Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 534 PM EDT Mon Mar 19 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 20/0000 UTC thru Mar 23/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Eastern U.S... A very dynamic southern stream shortwave will move through the Tennessee Valley today, inducing a widespread precipitation event from the Plains to the east coast...and some concentrated severe weather per the Storm Prediction Center Day 1 Convective Outlook - associated with a well defined frontal wave and the extensive back-bent occluded front. Areal average precipitation amounts will be greater north of the warm front and along the occlusion, where low level WAA/moisture convergence is maximized. Down along the front and into the warm sector storms will be moving quickly, with 70-80 knot westerly 500 mb flow...thus QPF will generally be less in the warm sector. Overall this still looks like more of a severe than flash flood event...with storms expected to be progressive in nature. However, still appears like isolated 2" amounts in an hour or two remain possible...which may approach FFG over the risk area. Could see some 1-1.5" totals further east into portions of NC and southern VA with embedded heavier convective cores...although instability here is weaker and elevated in nature. Thus not currently thinking rates will be high enough to pose a flash flood risk here. Further north across the Mid Atlantic and OH Valley precipitation will be stratiform in nature. Convection has expanded in coverage over the last few hours over the FL Panhandle...as suggested by earlier HRRR runs and the 12z runs of the ARW and ARW2. This activity is forming ahead of a weak mid level wave and on the nose of an enhanced axis of 850 mb moisture transport. Storms will remain quick moving, however a broad area of convective development will allow for some repeat convection. Some areas of the FL panhandle saw several inches of rain overnight, and with the potential for localized 1-4" additional rain today...some flash flood issues could arise in more susceptible locations. For QPF WPC utilized a multi model blend, incorporating both the latest global models and high res CAMs. The one consistent trend seen amongst all guidance was to push the northern extent of the precipitation shield over IN/OH further north. This makes sense given the track of the well defined shortwave currently over MO. QPF was also shifted a bit north across the northern Mid Atlantic given the consistent trend seen. From the southern Mid Atlantic into the southeast changes from continuity were more minor...with the general idea the same from our previous forecast...just some of the details changed based on the latest consensus. Over FL, preferred sticking closer to the ARW/ARW2...which both appeared to have a better handle of the ongoing convection over the Panhandle. ...CA... Rain will begin to move into central and southern CA by the end of the day 1 period...although the heavier rain should hold off until Tuesday night and last through mid week. Some model differences were already apparent by 0z Wednesday...with the GFS and high res CAMs quicker to bring in locally heavy rain onto the coast. Meanwhile the 0z ECMWF and 12z GEM regional were similar in depicting a slower and further north axis through day 1. For now opted to split the difference and go with a consensus in between the two solutions. Days 2/3... ...West Coast/Rockies/Great Basin... Ahead of a strong mid level system approaching the West Coast during Days 2 and 3, deep moisture associated with an atmospheric river will affect much of CA, especially from late day 2 and much of day 3. The deep moisture and strengthening upslope flow over portions of central and Southern CA could result in significant rainfall and the potential for flash flooding, as well as mudslides and debris flows in burn scar areas. While there is fairly good agreement among models with respect to the overall synoptic setup, there is still some spread concerning where the moisture becomes focused. In an attempt to mitigate some of the differences (mainly between the latest runs of the ECMWF/GFS), the WPC QPF was based on a blend of these models and some of the ensemble means, weighted more heavily toward the 12z ECMWF and NAM. Was able to incorporate some of the 12Z UKMET in the 00Z HPC QPF package after it shifted towards the EC/NAM/GFS consensus position. A closed mid level low located near 35N 135W at the start of Day 2 evolves into a positively tilted long wave trough as it moves closer to the CA coast by the end of the period. Ahead of the long wave trough, there is a multi model signal for deep moisture associated with a well advertised atmospheric river (a feature which the GFS has been showing for a few days now) to cross southwest CA. Precipitable water values near 1.25 inches (which is between four and five standard deviations above the mean) are ushered ashore by a 20 to 30 knot low level southwest flow. The flow become more orthogonal with time to the Transverse Ranges in Southern CA, resulting in a better upslope component by 21/12Z. Model soundings off the southwest CA coast showed marginal instability in the moisture plume, with the best synoptic scale lift expected late in the period. The ingredients are apparently coming together for a significant rainfall event for portions of Southern CA, which most 00z model solutions showing some version of the above occurring, yielding between 1.00 and 1.50 inches of qpf along the southwest CA coast. The strong positively tilted long wave trough off the West Coast approaches the Pacific Northwest during the first half of day 3. A strong mid level closed low forms off the WA coast, and the low level southwest flow ahead of the mid level and surface low transports 0.75 inch precipitable water air over the western WA/western OR. The best upslope flow occurs over the OR Cascades, where local 0.75 inch qpf amounts are expected. Some of the moisture survives the trip over the terrain into the Northern Rockies, where upslope flow produces local 0.25/0.50 inch qpf amounts over the Blue Mountains in OR, the Sawtooth Range in ID, and the Grand Tetons of WY. Further south, the low level southwest flow becomes focused on central and Southern CA ahead of a cold front associated with the low off the Pacific Northwest coast. The best lift associated with pieces of short wave energy in the mid level flow occur over the central CA coastal range into the Transverse Ranges in Southern CA after 22/00Z. The low level southwest flow increases to 30 to 40 knots, transporting 1.25/1.50 inch precipitable water air (which is approaching five standard deviations above the mean) from the central CA coast to the mountains north of LA. The low level flow becomes orthogonal to the terrain, resulting in strong upslope component, putting the ingredients in place for excessive rainfall over a large area. The model QPF and the signal for excessive rainfall remained solid so no changes were made either to the QPF or the previously-issued excessive rainfall outlook. ...OH Valley/Mid Atlantic/Northeast... Short wave energy tracking out of the Northern Plains and short wave energy in the southern stream crossing the Lower MS Valley aid in carving out a long wave trough extending from the OH Valley into the Mid Atlantic states during Day 2, then takes on a negative tilt over the Northeast by early day 3. The mid level lift supports the development of a pair of surface lows impacting the northern Mid Atlantic and Northeast. There is still a fair amount of model spread concerning the evolution of the mid level systems, but one consistent signal in the 19/12Z guidance was a northward/westward shift in the low track and associated QPF. Overall, the models have been struggling with the evolution of the system and it can not be ruled out that there will be another shift the other way. But nudging the WPC QPF that way should put the forecast in a position where subsequent changes should be fairly minor. Day 2... The first short wave in the northern stream tracks toward the Mid Atlantic coast by the start of the day 2 forecast. Banding of precipitation associated with the surface low looks as though it will remain along the DE/southern NJ coast, where an area of 0.50 to 1.25 inches of qpf was placed. Further west, a weakening surface low over OH/WV and its frontal boundary serves as a focus for moisture ahead of the short wave crossing the region, and a large area of 0.25 to 0.50 inches of qpf was placed over OH/KY/eastern TN across much of VA/MD. As the southern stream short wave reaches the Mid Atlantic states on day 2, it helps develop a negatively tilted long wave trough extending from the OH Valley to the Mid Atlantic coast at the start of day 3. A second surface low forms off the NC coast early in the period, then tracks northeast with a closing mid level system as to moves near the New England coast. The WPC QPF used the 12Z GFS/12Z NAM/00Z and 12Z ECMWF along with the ECMWF/GFS ensemble QPF. This resulted in a stripe of heaviest precipitation from parts of eastern PA northeastward across southern New England. Changes in the western edge of the more significant QPF could occur with subsequent model runs. ...Southeast... Deep moisture and strong instability streaming ahead of a cold front crossing FL early on day 2 that will feed late-day convection along and ahead of the front. Given that the models were in generally good agreement in terms of the timing of the front and its departure from the FL peninsula, a general model compromise was used. Because of the speed of the convection, flash flooding is not expected with the storms. Chenard/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml