Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 457 AM EDT Tue Apr 03 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 03/1200 UTC thru Apr 06/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...OH Valley into the central Appalachians... Low level southwesterly flow is expected to remain strong through Tuesday morning into a strengthening west to east oriented frontal zone across the OH Valley region. This will support the continued threat of organized overrunning convection along and north of this boundary in a region where pw values will be rising to 1.5 to 2+ standard deviations above the mean. The day 1 qpf leaned toward the hi res guidance of the arw...nmmb...nssl wrf and latest hrrr which all showed heavy rain potential in the vicinity of south central IN...southern OH...northern WV into far southwest PA. In these regions runoff risks will continue through most of Tuesday morning when training of cells in a west to easterly direction is possible. Uncertainties are with the axis of the heaviest precip...with the hi res guidance showing run to run and model to model differences. In areas of training---hourly rainfall totals of .50-1"+ and totals of 2-4" in a few hours are possible. A west to east oriented marginal and slight risk area will be maintained this period in the area of training potential. ...Upper MS Valley--Great Lakes into the northeast... There is fairly good model agreement with the deepening surface low pushing northeastward from the mid MS Valley into the Great Lakes this period. A well defined comma head/deformation precipitation band will be expanding east northeastward from the Upper MS Valley into the Great Lakes Tuesday and into southeast portions of Ontario/southwest QB Tuesday night-Wednesday morning. At the same time---strong isentropic lift to the northeast of the northeastward moving warm front will support expanding precipitation into the northeast from the northern Mid-Atlantic---across much of NY state and into New England. Early spring heavy snows likely in the comma head/deformation precipitation band from southern MN/northern IA---central WI---the northern L.P. of MI and into portions of southeast Ontario and southwest QB. Accumulating snows also likely on the northern edge of the warm advection precipitation across northern Maine. See the latest QPFHSD for additional winter weather information across these areas. ...Northeast TX---lower to mid MS Valley---lower OH Valley---TN Valley into the southern Appalachians... Late Tuesday afternoon frontal to pre-frontal convection should blossom along the trailing cold front from the above mentioned deepening low moving into the lower Great Lakes region in an increasingly unstable axis from eastern TX---the lower to mid MS Valley---lower TN Valley and lower OH Valley region. MUCAPE values of 1000-2000+ j/kg expected along with pw values rising to 1.5 to 2+ standard deviations above the mean along and ahead of this front during the 1800 UTC Tuesday to 0600 UTC Wednesday period. This frontal boundary is expected to be fairly progressive to the southeast---but widespread heavy rains possible late afternoon into early evening from northeast TX into portions of the lower MS Valley---lower TN Valley. 1"+ precip total possible across these areas---with lesser totals to the east and southeast toward the central Gulf Coast---mid to upper TN Valley where activity should be decreasing Tuesday night into early Wednesday morning. During the convective peak period late afternoon into evening---there will be some potential for runoff issues---with a marginal risk area depicted on the Excessive Rainfall Outlook for this period from northeast TX into portions of the lower MS Valley---lower TN Valley. ...Pacific Northwest/northern Rockies... A warm frontal boundary will be pressing slowly east northeastward toward the Pacific Northwest day 1. Increasingly moist west southwest low level flow with this boundary will support increasing showers late Tuesday afternoon into Tuesday night/early Wednesday from coastal Oregon/coastal Washington---inland to the Washington-Oregon Cascades. Model consensus is for light totals in the .10-.25" range across these areas. Light precip totals also likely inland across the northern Rockies from northwest WY into western MT and northern ID. Days 2/3 ...Pacific Northwest/Northern Rockies... Short wave energy approaching the Pacific Northwest late on Day 2 provides synoptic scale lift over a warm front just offshore. A deepening southwest flow of Pacific moisture ahead of additional short wave energy approaching the Pacific Northwest could result in locally heavy precipitation over the higher terrain during Day 3. There are some timing differences with respect to the short wave energy, so in an attempt to mitigate the differences, the WPC QPF was based on a blend of the 00Z ECMWF/GFS. Day 2... Short wave energy approaching the Pacific Northwest coast will usher another round of moisture onshore with 20 knots of low level southwest flow focuses 0.50/0.75 inch precipitable water air on the higher terrain over western WA. Further east, upslope flow becomes aligned ahead of an arctic front dropping into the Northern Rockies late. While moisture is scant, the upslope flow makes the most of the moisture in the column to produce local 0.25 inch qpf amounts over the Rocky Mountain Front Range in MT, as well as the Absaroka Range in WY/MT. Day 3... The arctic front becomes banked up against the Northern Rockies from western MT into central WY and as far east as western NE during Day 3. While moisture remains a limiting factor for qpf, upslope flow (augmented by low level convergence along the arctic front) and short wave energy from the Pacific Northwest makes the most of the moisture in the column. There was a multi model signal for some additional 0.25 to 0.50 inches amounts of precipitation still possible on day 3 across the Absaroka Range in MT/WY. The next piece of short wave energy in the mid level flow approaches the Pacific Northwest during the early to middle portion of Day 3. Ahead of the short wave (and behind the surface warm front which moves into southwest British Columbia), a 25/35 knot low level southwest flow transports 0.75/1.00 inch precipitable water air across northwest OR and western WA which should result in some local amounts of an inch or more over the Olympics and Cascades in WA. ...Great Lakes/Northeast... A deep area of low pressure will be moving north of the international boarder and into Canada early in the period. Moisture within the broad cyclonic circulation will keep generally light precipitation going over parts of the eastern Great Lakes to New England early on day 2. Precipitation should slowly taper off during the day on Thursday from west to east as the area of low pressure fills while it pulls away from the U.S.. ...Southeast into the far southern portion of the Mid Atlantic... Deep moisture and instability ahead of the frontal boundary associated with the system located over the northeastern U.S. will support some lingering convection over parts of Georgia and northern Florida northward to the eastern Carolinas early in the period. The front will continue to move off shore and precipitation comes to an end as drier air filters in from the west. Oravec/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml