Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 417 AM EDT Sat Mar 17 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 17/1200 UTC thru Mar 20/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Overview... The large scale features cold air and northwest flow locked in place over the Northeast / Mid-Atlantic. Mean large scale troughing is anchored along the West Coast. In between, the flow is semi-progressive with occasional lee-of-the-Rockies cyclogenesis. The cyclone that formed Friday will shear as it meets the blocked and confluent flow along the East Coast. Still, it will squeeze out a light wintry mix in parts of the mountains and Piedmont, and a swath of rain farther south, including some enhancement near a warm front in North Carolina and southern Virginia. The quasi-stationary front trailing this upper air system will linger over north Texas, and the gradual approach of renewed height falls and jet energy from the West should contribute to a few stronger thunderstorms breaking out near the Texas front and dryline this afternoon. ...Western U.S... Another spoke will rotate around the base of the mean Western U.S. trough, swinging into Arizona by Sunday morning. A separate shortwave will lift into Idaho and western Montana early in the day, and very cold upper air temperatures / cyclonic flow will remain in place over Oregon and Washington. The weather system as a whole remains separated from any substantial deep layer moisture. Locally respectable height falls, terrain enhancement, and steep lapse rates will, however, yield fairly wide coverage of measurable rain/snow west of the continental divide. The locations particularly favored for greater amounts, a half inch liquid or more, appear to be central Idaho near the lifting shortwave, and northern Arizona ahead of the shortwave swinging through the base. Still, we tried to err on the conservative side given PW values near or below climatology most areas. WPC QPF leaned toward the 00z WRF-NMMB...HREF blended mean...and held onto 40 percent continuity from the previous forecast. ...Eastern / Southern U.S... Precipitation should not be too well focused on the large scale, but there are some mesoscale QPF signals from the models, showing up in the upslope enhanced parts of the Virginias, along the warm front in NC/VA, and associated with the combination warm advection and surface-based deep convection event expected in parts of Texas. The SREF 24-hour QPF probabilities have done a little better job depicting some of these details than the GEFS and ECMWF ensemble have in the early spring season, at least on Day 1. We used these SREF probabilities as a guide, and this pointed us toward a QPF blend using the HREF and NAM CONUS Nest among other things east of the Tennessee Valley and along the Gulf Coast. For Texas we gave the WRF-NMMB, WRF-ARW, and GFS more weight, expecting that thunderstorms will form initially ahead of the slight dryline bulge in the Hill Country and then expand / evolve northeast from there. Days 2/3... ...Pacific Northwest/Northern Rockies... After a closed mid level low weakens over the Pacific Northwest early on day 2, a positively tilted long wave trough extends across the Pacific Northwest into the Northern Rockies. There was generally good model agreement with the overall synoptic setup, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. The low level flow ahead of the positively tilted long wave trough across the Pacific Northwest during day 2 transports 0.50 inch precipitable water air to the WA/OR coast. The lift associated with the mid level system is fairly modest, so QPF amounts in the upslope flow should remain below 0.25 inches. Further east, the moisture in the column is less than over the Pacific Northwest, but the upslope flow should make the most of the moisture left in the column. In addition, model soundings showed marginal instability across the Northern Rockies, which could support local 0.25 inch qpf amounts over southwest MT. ...Southern Rockies/Southwest... A long wave trough extending over the Great Basin and Southwest states early on day 2 tracks across the Southern Rockies before a closed mid level low forms on the based on the long wave trough by the end of the period. The combination of upslope flow and synoptic scale ascent is expected to produce locally heavy QPF over CO. In general, there was good model agreement with the synoptic scale system, though there were some differences in the placement of the QPF maxima. In an attempt to mitigate some of the placement differences, the WPC QPF was based on a blend of the 00z NAM/GFS/ECMWF. In advance of the long wave trough crossing UT/NV/AZ during the first part of day 2, surface low pressure forms on a low level baroclinic zone over southeast CO. The surface low deepens as it pulls into the Central and Southern Plains, as a closed mid level low forms at the base of the long wave trough. The moisture drawn northward ahead of the long wave trough becomes involved in the upslope flow across CO, where there was a multi model signal for a large area of 0.25/0.50 inches of qpf across much of the higher terrain in CO. There was also a multi model signal for local 1.25 inch qpf amounts over the Front Range in CO, where the upslope flow makes the most of the moisture in the column. Almost all of the QPF is expected to fall in the form of snow, so there is no flash flood threat with the higher qpf here. Across the plains of northeast CO, there was a multi model signal for 1.25 inches of qpf, but there could be some precipitation type issues here, so at this point the threat for flash flooding appears fairly low. ...Northern and Central Plains... A weakening long wave trough over central MT early on day 2 track toward ND by the end of the period. Ahead of the long wave trough, a low level flow transports moisture and marginal instabilty across portions of the central Plains, resulting in locally heavy rainfall. While there was generally good model agreement with respect to the placement of the synoptic scale systems, there were some differences in the placement of the qpf maxima. In an attempt to mitigate some of the placement differences, the WPC QPF was based on a blend of the 00z NAM/GFS/ECMWF. Day 2... As a weakening long wave trough tracks toward ND during day 2, a 25 knot low level southeast flow transports 0.50/0.75 inch precipitable water air along an inverted surface trough extending from north central KS across western NE into western ND/western SD. The best lift along the inverted trough occurs after 19/00z, when model soundings showed marginal instability in the low level southeast flow. The instability could allow for locally higher QPF amounts across central KS into far western NE, with a model consensus showing local 1.00/1.50 inches along this axis. While there were model solutions with higher QPF amounts (with the 00z GFS showing local 2.00+ inch amounts), the qpf maxima where in different locations along the inverted trough. It has been dry in this area lately, as evidenced by three hour flash flood guidance values above 2.50 inches. So, while there could be locally heavy rainfall associated with low topped convection, the threat for flash flooding appears to the fairly low. Day 3... The mid level trough crosses ND for the Upper Great Lakes during day 3, and moisture on the low level southerly flow could support an area of 0.10/0.25 inches of qpf over NE/ND/SD, with local 0.25+ inch amounts possible over central NE. ...Southern Plains/Lower MS Valley into the Southeast... A long wave trough over the Great Basin and Southwest states early on day 2 tracks across the Southern Rockies, and a closed mid level low forms at the base of the trough over southern KS/northern OK by the end of the period. A frontal boundary extending from surface low pressure crossing the Southern Plains becomes the focus for moisture and instability, which feeds convection would could produce heavy to excessive rainfall. The weakening mid level system moves east and weakens during day 3, reaching the Mid Atlantic coast by the end of the period. There is increasing spread concerning the placement of heavy rainfall associated with the mid level system, so the WPC QPF was based mostly on a multi model blend. Day 2... Ahead of a long wave trough tracking from the Southwest states toward the Southern Plains during day 2, surface low pressure forms over southeast CO, which moves toward southern KS/northern OK by the end of the period. A frontal boundary extending from the surface low across the lower MS valley and Gulf Coast into the Southeast becomes the focus for deep moisture and instability. A 25/35 knot low level southwest flow transports 1.50 inch precipitable water air (which is about two standard deviations above the mean) along and south of the front across central MS/central AL into west central GA. Along this axis, 1.50 inches of qpf was stretched, with the highest amounts expected over central MS, which is closest to the best instability (which maximizes over LA/MS around 19/00z). In general, there was a drop in the QPF amounts associated with the convection along the front, but wet antecedent conditions and the potential for training along the front (as 850-300 mb mean winds become better aligned with the propagation vectors) warrants a Slight Risk area across central MS into central AL. This axis was surrounded by a much larger Marginal Risk extending from the Lower MS Valley to the Southeast states. Day 3... The closed mid level low over northern OK early on day 3 weakens as it tracks across the TN/OH Valley to the Mid Atlantic Coast. As the mid level system weakens, its surface low tracks across the OH Valley, and the cold front extending from this low becomes the focus for moisture and instability. As mentioned earlier, there is a fair amount of model spread concerning how the surface low crosses the Southeast states, and whether the potential for training is realized. For now, an axis of 1.00 to 1.50 inches of QPF was extending across central GA into far southern SC. The 00z ECMWF is much more bullish with the QPF along this axis, but there is not a clear signal from other 00z models concerning a flash flood threat. Three hour flash flood guidance values over this area are generally above 3.00 inches, so at this time, flash flooding is not expected. However, if training does occur (as suggested by the 00z ECWMF), there is a low flash flood threat. For now, no excessive area was assigned, but if there is better model agreement on the placement of the maximum QPF, a Marginal Risk could be needed in subsequent forecasts. ...Mid MS Valley/OH Valley into the Mid Atlantic... A closed mid level low over the Southern Plains at the start of day 3 weakens as it crosses the TN/OH Valleys before reaching the Mid Atlantic coast by the end of the period. There is still a fair amount of spread concerning the movement of the mid level system, as well as the placement of a secondary surface low over the Mid Atlantic Coast late in the period. Because of the spread, the WPC QPF was based on a multi model blend. The track of the weakening closed mid level low over the Southern Plains early on day 3 as it moves over the TN/OH Valleys will be the key to the forecast during day 3 from the Mid MS Valley to the northern Mid Atlantic states. As mentioned earlier, there is a fair amount of spread concerning how the mid level and surface lows interact with deepening moisture and any elevated instability extending from east central MO/southern IL across the OH Valley into WV, especially after 20/00z. For now, a blended model approach was used to place the maximum QPF across the OH Valley into WV, but the model spread results in a below average confidence forecast. Using this approach, an axis of 1.00 to 1.50 inches of QPF was stretched from southeast MO across KY into southern WV and far southwest VA. Individual model solutions show the placement of maximum QPF (more or less) along this axis, with the 00z ECMWF showing the highest amounts over far southwest MO/far western KY, while the 00z GFS/NAM were closer to southern OH with the highest QPF amounts. Since there was some agreement on the placement of the maximum QPF, in areas with three hour flash flood guidance values as low as 1.50 inches (and even lower in WV), a large Marginal Risk was placed over much of the OH Valley into WV for day 3. Burke/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml