Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 459 AM EDT Thu May 03 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 03/1200 UTC thru May 06/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 Convection likely to remain active during the upcoming day 1 time period as the upper trof moving out of the Great Basin and into the Central to Southern Rockies early this morning pushes eastward into the central to southern High Plains Thursday---across the Central Plains into the Mid to Upper MS Valley Thursday night-early Friday. An axis of much above average pw values ranging from 1.5 to 3+ standard deviations above the mean will stretch from the Southern to Central Plains---Mid MS Valley---Lower Great Lakes and into the northern Mid-Atlantic and New England ahead of this northeastward ejecting trof---supporting the potential for widespread moderate to heavy precip totals. Confidence remains on the low side with the evolution of precipitation across these areas---not only with how current early Thursday morning convection will wane--but also on the next round of convection later Thursday into the early morning hours of Friday. ...Upper MS Valley---Lower Great Lakes---northern Mid-Atlantic int New England... The current organized convection along and to the south of the generally west to east oriented frontal boundary stretching from the Upper MS Valley into the Great Lakes will likely weaken by or shortly after 1200 UTC as it pushes downstream into the lower Great Lakes---northern Mid-Atlantic--NY state to New England region. Moderate to locally heavy rains likely in this much above average pw axis across these areas. Later Thursday---convection should begin to enhance along the above mentioned west to east front as a well defined surface wave pushes eastward ahead of the ejecting mid to upper level trof across the Central Plains. This will support another round of organized convection pushing east northeast from central to eastern IA into northern IL...southern WI and into the southern portion of the L.P. of MI. This next activity may push across areas that received heavy rains late Wednesday into early Thursday. This will support potential for runoff issues. The excessive rainfall potential outlook for this upcoming period will depict a slight risk along this west to east frontal zone and expected axis of heavy precipitation. ...Southern Plains---Lower AR River Valley---Mid MS Valley---Lower OH Valley... Low confidence also for the upcoming day 1 period with convection evolution along the trailing cold front from the low moving into the Upper MS Valley-Great Lakes region across the Lower OH Valley---Mid MS Valley into the Lower AR river valley and portions of the Southern Plains. Convection enhancing early this morning over portions of the Lower AR Valley will likely streak northeastward into the Mid MS Valley-Lower OH Valley region in the axis of moist southwesterly flow. The hrrr was showing potential for another round of convection forming Thursday morning in this moist southwest low level flow in the wake of the lead area moving into the Mid MS Valley-Lower OH Valley region Thursday morning. This may support heavy precip totals across areas that receive more than one round of precipitation--with the marginal risk area for excessive rainfall extended southwest across the Lower OH Valley into the Mid MS Valley-Lower AR Valley from the previous day 2 issuance. On the southern end of the much above average pw axis extending into the Southern Plains---there is model consensus for a southern convective max over south central TX in the vicinity of the intersection of the dry line and southward pushing cold front where the moist southerly flow will converge into. Strong boundary layer convergence and favorable right entrance region jet dynamics will support the heavy precip potential across this region of south central TX. No significant changes made to the previous excessive rainfall potential outlook here---with the marginal risk maintained. Days 2/3... ...Great Lakes to northern New England on Day 2... Some moderate rainfall should be on-going across parts of the Great Lakes region in a deformation zone of the system as it fills and pulls north of the international border. The fact that the 00Z GFS idea had the support from the ARW 2.5 and ARW 5 cores tipped the WPC preference towards their solutions. The implications are that the axis of precipitation will be south of the previous forecast with some rainfall amounts approaching an inch over the northern Lower Peninsula of Michigan and the adjacent portions of the Upper Peninsula. This model choice also has implications for parts of New England. The ARW cores and the GFS bring a swath of heavier precipitation from upstate New York eastward into the upslope region of western Maine. In reality, some intense rainfall rates are possible which may lead to some short-term flooding concerns. Given precipitable water values approaching 1.5 inches and the amount of upper level forcing...decided to hoist a marginal risk at this point. ...Frontal Boundary Extending From Southern New England/Mid Atlantic Region to Texas... A surface front associated with the above mentioned system will become oriented from northeast to southwest on day 2...extending from southern New England and the mid Atlantic region into southwest Texas. The front will be the focus for showers and thunderstorms...especially over parts of the Tennessee valley midway through Day 2 as an upper level speed max and mid level vorticity interact with the axis of highest precipitable water. The real activity along the front looks to be over southwest Texas. Rainfall will have been driven by proximity of the dry-line...but it evolves into showers and thunderstorms being focused by the front on Day 2. Between the difluence of a wave in the southern stream, precipitable water values rising to around 1.75 inches and the low level southeast flow into the complex terrain should work in favor of increasing coverage of showers and thunderstorms capable of producing some locally heavy rainfall. Made some minor adjustments to the boundary of the previously issued slight risk in this area. Note that the changes were made to better fit the latest guidance and did not reflect a fundamental shift in forecast reasoning. ...Northern Rockies to Upper Midwest... A trough over the eastern North Pacific ocean will result in shortwave ridging over the western part of North America. Weak shortwave energy over Canada will ridge over the top of the ridge and drop southeast into the northern portion of the Rockies, the adjacent portion of the western High Plains and propagates into the Upper Midwest by the end of the Day 3 forecast. Moisture is scant in this part of the country in wake of the large system over the Great Lakes. As a result, model QPF is light with little agreement in exactly where the precipitation will fall. The manual WPC precipitation graphics were weighted towards the 00Z runs of the GFS and ECMWF with much below confidence in any of the details about placement or amounts. ...South Florida and the Southeast U.S. Coast... Generally light precipitation amounts are forecast across the coast of southeast Florida on Day 2 due to a combination of onshore flow ahead of a wave making its way slowly towards the northwest from the Bahamas. Showery precipitation should become better organized on day 3 as the moisture increases (shown by precipitable water approaching 2 inches over the waters just off the Florida coast) and mid-level height falls moving eastward across the Gulf on Day 3. Some of the showers will start moving northeastward and may brush the coastline of the Southeast U.S. by late Day 3 as the wave approaches. Oravec/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml