Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 556 PM EST Mon Mar 05 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 06/0000 UTC thru Mar 09/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern Plains/Great Lakes... The negatively-tilted upper low currently centered over the northern plains is forecast to slide east-southeast into the mid Mississippi valley as it comes under the influence of a second closed system rotating south across western Ontario on Tue. Overall, models are in good agreement through the period, showing the system gradually weakening on Tue. Guidance continues to indicate two areas of heavier precipitation -- one within the area of enhanced upper divergence ahead of the low and the other along a trailing surface boundary extending from the low back into the Dakotas. WPC QPF generally followed a blend of the GFS and the HREF. Refer to the WPC Winter Weather products for details concerning additional heavy snow potential across this region. ...Gulf Coast/Deep South/Southeast... Cold front trailing the previously noted low is forecast to drop southeast from eastern OK and central TX toward the western Gulf Coast and lower Mississippi valley this afternoon. Moisture increasing ahead of the front through the afternoon into the evening hours (PWs climbing in excess of 1.5 inches from central Mississippi to the lower Texas coast), along with some moderate instability early on, will support showers and thunderstorms, producing moderate to locally heavy accumulations across the region this afternoon and evening. Rainfall rates are expected to diminish as instability wanes during the late evening into the overnight hours. Could see an increase in rainfall rates with the return of daytime heating, raising the potential for locally heavy amounts as the front pushes further east across the Southeast and eastern Gulf Coast region Tue afternoon and evening. WPC QPF relied in large part on the HREF across this region. Elsewhere, no hydrologically significant precipitation is expected. Days 2/3... ...Pacific Northwest/Northern Rockies... A closed mid level low approaches the Pacific Northwest coast late on day 2 into day 3. A Pacific moisture plume ahead of the mid level system, focused on a warm front, feeds the development of locally heavy rainfall, especially on day 3. There was generally good model agreement with the placement of the synoptic scale features, so the WPC QPF was based on a blend of the 12z ECMWF/GFS. Day 2... The best lift associated with a closed mid level low approaching the Pacific Northwest coast on day 2 is expected between 07/18z and 08/00z. Ahead of the closed mid level system, the low level southerly flow keeps much of the deeper Pacific moisture offshore during day 2, but some of the moisture is funneled along a warm front extending over southwest OR and northern CA. QPF amounts are expected to be modest, ranging between 0.10 and 0.25 inches. Day 3... The closed mid level low approaches the Pacific Northwest early on day 3, before weakening into a short wave that crosses OR/WA and the northern Rockies late in the period. Ahead of the mid level system, a 40 to 50 knot low level southwest flow transports 0.50 inch precipitable water air across the Pacific Northwest from WA into northern CA. Model soundings showed marginal instability associated with the weakening mid level system as it crosses the coast, which could briefly enhance rainfall rates. There was multi model signal for 1.00 to 2.00 inches of qpf extending from the coastal range in OR south into the northern Sierra Nevada range. The highest amounts are expected over southwest OR, where the upslope flow and moisture maximize. Hourly rainfall rates appear as though they will remain below 0.50 inches here on day 3, so outside of local runoff issues, flash flooding is not expected. Further east, as the mid level system crosses the Northern Rockies late on day 3, the combination of synoptic scale lift and upslope flow make the moisture of the paltry moisture in the column to produce local 0.35 inch qpf amounts over the higher peaks in southern and central ID. ...Great Lakes/Ohio Valley... A closed mid level low over IL early on day 2 crosses the OH valley, as another closed mid level low over the upper Great Lakes on day 3 weakens as it heads east. Dynamic lift and steepening lapse rates make the most of the moisture in the column to produce QPF each day. There was generally good model agreement with the placement of the mid level systems, so the WPC QPF was based on a blend of the 12z ECMWF/GFS. Day 2... As a closed mid level low tracks from IL to northeast OH on day 2, the lift and steepening mid level lapse rates associated with the mid level system will make the most of