Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 410 PM EDT Fri Apr 13 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 14/0000 UTC thru Apr 17/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Lower Arkansas Valley---Lower MS Valley---Mid to Lower TN Valley---Lower OH Valley The strong central Plains closed low and the trailing trof into the Southern Plains will be pushing steadily eastward day 1--with overall good agreement with the evolution of this system. Widespread heavy precipitation likely in the what will be an increasingly well defined convective component pushing out of the eastern portions of the Southern Plains into the lower MS Valley tonight--eastward into the TN Valley-central Gulf coastal region during the day on Saturday. With pw values expected to rise to 2 to 2.5+ standard deviations above the mean in an axis of strong low level south southwest flow--strong frontal convergence and well defined upper difluence---confidence is high for the heavy rainfall potential across these areas. The uncertain parts of the forecast continue to be the details with respect to the timing and exact evolution of this convection. The latest hi res guidance is showing a typical amount of variability run to run with how fast this convection moves downstream. Day 1 qpf leaned initially toward the arw and nssl wrf runs for details at the beginning of the day 1 period---and then more toward the href mean later in the period when some of the timing differences become more acute between each model cycle. Widespread 1-2" areal average precip values depicted across these regions with locally heavier totals where training may occur. At the moment---best chance of training may be at the beginning of this day 1 time period in the 0000 utc to 1200 utc Sat period. In areas of training totals in the 3-5"+ range are possible. ...Central to Northern Plains---Upper MS Valley---Great Lakes into northern NY state and northern New England... A well defined northern precipitation max likely to the north and northeast of the strong plains closed low in the comma head/deformation precip band to the north and northwest of the closed low along and north of the strong west to east frontal boundary stretching from the Mid MS Valley into the Great Lakes. Aside from some shorter term detail differences in the qpf---there is good agreement in the axis of what will likely be a very broad region of heavy precipitation oriented west to east across these regions. Heavy early season spring snows likely in a large portion of the comma head/deformation precip area over northwest KS---central to northeast NE---southeast SD into southern MN. Heavy snows also possible on the northern edge of the overrunning precip to the north of the strong west to east front across the northern U.P. of MI into far northern NY state and northern New England. ...Pacific Northwest into the Northern Rockies... A northeast Pacific cold front will be pressing slowly southeastward day 1 toward the Pacific Northwest coast. This will keep a sustained period of moist southwest low level flow into the Pacific Northwest--Northern Rockies region day 1. There is high confidence for heavy precip values through the Washington Cascades west to the northern Oregon coast range into the Olympic Range---with very good model agreement on amounts and distributions. Inland into the Northern Rockies---precip totals not expected to be as heavy---with model consensus for moderate to locally heavy totals from the Blues of northeast Oregon into northern ID. Days 2/3 ...West Coast... A closed low over the southeast Gulf of Alaska will continue to drop south generally parallel to the Northwest U.S. coast shedding a shortwave late Saturday night into Sunday morning which aids focus for moisture flux ashore across Western Washington into Oregon with .75" PW values. By late Sunday, the low will drop south before next shortwave and associated height falls reach northern California by 16/00z Monday, though moisture plume becomes generally severed and totals drop below .7 to .6" by Monday. Still, given orthogonal flow into Sierras areal 12hr QPF may reach up to 1" along the western slopes by midday Monday. WPC forecast is a general blend in small differences in timing between the ECMWF/GFS and UKMET. ...Upper Midwest across the Great Lakes to the Northeast... By Sunday, the potent closed low will be moving out of the Central Plains into the Upper Midwest but coupled jet orientation across the Great Lakes (at nearly 90 degrees) as well as a strengthening dry slot/upstream jet streak will support a negative tilt orientation of the closed low. Tropical moisture (TPWs up to 1.75" in Mid-MS valley at the start of the period) will continue to lift north into the TROWAL/left branch of the warm conveyor belt producing a strong comma head producing notable late season wintry conditions (please refer to QPFHSD for additional details). Model trends continue to strengthen the negative tilt, increasing southeasterly stretching of the circulation by Sunday...which has delayed northeastward progression compared to prior runs. The 12z GFS had slowed past the older 00z guidance but was further supported by the 12z ECMWF and 12z UKMET in the slowing process. This provided a bit higher confidence to trend slower with the QPF as well and reduce northward progression of precipitation shield across southeastern Canada into Monday in the vicinity of the strengthening high pressure area. ...Southeast to the Ohio Valley to the Mid-Atlantic and eastern Great Lakes... A broad axis of 1.5 inch PWATS narrowing to a 1.75 to 2.0+ inch axis, or roughly 2 to 3 standard deviations above normal, is expected to pivot over the southeast from LA to KY (and even IN/OH) as the mid-level trough swings into a neutral tilt Saturday. Heavy rainfall with unidirectional flow promoting periods of training can be expected to continue east of the MS river Saturday into early Sunday. Slightly stronger upstream jet energy will accentuate eastward progression of the height-falls leading to a trend originally in the Hi-Res CAMs (ARW/ARW2) leading to a bit of increased forward progression...however, also increasing the upper level divergence/diffluence pattern supporting continued efficient rainfall production of quasi-linear convective complex(s) into Mississippi/N and W Alabama. Additional backing low level flow along the Gulf Coast may further enhance convection to pose an increased thunderstorm activity with efficient rainfall production. A broad areal coverage of 3.5-4.5" QPF is forecast from New Orleans to the Alabama/Tennessee boarder as general blend of the 12z GFS/ARW/NMMB/ECMWF/UKMET and timing/axis help from the NAM/NAM-CONEST as well. Some guidance suggests 24hr QPF totals up to 6-7" are possible. As such after coordination with local NWS forecast offices, WPC has upgraded the Day 2 Excessive Rainfall Outlook (12z Sat-12z Sun) to a Moderate Risk; the Slight Risk was adjusted slightly further east to account for some of the faster guidance but also extends as far north as Louisville,KY. Into Day 3, as the upper low elongates...a second focus/center will focus moisture flux just west of the spine of the Appalachians leading to response across the Sargasso Sea...with return moisture (TPWs to 1.25" reaching the Carolinas by early Sunday morning) combining with deep moisture feed with the main warm conveyor leading to strong flux convergence along the southern Appalachians of NC/SA/N GA before slowly sliding northward into SW VA/W VA/W MD/Central PA by Monday leading to increased uptick in convective vigor and rainfall efficiency. Deep southerly flow ahead of the cold front will present ideal South to North training of thunderstorms before being orographically enhanced. As such areal QPF forecasts of 2-3" with some isolated higher totals up to 4-5" possible leading to an expansion of the Day 3 Excessive Rainfall Outlook (12z Sun-12z Mon) Slight Risk to stretch from Northern GA to Central PA. Given low FFG values in the region it is not out of the realm of possibility to increase to a Moderate Risk, especially if the 12z GFS is to come to fruition in Central PA...but confidence is not high enough given model spread at this time. A backdoor cold front/stationary front will be laid out across the northern Mid-Atlantic to Southern New England and will likely add additional focus to the south to north streaming convection leading to a secondary maxima of precipitation Monday across this region. Oravec/Gallina Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml