Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 455 PM EDT Wed Mar 28 2018 Prelim 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 continues to expand this afternoon across much of eastern TX. This activity is forming 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. CAPE should continue increasing to the south of this boundary...thus serving as a source of instability being advected into the boundary. Thus 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 day 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 better 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 saturating areas receive additional heavy rainfall. Thus the flash flood risk overnight may be even higher along a similar axis. Some uncertainty on exactly where the boundary ends up anchoring...and thus where the axis of most intense rainfall sets up. The 12z ARW and ARW2 are further south...with the NMMB and NAM NEST further north. Usually would prefer the ARWs in setups like this. However based on recent radar/observational trends...it appears like they are a tad too progressive in bringing the front towards the coast. Thus for now prefer to just go with a consensus of these 12z high res runs. Also, recent runs of the HRRR appear to be handling the evolution fairly well...so QPF from the 16z HRRR was incorporated into the WPC QPF. All the high res CAMs are in agreement with widespread 3"+ amounts...with a corridor of 4-7" (locally even higher). Based on the set up...think these are probably realistic depictions. Thus would appear the threat of significant flash flooding is increasing across portions of southeast TX into central/northern LA and possibly even into MS. Not confident enough in location/magnitude/coverage to introduce a HIGH risk...but this event seems to solidly fit into the MODERATE category...and will shift the moderate risk a bit southwest to include more of southeast TX to the Gulf Coast. Also trimmed back the northern extent of the moderate risk over AR and northern MS where rates are not expected to be as heavy. 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