the moisture in the column to produce a large area of 0.10 to 0.25 inches of qpf extending from eastern IN and MI into western PA. Areas of 0.25/0.50 inches of qpf are possible over southern MI/northern OH (close to the track of the mid level system) and across the higher terrain of WV (where upslope flow becomes a factor). Day 3... A closed mid level low over the upper Great Lakes weakens as it moves east during day 3, reaching a position over western PA/western NY state by the end of the period. Along the track of the closed mid level low, an axis of 0.25/0.50 inches of qpf was placed over western NY state. This area was surrounded by a much larger area of 0.10 to 0.25 inches of qpf covering much of MI into northwest PA. ...Mid Atlantic/Northeast... As a negatively tilted long wave trough tracks from the Great Lakes/Ohio Valley to the East Coast on day 2, a closed mid level low forms over NJ. Lift associated with the mid level systems induces surface cyclogenesis off the mid Atlantic coast, which tracks north along the coast with the mid level system during day 3. The 12z model suite has begun to converge on a solution for the tracks of the mid level and surface systems, with the 12z NAM seemingly a westward outlier. With this in mind, the WPC QPF was based on a blend of the 12z ECMWF/GFS. Day 2... A negatively tilted long wave trough moving from the Great Lakes toward the East Coast early on day 2 spawns a closed mid level closed low over the northern Delmarva/NJ before the end of the period. The lift associated with the newly developing mid level system induces surface cyclogenesis off the NC coast early on day 2, which tracks of the NJ coast by the end of the day. Ahead of the developing mid level and surface systems, a 40/50 knot low level easterly flow transports 0.50/0.75 inch precipitable water across the northern Mid Atlantic states into southern New England, with the best mid level lifted centered on 07/18z. Model soundings showed elevated instability in the moisture plume reaching into the southeast PA and northeast MD, as well as much of NJ. The positioning of the low level flow suggested a QPF maximum of near 1.30 inches over southeast PA, a placement supported well by the 12z GFS. Further north...the best lift and moisture in the low level flow is expected to produce an area of 0.75/1.25 inches of qpf extending across eastern PA/much of NJ/southeast NY state into CT/MA/RI before 08/00z. The highest amounts are expected closer to the coast, where model soundings showed the best potential for elevated instability. While locally heavy rainfall could result in local runoff issues, flash flooding is not expected on day 2. Day 3... The surface and mid level systems track from off the central NJ coast to a position near the ME coast during day 3. The 40/50 knot low level easterly flow moves north ahead of the surface low, affecting much of northern New England. Frontogenetic banding is expected northwest of the mid level system, and there is a multi model signal for 1.75 to 2.50 inches of qpf extending from eastern MA across southeast NH into coastal southwest and central ME. Only the 12z NAM is further west that this line, and that model appears to be a western outlier at this point. Locally higher amounts are possible where lift interacts with a developing coastal front, but model soundings suggest that any elevated instability remains offshore. While there is still some spread in the thermal fields for day 3 across New England, it appears as though at least some of the QPF will occur as rain. After collaborating with WFOs BOX/GYX, it was decided that a Marginal Risk was not needed at this time for day 3. Heavy snowfall is possible on days 2 and 3, especially over interior portions of New England. Please refer to the most recent suite of WPC Winter Weather products for more information. ...South TX/FL... A cold front extending from the Southeast across south TX during day 2 becomes the focus for deep moisture and instability to produce locally heavy QPF amounts. There was generally good model agreement with the timing of the front, and the placement of the maximum QPF amounts, so the WPC QPF was based on a multi model blend. A cold front extending from low pressure over the East Coast on day 2 crosses FL and far south TX. Ahead of the front, precipitable water values near 1.50 inches and marginal to locally moderate instability will fuel convection that produces areas of 0.25/0.50 inches of qpf across northern and central FL, well as far south TX. There should be sufficient movement with the convective cells to mitigate the threat for flash flooding, though locally higher rainfall amounts are possible. Pereira/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml