502 FOUS30 KWBC 250805 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 404 AM EDT Sat May 25 2019 Day 1 Valid 12Z Sat May 25 2019 - 12Z Sun May 26 2019 ...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS FOR MUCH OF KANSAS AND NEAR NORTHERN MISSOURI... ...Plains/Midwest... Periods of training convection near the northern KS/MO border could be ongoing as the period begins, but should be on the wane, weakening by mid-morning. The significant action is expected during the late afternoon and evening hours, as a complicated scenario is envisioned which lowers confidence in the overall forecast. Precipitable water values (PWs) of 1.75" are available at lower elevations of the Plains while the High Plains advect in 1-1.5" PWs. Dryline convection is expected to initiate across the Rolling Plains of northwest TX Friday afternoon/evening where the mid-level capping inversion is weak not far from where heavy rain has fallen the past 36 hours near Lubbock. The flow backs just prior to 00z which would make it difficult for storms to forward propagate initially. Stalling storms could drop very heavy rain between Lubbock and Childress TX -- up to 2.5" an hour -- before forward propagating after 00z. The guidance, to varying degrees, allows this activity to set off outflow boundaries that forward propagate through a significant instability pool which is more capped across portions of OK into KS which severely muddies the waters from a QPF perspective. Meanwhile, evening/overnight convection is expected to develop across northwest KS closer to a front. Whether or not this northern convection forms depends upon whether or not the more southern convection gobbles up the available instability to its south, which has led to a variety of QPF patterns across northwest TX, OK, and KS amongst the guidance. No matter what transpires, the congealed blob of convection/MCS/possible MCC should merge across eastern KS which should then navigate around the northern side of the mid-level cap into northern MO. Think the 00z NAM CONEST has a good idea here, though its magnitudes are probably too extreme. Warm advection precipitation should develop off to the east across portions of MO, IL, and IN while the mergers occur to the west. The QPF template was based on a blend of WPC continuity, the National Blend of Models, the in-house bias-corrected QPF, and 00z HREF. The mesoscale guidance has a strong signal for local amounts of 3-5" near Childress TX and closer to Kansas City MO. The region has been saturated by heavy rainfall for weeks now, so soils are saturated and rivers are in flood as stream flow is extremely high. This led to a large Moderate Risk area covering much of KS and northern MO, as well as adjacent portions of IA and IL, with surrounding slight and marginal risk areas to encompass the wider range of heavy rain signals. ...Northern Mid-Atlantic States... Deepening moisture and instability couple to support convection across the northern Mid-Atlantic States into northwest New England. Ahead of embedded short wave energy in a fast mid level flow, a low level west southwest flow transports 1.50"+ PWs (2-3 standard deviations above the mean) over NY state and northern New England, peaking between 25/21z and 26/06z. Behind a warm front moving toward New England a slow moving cold front crossing the Upper Great Lakes, an axis of up to 3000 ML CAPE extends across the central Appalachians into into far western NY state, generally before 26/00z. The convection is expected to produce hourly rainfall rates to 2" and local amounts of 3-4" (per the mesoscale guidance) in the deep moisture in place over these areas, particularly where cells merge and/or train. The convection is expected to be progressive, with storms motions from northwest to southeast at 10 to 15 knots. However, three hour flash flood guidance values here are as low as 1.50" in spots, and any short term training could pose a flash flood threat. The Slight Risk in place was expanded into western PA to accommodate the most recent model guidance. After the convection over western PA/western NY weakens during the evening hours, short wave energy in the fast zonal flow interacts with marginal instability associated with a weakening warm front crossing northern New England between 26/03z and 26/09z. Moisture is deep across the region, but as the mean 850-300 mb flow becomes better aligned with the propagation vectors, training becomes a possibility. Three hour flash flood guidance values are as low as 1.50-2", since 14 day rainfall amounts of have 150-250% of normal. With marginally wet antecedent conditions over this area, a Marginal Risk remains here. ...Western U.S... A mid level system over OR weakens as it drops south into Northern CA during the first half of day, followed by a much stronger mid level system, reaching northwest CA by the end of the period. Steepening lapse rates are expected to support 500-1000 J/KG of ML CAPE across much of the higher terrain of Sierra Nevada mountains, with the best instability between 25/18z and 26/03z, with the instability becoming consumed roughly around 07z. A low level westerly flow aids in pooling 0.75-1" precipitable water air (which is between two and three standard deviations above the mean) over this area. The combination of moisture and instability should support in at least scattered convection over the higher terrain initially, which drops into the valleys just after peak heating. In this airmass, hourly rainfall totals to 1.5" are possible, especially over the northern Sierra Nevada Mountains into the Shasta Trinity Mountains. The Marginal Risk placed over the Sierra Nevada range is meant for areas below the freezing level, for elevations mainly below 7500 feet. Roth/Burke/Hayes Day 2 Valid 12Z Sun May 26 2019 - 12Z Mon May 27 2019 ...THERE IS MODERATE RISK OF FLASH FLOODING ACROSS PORTIONS OF CENTRAL KANSAS... ...Update 08Z... Slight Risk area was expanded north into eastern NE, far southeastern NE and west IA with a Moderate Risk introduced across portions of central/southern Kansas (DDC/ICT). This is to account for the widespread convection expected east of the dryline with instability and moisture pooling across much of the central Plains. With ample upper level support arriving atop of strong instability and moisture convergence, felt there should be convective activity riding the instability gradient which would produce pockets of localized higher amount of rainfall; though these locations could be scattered in nature. Given the mesoscale dynamics will largely dictate convective interaction lending to some uncertainty coupled with 300-400% above normal precipitation the past 2 weeks, felt expanding the Slight Risk area and introducing the Moderate Risk was warranted. In addition, the Moderate Risk was discussed with local offices who agreed any additional rain (beyond what is expected on Day 1) will likely exacerbate the flood threat. The Marginal Risk over portions of western/central Montana remain with a Slight Risk area introduced along the higher terrain (Rockies Front Range down toward Bear Tooth) which was in careful coordination with the local offices (MSO/TFX/BYX). With strong upslope enhancement, strong 700mb east wind, 0.75 inches precipitable waters and high snow levels, expect QPF values to climb to around 1.5-2.25+ inches in some locations. High resolution model QPF totals are quite impressive as seen by the HREF ensemble members. Not to mention, the more large scale synoptic models hitting this region hard with QPF amounts as well. With the potential for snow-melt and lower FFG due to terrain/soil, expect flash flooding to be a concern. The Marginal/Slight Risk across portions of the Ohio Valley remains as convection rides a boundary aided by mid-level vorticity. With pockets of 2000+ J/kg MUCAPE in place, surface convergence from the sagging cold front will assist thunderstorm development. Convective activity may develop into an organized complex given deep layer shear in place. ...Previous Discussion with Modifications Below... ...Southern and Central Plains... Short wave energy ejected from a strong closed low over southern NV taps deep moisture and strong instability along a frontal boundary extending from the Central Plains into the Mid MS Valley to support convection producing heavy rainfall over wet soils. While there was decent model agreement concerning the placement of the synoptic scale system, there was some spread in the placement of the highest rainfall amounts (not surprising given the convective nature of the QPF). With this in mind, the WPC QPF and Excessive Rainfall Outlook were based on a multi model blend. Convection is expected to initiate along the dryline/cold front extending from central NE into west TX, mainly after 26/18z, in an axis of 2500/3500 J/KG of SBCAPE. Given the amount of instability in place, the storms are expected to be predominately hail and wind producers, especially through 27/00z. However, short term training or cell mergers along the dryline/cold front could pose a short term flash flood before that time. The convection is expected to increase in coverage as difluence increases ahead of the closed mid level system over the southern Great Basin during the second half of the period. A strengthening low level jet transports 1.50/1.75 inch precipitable water air across the Southern and Central Plains, peaking near 27/06z. By that time, the propagation vectors slow to less than 10 knots across portion of central KS into southeast NE, which could allow storms or clusters to train as they move ahead of the front. As mentioned earlier, there is some spread in the placement of the highest rainfall amounts, which is typical in an environment dominated (at least initially) by strong instability. However, there was enough commonality to place the highest rainfall amounts from west central OK across central KS into eastern NE and western IA, as the storms follow the best low level moisture plume. Therefore, the Slight Risk was expanded across this region. Another round of consideration with regard to a Moderate Risk area across portions of central KS, where soils are fairly wet due to convection earlier this week. However, given the spread in the placement of the highest rainfall amounts, and after collaborating with WFOS ICT/DDC, it was decided to hold off the Moderate Risk for now. However, if there is better agreement on the placement of the highest rainfall amounts, a Moderate Risk could be needed in later forecasts. ...Ohio Valley... Short wave energy tracking along a frontal boundary extending from the Central Plains into the Ohio Valley interacts with deepening moisture and instability to support convection capable of producing excessive rainfall, especially over southern OH/northern WV into far southwest PA. While there was generally good model agreement with respect to the overall synoptic setup (including the frontal position), there are some differences in the amount of QPF in these areas. In an attempt to mitigate some of the differences, the WPC QPF and Excessive Rainfall Outlook was based on a multi model blend. Convection weakening over the Mid MS Valley early in the period rides along a frontal boundary from IL into southern IN and southern OH. Short wave energy (or MCV) riding along the frontal boundary provides synoptic scale ascent over an area with 2000-3000 J/KG of MLCAPE extending along and south of the front, which should result in renewed convective activity, mainly after 26/18z. Storms will also tap into the 1.50 inch precipitable water moisture plume in place. As the low level moisture flux peaks between 26/21z and 27/03z, storms (or convective clusters, as the 12z GFS simulated IR images showed cooling tops aligned in clusters) could train as the mid level flow parallels the front. Hourly rainfall rates could approach an inch, especially where convection trains, with local 1.50 inch amounts possible through 27/06z. Three hour flash flood guidance values are as low as 1.00/1.50 inches over portions of southern OH/much of WV into southwest PA, and these amounts could easily be exceeded by training clusters. Based on the above, the Slight Risk was left in place, with just some adjustments due to model trends. ...Montana... As a strong closed mid level low drops south from Vancouver Island into Northern CA moisture is transported across portions of MT and far northern WY during Day 3. Steepening lapse rates in conjunction with the mid level low could support 250/500 J/KG MUCAPE values over much of central MT, where the low level flow transports 0.50/0.75 inch precipitable water air (which approaches two standard deviations above the mean). The combination of moisture and instability could support convection capable of producing hourly rainfall rates near 0.50 inches in this area. There is a strong model signal for 1.50 to over 2 inches of rainfall, especially over south central MT and along the Lewis Range, along and just east of the divide in the upslope flow. Three hour flash flood guidance values are as low as 1.00/1.50 inches, which are attainable where storms train. The Marginal Risk was left largely intact for Day 3, and if there is a better convective profile near the divide in later forecasts, a Slight Risk could be needed. ...California/Nevada... A strong closed mid level system tracks from northwest CA early on Day 3 to a position over southern NV by the end of the day. Steepening lapse rates is expected to result in pockets of 250/500 J/KG of SBCAPE over the northern and central Sierra Nevada Mountains into nearby sections of western NV. Scattered convection forming in the afternoon will have access to 0.75 inch precipitable water air to produce locally heavy rainfall through the early evening hours. As the mid level system passes overhead, steering currents will be weak (generally under 10 knots), which could support local training and cell mergers. Hourly rainfall rates in this environment could approach 0.50 inches (as indicated by the 12z GFS), with local 1.25 inch precipitation amounts possible where training or mergers occur. Three hour flash flood guidance values are as low as 1.50 inches over portions of the Sierra Nevada Range, and these values could be approached. It should be noted that snow levels drop to around 7000 feet with the mid level system, so the potential for runoff issues should be limited to elevations below this. The environment could be borderline for a low end flash flood threat, especially as snow level drop. For now, no excessive area was assigned. Hayes/Pagano Day 3 Valid 12Z Mon May 27 2019 - 12Z Tue May 28 2019 ...THERE IS SLIGHT RISK OF FLASH FLOODING ACROSS PORTIONS OF THE NORTHERN PLAINS... ...Northern Plains... As the upper level low continues to track across NV toward UT, a surface low will develop amid height falls and better diffluence aloft. The progression of this surface low is what will set the stage for potential flash flooding across portions of eastern WY, far southern MT into southwestern SD. As the surface low develops across far northeast CO, it will slowly meander into NE and become situated in this region for 9-18 hours allowing easterly winds to enhance upslope across portions of the Black Hills, Laramie and Big Horn Mountains. While precipitable water values of 0.75 inches may seem modest, for this region it is sufficient (around 1 standard deviation above the mean), especially given it will be aided by 35+ knot winds for a long duration. Combine this feature with moisture flux, strengthening divergence aloft and mid-level impulses and widespread precipitation will result. While these synoptic scale features are well aligned, instability is nearly non-existent north of the surface low, though some instability aloft is noted early. Based on this, expect 1.5 to nearly 3 inches of rainfall is possible along the upslope regions of the higher terrain. Locally higher amounts are possible. With FFG around 1-1.5 inches in 6 hours, we will likely see QPF approach these values in some locations. Therefore, felt a Slight Risk was warranted in this region. Meanwhile a Marginal Risk is extended to the east along a stationary front where precipitation from an earlier system exited toward the Upper Mid-West/Great Lakes region with strong instability noted along this boundary overnight. ...Upper Mid-West/Upper Great Lakes Region... A Marginal Risk has been extended across this region to account for convection on Day 2 lifting northeast aided by mid/upper level flow. As rich moisture transported by southerly flow converges along a stationary boundary with strong instability (>2000 J/kg MUCAPE), expect thunderstorm development to become reinvigorated Monday afternoon/evening. Areal average precipitation of 1 to near 2 inches can be expected with higher amounts possible aligned with better instability and possible training. With three hour flash flood guidance as low as 1 inch, felt with the aforementioned ingredients, QPF may approach these values. Therefore, a Marginal Risk was introduced. With multi-model solutions starting to hint at values climbing above 2 inches, we may need to consider increasing to a Slight Risk across a portion of this region. With the soils across southern MN and WI a bit more sensitive to additional rainfall, these locations will likely be closely evaluated in the coming days. Pagano Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$