Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 459 PM EDT Mon Mar 12 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 13/0000 UTC thru Mar 16/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...California... Precipitation moves into northern CA late tonight as a cold front with good low- to mid-level frontogenesis moves ashore and moves steadily into central CA with time. Given the well defined nature of the front and good mid/upper level forcing forecast...would expect to see a broad area of steady precipitation. The combination of precipitable water values of just over 1" and MU CAPE values of 200-300 J/kg should yield hourly precipitation totals up to 0.6" across the favored terrain of northern CA into the Sierras late tonight into Tuesday morning. The 12z HREF probabilities of 0.5" an hour agree with the rule of thumb, with a spike above 50% in the northwest CA coastal ranges 07-08z and across the Siskiyou and Sierra Nevada intermittently from 06z-18z. Model agreement was good, allowing for a multi model blend for QPF (used a 12z GFS/00z and 12z UKMET/12z ARW/00z ECMWF compromise). This keeps reasonably good continuity from WPC's previous forecast. See our winter weather suite of products for more concerning this system. ...Mid Atlantic into the Northeast... A developing coastal low intensifies as it moves offshore the Mid Atlantic coast. Frontogenesis in the 650-850 hPa layer, eventually within the system's comma head, is expected to be the main player for heavy precipitation with this system. The combination of 100-200 J/kg of MU CAPE and 0.5-0.7" precipitable water values late tonight and early Tuesday morning should be able to yield hourly precipitable totals up to 0.3-0.4". The 12z HREF probabilities of 0.25" agree, showing 50-90%+ chances of such from 06z-20z. With the best frontogenesis stuck at any one spot of southeast MA for 9-12 hours, this should be able to achieve total liquid equivalent above 2" with local amounts of 3-4" possible. The QPF was based on a blend of the latest GFS/UKMET/ARW/ECMWF which allowed for some sense of continuity across southeast New England while scaling back amounts across Maine. See our winter weather suite of products for more concerning this storm. Days 2/3... ...The West... Deep upper-level trough/closed low will move into the West Coast with height falls moving eastward across the region. The moisture will drop off for a while with precipitable water values starting at 0.75 inches dropping below 0.50 inches by Wednesday morning. The precipitable water values will come back up to 0.50 inches on Wednesday evening into Thursday evening. The system will produce maximum qpf amounts ranging from 1.75 to 2.15 inches over the Sierras with a secondary maximum over the Blue Mountains into the Wasatch ranging from 1.10 to 0.75 inches on Tuesday evening into Wednesday evening. On Wednesday evening into Thursday evening the qpf amounts will drop off over the Sierras with maximum qpf amounts ranging from 0.25 to 0.50 inches. The maximum qpf amounts on Wednesday evening into Thursday evening are over the Sawtooth Mountains into parts of the Bitterroot Mountains with maximum qpf amounts ranging 0.50 to 0.75 inches. WPC went with a blend of the ECMWF and the UKMET on Tuesday evening into Wednesday evening as the starting point for the manual graphics. On Wednesday evening into Thursday evening, WPC went with a blend of the ECMWF and the UKMET a long with SREFMEAN. The GFS was not included due to having heavier qpf amounts over the Sierras. ...Northeast... A deep surface low off New England Coast will slowly move into the Canadian Maritimes and slowly fill from Tuesday evening into Thursday evening. Circulation around the associated upper-level low will stream moisture into New England with precipitable water values of 0.50 inches with the low-level flow of 50 to 55 knots out of the east with the flow dropping 35 to 30 knots as the precipitable water values drop below 0.50 inches by Wednesday morning. The low-level flow will continue to weaken coming out of the north at 15 to 10 knots by Wednesday evening and becoming westerly, offshore, at 35 to 30 knots by Thursday evening. The storm will produce a maximum qpf amounts ranging from 1.25 to 1.35 inches over Maine on Tuesday evening into Wednesday evening. The qpf will drop off on Wednesday evening into Thursday evening with the maximum qpf amounts ranging from 0.25 to 0.40 inches over parts of Northern New England. ...Big Bend of Texas... A short wave off of Northwest Mexico will move inland moving across Southern Texas through Thursday evening. Northwesterly flow off the Western Gulf of Mexico will stream moisture into the Big Bend region with precipitable water values of 0.50 inches with the flow becoming westerly on Thursday into Thursday evening. QPF amounts will be low with qpf amounts near 0.10 inches on Tuesday evening into Wednesday evening and more modest on Wednesday into Thursday evening near 0.01 inches. Roth/Ziegenfelder Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml