Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 403 AM EST Wed Mar 07 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 07/1200 UTC through Mar 10/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Great Lakes/Northeast/FL... Dynamics from an upper level trough have induced a cyclone which is deepening as it moves east of Kitty Hawk, North Carolina. The low is expected deepen more rapidly as it moves up the coast offshore New England. As it strengthens, frontogenesis in the 850-650 hPa layer strengthens across the northern Mid-Atlantic and coastal sections of New England, which will be the main focus for heavy precipitation, mostly wintry. The guidance is disagreeable concerning how far north and northwest to extend heavy precipitation across northern New England and eastern NY, with the mesoscale guidance a bit more northerly than the global/NAM guidance. For now, maintained reasonable continuity and left the 00z ARW out of the mix. Amounts across eastern NY and central PA were trimmed back a little from WPC continuity as the 00z ECMWF shifted a touch to the east. Another highlight is a deep layer circulation over MI/Lake Huron where a combination of low level frontogenesis and lake effect/enhancement should be able to afford respectable totals near the Tip of the Mitt (northern Lower Peninsula) with local 0.25"+ spots across portions of the Upper Peninsula of MI. No changes to continuity were necessary here. Down south across FL, modest precipitation is expected as the system's cold front moves through the central into the southern peninsula. Went closest to the 00z WRF NSSL for amounts and placement in southwest FL. The day 1 QPF was composed of a blend of WPC continuity, the 00z NAM, the 00z GFS, the 12z ECMWF, the 00z NMMB, and the 02z National Blend of Models with some components of the 00-06z in-house bias corrected QPF, 00z UKMET, and 00z ECMWF factored in manually. For winter weather/snow impacts, consult the WPC suite of winter weather products. ...Northern CA/Northwest... Onshore flow with precipitable water values of 0.5-0.75" near and ahead of a frontal boundary is expected to lead to modest totals across northern CA and light amounts across the Northwest this period. Used a blend of model guidance and WPC continuity to come up with the QPF in this region, which ended up best resembling the 00z GFS. Days 2/3... ...Pacific Northwest and Northern/Central California into Northern Rockies... A weakening closed middle-level low approaching the Pacific Northwest Coast will eject multiple short waves into the Northwest. This stream of shortwave energy will help beat-down an upper-level ridge initially in place over the region. Onshore flow of 35 to 45 knots across the Northwest/California will draw an airmass with precipitable water values over 0.50 inch. The result will be increasing coverage of precipitation along the coastal ranges of WA, OR, and northern CA that spreads into portions of the Great Basin. Heaviest precipitation amounts are expected in the coastal ranges, and the Cascades as the moisture plume encounters the complex terrain and is forced to ascend as a result of orographics. In due time, precipitation should spread across the Bitterroot and Sawtooth ranges into the northern Rockies by the end of day 3. There was a general uptick in the model QPF that WPC followed. WPC used a blend of the GFS/ECMWF with some input from the higher resolution version of the GFS to help depict precipitation amounts/placement in the mountains. ...Great Lakes into the Central Appalachians into the Northeast... The primary area of low pressure will be starting the day 2 period over the Gulf of Maine and moving towards the north northeast. The system should then begin to weaken as the center of the low moves inland over Maine. There should still be a period of strong onshore flow to the east of the low center...although the best low H85 warm advection and differential vorticity advection have been shunted north and east of Maine. The 07/00Z GFS does show a period when the upper level divergence increases across much of the northeast on day 2...so tended to give credence to the somewhat higher QPF shown by the GFS/ECMWF that lingers into early day 3. This area of low pressure and another surface low pressure center dropping south over the eastern Great Lakes region by a trough of low pressure at the surface and by a broad cyclonic circulation over the northeastern part of the country. Generally light precipitation should linger across parts of the northeastern U.S. along this surface trough and in association with the Great Lakes surface low. While there should be a broad shield of light precipitation over the Great Lakes that gradually tapers off with time as the upper system weakens, precipitation should linger over parts of the west-facing Appalachians on both day 2 and day 3 as cold air with high relative humidity is forced upslope by broadly cyclonic flow. ...Western Gulf Coast/Southern Plains... Return flow off the Western Gulf of Mexico will begin to flow northward over the Plains on Thursday morning with precipitable water values of 0.75 inches that will increase to 1.25 to 1.50 inches over the Western Gulf Coast. The resulting QPF looks to be light through day 3 without substantial upper support or a low level boundary to work with. A consensus value of the ECMWF and GFS was used in preparing the WPC manual QPF graphics. Roth/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml