Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 455 AM EDT Mon Apr 09 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 09/1200 UTC thru Apr 12/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Pacific Northwest... Short wave energy at the base of a long wave trough off the West Coast early on Day 1 takes on a negative tilt as it approaches the OR coast late. Ahead of the short wave and its associated cold front, an increasing low level south southwest flow transports moisture to the Pacific Northwest coast. There was generally good model agreement with timing of the short wave and the front, so the WPC QPF was based on a blend of the 00z GFS/12z ECMWF. A short wave at the base of a long wave trough, located at 36N 138W early on Day 1, tracks toward the Pacific Northwest during the period, reaching a position just off the OR coast before 10/12z. Ahead of the short wave (and its associated surface front), a 35/45 knot low level southwest flow transports 0.75 inch precipitable water air to the WA/OR coast between 10/06z and 10/12z. The low level flow is almost parallel to the coastal terrain, so qpf amounts are expected to remain below 0.25 inches along the OR/WA coast by the end of Day 1. ...Rockies/Central Plains... Short wave energy over the CO Rockies early on Day 1 tracks across the Central and Southern Plains, providing synoptic scale ascent over a frontal boundary dropping south over the Southern Plains. Mainly light qpf is expected with this system, and the WPC QPF was based mainly on a multi model blend. Lift associated with short wave energy tracking across western CO early this morning is expected to interact with a dwindling moisture feed to produce areas of 0.15 inches of qpf over portions of northwest into central KS, as well as the higher terrain of the CO Front range. As the lift exits, the precipitation shield is expected to dissipate. ...Texas... Short wave energy tracking from the CO Rockies to the Southern Plains on Day 1 interacts with moisture and instability along a frontal boundary dropping south across TX. Convection across far southern TX could produce locally heavy rainfall during the second half of the period. For the most part, there was good model agreement with respect to the evolution of the synoptic scale systems, and the WPC QPF was based mainly on a blend of the 00z GFS/12z ECMWF. Ahead of a cold front dropping south across TX, a low level southwest flow aids in transporting 1.50 inch precipitable water air across far South TX during the second half of Day 1. Model soundings showed moderate instability ahead of the front through 10/00z, but as the front drops south, the instability is suppressed to the Rio Grande River. The combination of moisture and instability is expected to feed convection that produces a swath of 0.25/0.50 inches of qpf near the Rio Grande River region. ...Mid Atlantic/Southeast/Florida... Weakening short wave energy crossing the OH Valley and Northeast during the first half of Day 1 spins up a surface low over a frontal boundary extending off the Mid Atlantic coast. The southern end of the front becomes quasi stationary over north central FL, and deep moisture and instability focused along the front will feed convection that may produce heavy to locally excessive rainfall. While there was generally good model agreement with respect to the overall synoptic setup, there are some differences in the magnitude and placement of the best convection and highest qpf amounts. With this in mind, the WPC QPF across FL was based on a blend of the 00z HREF mean/00z WRF ARW and 00z NSSL WRF, in an attempt to better depict the placement of the highest qpf. A frontal boundary extending across north central FL early on Day 1 becomes the focus for deep moisture and instability for convection. Waves on the front during the first part of the period are expected to provide low level moisture convergence, fueling convective elements over the eastern Gulf of Mexico. While the best mid level support is not expected until the second half of Day 1, there is a solid high resolution model signature for convection along the front to fire before 09/15z off the coast of the FL Big Bend. The convection is then expected to track south of the front, and produce locally heavy rainfall across north central FL between 09/15z and 10/00z. During this time period, some of the high resolution guidance (including the most recent HRRR) suggests the potential for 3.00+ inches of qpf here. As a 25 knot low level southwest flow focuses 1.75 inch precipitable water air (which is about two standard deviations above the mean) and moderate instability along the front, these amounts look plausible. It should be noted that the regional/global guidance values are not nearly this high, nor is the 00z HREF mean. The convection is expected to produce a sizable cold pool, which pushes the convection and front south closer to the Tampa Bay area, between 10/00z and 10/06z. The moisture and instability profiles remain similar in magnitude, which could push the 2.00+ inch qpf amounts south to the northern Tampa Bay area. After the surface wave passes, the front again becomes quasi stationary through about 10/09z, after which time low pressure off the NC coast drags the front southeast toward 10/12z. Further north, short wave energy over the OH Valley tracks across southern New England before 10/00z. The airmass ahead of the short wave is fairly dry, with precipitable water values below 0.20 inches over much of the northern Mid Atlantic north into southern New England. Much of the moisture associated with the short wave will moisten the column first, so qpf amounts should be fairly light. The short wave provides sufficient lift to spin up a surface wave on the frontal boundary off the Mid Atlantic coast. The western and northern edge of the precipitation shield associated with surface low is expected to be limited by the dry air in place, with 0.10+ inch qpf amounts placed over the Mid Atlantic coast. Days 2/3 ...Pacific Northwest to the Great Lakes... A shortwave trough axis rounding a Gulf of Alaska low will come ashore over western WA Tuesday morning. 0.75 to 1 inch PW streams into the PAC NW coast north from Mendocino County CA to the Olympics of WA immediately ahead of this trough which swings into BC and the ridge over the western CONUS. The resultant zonal flow pushes shortwave energy across the northern Rockies and onto the far northern plains. The swath of precip is enhanced as it reaches MN as shortwave energy over Manitoba adds some energy. The track of the surface low associated with this swath is similar among the 00Z GFS and ECMWF which a blend of two was the preferred precip axis that crosses the northern lower peninsula of MI. The QPF varies among the models with a non-NAM blend preferred for QPF. A more powerful shortwave swings from the Gulf of Alaska and across much of the west coast Wednesday night. PW will be less with this system, (0.5 to 0.75 inches - so QPF magnitude will be a little less while the areal coverage is much larger than the previous low passage. The 00Z ECMWF is slightly more amplified than the 00Z GFS. A blend of the two was included for the Day 3 QPF. ...Florida... Zonal flow aloft will persist ahead of an approaching shortwave, keeping the cold front nearly stationary across the north-central portion of the FL peninsula through Tuesday. 1.75 inch PW is 2 standard deviations above normal which combined with 1000 j/kg MUCAPE and bulk shear of 25 to 30 kt will allow heavy convective thunderstorms to persist over this area. An additional areal average of 2 to 3 inches can be expected with several inches possible storm total. FFG in this area is quite high, so only the most extreme scenarios of repeating thunderstorms/upwind propagating activity would lead to life threatening issues. Furthermore, the length of the event (roughly 18Z Mon-00Z Wed) is notable. A marginal risk for excessive rainfall was added to the north-central FL peninsula for Day 2 (12Z Tue-12Z Wed). The 00Z NAM is most robust with QPF through this event, but it is also the sharpest with the upper trough that approaches from the west. Despite the slight outlier appearance for Day 2, the NAM was included in the general model blend of 00Z guidance that was the basis for QPF. The latter portion of this event is just getting into range of CAMs which should help with estimates of storm total rainfall. ...Lower Rio Grande Valley... A cold front will push south of the Rio Grande/southern tip of TX Tuesday morning. PW of 1.75 inch (1.5 standard deviations above normal) will be present through the mid morning. Convective activity continues to be progged mainly just south/west of this lowest portion of the Rio Grande where terrain enhances the activity. Hayes/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml