (downwind) propagation, especially after midnight as the LLJ orients more parallel to the mean 850-300 mb flow. Thus as with last night across portions of the lower MS Valley (central AR in particular), expect a heightened risk for training/repetitive convection across portions of the TN Valley (AL into eastern TN), as moisture parameters remain impressive with strong 850 mb moisture transport and PWATs approaching climatological max values. The one limiting factor in terms of the flash flood threat would be the anticipated rain rates/amounts -- both of which would likely be curtailed by the more limited instability (500 mb j/kg or less). WPC QPF for the warm sector of this system generally blended the HRW ARW, ARW2 and the HREF Mean. The HRRR was also heavily used early in the period, as it was the best match of the high res guidance to the current radar. The ARW was also a good match, as the previous (00Z) version had become a bit too progressive by this afternoon. Given that organized convection is ongoing and will maintain through the day, generally stuck with the high res guidance for this warm sector QPF, as it is situations such as this when they typically are much more useful than the global guidance. ...Upper MS Valley---Great Lakes into northern NY and northern New England... A surface low across the central Plains-mid MS Valley will track east into the OH Valley tonight. To the north and northwest of this low will see continued wintry precipitation across MN/IA/WI/MI and far northern IL/IN. Strong 850 mb moisture transport ahead of the low will transport convection up and over the warm front into MI tonight, although rainfall intensity north of the warm front should be tempered by the lack of instability. In general, models are in pretty good agreement, with the latest WPCQPF exhibiting good continuity from our previous forecast (especially along the TROWAL). Days 2/3 ...West Coast/Northern Rockies into Northern High Plains... Upper low off the Pacific Northwest Coast by Monday swings shortwave around the base of the trof as the strong southwesterly upper-level jet slackens reducing forcing/moisture flux for a short stint before the shortwave energy begins to amplify back into a closed low feature. This low will progress across into the Northern Rockies by late Monday into Tues, though fairly moisture starved...higher terrain in UT/WA/ID/WY will see increased QPF. By day 3, timing differences become much more marked with the 12z GFS starting to outpace its global brethren though the 12z GEFS was a bit more useful toward a preferred blend. The 12z ECMWF while preferable in QPF magnitude and wind orientation to favored terrain...it still appeared much slower; this is opposed to the UKMET which was timed ideally but had too much QPF (as it is apt to do). So WPC was a compromise of the ECMWF/UKMET/GEFS and ECENS means. ...Great Lakes to Southeastern Canada... By Monday, the deep upper low was fully mature and starting to show signs of stretching from NW to SE across the eastern third of the North American continent centered near Southern Lake Michigan. A break down into two smaller centers on opposite sides of the larger vortex begin to drive the forcing for the remainder of the period late Monday into Tuesday. Reduced flux within the comma head having the eastern lobe severing the moisture connection by Tuesday limits the draw westward across to the western lobe of vorticity over the NW Great Lakes, Tuesday...and so QPF is limited to NW and W flow lake effect streamers from Superior, Michigan and eventually Erie/Ontario by the end of the forecast period. The eastern lobe will continue to support modest moisture flux through the warm conveyor belt though it will generally be directed into Southeast Ontario and Quebec into the Day 2/3 time period, with a small exception of up to .1-.2" of QPF for northern Maine/northern New Hampshire late on Tuesday (Day 3) in the wrap-back/comma head as the eastern lobe/closed low slides up the coast. Please refer to QPFHSD for additional details regarding snow/icy conditions. ...Mid-Atlantic and Coastal New England... By Monday, height-falls swinging through the South and Southeast will stretch the upper low toward the southeast with negative tilt orientation. Additionally, this will help recenter the low through the upper Ohio Valley/Mid-Atlantic and spur a secondary surface low at the triple-point in the lee of the VA Appalachians intersecting with a backdoor cold front that bisects the Mid-Atlantic by this time period. Deep mid-level moisture will be in place from the warm conveyor/ongoing convection through the upper TN valley/Central Gulf States, but with strong upstream height falls as already lead to a broad axis of return low level moisture from the Western Atlantic...alone providing 1.25" to the Total PW. Once combined early Monday, will support TPWs of 1.5" potentially as high as 1.75" which is well over 2 StDev of normal though strong LLJ up to 60-65kts suggest IVT anomalies in the 6-7 range across the Piedmont of the Carolinas into the southern Mid-Atlantic. Much of the low level flow is directed to the deepening surface low and given fairly deep southerly wind profiles...modest instability and low capping...Hi-Res CAMs suggest a few N-S orientated confluence boundaries in the broad return flow/warm sector that may weakly activate with some training northward. The main focus/greatest ascent will be associated near the triple point/returning warm front Monday as well as intersection of flow with the Blue Ridge and eventually higher terrain of the Appalachians across E WV/W PA. A split in guidance was noted with this scenario...as Hi-Res CAMs best represented by the ARW/ARW2 suggested more activity in the warm sector over the Piedmont of VA into MD/E PA/Delmarva versus the Global guidance represented by the GFS/ECMWF/UKMET that forgo the warm sector...focusing near the triple point and strong overrunning in the TROWAL across E WV/W PA eventually shifting toward SE PA. WPC QPF was a blend compromise of these two camps...hedging toward the stronger moisture flux/IVT solutions from the global guidance though not as far north and west. Still, high rates of 2-3" in 6hrs warrants a Slight Risk of Excessive Rainfall in the lower FFG values across the higher terrain stretching from NC to SW/S Central PA encompassing E WV Appalachians and the Blue Ridge of VA. A Marginal Risk extends further NW into NE Ohio/NW PA and over to NJ and the I-95 corridor of MD/NoVA and I-85 corridor of SoVA/NC. Into late Monday/Tuesday, the triple point/surface low will trek east across southern PA toward the lower Hudson River Valley with continued strong deep southerly flow ahead of it slowly returning the front north. However...the front generally hangs up near/along the southern coast of New England and will support continued banded convective elements in the warm sector to expand but become elevated with heavy rainfall and training leading to areal QPF forecasts of 1.5-2.25" from NYC to Downeast Maine. While instability will be limited to 100-250 J/KG aloft...the rates and eventual inundation totals may lead to some flooding concerns and so introduced a Marginal Risk of Excessive Rains for 12z Mon-12z Tues for this area. Hurley/Gallina Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml