Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 310 PM EDT Tue Mar 27 2018 Prelim Day 1 QPF Discussion Valid Mar 28/0000 UTC thru Mar 29/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central and Eastern U.S... The main focus for high rainfall rates and more significant flash flooding risk is shifting southwest...with the main axis now expected from northeast TX into southern AR through 12z Wednesday. Showers and embedded thunderstorms with the lead wave continues to move northeast across AR into portions of MO/IL. This activity will still pose some flash flood threat...but in general rates will be lower where the duration is longer...and the duration will be shorter where the rates are higher (central AR). Thus think a slight risk should suffice for this activity...and went ahead and removed the northern half of the moderate risk with this update. Attention then turns to the nearly stationary (only slowly pushing southeast) boundary stretching from east central TX into southwest AR. Overrunning of this boundary will continue to result in convection to its north. This convection will have a large component of its motion parallel to the boundary...suggesting some training is likely. Given the boundary will only slowly move southeast...repeat convection in this southwest to northeast corridor will increasingly become a concern through the night. This activity may stay rather disorganized into the evening hours limiting the flash flood threat somewhat through that period. However, overnight we should see synoptic forcing increase with height falls moving in from the east...along with an increase in 850 mb moisture transport into and over the boundary. This should result in an increase in convective intensity and organization tonight along the aforementioned southwest to northeast corridor. This evolution is supported by a majority of the 12z high res guidance and recent runs of both the HRRR and experimental HRRR. Overall the consensus suggests totals upwards of 3-6" from northeast TX into portions of southern AR through 12z Wednesday. Given these model trends and the favorable setup descried above...opted to expand the day 1 slight and moderate risk areas further southwest into TX than our previous issuance. Convection should be undergoing a weakening trend Wednesday morning as it outruns the better forcing/instability...although some locally heavy rains should continue from portions of east TX into northern LA...southern AR and western TN. However by late morning into the afternoon, should see convection regenerate across east TX into western LA. This convection will have strong support synoptically as the southwestern U.S. shortwave finally begins to eject east. Some uncertainty whether this convection forms along the morning outflow closer to the TX Gulf Coast or further northwest along the actual cold front...or whether both boundaries eventually become active. Given these uncertainties preferred to stay close to a consensus of the high res ARW/ARW2 and the GFS/UKMET...which seemed to offer a good middle ground solution. Either way some heavy totals will be possible from east TX into LA...with some solutions suggesting a period where convection could train. Thus likely to see an increasing flash flood risk again by Wednesday afternoon in this area...with future model trends hopefully giving more insight as to whether the highest threat ends up closer to the coast or a bit inland. Chenard