Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 505 AM EDT Sun Apr 08 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 08/1200 UTC thru Apr 11/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 For the 12z QPF issuance: no significant changes were made. ...Pacific Northwest/Rockies... A closed mid level low over Vancouver Island at the start of Day weakens as it tracks southeast, reaching the Central and Southern Rockies by the end of the period. Upslope flow is expected to produce heavy rainfall across the higher terrain of the Pacific Northwest, and locally heavy snowfall over the higher terrain of the Rockies. There was generally good model agreement with the overall synoptic setup, so the WPC QPF was based primarily on a blend of the 00z GFS/12z ECMWF, with some 00z HREF mean/00z WRF ARW used to better delineate higher terrain qpf maxima. As the strong closed mid level low near southern Vancouver Island tracks slowly eastward toward the WA coast early on Day 1, it weakens and opens into a positively tilted sort wave. Ahead of the weakening closed mid level system, a 35 knot low level westerly flow focuses 0.75 inch precipitable water air on the Olympic Range in WA/northern OR Coastal Range, as well as the WA/OR Cascades. The best mid level lift occurs just after the best upslope flow, and there is a better model signal for an axis of 1.50/2.25 inches of qpf stretching across the WA Cascades, with lesser amounts (generally between 1.25/1.50 inches) over the WA/OR coastal terrain. Some of the Pacific moisture plume survives into the Northern Rockies after 08/18z, and model soundings across ID/western MT/WY into eastern CO showed marginal instability (due at least in part to steepening lapse rates). The convection is expected to be tied to the terrain initially, before it moves into the valleys toward 09/00z. There was a strong high resolution model signal for 1.00/1.50 inches of qpf across the Blue Mountains in OR and the Bitterroot Range in ID, with lesser amounts (between 0.50 and 1.00 inches) over the Grand Tetons and Big Horn mountains in WY. Snow levels will drop during the day, but at least some of the qpf, especially in the lower elevations, is expected to fall in the form of rain. The combination of snowmelt and runoff from heavy rainfall could result in localized flooding issues, but it appears to be more of a flood versus flash flood threat. Locally heavy snowfall is expected across the WA Cascades, as well as the higher terrain of ID/western MT and north central WY. Please refer to the most recent suite of WPC Winter Weather products for more information. ...Northern Plains/Mid MS Valley into the OH Valley... Short wave energy tracking from eastern MT across the Upper MS Valley interacts with moisture to produce a stripe of qpf extending from the Northern Plains into the Mid and Upper Ms Valley during the first half of Day 1. The short wave and its attendant surface low weaken as they crosses the OH valley toward the end of the period. There were some minor differences with respect to the placement of the axis of maximum qpf, so the WPC QPF was based on a blend of the 00z GFS/12z ECMWF. Ahead of short wave energy tracking across eastern MT early on Day 1, a 30 to 40 knot low level southerly flow transports 0.50 inch precipitable water air across the Northern Plains. The moisture is lifted isentropically over southern ND/eastern SD into southwest MN and central IA between 08/12z and 08/18z. Banded precipitation is possible ahead of the short wave (and its weakening attendant surface low) over these areas. There was a multi model signal for a long axis of 0.25/0.50 inches of qpf across this area. with each 00z model solutions showing roughly the same placement. Local 0.50+ inch amounts are possible over eastern SD into far southwest MN, where the best chance for banding occurs. The short wave weakens as it crosses the Upper Great Lakes into the OH valley during the second half of the period. The moisture source wanes as it is shunted to the south, and for the most part, qpf amounts from central IL into eastern WV and eastern TN/westernmost VA are expected to remain in the 0.10/0.25 inch range. A swath of snow is expected across the Northern Plains into the Upper MS Valley during Day 1. Please refer to the most recent suite of WPC Winter Weather products for more information. ...Florida... A frontal boundary stalls across central FL during Day 1, before moving back north as a warm front late in the period. Moisture and instability focused on the front feed convection that produces locally heavy rainfall, especially across central FL. While there were some minor differences in the placement of the maximum qpf, there was enough model agreement to use a multi model blend (which included the 00z HREF mean) as a basis for the WPC QPF. After a morning minimum of convection ahead of a cold front dropping into northern FL early on Day 1, convection is expected to fire ahead of the slowing front across central FL, mainly after 08/18z. The front becomes quasi stationary into the early evening hours, as 1.50 inch precipitable water air and marginal instability feed low topped storms into the early evening. For the most part, there was good model agreement with the placement of the front as it briefly becomes stationary, so the qpf along the front was based primarily on a blend of the 00z HREF mean and the 00z WRF ARW. This combination placed an axis of 0.50 to 1.00 inches of qpf over east central FL, with the highest amounts between KVRB and KLNA. Locally higher amounts are possible if training occurs along the the nearly stalled front, but much of the available high resolution guidance suggests maximum qpf amounts remains below 2.00 inches. Three hour flash flood guidance values are fairly high, so while locally heavy rainfall is possible, flash flooding is not expected. Days 2/3 ...Pacific Northwest to the northern Rockies... A cold front and mid level shortwave energy will push into the Pacific Northwest (north from Humboldt County, CA) Tuesday, reaching the northern Rockies Tuesday night. This return of precipitation is after a lull Monday. 0.75 to 1 inch PW will cause some locally heavy precip in preferred western slopes of the Coast Ranges and Cascades. Some timing differences persist in 00Z guidance, but still preferred a general model blend for QPF...with the 00Z GFS/ECMWF/NAM/UKMET. ...Central Plains to the Mid Atlantic... Several pieces of weak shortwave forcing will shift east-southeast from the Great Plains and Great Lakes Monday resulting in patches of light precip. The GFS continues to have stronger mid and low level forcing with a shortwave over NE/IA that results in a swath of precip across central NE. This feature is weaker and in western NE/central KS in other 00Z guidance, so the GFS was discounted. Elsewhere, remnants of a shortwave trough will shift east from the upper Midwest to PA/NY Monday with QPF generally under a tenth inch expected. ...Far Southeast/Florida... A zonal jet will stall a cold front along the Gulf Coast/north-central FL Monday through Tuesday. 1.75 inch PW along this front over FL will result in locally heavy rain. Little antecedent rain will limit the flood risk, though since PW is 2 standard deviations above normal and the front is stalled, an excessive rainfall risk may be needed eventually for north-central Florida. 00Z guidance features decent confidence in the position of the front through both days, with magnitude differences noted. A general 00Z model blend was used for WPC QPF. ...South TX... A southward shifting frontal boundary will become convectively active in PW 2 to 2.5 standard deviations above normal Monday night and Tuesday morning across northeast Mexico near the border with south Texas before shifting well south by evening. 00Z guidance consensus is for the heavier totals to remain south/west of the border. Hayes/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml