Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 532 PM EDT Tue Mar 27 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 28/0000 UTC thru Mar 31/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central and Eastern U.S... 2030z update: Will be watching trends in the observations and in the HRRR forecasts which suggest heavy rain breaking out in the TX Hill Country overnight may become a bit more west to east oriented, therefore more persistent and producing heavier rainfall. It seems this is where the outflow / effective front is settling from the north, and strong deep layer forcing will kick out along the boundary tonight. This trend did not develop in time for the final QPF package. The going forecast still looks good elsewhere, but may be somewhat underdone over the Hill Country. Previous discussion: 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 of excessive rainfall should suffice for this activity...and we 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 move only slowly southeastward...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 west...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...we opted to expand the Day 1 Slight and Moderate Risk areas farther 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. By late morning into the afternoon, we 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 farther northwest along the actual cold front...or whether both boundaries eventually become active. Given these uncertainties we 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 when convection could train. Thus we will likely see an increasing flash flood risk again 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. Days 2 & 3 ...Lower MS Valley/Gulf Coast/TN & OH Valleys... A shortwave trough rounds the long wave trough axis drifting east across the central CONUS, moving northeast from west TX Wednesday evening to IL by Thursday evening. This northeast motion both aloft and with the surface low over ArkTxOma will maintain the advection of a plume of Gulf moisture over east TX and up the lower MS valley into Thursday morning. PW of 1.5 inches streams across this area with increasing jet dynamics overhead in the right entrance region of the jet enhancing over this area through this time. The lower MS valley is typically quite resilient to flash flooding due to soil type and land use (forest and field) and there has not been rainfall in this area in the past week. However, the elevated PW is 2 to 2.5 standard deviations above normal and the slow progression will make for a flash flood risk from far east TX up to west-central TN. The moderate risk for excessive rain was expanded a bit SW to NE where 3-5 inches of rainfall is expected on an areal average. The slight risk is probably too broad, but one concern which is noted in 12Z hi-res ARW and NAM CONEST runs is uncertainty with the eastward progression of the front once the surface low accelerates to the Midwest/OH Valley Thursday. Therefore the cushion on the east side of the Day 2 (12Z Wed-12Z Thu) was maintained. Instability also decreases farther NE across TN and into KY, though FFG decreases as well. Therefore the Slight Risk was maintained to the KY/WV border. The low lifts from the OH Valley Thursday evening to the St. Lawrence Valley Friday. The progression of the cold front trailing the low will pick up with increasing zonal movement to the gulf moisture plume and associated rain over the southeastern states. The Slight risk for excessive rainfall on Day 3 (12Z Thu-12Z Fri) was maintained SW from KY, but this may be able to be downgraded to a Marginal Risk depending on the progression particularly later Thursday morning. WPC QPF was based on a preference for the 12Z GFS for both Days 2 & 3 along with some hi-res ARW and NAM Conest on Day 2 with GEFS mean incorporated for Day 3. ...Northeastern U.S... Low pressure slowly develops as it moves northeast from the Mid-South Thursday to the St. Lawrence Valley Friday. One inch PW along and east of this low track will spread from the eastern Midwest (2 standard deviations above normal) all the way into Maine (3 standard deviations above normal) Friday. However, activity will be progressive with this system given low level flow of 50 to 60 knots and the jet increasing above 150 kt along the northeastern seaboard, so one inch areal average QPF over the eastern Midwest later Thursday through Thursday night give way to half to three quarter inch areal average QPF over upstate NY and New England through Friday. The 12Z ECMWF slowed progression of the low compared to the 00Z, but the 12Z GFS maintained greatest preference with this QPF package. ...Northern to Central Rockies and across the northern Plains... High pressure building down the northern plains Wednesday night into Thursday will provide an upslope easterly flow to the eastern slopes of the central and northern Rockies with areal averages of a tenth of quarter inch for the northern Rockies and around a quarter inch for the CO Rockies and adjacent high plains. A broad shortwave trough pushes southeast across BC Friday with NW flow allowing a swath of precip to develop from MT across the northern plains to northern MN/WI. This baroclinic zone will enable snow which will be discussed more in the QPFHSD. ...Western WA... Northwesterly onshore flow south of shortwave trough activity over western Canada will maintain light to moderate precip across the Olympic peninsula and northern WA Cascades for both Days 2 & 3. Snow elevations will be high on the south side of the activity, but rain rates are not expected to be excessive. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml