Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 537 AM EDT Mon May 21 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 21/1200 UTC thru May 24/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...New Mexico into Texas... Convection will develop this afternoon across western NM along a tightening low level convergence axis and broadly divergent upper level flow. Activity should be rather widespread in nature, and will probably form into one or more linear segments as it propagates eastward with time. Storms are expected to generally be progressive in nature, although two periods of slower movement/training are possible. First near initiation time over central NM, where storms may linger near the terrain features that help initiate them. Then later at night an increase in southeasterly 850 mb moisture transport is expected. As this feeds into the eastward moving convection, storms may try build southeast into the low level flow and better instability. This motion may allow for some brief training to occur. PWATs increase into the very above normal range across this area, so any repeat cell or training could result in some flash flood concerns. Will maintain a Slight risk across portions of southeast NM into far west TX. Some lingering uncertainty with the magnitude of the southeasterly 850 mb moisture transport into this area, which will likely impact how organized and persistent convection is as it moves eastward...so will need to monitor trends through the day. Will also carry a Marginal risk across portions of southeast TX. A good model signal that convection will develop with diurnal heating along a weak boundary forecast just inland from the coast. Not expecting much organization to this convection given weak flow. However PWATs will remain high, and the weak flow will allow storms to be slow moving, with the boundary possibly focusing cells long enough for some higher totals. Not expecting widespread flash flood issues, but a localized threat is possible, especially across areas that saw some heavy rains yesterday locally lowering FFG. ...Lower MS Valley into the TN/OH Valley and Great Lakes region... Another day of active convective weather stretching from the Lower MS Valley northeast into the Great Lakes region. This activity will develop in a broadly convergent low level pattern, with ample moisture and instability for locally heavy rains. Across portions of MS, eastern AR, and western TN/KY flow will generally be weak, and thus convection should generally remain unorganized and more pulse in nature. With that said, this will be the axis of greatest instability (> 2000 j/kg in spots), and thus anticipate some strong convective cores to develop. As we have seen the past few days, while most of these storms will be short lived and brief, some cell mergers along outflows will likely occur...leading to localized areas of 3"+ rainfall and isolated flash flood concerns. Will carry a Marginal risk here. Further north across portions of IN/OH, flow and forcing will be stronger. This area will be closer to the mid level shortwave and upper jet to the north, and deep layer wind shear increases enough to suggest at least some convective organization is likely. While not as high as areas further south, instability should still become sufficient for convection along and south of the warm front stretching across the region. Overall think convection will be progressive in nature, although some brief backbuilding/training is a possibility along/near the warm front. Overall this setup, combined with PWATs above average, suggests the potential for some heavy rainfall totals locally exceeding 3", and thus will carry a Marginal risk. WPC QPF generally favored a consensus of the 0z HREF members and the 0z Experimental HRRR for this region. ...Mid Atlantic... Another active day expected across portions of WV, into southwest VA, and northern NC. A backdoor front will likely act as a focus for convective development this afternoon/evening across this area. PWATs, while not extreme, will remain solidly above normal, and thus locally heavy rates are possible. Convection may first develop across portions of eastern KY/TN into southern WV, southwest VA, and northwest NC...where differential heating in the terrain may act as an additional trigger for storms on top of the boundary. Slightly stronger southerly flow across this region should result in storms drifting off to the north. Instability is high enough that will probably have some brief cell mergers along outflows, and also may see some brief training at cell initiation on/near the terrain. Thus, while in general storms will both be moving enough and unorganized enough to prevent widespread heavy totals, likely to see some localized heavy amounts. And given the saturated conditions, these isolated heavier totals will result in at least a localized flash flood risk today. Contemplated a Slight risk given the saturated conditions, but opted to just go with a marginal for now given the expected unorganized nature of convection and some chance the higher amounts end up west of the most saturated soil. Will however need to closely monitor through the day, as do anticipate at least localized flash flood issues will arise. Further east storms will fire along/near the stationary boundary near the NC/VA border. Instability is forecast to be even higher here, and mean flow is westerly, parallel to the boundary. Thus even though shear is weak and suggestive of pulse convection, a bit more concerned about heavier rains here. Think convection may try to organize along the boundary, with some regeneration along outflows allowing the storms to slowly drift south. Thus seems like a pretty good chance we will see at least localized 3"+ amounts from near the VA/NC border south into northeast NC. These areas have not been quite as wet as locations further northwest, so could likely handle a bit more rain before flash flooding becomes a concern. Nonetheless, the setup suggests at least a localized flash flood risk is probable and a Marginal risk was issued. ...Florida and Southeast... A weak vort and low is currently moving into the Florida panhandle this morning. This system will bring locally heavy rains as it moves northward. The 0z high res models are pretty much unanimous in depicting some very heavy totals in narrow bands with this feature. Indeed significant amounts are verifying early this morning in a very localized band, with a gauge on the Ochlockonee River at Curtis Mill reporting 7.59" as of 09z. As the low/wave drifts northward, it should act as a focus for diurnally driven convection into portions of southeast AL and southwest GA. Instability will probably be a limiting factor as we go inland, although still likely to see some locally heavy convective cores. Think the high res models are probably overdone with amounts, but localized totals exceeding 3" seems likely, with at least a localized flash flood threat given PWATs increasing to above the climatological 90th percentile. Higher FFG and the question of instability keeps the threat here at Marginal for now, but will need to monitor through the day. Should instability become high enough, the setup does have the potential to produce a more organized flash flood threat and warrant a Slight risk. ...West... Troughing and above normal PWATs will lead to another showery day across portions of the west. Best focus seems to be across the central Sierras into NV and also across central MT. Instability may never get high enough over CA/NV to generate rates high enough for a flash flood concern. Slightly higher instability is possible over MT, so will need to monitor for localized heavier rates there. Days 2/3... ...Southeast/eastern Gulf Coast/FL... Deep moisture within confluent flow with corresponding lift provided by a weak 850 mb circulation drifting inland across a combination of AL/GA will support bands of showers over the eastern Gulf coast....including the FL panhandle inland to AL/GA on day 2 and 3. 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 Tuesday and Wednesday. 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 progs blended continuity with the GFS/ECMWF/UKMET. Model to model differences as well as run to run differences in rainfall continue, so confidence in the details remains low. Marginal risks for excessive rainfall were shown covering parts of Alabama and Georgia in the proximity to the low-mid level circulation, to the adjacent Florida panhandle. ...Great Basin/Northern Rockies/Northern Plains... A closed upper level low will will slowly move northeast across NV into UT on day 2, and then WY and eastern MT on day 3/Wed. On day 2/Tue, the focus of rainfall develops with upper divergence maxima and moisture advection wrapping around the low across northern NV and then the CA Sierra Nevada mountains. On Tue as the upper low ejects north across Utah to the ID border, low-mid level warm/moisture advection develops and impinges on the ranges of Wyoming, including the Wind River and Bighorn Mountains. Further east on the northern Plains, the upper divergence maxima eject northeast across the northern Plains Tue night, to early Wed and a round of showers/storms develops in southeast SD to northwest IA and southern MN. As an area of warm advection streams north across southern MN Wed, additional showers/storms develop. The 00z ECMWF develops a pool of Predictable Water Values near 1.5 inches, so heavier cells may develop. The guidance spread grows as to timing/location with successive vertical velocity maxima, so confidence wanes again as Wed progresses. The NAM intensifies the 300 mb divergence with a bulls eye near the North Dakota/MT border in response to convective heating, usually a sign of convective/grid scale feedback from convection. Consequently, the higher NAM total rains Tue night to Wed night are viewed suspiciously. 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 00z GEFS Mean/ECMWF/UKMET. ...Great Lakes to the Northeast/Ohio Valley/Appalachians/mid Atlantic... The models show a progressive 700 mb wave crossing from the Great Lakes across the northeast. The 850 mb warm front crosses New York and New England and trailing cold front crosses the Oh Valley and Appalachians, followed by the mid Atlantic. The well defined 850-700 convergence and theta-e advection maxima crossing New York and New England has led to multiple models increasing QPF amounts across the region. Manual progs blended the 18-00z NAM/00z GFS/21z SREF Mean and 12 ECMWF Ensemble Mean for QPF amounts. Showers wane in New England and the mid Atlantic late Wed as the wave moves off the coast and trailing front passes, leading to drier air advecting into the areas. The QPF becomes more focused in pre-frontal convergence in Carolinas, with less trough development aiding in low level convergence and shower initiation. Lee trough development also aids showers/storms in MD on Tuesday afternoon and evening, focused on the Bay and eastern shore during the evening before moving offshore. ...West Texas/eastern NM/TX-OK panhandles into southwest KS... Initial south to southeasterly confluent low level flow over Eastern NM and across much of west Texas will advect moisture north across the region. Day 2/Tue QPF becomes focused with a progressive wave moving north/northeast across the region. The 00z GFS at the end of day 1 has a large bulls eye in convergence and ascent indicative of convective/grid sale feedback, which carries over into day 2 with high amounts near the TX/NM border. The more conservative GEFS Mean amounts were used instead in conjunction with the NAM/UKMET/21z SREF Mean/continuity. On Wed, the guidance shows persistent lighter amounts with the GFS again being on the heavier side of the guidance suite with manual progs used a multi-model consensus. The 700 mb theta-e ridge moves downstream towards the central Plains on day 3, with shower/storms developing in southwest KS to the TX/OK panhandles. The 00z NAM develops temporary bulls eye in southwest KS so its amounts were lowered to be closer to other models. Chenard/Petersen Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml