Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 651 PM EST Mon Feb 26 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Feb 27/0000 UTC thru Mar 02/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 update... Made some minor adjustments to the evening and overnight periods based on recent runs of the HRRR and ensemble guidance, otherwise no significant changes were made to the prelim Day 1 issuance. Pereira Day 1... Southern Plains/Western Gulf coast... A returning warm front to the east of an upper cyclone closing off across the Southwest will move northward across Texas and Louisiana, leading to initially scattered convection across Texas through tonight. Amounts and coverage are expected to ramp up as 850 hPa inflow increases to 25-40 kts, 0-6 km bulk shear rises to ~50 kts, precipitable water values rise to at or above 1.75", and MUCAPE of 500+ J/kg is projected to lie within the system's warm sector. A bulk of the heavy precipitation is expected to lie in the Tuesday afternoon time frame rather closely to the warm front. Hourly rain totals up to 2" are possible at that time. Mass fields from the 06z GFS and 12z NAM, along with our in-house bias-corrected QPF and the 12z-13z National Blend of Models suggest that a solution most like a compromise between a 00z/12z ARW, 12z Canadian, 12z NSSL WRF, and 06z/12z GFS blend is most plausible, so adjusted the WPC QPF to match. While this is substantially wetter than WPC continuity, it still lags the 12z in-house experimental bias corrected QPF. California, Great Basin, Wyoming Rockies... A closed low over California combined with modest onshore flow and seasonal precipitable water values should lead to locally moderate to heavy rain across southern California. The best chance for any locally heavy totals over an inch will be in the transverse and peninsular Ranges. To the east and northeast of this developing closed low---broadly upper diffluent flow will support widespread scattered precip activity through the Great Basin and into Wyoming Rockies within a region of good low- to mid-level frontogenesis. Using 06z GFS mass fields as a guide, WPC QPF was adjusted to better match the agreeable model consensus. Days 2/3... ...Southern Plains...Lower MS/TN/OH Valleys to Southern Appalachians... ...SLIGHT RISK for excessive rain on Day 2 (12Z Tue-12Z Wed) from ArkLaTex to far northwestern AL... ...MODERATE RISK for excessive rain on Day 3 (12Z Wed-12Z Thu) from southern AR to central TN... Closed low pressure along the southern CA coast opens and ejects east Tuesday night...reaching OK Wednesday night. A strong WSW jet will shift east ahead of the trough axis. Southerly flow ahead of the system (in conjunction with high pressure along the SE CONUS coast) will have brought gulf moisture across from east TX up to the MO/AR border by Tuesday evening which will spread east to the southern Appalachians through the day Wednesday. Surface low pressure in the lee of the Rockies near the CO/NM border Tuesday evening shifts east...developing as it crosses the OH Valley Thursday. Day 2... A warm front ahead of the surface low will serve as focus for shower and thunderstorms development as it lifts north across AR Tuesday night Strengthening low-level cross-frontal flow along and minor PVA in the WSW jet are expected to be the primary forcing mechanisms for convection. PW 1 to 1.5 inches with some CAPE feeding northward across the warm front in a 40-50 kt low-level jet should support rain rates of 1-1.5 in/hr in the heavier convection. The potential for training/repeat cell development over the same areas could result localized areas exceeding 2 inches of rain on day 2, with areal averaged amounts of 1-1.50 inches. Day 2 WPC QPF was based on an even blend of the Hi-Res ARW/ARW2, 12Z GFS, and 12Z ECMWF. The SLIGHT RISK for excessive rain was maintained day 2 for portions of the lower MS valley and slightly narrowed given the aforementioned guidance inputs. Day 3... The synoptic pattern becomes more favorable as strong upper height falls move into the south-central CONUS in association with an amplifying trough, supporting a deepening surface cyclone traversing from the central plains to the Midwest. A surface boundary will be laid down from the Tuesday night/Wednesday morning convection across the TN valley/mid-south, which will serve to focus convection once again wed afternoon/night. As the surface cyclone deepens across the central CONUS, a 40-50 kt low-level jet is expected to once again develop, transporting moist and unstable air north and lifting it across the surface boundary. This scenario would result in another round of convection with possible training over the same areas which received prior rains, further increasing the potential for flash flooding. The MODERATE RISK for excessive rainfall for Day 3 (12Z Thu-12Z Fri) was widened to span from Texarkana to central TN, with a surrounding SLIGHT RISK which extends to the NC/TN border. Maintained a MARGINAL RISK in the western OH Valley given the potential for locally heavy rain immediately ahead of the surface low and particularly low FFG. The MODERATE RISK covers the heaviest swath by 12Z ECMWF/NAM which are in good agreement and includes a southern buffer to account for the 12Z GFS which has low level boundaries south of TN. The 12Z GEFS ensemble mean is a bit north of the operational, so the southern line of the MODERATE RISK is the axis of heaviest QPF from the 12Z GFS operational east to northern AL. East of Northern AL features increasing FFG...so a SLIGHT RISK was maintained there. Western US... The aforementioned upper low along the southern CA coast Tuesday evening shifts east with light to locally moderate precip from southern CA to AZ/NM through Wednesday. A stronger low will drop into the PAC NW and northern CA Wednesday night through Thursday from the gulf of Alaska. Strengthening low-level flow perpendicular to the coast could produce some heavy rainfall amounts along the Coastal Ranges of southern OR/northern CA, and CA Cascades/northern Sierra Nevada where 1-2 inches are forecast on Day 3. There was good model agreement and the WPC QPF was based on an even the 12Z ECMWF/GFS/NAM. The heaviest inland precip will be at higher elevations. See WPC WWD products for further information on snow. Roth/Jackson Graphics available on the web at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml