Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 537 PM EDT Thu Mar 29 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 30/0000 UTC thru Apr 02/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central Gulf Coast into northern Florida... Lead frontal system associated with the ongoing convection moving through the central Gulf states has picked up momentum since yesterday and will lift northeastward this evening and overnight. Upper dynamics have been waning but lingering inflow from the Gulf will maintain precipitable water values 1.25-1.50" (about +1.5 to +2 sigma) ahead of the front. This supports 0.5-1" QPF along and north of I-10 (local max perhaps twice that) from about Mobile eastward after 00Z. Upper jet elongates and strengthens to the northeast, stretching out the precip overnight into tomorrow. Tail end right entrance region may support another area of heavier rain into north central Florida tomorrow afternoon as the front approaches. Slight risk of excessive rainfall was maintained over southeastern Louisiana eastward to the western Florida panhandle overnight with embedded heavier convection, especially affecting urban areas. ...Ohio valley into the eastern Great Lakes... Boundary atop the front in the southeast will consolidate into one entity with the front to the south by tomorrow as the surface low lifts northeastward through the upper Ohio valley and into the northeast by tomorrow morning. Comma head/deformation precipitation will continue northeastward into the eastern Great Lakes this evening and across northern NY state into northern New England tonight/early Friday. Models still differ on timing of the front eastward to the mid-Atlantic coast and how quickly the precipitation shield moves along with it, but generally favored the HREF mean for placement with adjustments on amounts per the other guidance. Precip amounts were generally in the .50-1" range across these regions with the most coverage across northern New England. ...Northern Rockies into the Northern Plains... Cold front pushing through the Dakotas/Minnesota will support a lead area of light snow tonight while the more significant area lags behind with an approaching surface low. Left exit region upper jet will help expand snow across eastern MT through ND into MN tomorrow with mix/light rain to the south. North-south differences were still unresolved in the guidance, but a middle ground seemed plausible given the split in the hi-res and global guidance with amounts held to just under 0.5" through 00Z/31. ...Pacific Northwest/southwest Canada... Mid-level shortwave will push into southern BC as a surface cold front pushes into western Washington early Friday. Heaviest precip totals likely to the north of the U.S./Canadian border as the jet streak slides into northern ID and northwestern MT but .1-.25" totals will affect the western Olympic peninsula/Olympic range and .5-1"+ in the Cascades liquid equivalent. HREF mean offered a good starting point. Days 2 & 3 ...Pacific Northwest/Northern and Central Rockies/Central Plains... Short wave energy tracking from northern ID/western MT into WY during Day 2 provides synoptic scale lift over an arctic front stretching from western MT into eastern CO. The short wave then crosses the central Plains early during Day 3 as the arctic front moves eastward. A closed mid level low over Vancouver Island on early on Day 3 drops into northern WA by the end of the period. There was generally good model agreement with the overall synoptic setup, so the WPC QPF was based on a blend of the 12z ECMWF/GFS. Day 2... Short wave energy moving over ID/MT early on Day 2 provides synoptic scale lift over the Northern and Central Rockies as an arctic front extends from western MT across central WY into northeast CO. Moisture in the column is lacking (with precipitable water values at or below 0.25 inches), but the combination of mid level lift and upslope flow (augmented by low level convergence along the arctic front) will make the most of the moisture in the column. There was a multi model signal for 0.25/0.50 inches of qpf over the Absaroka Range in northwest WY/south central MT, the Big Horn Range in WY and the Rockies in southeast WY. Day 3... The arctic boundary moves east and weakens during Day 3, as the short wave energy crosses the Central Rockies and the Central Plains. Once again, moisture is a limiting factor for QPF, but the 12z EMCWF/GFS showed an axes of 0.25/0.50 inches of qpf extending from WY across western NE, where local level convergence combines with mid level lift to overcome the dry airmass in place. Further west, a closed low over the northern coast of British Columbia drops southward into northernmost WA during Day 3. Ahead of the short wave, a 20 knot low level west northwest flow focuses 0.75 inch precipitable water air on the higher terrain of western WA/western OR. There was a multi model signal for 0.25/0.50 inches of qpf across the higher terrain of the WA/OR Cascades. ...Upper MS Valley/Upper Great Lakes/OH Valley... Short wave energy tracking from northeast MT across MN/WI to the UP of MI during Day 2 pushes along a surface low and an axis of heavier QPF is expected north of the surface low. There are still some model differences with the track of the surface low, with the 12z ECWMF/12z NAM further north and the 12z GFS/UKMET further south. The GFS has been more consistent with the track of the surface low, so the 12z GFS was blended with the 12z ECMWF to create the WPC QPF, with more weight given to the 12z GFS. As short wave energy moves from MT across the Upper MS Valley during Day 2, it aids in deepening surface low pressure that tracks from SD across the Upper Great Lakes into southeast Ontario. Ahead of the short wave, a 65 knot low level southwest flow transports 0.50 inch precipitable water air north of the surface track from eastern ND across central MN into northern WI/UP of MI. The increased moisture is lifted in the upglide could be arranged into banded precipitation in the left front quadrant of an approaching 130 knot jet streak. The banded precipitation is expected to produce an axis of 0.50/0.75 inches of qpf over this area. As mentioned earlier, there are still some latitudinal differences in the placement of the maximum qpf axis, and the WPC QPF was based most closely on the placement offered by the 12z GFS, given its consistent depiction of the track of the surface low. Heavy snowfall is expected during Day 2 across portions of the Upper MS Valley and Upper Great Lakes. Please refer to the most recent suite of WPC Winter Weather products for more information. ...Southern Plains into the Lower MS Valley... Short wave energy in the fast mid level flow provides synoptic scale lift over a frontal boundary extending across the Southern Plains into the Lower MS Valley during Day 3. Moisture and instability combine to produce convection with locally heavy rainfall along the front. There was generally good model agreement with the placement of the frontal boundary, so the WPC QPF was based on a blend of the 12z ECMWF/GFS. Short wave energy ejected from the Rockies early on Day 3 crosses the Southern Plains and Lower Ms Valley, providing synoptic scale lift over a frontal boundary extending from eastern OK across northern AR into western TN/western KY. The boundary becomes quasi stationary as it becomes parallel to the mid level flow. A 20 knot low level south southwest flow transports 1.00/1.25 inch precipitable water air and marginal instability (some of which is elevated, based on 12z GFS soundings in the region) along and south of the front, mainly after 01/12z. The resultant convection is expected to produce an axis of 0.50/1.00 inches of qpf stretching from far eastern OK across northern AR and far western TN, with lesser amounts further south and east. As the 850-300 mb mean flow becomes better aligned with propagation vectors along and south of the front, the potential for short term training increases. This could allow for higher QPF amounts along the front (both the 12z ECMWF/GFS showed maximum QPF amounts closer to 1.25 inches along the front, but in different locations). These areas received heavy rainfall earlier this week, and wet antecedent conditions could present a flash flood risk with the expected convection. With this in mind, the Marginal Risk was allowed to remain in place for Day 3. ...Northeast... Short wave energy tracks from the LP of MI across northeast NY state into eastern Canada during Day 3, pushing a surface low into Quebec. A frontal boundary extending from the surface low focuses moisture across portions of the Northeast. There was good model agreement with the synoptic scale features, so the WPC QPF was based on a blend of the 12z ECMWF/GFS. Ahead of the short wave and surface low tracking across northern NY state into Northern New England and nearby eastern Canada, a low level southwest flow transports 0.50 inch precipitable water air from WV across PA/NY state into much of northern and western New England. Model soundings showed little in the way of instability (surface based or elevated), which mitigates the potential for locally heavy rainfall. There was good model agreement for a large area of 0.10/0.25 inches of qpf over the aforementioned areas. Local 0.25/0.50 inch qpf amounts are possible eastern of Lake Ontario, where conditions become more conducive for lake enhanced precipitation. ...FL... Weak short wave riding over an inverted surface trough provides synoptic scale lift for convection on Days 2 and 3. There was good model agreement with the placement of the inverted trough and maximum QPF, so the WPC QPF was based on a multi model blend. Day 2... Broad and generally weak short wave energy in the relatively fast mid level flow crosses central and southern Fl late on Day 2. The short wave energy produces broad synoptic scale lift over an inverted trough/weakening frontal boundary extending over central and South FL. Ahead of the surface trough, a low level southwest flow transports 1.50/1.75 inch precipitable water air and marginal to moderate instability across central and South Fl, and the resulting convection is expected to produce an axis of 0.50 inches of qpf here. Locally higher amounts are possible where short term training occurs, but overall the threat for flash flooding very low for Day 2. Day 3... The surface trough meanders across central Fl during Day 3, as additional short wave energy in the mid levels crosses FL. Moisture and instability profiles are similar to those of Day 2, and similar QPF amounts are expected over central FL. Once again, though locally higher rainfall amounts are possible. flash flooding is not expected on Day 3. Fracasso/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml