Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 451 PM EDT Wed Mar 21 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 22/0000 UTC thru Mar 25/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Western U.S... On Day 1, the models target California for the heaviest precipitation with several inches of liquid equivalent precipitation in the coastal ranges of southern CA and along the spine of the CA Sierra Nevada range. One distinct area is within the plume of deepest moisture streaming onshore into southern CA into the southern CA Sierra Nevada ranges. The integrated water vapor imagery is well above normal and the slow movement of the max precipitable water values in the streaming streaming onshore in the counties north of Los Angeles combines with 850 mb convergence and 300 mb divergence to produce areas of heavy rain, with ascent aided by windward terrain. The continuation o9f the high moisture and water vapor transport into interior southern Ca leads to maxima along the foothills and mountains of interior CA with heavy Sierra Nevada snows, with several feet likely over the next day and a half. A second area covers northern CA to the OR border as the northeast Pacific upper low and trough drift towards the coast...with a jet max streaming onshore and inland into the ranges of ID and adjacent western MT. The upper jet moving into NV Thu afternoon supports upper divergence maxima streaming northeast from the CA Sierra across NV into UT with a maxima near the northern Wasatch/Uintas of UT, driving precipitation in windward terrain. QPF was derived from a combination of the 06z high resolution ensemble forecast mean along with the 12z runs of the wrf ARW/nmmb/arw 2/nam/gfs. See the winter weather forecast amounts and snow probabilities for heavy snow potential in the mountains of CA ...Northeastern U.S... The model indicate that as the low pressure off the coast moves slowly northeastward overnight. the heavy snow event in the mid Atlantic states crosses Long Island New York and southeast New England before heading north into the Canadian maritimes. The 850-700 mb front and convergence along it should persist as a focus for heavier, banded precipitation, creating a swath of maximum QPF along its path in Long Island to eastern MA this evening and then tapering in Long Island as the front departs. An enhancement develops as easterly flow onshore into eastern MA produce coastal convergence and aids synoptic scale lift tonight in producing ascent and snow. As the circulation continues northeast Thursday, the winds back and then the onshore flow of mid level moisture and lift over the 850-700 mb front occurs up the coast in downeast Maine. Soundings and surface observations in interior New York and New England indicated very dry air in place, which has provided for a sharp northern edge to the snow. The dry air counter acts the moisture fluxes to prevent heavy snow from extending further inland into Maine. The models are not in total agreement yet, as the 12z nam was heavier than other forecasts. Given the WRF ARW/ARW2/NMMB/Canadian regional GEM are drier, manual QPF cuts back on NAM amounts in eastern Maine on Thu. WPC favored a QPF blend of the HREF mean with the global model consensus. See WPC Heavy Snow graphics and discussion for potential amounts and probabilities. Day 2 California The axis of much above average pw values---4 to 5+ standard deviations above the mean---will gradually be forced to the south and east of California from late Thursday into Friday. Before its departure, moderate to heavy rainfall will be ongoing at the start of the forecast period. Precipitation rates will be lessening by the start of the day 2 forecast period at 23/00Z as the axis of the anomalous moisture flux and PW axis pushes eastward. Runoff issues remain a great concern into Thursday evening with any additional rain falling over the recent burn areas across the Transverse range including the Canyon--Sherpa---Whittier---Rey---Thomas---Sand---LaTuna---Bluecut and Pilot Burn area. The day 2 excessive rainfall outlook--including the high risk area was maintained over the western portions of the Transverse range---with the moderate area covering the above mentioned burn scars. Heavy snows likely through the Sierra. See the latest QPFHSD for additional winter weather information. Great Basin---Northern Rockies The inland moving height falls through the Great Basin---central to northern Rockies will spread the above mentioned axis of much above average pw values northeastward day 2. This will support widespread moderate to locally heavy precipitation through the Great Basin and into the Northern Rockies. Heaviest totals expected from the Wasatch/Uintas into the northern Rockies from far eastern ID/western WY into central to northern ID. Pacific Northwest The low level flow that is forecast to have a mostly terrain parallel trajectory day 1 across the Pacific northwest is forecast to veer and take on an onshore direction as height falls rotate inland on the south side of the south and southeast side of the strong closed low forecast off the B.C. coast. Widespread moderate to locally heavy precip likely from the Washington-Oregon Cascades west to the northwest California coast range---Oregon coast range into the Olympic range. A general model compromise worked well here given comparable placement and precipitation forecasts from the NCEP and non-NCEP global models. Northern plains Strong isentropic lift will be spreading eastward into the Northern Plains by the end of the day 2 time period associated with the above mentioned height falls pushing through the Great Basin. PW values will be rising to much above average levels at this time---supporting increasing precipitation coverage from northeast MT into western portions of ND and SD. Model consensus is for moderate totals across these areas during the day 2 period---with the WPC qpf finding no reason to deviating from this. Eastern New England The comma head/deformation precip band with the storm storm will be pulling away from the coast with a corresponding decrease in areal coverage and intensity of precipitation. Some light precipitation will be lingering in the cold, cyclonic flow around the storm that should result in some enhancement by the Great Lakes. Day 3 Northern California into the Pacific Northwest and Great Basin to the Central Rockies The strong closed low off the B.C. coast will sink slowly southeastward to off the Pacific Northwest coast from late day 2 into day 3. This will maintain deep layered cyclonic flow into the Pacific Northwest and northern California. PW values not expected to be anomalous in this deep layered cyclonic flow---with values below seasonal norms. Heaviest precip totals expected over parts of the Sierra---northwest California coast range into the southwest Oregon coast range where PW values will be greatest and the upslope component the strongest. Aerial average 1-2"+ totals expected across this area--with aerial average amounts in the .25-50"+ range north of this into the Pacific Northwest. Precipitation will be waning during the latter part of the day 2 time period from the Southwest---Great Basin into the central Rockies as height falls press northeastward from these areas and into the Central to Northern Plains. For the day 3 period---additional light to moderate totals possible Northern-Central Plains---Lower MO Valley---Mid to Upper MS Valley---TN Valley The above mentioned height falls exiting the Great Basin/central Rockies region and pushing into the central to northern Plains will support an expanding area of precipitation across portions of the Northern to Central Plains into the lower MO---mid to upper MS valley region. WPC continued to favored a slightly farther south qpf axis---especially once the system reaches the Tennessee Valley late on day 3. The GFS fit in between the slower/drier ECMWF and the much faster 21/12Z runs of the NAM. Both the GFS and ECMWF continue to show convection developing in an axis of instability Friday night/early Saturday over eastern portions of the Central Plains---Lower MO Valley-Mid MS Valley region. Accumulating snows possible on the northern portion of this broad precipitation shield--see the latest QPFHSD for additional winter weather information. Petersen/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml