Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Tue May 22 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 22/1200 UTC thru May 25/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...East... The wet pattern will continue today across much of the east. A moderately unstable airmass, moderately anomalous PWATs, and broad troughing will support scattered to widespread convective activity. A few forcing mechanisms within the broad troughing will help focus convection. Will have stronger synoptic forcing and wind shear across the central and northern Mid Atlantic, being closer to the mid level shortwave and upper jet to the north. Thus would anticipate convection will be a bit more organized here, probably generally in small linear segments. Will also have whats left of the backdoor front that is lifting north as a warm front, which should locally enhance convergence. Instability will drop off north of the front, which should cap rates there, however along and south of the front will have ample instability to produce locally heavy rates across portions of northern VA into MD. Storms should form near the terrain of WV/VA/MD this afternoon and progress eastward. Generally expect storms to be progressive in nature, limiting the overall flash flood threat. Although a localized threat probably still exists given the saturated conditions over portions of the central Mid Atlantic. Also likely to have storms form along the actual cold front further west over the OH Valley, and move eastward into portions of western PA. These should weaken after sunset as they head into the terrain. Across the southeast a lingering mid level shortwave will once again act as a focusing mechanism for storms this afternoon. This feature is currently over southern AL, and is responsible for the small scale training shower activity over southeast AL. With diurnal heating, anticipate a broader area of scattered convection to form around this wave across portions of AL and GA. As with past days, not expecting this activity to be organized, and cells should generally be pulse in nature and thus short lived. However briefly intense rates are likely, and where cells are able to merge extending rainfall duration, a localized flash flood risk exists. Activity should diminish after sunset, with a more localized band of showers likely near the center of the wave (similar to this morning). In between these two areas, the mid/upper level focus for convection is less evident. However seems likely that we will again see cells form over the terrain of the western Carolinas, eastern TN and southwest VA with diurnal differential heating. These cells will tend to drift eastward off the terrain with time. Normally this activity would not really pose a flash flood risk, but given the saturated conditions over portions of this area, a localized threat likely exists once again. May need to watch northeast GA into western SC and southwest NC a bit closer this afternoon. Some chance that we see a bit more enhanced cell merger threat here, with cells moving northward on the periphery of the wave over AL/GA possibly merging with cells moving east off the higher terrain. If this were to occur then may see a bit more focused flash flood risk develop. Confidence is not high on this though, and will just need to monitor through the day. A separate weak shortwave over the Lower MS Valley may also act to focus some convection over portions of AR/LA/MS today. The QPF signal amongst the 0z HREF member suggest this activity will be even more isolated and short lived than the activity further east. Thus while very localized heavy totals are possible, areal averaged rainfall should not be all that high. ...Central and Northern Rockies into the Plains... The approach of the mid/upper level trough will increase convective chances across the Rockies into the Plains. Upper level flow will become very divergent over WY by this afternoon, which combined with increasing instability,and the presence of a low level boundary, will result in scattered convective development. Instability should be high enough to fully take advantage of the anomalous PWATs and produce some locally intense rates. Storms will generally be progressive in nature off to the north, however could see multiple rounds of storms given the favorable synoptic environment. Thus at least a localized flash flood risk probably exists. Eventually these cells should take a turn to the right and move into the northern plains, with one or more convective clusters likely. Low confidence on how this evolves, but locally heavy rains are possible into SD/ND as convection moves across. Storms should generally be progressive, although one area to watch may be southwest SD. Some signs that storms that initiate off the Black Hills could merge with convection moving northeast out of WY/NE. Should this happen a more focused flash flood risk could develop. As we head into the overnight hours appears to be a pretty good setup for an axis of elevated convection across portions eastern SD into southwest MN and northwest IA. 850 mb moisture transport increases and persists through the night, with this elevated activity likely focusing on the nose of this feature. Storm motion vectors appear to have a large component parallel to this moisture convergence axis...suggesting that storms may tend to regenerate and train for a period from northwest to southeast. Would appear that both instability and PWATs would be high enough to produce some pretty heavy amounts should this materialize. Still a good amount of uncertainty though with both the organization of this activity and the latitude it sets up at. Generally favored an axis close to the 0z nam nest and ARW2 for the WPC QPF, although likely will need to make some adjustments to this as things become clearer by later today. Either way, something to watch, as certainly could see some flash flood potential with this setup. Days 2/3... ...Southeast/eastern to central Gulf Coast/FL... Deep moisture within confluent flow and a weakening cold front across the southeast combines with a remnant circulation now in Alabama to support bands of showers over the region. Elsewhere, showers should form in areas of diurnal instability and sea/lake breeze initiated lift and convergence near the front. The models also depicted persistent, deep flow of moisture from the Bahamas into southeast FL with peak precipitable water values near 2 inches so showery conditions are expected along the southeast coast. The pattern favors inland penetration of the sea breeze from the east coast so peak afternoon convection should occur over interior portions of the FL peninsula to the west coast. Manual QPF was based on the 00z GFS/ECMWF. The 00z NAM was an outlier bringing low pressure closer to the central Gulf coast with other solutions much further south so the NAM was given no weight. ...Great Basin/Northern Rockies/Northern High Plains... An opening upper level low and corresponding 700 mb low near the ID/MT/WY border Wed morning will lift northeast across eastern MT Tuesday night through Wednesday and into the Canadian Prairies Thursday. Instability under the trough will allow convective showers and 0.75 inch PW is above normal, so amounts are higher than you typically see with a late May system. Showers occur in the area of low-mid level convergence and ascent near the low, which leads to reduced coverage/amounts once the low moves into Canada Thu. Manual progs blended continuity with the GFS/ECMWF/NAM/21z SREF Mean. ...Upper Midwest... A warm front will focus convection on a plume of moisture with the ECMWF forecasting near 1.5 inches precipitable water at its peak as it lifts north across MN Wednesday and Thursday. Once the front has departed activity will start to focus ahead of a lee trough in the high Plains, with another plume of enhanced moisture forecast to form ahead of the trough and approaching cold front Thu evening in MN. Clusters of showers and storms are expected again with convergence on the nose of the low level jet progressing across MN, areas near the SD/IA border, and eastern Nebraska. Manual QPF blended the 00z ECMWF/GFS/NAM. ...West Texas/eastern NM/TX-OK panhandles into west KS... A broad upper trough persists over the southwest on Wed. The ECMWF indicates upper divergence maxima Wed crossing central to eastern NM and then into the TX panhandle. With continuing low level southerly flow enforcing the existing moist air mass and trough into enhance low level convergence, widespread showers/storms are expected again Wed. Another deepening low approaches CA Thursday. The trough over NM lifts out is replaced by a building mid-upper level ridge. Isolate to scattered showers/storms are possible but with less coverage due to weakened low level convergence and lift. Manual QPF blended the NAM/GFS/21z SREF Mean. ...Pacific Northwest/Northwest CA... Showers are expected on Wed across northwest CA and northern NV and OR as 700 mb relative humidity remains high across the region and afternoon instability in an airmass with lapse rates near 7 deg C/KM triggers the development of showers and storms. Additional showers should form on Thu as moisture remains in place and a spoke of mid-upper cyclonic vorticity progresses across northwest CA and then into OR, generating more showers. The models are in good agreement on showers developing with a typical spread developing, with the GFS on the wet side and displaced further east in OR than the other models and the GEFS Mean. More weighting was given to the majority cluster, including the NAM/Canadian Global/ECMWF. The UKMET was on the dry side and given less weight, given favorable moisture and lapse rates in place. Chenard/Petersen Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml