Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 355 AM EST Fri Mar 09 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 09/1200 UTC thru Mar 12/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Eastern Great Lakes - Northeast... Snow showers will continue across much of central and northern New England and NY through the day 1 period. Anticipate one local maximum across northern Maine as the closed mid level low tucks an area of surface low pressure westward into southern Canada. Elsewhere more showery conditions underneath the closed low in the northwesterly flow can be expected. Local maximums will occur where lake enhancement and orographic impacts are maximized. Models were close enough that WPC utilized a blend of the high res and global guidance...with more weight given to the high res guidance where lake effect and orographic enhancement are expected. ...Northern CA and Pacific Northwest eastward into the Northern Rockies... Moderate showers will continue this morning across portions of northern CA and southern OR as a weak wave moves ashore. However anticipate mid level ridging to build into the pacific northwest through the day...which should result in a decrease in shower coverage and intensity as we go through the morning hours. The focus for light QPF amounts will shift east into the northern Rockies and portions of Montana. A pretty compact wave will move across these areas...but limited moisture and progressive forcing will limit QPF magnitudes. ...MS valley into the OH and TN Valley... Mainly light QPF amounts are expected through the day 1 period from the central Gulf Coast north northeastward into Kentucky. Precipitation should break out this evening across portions of the Ohio Valley...shifting east into portions of southern WV and southwest VA. Note a pretty good upper jet moving southeastward placing the region in the favorable left exit region...along with an increase in 850mb moisture transport. Do note some model differences...with the 12z ECMWF and 0z UKMET quite a bit drier than the GFS. The 0z suite of high res models are more in line with the wetter GFS. Went with a compromise solution...although given the upper jet and frontogenesis forecast...did lean more towards some of the wetter solutions. The approach of the exit region of an upper jet quickly moving east out of the Pacific, and a weak mid level wave...will result in increasing southerly flow across the lower and mid MS valley by tonight. This should result in the development of some light showers...with any heavier convection expected to remain offshore in the Gulf through the day 1 period. Days 2/3... ...California...Great Basin...Desert Southwest... A very positively tilted upper trough currently over the eastern Pacific contracts east as a shortwave trough Friday night and reaches central/southern CA on Saturday before shearing-out downstream into the Great Basin/Southwest on Sunday. A plume of anomalous pacific moisture (PW around 1 inch; 2 to 2.5 standard deviations above normal) content from the subtropics will surge into Central/Southern CA on Saturday and this onshore component will linger into Saturday night, as the upper dynamics fade while advancing downstream. Slight spread on progression of this feature persists with 00Z guidance including the ECMWF continuing to be more progressive than GFS. WPC followed a blend compromise of 00Z guidance, with moisture spreading into Central/Southern CA on Saturday. Maintained a slight risk for excessive rainfall on Day 2 (12Z Sat-12Z Sun) where areal averages are 1 to 1.5 inch. A focus more toward the central CA coast allowed all of the Bay area metro to be removed from this risk (except for the Santa Cruz mountains). Burn scars in southern California are in the risk area with the best forcing/fetch coming ashore Saturday night. ...Central/Southern Plains and Mid/Lower MS Valley into South/Southeast and Mid-Atlantic... High variance in the 00Z guidance suite with the southeastern CONUS low development Day 2 through Day 3. All guidance has some semblance of a surface low development with meridional differences based on presence of an degree of phasing northern and southern stream energy. The 00Z ECMWF and 00Z UKMET track farthest south with the least phasing while the 00Z GFS maintains a more robust phase with a farther north resultant low that tracks well into the Mid-Atlantic on Day 3. The 00Z Canadian seemed to be a decent operational compromise for QPF placement, but given the spread in the whole 00Z suite, GFS and ECMWF ensemble means were heavily weighed. This results in a more disperse QPF, so some magnitude from the 00Z GFS/Canadian were incorporated during QPF creation. The Day 2 (12Z Sat-12Z Sun) Slight Risk for excessive rainfall over the Deep South was maintained given convergent signals Saturday night over AR to AL. Wide variance even among the Day 2 solutions (mainly involving the presence/strength of southern stream energy) made for a below average confidence forecast. 1.25 inch PW is about 1.5 standard deviations above normal. The key will be what the moisture can converge on and the stronger 00Z GFS produces more QPF than the weaker 00Z ECMWF. Elements associated with the Day 3 (12Z Sun-12Z Mon) low in the southeastern CONUS are just coming ashore, so hopefully the 12Z guidance suite trends toward a more apparent solution. The location and strength of a cold front associated with the low will be the key feature for a flash flood threat on Day 3 in the far southeastern CONUS/FL where 1.25 to 1.5 inch PW will reside. Given the disparate solutions, no excessive rain risk was raised at this time for Day 3. ...New England... A blocking ridge stretching from Newfoundland to Labrador keeps low pressure over Maine/the Canadian Maritimes Saturday with wrap around precip over northern New England. Please see the HSDQPF for further information on snow in this region. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml