Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 338 PM EST Sat Mar 03 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 04/0000 UTC thru Mar 07/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Western U.S and Northern Plains... Two shortwave troughs were pinwheeling through the mean large scale trough in the Western United States. The lead wave had laid out the surface to 700 mb baroclinic zone, analyzed per the 700 mb layer to lie from north of Las Vegas to near Salt Lake City to Glasgow Montana. The mean trough had finished digging, so should not achieve any more southerly a latitude, but the dance of the two shortwaves will sling-shot the southern member from a position along the West Coast this evening toward the Rockies front range and northern High Plains by Sunday evening. The result is a negative tilt wave and strong cyclogenesis to ensue downstream on Day 2. In the meantime, however, substantial snowfall, strong winds, winter storm conditions will be observed through the Intermountain West, especially along the the elevated frontal zone. Much of the model QPF for the Day 1 period had been trending that direction, slightly back to the west of earlier cycles, which is most noticeable over Montana and the Dakotas. Forecast amounts increased somewhat, as well, especially central to eastern Montana and in the orographically enhanced zone from southeast Nevada across much of Utah through western Wyoming. Areal average liquid equivalent of 0.50 to just under 1.00 inch is expected in this zone. The central Rockies become largely dry-slotted during this multi-day event, as the initial corridor follows the pre-established frontal zone, and then gradients reform over the Plains when the jet ejects past the mountains. The model QPFs form a fairly tight consensus as of the 12z cycle. For the details, we preferred a blend that took the WPC continuity forecast toward the 12z NAM CONUS Nest and WRF-NMM which was oddly more generous with contiguous precipitation areas than the ARW runs. The 12z ECMWF trended right in line with these choices, which are also supported by the GEFS re-forecast data based on analog events. See the heavy snow discussion, QPFHSD, for more details. ...Southern Plains... Return moisture be released in the form of showers and a few thunderstorms as upglide occurs on the 295-300 K isentropic surface. The upper flow also becomes increasingly difluent atop the moist/CAPE axis across central to east Texas and adjacent areas. We were therefore generous, within reason, with respect to areal coverage, using the GEFS and ECMWF ensemble QPF probabilities to help determine the 6-hourly details. This approach increased amounts over the ARKLATEX region, with areal average amounts climbing well above a half inch now through Sunday afternoon. Although not intense, this rainfall may lead to some local concern over renewed flooding or rapid surface runoff given saturated soils. Similar to the western U.S., our QPF here leaned toward the NAM CONUS Nest and WRF-NMMB. Days 2/3... ...Northern and Central Rockies across the northern Great Plains... An upper trough swinging negatively tilted will be over the northern and central Rockies Sunday evening with surface low development well underway over the central high plains. The upper portions close into a low through Sunday night and occlude over the northern Great Plains Monday. This occlusion will stall the motion of the surface low center over southeastern SD into Tuesday night and allow the occluded and cold front wi swing east over the Midwest and arc south through the TN Valley to the western Gulf coast. 0.75 inch PW will advect into the northern plains on a 45-55kt southerly jet ahead of the low Sunday night into Monday. Warm rainfall with minor instability aiding moderate rates over the snowpack (estimated by NOHRSC to the 5+inches) of southern MN may cause areal flooding concerns...but not flash flooding as of this time. A deformation zone sets up on the north side of the low from WI into lower MI Monday night...but with occlusion comes weakening and decreasing precip rates for Day 3 across this area. Precip type is of concern with this system...please refer to WPC's Heavy Snow Discussion (QPFHSD) for more details on winter weather. WPC QPF was based on a blend of the 12Z GFS/ECMWF with some consideration of the 12Z NAM. ...Central Plains to Midwest... Moderate rainfall is likely from the central plains across the Midwest with the frontal system pushing ahead from the stalled occluding low over SD Sunday night into Monday night. Given the excessive rain recently in the Mid-MS Valley up the OH Valley, some concern was given to the rainfall with this frontal system. However, it appears progressive enough to keep areal averages below an inch, particularly east of the MS river where antecedent conditions are wetter. ...Gulf Coast and Deep South... The cold front ahead of this low drapes from east TX and pivots to a more east-west orientation through Monday night as it spreads to GA/SC. PW of 1.75 inches is two standard deviations above normal which may allow a narrow swath of areally averaged one to two inches of rainfall near the gulf coast. Some spread was evident for Day 3 with 12Z NAM/UKMET farther inland than the 12Z GFS/ECMWF which are near the coast. WPC QPF was based on a blend of the 12Z GFS/ECMWF. FFG is quite high in this area with decreasing rain from the past week toward the Gulf Coast. A GFS/ECMWF solution does not warrant an excessive rainfall risk as of this time...however any inland shift would raise the specter of flooding. ...Northeast... A shortwave trough pushes south from New England Sunday night. Up to a quarter inch areal average is expected for Cape Cod Sunday night which is on the heels of the recent excessive rainfall. Burke/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml