Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 551 PM EDT Tue Mar 13 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 14/0000 UTC thru Mar 17/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Afternoon update... Few changes were needed to the previous day 1 forecast. Some of the global and hi-res/CAM models showed enough of a consistent signal for us to trim the back edge of precipitation in New England in the 6-hourly forecasts. The differences summed up over 24 hours, though, was minimal. Tweaked amounts over the western U.S. but the WPC QPF remained close to the tightly-clustered solutions. New England---northern to western NY state---eastern Great Lakes into the central to southern Appalachians The rapidly deepening winter storm off the southeast New England coast this afternoon will reach its lowest pressure near the beginning of the upcoming day 1 time period---weakening after this into the day on Wednesday as it pushes across Nova Scotia---P.E.I. and New Brunswick. The emphasis for the heaviest snows in the well defined comma head/deformation precip band will be shifting northward at the beginning of the day 1 period into primarily northern NY state into northern New England from northern VT---northern NH into much of the state of ME. Model qpf solutions are tightly clustered across these areas---supporting heavy snow potential in the 6-12"+ range. To the west of the primary deformation precip band---the upstream closed low will be sinking east southeastward from the lower lakes into the northern Mid Atlantic and into southern New England. In the wake of this system---the west northwesterly low level flow off of lakes Erie and Ontario will be strengthening. This will support lake effect/lake enhanced heavy snow potential from far northern OH---northwestern PA into western NY to the south of lake Ontario and east of lake Erie. Lighter snows expected southward into the central to southern Appalachians in a region of persistent northwesterly to westerly upslope flow. See the latest qpfhsd for additional winter weather information across these areas. California---Great Basin---northern Rockies---Pacific Northwest A series of height falls expected to rotate through the base of the full latitude mid to upper level trof off the west coast day 1. The lead area of height falls will push inland this evening through California into the southwest--across the Great Basin and into the northern Rockies. This will push the primary frontal boundary and axis of much above average pw values inland across these areas. This will be followed by additional post frontal height falls rotating inland into northern to central California during the day on Wednesday. The net effect will be to support a rather large area of moderate to locally heavy precip totals. The best chance of heavy precip will be through the Sierra where the greatest upslope flow is expected. Areal average 1-2"+ totals expected in the Sierra with generally .10-.25"+ totals elsewhere from the Pacific Northwest---northern Rockies---through the Great Basin and across the remainder of California. Days 2/3... ...The West... A pair of mid- to upper-level lows move through the region this period, with the system near the OR/CA border on Friday into Saturday the stronger of the pair. Precipitable water values are expected to be 0.5"+ through the period, which along with ~500 J/kg of CAPE forming each afternoon in the Sacramento Valley should increase totals in the northern Sierra Nevada. The big question for the Sierra Nevada Wednesday into Thursday morning is how much precipitation is expected. The 00z ECMWF ensemble mean was significantly wetter than the other guidance in the Sierra. Because of this, the amounts in the mountain chain resemble the 00z GEFS-based re-forecast/12z Canadian output as a compromise solution. Across the Great Basin, low- to mid-level frontogenesis is expected to lead to modest totals. In general, used a compromise of the 14z National Blend of Models, WPC QPF continuity, 00z/12z ECMWF, and the 06z-12z GFS to lead to the precipitation pattern depicted here. With higher freezing levels and somewhat higher amounts of precipitation expected between March 15-16 12z, added a marginal risk for excessive rainfall to account for the modest increase in 850 hPa and forecast instability. As usual, burn scars would see the highest concern for any heavy rains. ...Central Plains/Front Range of Rockies/Mid-South... A system aloft is expected to come ashore over southern CA Thursday and then cross southern CO Thursday night. Surface low pressure ahead of this shortwave will develop over KS Thursday night. The combination of low-level moisture from the Gulf of Mexico and mid- to upper-level moisture from the Pacific offshore southwest Mexico is expected to saturate the atmosphere region-wide. An easterly component north of the low looks to make a max precip area Thursday night across eastern WY where upslope and upper level forcing combine to enhance precip. Adding to the mischief are pockets of instability (500-1000 J/kg) near the WY/CO/NE border Thursday afternoon and another pocket moving across KS from Thursday night through Friday which should aid precipitation production. While the manual QPF started with a combination of WPC QPF continuity, the 14z National Blend of Models output, 00z-12z ECMWF, and 06z-12z GFS ideas, using 06z GFS output led to a compromise with position of the maximum across eastern WY and a more ECMWF-like pattern in the central Plains. The pattern across the Mid-South more closely resembled a compromise of the 12z GFS/12Z ECMWF. ...Western Gulf coast... Return moisture flowing over a stubborn boundary near the LA and Upper TX coasts should lead to showers and thunderstorms from Thursday night into Friday as instability builds within the warm sector of a spring-like storm system. Temperatures at 700 hPa of 6C+ appear constrained to south TX, which should lead to a minimally capped atmosphere along the upper TX coast and across LA. Broad 850 hPa inflow of 25-40 kts combined with MU CAPE values ascending towards 2500 J/kg and 35-50 kts of 0-6 km bulk shear will attempt to organize convection into short bands as the flow through the troposphere is fairly unidirectional out of the west-southwest to the southeast of a deep layer low moving across the Central Plains. Hourly rain totals up to 1.75" should be possible within this environment. With such a favorable environment for precipitation, it is a surprise (if not disappointing) that the guidance is so dry, particularly the 12z GFS. Used ideas from the 06z-12z GFS mass fields and QPF output of the 00z-12z ECMWF, 12z UKMET, 12z NAM, and 12z Canadian to help come up with the pattern in this region, staying slightly wetter than those pieces of the guidance which was hinted at by our in-house experimental bias corrected QPF. There is some concern that the precipitation could try to streak out quicker like the 12z Canadian advertises. ...Northeast/Great Lakes... Cyclonic flow combined with cold air aloft and relatively mild lake temperatures are expected to lead to skirmishes of lake effect precipitation downstream of the Lakes with time as a system moves by the region. A compromise of the available guidance was used here. Oravec/Bann/Roth Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml