Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 402 PM EST Tue Feb 27 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Feb 28/0000 UTC thru Mar 03/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Southern / Eastern U.S... We are setting up for another heavy rain event going into Tue night and Wed. This is initially driven by warm advection and a pocket of rich moisture that had pooled along the tail end of the frontal boundary in the western Gulf. Stronger deep layer forcing will not arrive until Wednesday, but the region from the TX Gulf Coast up though Illinois was already upstream of shortwave ridging, and experiencing gradual height falls along with strengthening warm advection. Upglide on the 295-300 K surface, along with relatively steep mid level lapse rates owing to a strong westerly component to the flow and an unseasonably mild column overall, will yield swaths of convectively enhanced rainfall. Widespread convection was initiating as of 18z in the central U.S., and will spread eastward. There is fairly strong model agreement as to an axis of heaviest amounts, areal average 1 to 3 inches. from the ARKLATEX through central Tennessee through 00z Thursday, and a secondary axis farther up the sloped frontal surface extending from eastern Kansas to southern Illinois. With convection being elevated, and the pattern suggestive of progressive warm advection that quickly washes through a given area, it may be difficult for much training to set up anywhere until later on Wednesday when frontogenetical forcing and upper jet dynamics will become more persistent ahead of the arrival of the longer wavelength trough out of the Southwest states. Through this evening, however, persistent / training convection may set up along the edge of deeper moisture moving into northeast Texas and northern Louisiana. WPC favored, in particular, the WRF-ARW2 and NSSL WRF solutions. Both models depict hourly accumulation rates that could briefly exceed 0.75 inches. This presents the possibility of renewed flash flooding, given recently saturated ground. ...Great Lakes and Northeast... While the primary jet axis is farther south, there is a distinct northern corridor of enhanced gradients / stronger flow in up through the mid levels. As moisture returns northward along the 295 K isentropic surface, it should produce measurable precipitation over much of the lower Great Lakes tonight, and then again in a heavier fashion beginning Wednesday night / Day 2 / as troughing phases and forcing strengthens. In the Day 1 period the ECMWF ensemble probabilities and SREF were in better agreement with the overall consensus, allowing us to introduce a stripe of 0.10" and greater QPF in multiple swaths over WI/MI and northern IL/IN. ...Western U.S... A well defined closed low over southern California will open up and push steadily eastward. With no real connection to sub-tropical moisture, its effects will be limited, but a showery regime will persist under the upper low through evening, and rain/snow is expected out ahead of the trough through the 4-corners area, although accumulations will be somewhat minimal as the system weakens / shears out on its way downstream. Days 2/3... ...ArkLaTex/Mid MS Valley to Southern Mid-Atlantic... ...MODERATE RISK for excessive rainfall on Day 2 (12Z Wed-12Z Thu) from northeastern TX through south-central AR across southern TN, northern MS/AL/GA to the southern TN/NC border... A dynamic southern stream closed low, currently over southern CA, will open into a positively tilted trough and eject east...reaching southern NM by Wednesday evening and the central Appalachians by Thursday evening. Gulf moisture is building north ahead of surface low pressure beginning to develop in the four corners area to KS. PW of 1.5 inches persists over a stripe from northeast TX ENE to TN Wednesday into Wednesday night along a warm front and persist until the cold front in the wake of the low shifts east over the area. WPC QPF for Day 2 was based on a blend of the 12Z GFS/ECMWF which were in good agreement. Hi-Res WRF ARW2 was incorporated from 00Z to 12Z Thursday. Both models displayed an increase of QPF along this stripe of high PW (2.5 to 3 standard deviations above normal) with very little north-south discontinuity. ...Midwest to Northeast... Low pressure passes through the Midwest Thursday. A mix of rain and snow is generally expected (see QPFHSD for further discussion of Midwest/southern Great Lakes snow). Just south of this low track may be moderate rainfall which is normally not very noteworthy, but this area (along the Ohio River) is quite sensitive to flooding at this time. Therefore, the marginal risk for excessive rain was maintained with a QPF up to one inch. The key to the forecast for the northeast Thursday night and Friday is the degree of phasing between the southern stream low and northern stream trough and resultant low level boundary/low center movement. The 12Z global model suite was quite varied with the 12Z GFS as an major outlier. WPC QPF was based on a blend mostly of the 12Z EMCWF with some 12Z NAM and a little 12Z UKMET. Please see the PMDHMD for model discussion of this storm. WPC continued a marginal threat for excessive rain for parts of NJ/DE through PA, as FFGs are extremely low due to saturated conditions with moderate rainfall ahead of the cold front. Discussion of snow in the area north of this marginal risk...upstate NY to interior New England is available in the QPFHSD. ...West... ...SLIGHT RISK for excessive rainfall on Day 3 (12Z Thu-12Z Fri) for the CA coast from the Santa Cruz mountains to Ventura County... A phased cyclone will drop down the West Coast to northern CA through Thursday night. The associated cold front will then push to southern CA Friday. Ahead of the low...expect 0.5 inch PW to push ashore producing heavy mountain snow and heavy rainfall along coastal sections of WA to CA. Expect 1 to 3 inch areal avg amounts for the coast/coastal ranges/Cascades of CA and the Sierra Nevada Thursday and Friday. The entire closed low will continue to dig south on Thurs, the associated frontal zone will sweep inland and the slight anomalous plume of Pacific moisture will drop down almost the entire CA coastline. Thus a slight risk for excessive rain is in effect for coastal ranges down to the Thomas burn scar area of Ventura and Santa Barbara counties for runoff issues. Otherwise, most of the moisture will flourish in the Sierra with ideal for orographics and heavy mountain snow. There was excellent model agreement with this system and WPC QPF was based on the 12Z ECMWF/GFS/NAM/UKMET. Burke/Jackson Graphics available on the web at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml