Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 700 PM EDT Sun May 20 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 21/0000 UTC thru May 24/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Central-Northern Intermountain West... The highly-amplified, initially negatively tilted upper trough along the west coast early will evolve toward a deepening/closed upper low over southern CA-NV on Monday as an upper level jet streak moves through the trough base. During this time, shortwave ridging builds across southwest Canada, thus putting the northern Great Basin within a 'col' region both aloft and at lower levels. Given the broadly difluent upper flow, along with the departing area of confluent upper flow (toward south-central Canada), the models continue to depict focused areas of upper level divergence during the day 1 period from the Sierra Nevada northeast to the northern Rockies to include the intermountain region. Modest deep-layer instability (MUCAPES 1000-1500 j/kg) along with the moist environment (PW anomalies ~2 standard deviations above normal) will generate widespread additional areal-average totals between 0.25-0.50" this afternoon through Monday, with locally higher totals between 1-1.5" per the high res CAMs. ...Texas... In the 2030Z final issuance we moved things along toward the south more quickly this evening based on trends, and using primarily the mid afternoon HRRR runs. Confidence in the QPF details lowers considerably after 06Z Mon as departing MCV energy and ensuing shortwave ridging would favor less convective coverage on Monday (mainly confined along the Gulf coast where the axis of higher PWs will linger). ...Lower-Mid Mississippi Valley into the Tennessee and Ohio Valleys and Mid Atlantic... Another region where the model guidance continues to lag the observational/mesoanalysis trends. Leaned more toward the experimental HRRR early in the period (through the overnight) with the QPF ahead of the mesoscale-convective aided vort centers across western NE and eastern OK-KS early this afternoon. WPC focused the highest areal-average totals where the airmass was able to destabilize Sun afternoon (following the thinning/dissipation of nocturnal debris from last night's convection) Moderate to high instability will develop, and will generally be uncapped given the broad troughing remaining over the area. PWATs, while not extreme, will continue to run above normal. Multi-cellular convection will be most organized downstream of the aforementioned shortwaves, with the modest deep-layer instability in place. The activity farther north will likely become better organized given the 30-40 kt 0-6km bulk shear; however, slower cell motions (Corfidi vectors 5kts or less) would support locally heavy rainfall into central and southern MO-IL where bulk shear values are 20-25 kts or less. ...Florida... Will be another wet period across Florida, with moisture remaining high. In the 2030Z final issuance we did make some stark changes to the expected overnight rainfall, moving it westward in accordance with the low level convergence signal in the RAP and associated heavy rain signal in the HRRR. This makes some sense as the global models predict a slight retrogression of the weakness over the eastern Gulf tonight. This would place heaviest overnight totals in the Everglades, parts of the Keys, and possibly up toward Naples. At least this change represents a reduction of QPF over southeast Florida where very heavy rain fell earlier today. Still, we will maintain a Marginal Risk of excessive rainfall stretched across all of south Florida overnight. Some signs another weak mid level shortwave wave may be approaching the state again on Monday, as the southeasterly 850mb flow will remains strong enough to support the persistence of organized convection shall it develop. Days 2/3... ...Southeast/eastern Gulf Coast/FL... Deep moisture within confluent flow with corresponding lift provided by the passage on an inverted 850 mb trough will support bands of showers over the eastern Gulf coast....including the FL panhandle inland to AL/GA on day 2 and 3. The NAM/UKMET/GFS showed a closed low developing which led to strong low level wind fields and resultant convergence/lift. The inflow off the gulf advects the high moisture into the Florida panhandle across into southern Alabama and Georgia, so combined with diurnal instability and sea breeze initiated lift combined with convergence near the low, the higher amounts are shown in these areas for Monday and Tuesday. The models also depicted persistent, deep flow of moisture from the Atlantic over portions of the eastern and central peninsula. Onshore speeds are not terribly great, but the combination of PW values approaching 2 inches and the shoreline convergence suggests that some locally heavy rainfall amounts are possible from the eastern FL peninsula up into coastal South Carolina. Manual progs blended continuity with the GFS/ECMWF/UKMET. There certainly model to model differences as well as run to run differences within each model, so confidence in the details here are below average. Marginal risks for excessive rainfall were shown covering parts of Alabama and Georgia in the proximity to the low-mid level circulation. The portion of the marginal risk in the Florida panhandle was expanded to include nearly all of the peninsula. ...Great Basin/Northern Rockies/Northern Plains... A long wave trough will slowly move east out of OR and WA across the northern Great Basin and the Northern Rockies by the end of the forecast period. The focus of rainfall develops with upper divergence maxima just downstream from the trough/near the low level front in the ranges of ID and western/central MT and northwest WY on Mon/Mon night. Further south, showers continue in the proximity of the upper trough and embedded closed low as it drifts out of CA towards NV...with QPF maxima in the CA Sierra Nevada and favored terrain of northern NV. On Tue as the upper low ejects north across Utah to the ID border, southerly deep layer flow advection moisture north across Wyoming and the northern Plains. The warm/moisture advection impinges on the ranges of northeast Wyoming, including the Bighorn Mountains. Once again, the upper divergence maxima eject northeast across the northern Plains Tue night, another round of showers/storms develops. with the NAM/UKMET/Canadian showing higher amounts over a larger area. T The NAM intensifies the 700 mb low over North Dakota in response to convective heating, usually a sign of convective/grid scale feedback from convection, in this case in southeast ND. As a result, the NAM solution was given least amount of consideration once again in this forecast cycle. Otherwise, the manual QPF from WPC was more in line with the latest GFS/ECMW/UKMET. ...West Texas/eastern NM... East of the developing western U.S. trough, southeasterly confluent low level flow over Eastern NM and across much of Texas will advect moisture off the Gulf of Mexico inland. The airmass will be characterized by a precipitable water value around 1.25-1.5 inches on Monday...which is 2 standard deviations above climatology. This, combined with high CAPE and low shear, favors slow moving storms. On day 2 with a couple of the models, including the 00z GFS, show locally 2-3 inches of rain. The excessive rainfall outlook was maintained at a slight risk category on day 2, but not forecast on day 3 following the lower amounts forecast by the models on Day 3/Tue. WPC manual graphics tended to put more weight on the ARW core and GFS for the QPF here early on day 2...with GFS/NAM getting the nod once the CAM guidance ended. ...Great Lakes to the Ohio Valley/Central Appalachians/mid Atlantic... A low-mid level wave over eastern Iowa on Sunday evening moves towards the Great Lakes, with the deformation zone focus over eastern Iowa/northern Illinois/northern Indiana. The threat for showers in northern IL across southern Lake MI to southwest lower MI will gradually shift towards the east during the 2-day period, with slow moving cells possible near the elongated cyclonic circulation. More progressive showers/storms develop in an axis of the low level jet with well defined 700 mb convergence near the jet maxima crossing Ohio into PA Monday, supported by upper divergence maxima. This continues downstream into NJ/southern NY in the zone of warm/moist advection at 700 mb on Monday evening. Bands of low level moisture convergence stream across much of Pennsylvania and MD on Tuesday afternoon and evening, with diurnal instability contributing to showers/storms. Manual progs used a blend of the latest available ECMWF, the 12Z NAM and GFS. Hurley/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml