Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 700 PM EDT Wed Mar 28 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 29/0000 UTC thru Apr 01/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central and Eastern U.S... Convection had expanded in the afternoon hours across eastern TX / northern LA / western MS. This activity formed along and just north of a well defined outflow boundary and just ahead of an advancing cold front. The cold front should help push this outflow boundary slowly southward with time into this evening...before eventually stalling through tonight. This axis should be the focus for continued convective development through the overnight period. Given available CAPE it seems likely that convection will be able to maintain intensity on the southern edge of the merged boundary...with multiple rounds of storms through the night. And with limited boundary movement and storm motions exhibiting a large component parallel to the boundary...training is likely. A flash flood risk exists through the night in the vicinity of this boundary from southeast TX into central/northern LA and MS. Anticipate convection will even increase in intensity and organization some tonight as the stronger height falls move in from the west indicative of stronger synoptic support. This will help eventually push the convection more progressively off to the east...but not before already saturated areas receive additional heavy rainfall. Our expectations were shaped by the 12z WRF-NMMB and NAM CONUS Nest, which appeared to have the best handle on convection occurring this afternoon. The ARW models were too generous with cold pool production and propagation out toward the coast. This made for some slight changes to the final QPF issued at 2030z, in which we pulled the overnight axis of heavier rains father inland, catching more of the southern and eastern portions of Houston with a swath of 2-5 inch areal average rainfall. Certainly some of this was ongoing on the northern side of the city at 22z, but this activity was tied to the entrance region of the synoptic jet that was supporting 2 inch per hour rain rates northward across Louisiana. Visible satellite and surface observations indicated sufficient warming ahead of the leading line to sustain a slowly progressing MCS into Mississippi, and perhaps lifting into portions of TN/AL this evening. Enhanced 850 mb wind speeds and confluent lower level flow is forecast to remain in place along this zone from east of Houston to western Mississippi through the night, leading to repeated rounds of convection as upstream forcing ejects from the base of the large scale trough. In addition to the somewhat continual storms along this zone, models strongly indicate a separate batch of strong convection that will initiate over south Texas as height falls eject, and this activity may slow and begin to train more as it enters the deeper low level convergence along the upper Texas coast late in the night. All indications are that the convection will become more progressive by 12z Thursday as the shortwave ejects eastward giving the cold front a push. Thus the flash flood threat should be on the decrease through the day Thursday...although locally heavy rainfall will still be likely along the Gulf Coast from southern LA into southern MS/AL. Convection should be intense enough over these areas to produce locally heavy rates...but the progressive nature of storms should limit any flash flood threat to only the more susceptible locations. Will probably end up with a split in the QPF distribution during the day Thursday. One max towards the Gulf Coast along the front where instability is greatest...and another across the OH Valley closer to the stronger mid/upper level forcing. In between should end up with a relative minimum...with these areas split in between the better forcing to the north and instability to the south. WPC QPF during this period relied on a a global and high res model blend to handle mesoscale differences...with generally good agreement seen on the large scale. Days 2 & 3 The preferred QPF source was the 12Z GFS which made up a majority of the blends. Some ARW was included for the first half of Day 2. The 12Z ECMWF was unavailable for these times at issuance due to issues on their end. Some 00Z ECMWF and 12Z NAM was included in the QPF blends. ...Gulf Coast/Southeast up to the OH Valley... A cold front associated with a low moving up the northeast will sweep across the southeastern CONUS Thursday night through Friday. Elevated PW of 1.5 to 1.75 inches along the eastern Gulf Coast through Georgia is 2 standard deviations above normal which will allow lines of heavy convective thunderstorms. These will be progressive with nocturnal decreasing intensity trends, so the excessive rain risk is marginal for AL into GA with no risk farther east. The excessive rainfall risk for Day 2 (12Z Thu-12Z Fri) was reduced from Slight to Marginal south from the OH Valley except for the central Gulf Coast where low level convergence is best under the right entrance region of the jet. ...Northeast... Surface low pressure shifts northeast from the central Appalachians Thursday evening to New Brunswick Friday. Anomalously high PW of 1 to 1.25 inches along/ahead of this low (2.5 to 3 standard deviations above normal across the northeast) will allow locally heavy rainfall. However, the activity will be progressive and not convective enough to warrant an excessive rain outlook northeast of the OH Valley. ...Upper Intermountain West/Northern to Central Rockies/Northern Plains/Upper Midwest... A series of shortwave troughs will slip underneath a closed upper anticyclone across the Yukon/Northwest Territories across British Columbia and into the northern Rockies through this weekend. The most notable trough pushes southeast across BC Friday and across the northern Great Plains Friday night. Surface low pressure develops along a surface trough across the Dakotas Friday evening reaching the Great Lakes Saturday. A swath of precip along this baroclinic zone will develop and spread southeast Thursday night through Friday night. The 12Z GFS was the preferred guidance for WPC QPF here while the 12Z NAM and 00Z ECMWF are farther north with the axis of precip which will be a quarter to half inch max for the areal average. Please see the QPFHSD for further info on snow across this area. ...Western WA... A steady plume of Pacific moisture with PW around 0.75 inches will continue streaming into the western coast of WA through the period. Shortwave trough activity over western Canada will aide periods of precipitation over the Olympics and WA Cascades. A shortwave trough approaches from the west Saturday which will shift precip down to the WA/OR border. Snow levels will be high south of/ahead of the troughs, but rain rates will be below concerning intensity. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml