Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 700 PM EDT Mon Apr 30 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 01/0000 UTC thru May 04/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Late Afternoon Update... In the 2030z final issuance of the day shift QPF package we made a few minor tweaks. Went with less areal coverage down the dryline into southwest Texas where forcing falls off after 00z - and based on the NSSL WRF and NMMB forecasts. These models were doing a good job placing more organized convection near the caprock. We used these and multi-model ensemble trends to nudge or expand precipitation eastward overnight in Nebraska/Iowa and from the Dakotas into northwest Minnesota. ...Full Discussion... ...Northeast... The strong New England mid to upper level closed low will be pulling offshore tonight and passing south of Nova Scotia during Tuesday. The precipitation effects from this system will continue to diminish across the northeast tonight with only light additional precip totals expected after 0000 UTC Tuesday. ...Northern Rockies into the Northern Plains / Upper MS Valley... A lead area of height falls will be ejecting northeastward out of the mean Rockies/Great Basin upper trof---pushing across portions of the Northern Plains into the Upper MS Valley. This will accentuate large scale lift from the upslope region of the Northern Rockies across southwest MT into northwest WY---northeastward along and north of the surface front lying northeastward into the Upper MS Valley---southwest to central Ontario. Model consensus is for moderate to isolated heavy totals across these areas--with heavy spring snows possible across northwest to north central WY. See the latest QPFHSD for additional winter weather information. ...Central Plains... Convection may flourish this evening across portions of the Central Plains in the vicinity of central to eastern Nebraska in a region of strong frontal convergence/well defined upper difluence ahead of the above mentioned northeastward moving height falls. There may be a brief period of training of cells between 0000-0600 UTC over northeast Nebraska--supporting locally heavy rainfall potential across this area. ...Central to Southern High Plains... Farther to the south along the dry line stretching across the Central to Southern High Plains---scattered convection is possible early in the period in the broadly difluent upper flow downstream of the amplifying Southwest upper trof. Low confidence on QPF details here---with light areal average amounts depicted---and we are unlikely to see isolated moderate or heavy totals. ...Sierra---southern California into the Southern Great Basin... In the wake of the lead area of height falls ejecting out of the mean western trof----additional upstream height falls will be dropping south and southeastward, re-amplifying the upper low over southern California Tuesday. A broad region of scattered precipitation likely downstream of this amplifying upper low from the Sierra--into southern California and the southern Great Basin. The day 1 QPF did not deviate from the model consensus of light to moderate areal average totals across these regions. Days 2/3... ...Western United States... A series of three shortwave troughs will track over CA along the positively-tilted mean western CONUS trough as it amplifies to southern CA through Tuesday night. Continental air with this interior trough will limit moisture under the forcing from the shortwave passages over the Sierra Nevada and southern CA. QPF is limited each day to around a quarter inch for preferred higher terrain. As the trough amplifies over the western CONUS will wrap Pacific-sourced moisture across the Intermountain West to the Rockies including along a stalled surface front over NV/UT/CO that sets up from Tuesday into Wednesday. Precipitable water values around half an inch shown by the NCEP guidance 0.5 inch PW is two standard deviations above normal for the Great Basin which enables moderate precip over preferred terrain with areal average QPF max over the Wasatch of Utah. Model guidance from the mass fields were not too dissimilar from each other, but their respective QPF varied quite a bit. The manual WPC QPF was largely based on a GFS/NAM/ECMWF blend...with the ECMWF getting least weighting until the end of the day 3 period. The ECMWF seemed to be showing its dry bias. By the end of day 3, the GFS appeared to be too fast. Confidence in the details of the forecast was low. ...Central Plains... An amplified trough across the western CONUS will stall and close briefly Tuesday night with the trough axis remaining as shortwave energy ejects north across the Four Corners region Wednesday night and the central plains Thursday. A stationary front will set up over the central plains Tuesday night as the positively-tilted trough ejects shortwave energy over the central CONUS. This provides a focus for moisture convergence and instability (1.25 PW is 2 standard deviations above normal) near the borders of NE/IA/MO. Maintained a slight and marginal risk for excessive rainfall mainly over NE/IA with a slight shift back east per 12Z guidance. An areal average of up to two inches is forecast in the Day 2 (12Z Tue-12Z Wed) time frame. QPF was based on a blend of the 12Z GFS/ECMWF with some inclusion of the NAM for both days and hi-res models like the ARW2 for Tuesday night. ...Southern Plains... Continued a marginal risk on day 3 (12Z Wed-12Z Thu) for west TX from the Rio Grande to the Red River. An ejecting upper low over the desert SW will allow surface low pressure to persist over northeastern NM/the TX panhandle with a dry line shifting east across west TX. 1.5 inch PW from gulf moisture is 2 standard deviations above normal for west TX which along with ample instability allows 1 to 2 inch areal average QPF in thunderstorms particularly Wednesday evening. ...Central High Plains to Front Range of Colorado... Low pressure will slowly traverse a surface trough/stationary front from southern CO to KS Tuesday through Thursday. Easterly flow on the north side of the low will allow topographically enhanced rainfall for northeastern CO/western NE and southeastern WY. Precip rates increase from Day 2 to 3 with an areal average of 1.25 for Day 3 (00Z Thu-00Z Fri). ...Great Lakes to Northeast... A plume of Gulf moisture shifts east from the central plains on southwest flow ahead of a shortwave trough from a positively tilted trough over the Desert Southwest. Widespread 1.25 inch PW across the Great Lakes and into the Northeast is 2 to 2.5 standard deviations above normal with moderate to locally rainfall expected. A marginal Day 3 excessive rainfall outlook (12Z Wed-12Z Thu) was issued for western portions of the Midwest where areal averaged QPF is 1 to 1.5 inches per a blend of mainly the 12Z GFS and ECMWF. Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml