Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 626 PM EDT Sun Mar 18 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 19/0000 UTC thru Mar 22/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central / Southern U.S... A rather active Sat evening through Sun from the Central Plains to parts of the South/Southeast into northern FL. Multiple streaks of heavier rainfall is anticipated over the next 24 hrs, as a lead subtropical short wave streams through the southern tier of the country, only to be followed up by a dynamic southern stream trough. The combination of the two features will produce widespread qpf from KS to FL/GA. The subtropical short wave has already produced clusters of showers and storms across LA into MS and southern AL today. Rounds of convective clusters should persist, as the short wave arrives and moisture pools near a surface stationary front Sat evening into perhaps the overnight hrs. The latest global models and hi-res suite indicate the prime spot being southern GA/FL panhandle where increasing instability is likely and possible areal avg 1-2 inch amounts with locally higher values. Meanwhile, a vigorous short wave, moving through the Four Corners region today, will eject overnight into the central/southern Plains. The guidance indicates broad swrly upper difluent flow ahead of the negative tilt short wave for synoptic scale heavy rains across eastern/central KS. This area of enhanced vertical lift invof of the 500mb vort will progress downstream through central/southern MO into the TN valley before tomorrow aftn and possible active convection near the wedging frontal zone from the TN Valley into AL/western GA. Overall the guidance was very similar with mass fields and WPC felt comfortable using a blend of 60% hi-res guidance, 20% global models (GFS/ECMWF) and 20% NBM (National Blend of Models) ...Western U.S. Including the Central Rockies up through the Dakotas... Scattered areas of mainly light qpf is anticipated from the Northwest through the northern Inter-Mountain west into the northern Plains and Upper Midwest. Not much of an upper flow regime with a closed low hovering just off the PacNW coast and over the northern Plains, while an inverted trough extends from the developing Plains cyclone into NE and the Dakotas. This is where the most organized qpf is anticipated of up to a .50 inch, some snow and rain mixed with snow. Overall WPC just followed a blend of the latest model guidance. Days 2/3... ...California into the Pacific Northwest.. A powerful upper low across the open waters of the eastern Pacific will feature 500-mb height anomalies around 4 to 5 sigma below climatology. Broad southwesterly flow should extend downstream into the West Coast, while being accompanied by a well defined subtropical moisture plume. This atmospheric river setup has the potential to bring prolific rainfall amounts into Southern California as well as over the Sierra Nevada ranges. Considering the GFS modeled water vapor transport forecast, the IVT values exceed 800 kg/m/s on Wednesday morning with the best moisture fluxes taking aim at southern sections of California. Based on the latest guidance, there is solid low-level support across the region with a 35 to 40 knot 850-mb jet advecting 1.50 inch precipitable water values toward the coastline. Additionally, such support exists in the upper levels of the troposphere as the southwestern U.S. will be on the right entrance region of a powerful 250-mb jet streak. All and all, most parameters are in place for a heavy rainfall producer on Day 3 onward. Multiple inches of rain are likely across Southern California which would easily be up to 25 percent of many locations annual rainfall budget. This will sufficiently enhance the threat for flash flooding and mudslides, particularly over the local burn scars. As such, a slight risk area continues to be advertised on Day 3, ending 21/1200Z across this region with a likely upgrade in future days. The overall preference was primarily focused on impacts over California although light precipitation does break over the Pacific Northwest as well. The past several runs of the GFS appear to be plagued by feedback issues, per the 2 to 2.50 inch amounts in 6-hours which seems highly unreasonable. Generally took a combination of the 00Z ECMWF/UKMET solutions which were slower and a bit more conservative in comparison. ...Northern to Central Plains/Middle and Upper MS valley... Multiple weak shortwaves are forecast to slip eastward across the Dakotas into the Upper Midwest on Monday evening into Tuesday. A broad area of light to locally moderate snowfall will likely break out along a north/south oriented inverted trough extending into the north-central U.S. While expected amounts are on the lighter side of the equation relative to locations across west and east coasts, respectively, multi-cycle trends show an uptick in the overall intensity across the Upper Midwest into the Middle Mississippi Valley. This allowed amounts to be bolstered a tad relative to continuity. A blend of the 06Z GFS and 00Z ECMWF was sufficient to handle such trends favoring a 24-hour QPF above 0.25 inches across southwestern Minnesota ending Tuesday evening. ...OH and TN Valleys/Mid Atlantic/Northeast... A pair of systems will need to be monitored during the period with the initial one being a flattening wave migrating from the Ohio Valley into the mid-Atlantic on Tuesday morning. In spite of the impulse being in the process of deamplification, a well defined area of low pressure will sweep eastward along the Virginia/North Carolina border with models showing better clustering than previous days. Strong lift with the attendant shortwave will drive a band of quick hitting precipitation through the mid-Atlantic on Tuesday with some wintry p-types expected given appreciable low-level cold air streaming northward from a dome of high pressure to the north-northwest. The most concentrated activity of heavy rainfall should be across southeastern Virginia, generally closer to the parent surface low center. The approach here was to follow a consensus of the 12Z GFS and 12Z ECMWF/UKMET with the latter pair of solutions being a tad slower though. On the heels of this system exiting the Delmarva Peninsula, the more robust trough ejecting out of the southeastern U.S. has proven to be a system to watch given large trends in recent model cycles. Over the past day of runs, a vast majority of the operational/ensemble guidance have made large west/north shifts in their low track and corresponding QPF output. This particular trend of course now brings measurable precipitation into the I-95 corridor across the northeastern U.S. Given plenty of cold air already in place at the surface, wintry precipitation will be an issue over this region. Please view the latest heavy snow/icing discussion (QPFHSD) for more information on this threat. Decided to make a northward adjustment in the updated QPF package which nudged the activity northward as noted in the 12Z ECMWF/UKMET solutions. A lot can change in the upcoming packages given how quickly the models have navigated about this storm. ...Southeastern U.S... Modest height falls sweeping through the Red River on Monday evening will accelerate east-southeastward toward the Florida panhandle during the following 24 hours. A marked increase in amplitude of this shortwave trough is anticipated with seasonably strong 500-mb winds in excess of 80 knots sweeping across the Sunshine state. Strong dynamical lift with this system in conjunction with ample buoyancy will support organized thunderstorms pushing through the southeastern U.S. on Tuesday. Based on the Day 3 outlook from the Storm Prediction Center, some of these storms may be severe in nature with an enhanced risk currently in place. While damaging winds should be a major issue with cells that ignite, a heavy rainfall threat will also ensue given the high precipitable water environment these storms will have access to. Given the individual clusters should be booking, a flash flood threat is not likely unless there are instances of repeat convection. The manual QPF across the region was generally driven by a split of the 06Z GFS/00Z ECMWF given agreed upon timing and the preference to be on the wetter side of the guidance. Musher/Rubin-Oster Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml