Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 658 PM EDT Thu May 17 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 18/0000 UTC thru May 21/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Great Plains/Northern to Central Rockies... At the start of the forecast, a mean mid-level ridge will be anchored over the Great Plains while an upstream longwave trough remains in place over the western United States. Models continue to show erosion of this mentioned ridge as strengthening southwesterly flow draws a series of perturbations into the northern High Plains. This will all occur in accordance with a persistent southerly influx of Gulf Moisture as shown by 12Z RAOBs as well as suggested by current VAD wind profilers. The low-level flow should continue to strengthen toward 18/0000Z with many moisture transport vectors suggesting an upslope component into the eastward extension of the northern/central Rockies. As the evolving convection pushes off the higher terrain, it will likely become better organized within the high theta-e environment over South Dakota and western Nebraska. By Friday convection should take on a more progressive storm mode once the parent cold front begins to accelerate downstream. Based on the latest forecast models, the 18/0000Z-0600Z window should contain the heaviest rainfall which may lead to some flash flood concerns with a Slight Risk indicated over central North Dakota. The forecast was heavily driven by the overnight continuity with some minor/modest adjustments via the latest CAM data as well as the global guidance. Farther south, the central U.S. ridge should also buckle somewhat across the Southern Plains as a mid-level speed max sweeps through the TX/OK panhandles by the late afternoon/early evening hours. In addition to diurnally forced dry line convection across west TX, some further development is likely for locations downstream as the shortwaves traverse a moisture-laden environment with appreciable buoyancy profiles. Areal coverage of convection will be much lower given weaker support aloft as compared to locations farther north. ...Southeastern U.S./Tennessee and Ohio Valleys/Mid-Atlantic... A wavy stalled frontal zone remains generally over the mid-Atlantic, just south of the D.C. metro area, south-southwestward back toward the lower Ohio/upper Tennessee valleys. Aloft, a persistent elongated upper trough continues to sit over the north-central Gulf of Mexico which remains a player in drawing copious sub-tropical moisture along the spine of the Appalachians and into the mid-Atlantic. Low-level moisture is quite impressive as shown by morning dew point temperatures with numbers into the upper 60s. Little change in this overall synoptic pattern is expected over the course of the next 24 hours with right entrance region upper jet dynamics focused over the mid-Atlantic and the adjacent Delmarva Peninsula. Additionally, the 12Z GFS shows only minor adjustments in the position of the west-east oriented frontal zone so it should remain an anchor for continued moderate and occasionally heavy rainfall, particularly as embedded mid-level shortwaves lift northward along the Eastern Seaboard. Generally speaking, we left much of what was inherited from the overnight crew which favored the heaviest amounts from southwestern Virginia northeastward to just south of the DC metro area into the Delmarva Peninsula. Two to 3 inches of rainfall is in the forecast, including what has already occurred early this evening. With this occurring atop saturated soils there remains a Slight Risk of excessive rainfall. ...Northwestern U.S./Central Great Basin/Sierra Nevada... A disorganized/chaotic band of shortwaves continue to rotate cyclonically about the central Great Basin. Such mid/upper-level impulses will remain foci for patches of semi-organized shower and thunderstorm activity. There is limited predictability of the placement of these embedded features so confidence is below average in placing the convective maxima across the region. However, there should be the usual focus across the higher terrain and regions of enhanced upslope. It appears Montana could see a bit more storm organization as the upper-level flow is much more diffluent in nature as shown by current 12Z upper air analysis and forecast streamlines. Overall, we attempted to maintain continuity as much as possible given the low predictability of such convective elements. Rubin-Oster Days 2/3... ...Pacific Northwest/Northern Rockies/Great Basin... A long wave trough will slowly amplify along the west coast Friday night through Sunday as low pressure shifts east from the north-central Rockies. The focus of rainfall in this region will shift west from the Northern Rockies on Day 2 as the low departs to the northern Great Basin on Day 3 from the trough axis down the coast. Areal average QPF around a half inch, combined with snowmelt, warranted maintenance of the marginal excessive rainfall risk centered on northern ID on Day 2 (12Z Fri-12Z Sat). QPF is based on a blend of the 12Z GFS/ECMWF with some inclusion of both of those ensemble means. ...Northern and Central Plains/Mid and Upper MS Valley... Low pressure develops in the lee of the WY/CO Rockies Saturday with surface low development over KS. Gulf moisture wraps around the low and is constrained by a surface high centered over the MT/ND border. This northeast flow near the NE/WY/CO common border in the deformation zone also encounters topographical lift up the high plains. PW of 1 inch in this elevated area is 2 standard deviations above normal and should result in heavy rainfall with max QPF 2-3 inches. Strength of the low is still uncertain with the 12Z ECMWF and UKMET still the weakest solutions (barely closing the low at 500mb). The Day 2 (12Z Fri-12Z Sat) Slight risk was maintained with a slight shift south into CO where some sub two inch 6hr FFG exists. Absolute instability may be lacking in the current Slight risk area, so consideration will need to be made on future shifts on the area farther east over southwestern NE which should have greater instability (but also higher FFG) on Day 2. Day 2 QPF was based on the 12Z GFS/ECMWF with some 12Z 3km NAM and ensemble means. The low shifts east Saturday through Sunday with the heavy rain/deformation zone focus over eastern NE/western IA on Day 3 where the areal max QPF is 2 inches. The Day 3 (12Z Sat-12Z Sun) Slight risk was also maintained with a south shift given 12Z guidance consensus. Day 3 QPF was based on a blend of the 12Z GFS/ECMWF and their ensemble means. ...OH Valley/Mid Atlantic/Northeast... Deep tropical moisture streaming northward from the Southeast states and FL rides up and over a surface frontal boundary extending from the OH Valley across the central Mid-Atlantic states during Day 2 then lifts across the coastal northeast on Day 3. PW in this plume of 1.75 to 2 inches is 2-3 standard deviations above normal which warrants large areas of Marginal with an embedded Slight excessive rain risk on Day 2 (12Z Fri-12Z Sat) for the central Mid-Atlantic which is in the midst of a multi-day excessive rain event. Instability will be contained south of the surface trough/front which finally ejects north from the central Mid-Atlantic (after multiple days there) to New England on Saturday. The broad marginal excessive risk for the northeast on Day 3 (12Z Sat-12Z Sun) may not be necessary for New England depending on the progress of the surface front, but the widespread areal average of one inch QPF is notable. ...Southeast/FL... Deep moisture ahead of a weakening long wave trough over the Southeast states and a Bermuda high will continue to feed convection riding north across the east coast of FL into the Southeast during Day 2/3. The combination of moisture and instability should continue to result in heavy to locally excessive rainfall, especially across the east coast of FL and into eastern NC. WPC QPF was based on the 12Z GFS/ECMWF as well as their ensemble means - and the 3km NAM for Day 2. A slight risk was raised for southern portions of eastern NC for both Days 2/3 as agreement increases for a narrow persistent band of heavy rain Friday into Saturday. The size of this area was limited due to a lack of absolute instability, particularly inland. Otherwise the marginal risk was expanded to Atlanta per 12Z consensus and maintained east from there over NC as well as southern FL. Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml