403 FOUS30 KWBC 192031 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 PM EDT Thu Jul 19 2018 Day 1 Valid 1850Z Thu Jul 19 2018 - 12Z Fri Jul 20 2018 19/1900Z update... Made an adjustment to the Marginal Risk extending it back across west-central Missouri into north-central Kansas. This is in response to developing convection the past few hours as noted in GOES-16 visible satellite imagery and radar trends. It has generally focused in a northwest-southeast fashion, just to the south of a west-east baroclinic zone. 12Z CAMs including the hourly update variety continue to struggle with this convective feature. Do not anticipate flash flooding but the orientation the convection has attained at least warrants a Marginal Risk over the region. Elsewhere, no discernible changes were necessary as the latest global models and CAMs have been consistent in painting heavier QPF amounts across sections of the Upper Midwest down into the Middle Mississippi Valley. This will particularly be the case through 20/0000Z. Rubin-Oster ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER EASTERN PORTIONS OF THE NORTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY... ...Eastern portions of the Northern Plains into the Upper Mississippi Valley... There are no significant model mass field differences with the Northern Plains closed low that is expected to push slowly eastward Thursday morning from eastern portions of the Northern Plains and into the Upper Mississippi Valley Thursday night into early Friday. There is strong model signal for heavy precipitation potential in the vicinity of this upper center and to its southeast in a region of defined upper difluence and above average pw values from across eastern portions of the Northern Plains into the Upper Mississippi Valley. Changes to the previous outlook for this period were minor with only a slight southwestward extension of the slight risk area into far northeast South Dakota and far southwest Minnesota and slight extension to the slight risk area into northwest Illinois. Both of these changes were to cover the model spread with the max qpf axes. Slowest moving cells likely near and just to the north and northwest of the mid to upper level center from far southeast ND into far northeast SD...southwest to central Minnesota where precipitation areas may pivot in the associated comma head/deformation precipitation area. Precipitation likely to be more progressive to the southeast of the mid to upper level closed low across Iowa into northern Illinois where greater instability will counteract the expected more progressive motion of the precip areas. ...Central to Eastern Gulf Coast into the coastal Southeast... A surface frontal boundary is expected to remain stationary this period from the Lower Mississippi Valley eastward into the Southeast. Unstable air along and to the south of this front..mu-cape values 1000-2000 j/kg will persist Thursday into Thursday evening across the Central to Eastern Gulf coastal region into the coastal Southeast. This will support widespread scattered convection in the axis of much above average pw values...2 to 2.5 standard deviations above the mean. Heavy to isolated excessive rainfall amounts possible with slow moving cells across these areas...with hourly rainfall rates of 2"+ per hour possible. Changes to the previous outlook were to extend the marginal risk area farther westward into the central Gulf coastal region from southern Georgia...the Florida Panhandle..southern Alabama into far southern Mississippi. ...Southwest... No significant changes made to the previous marginal risk area extending across portions of the Southwest. With the mid to upper level ridge remaining in place this period across this area and continued much above average pw values...additional widespread scattered convection likely Thursday afternoon into Thursday evening...especially along the high terrain from west-central New Mexico westward along the Mogollon Rim of Arizona...into southern Utah and southern Nevada. Oravec Day 2 Valid 12Z Fri Jul 20 2018 - 12Z Sat Jul 21 2018 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE OH/TN VALLEY... ...Lower Great Lakes / Ohio Valley / Tennessee Valley... Showers and thunderstorms are likely on Friday into Friday night from portions of the Great Lakes south through the TN Valley. A developing deep layer low will slowly push east into the Great Lakes, with a cold front extending to its south. Moderate, to even high, instability is forecast ahead of this front, with PWATs above 1.5". This region will also be well positioned in the left exit region of the 250 mb jet, increasing upper level divergence. Thus seems likely that scattered to widespread convective activity will develop through the day Friday. The most intense activity is likely to focus across portions of KY/TN into northern MS/AL. It is here where the best overlap of instability and deep layer wind shear exists. Thus would anticipate to see one or more organized convective clusters across this region. This activity may very well forward propagate off to the south into the instability pool...with these fast storm motions potentially limiting the flash flood risk. However, will maintain the Slight Risk for now, as the combination of favorable forcing, high PWATs and high instability supports the potential for short duration heavy rates...and some of this area had been anomalously wet of late. Also note several of the 12z HREF members depicting localized 3"+ amounts. Further north into OH/IN note less shear and lower (although still decent) instability. This suggests that activity here may not be quite as organized/intense...however also favors slightly slower cell motions. Also the closer proximity to the deep layer low suggests that multiple rounds of storms are possible. This combined with with several 12z HREF members depicting 3"+ across this area warrants an extension north of the Slight Risk. Further north into MI/WI instability will be lower. Although cool mid level temperatures underneath the low, and weak mean flow suggests slow moving cells producing brief period of heavy rainfall are possible. Thus will extend a Marginal Risk into this area. ...Coastal GA/SC/NC... A shortwave along the coast combined with increasing mid/upper level forcing with the approach of the longwave trough...supports the potential development of an area of low pressure off the southeast coast during the day Friday. Plenty of moisture will be in place along the coast...with PWATs greater than 2". Instability will thus be the main question with regards to the heavy rainfall and flash flood potential. This will be dependent on exactly where the low forms, which will help impact the location of the coastal front. The 12z GFS and a majority of the 12z HREF members bring the coastal front inland enough to bring heavy rainfall ashore. In fact, several HREF members show the potential for some 5"+ amounts along the coast. On the other hand, the ECMWF and UKMET have been further east...generally keeping the instability and heaviest rainfall amounts offshore. Confidence in this forecast is below average and will thus need to closely monitor. For now will stay close to continuity and favor the heaviest rains staying just offshore...and thus maintaining just a Marginal Risk. However trends overnight into early tomorrow should make this forecast more clear...and could certainly see a more organized flash flood risk develop should the low become organized enough to push the coastal front inland. ...Northern FL... Will maintain a marginal Risk for portions of northern FL. With the area of low pressure potentially developing off the southeast coast...a trailing front will extend southwest across the FL peninsula. Sometimes the tail of the fronts in these situations can act as a focus for heavy rainfall...with a bit of enhanced flow interacting with the tropical PWAT plume. Note several of the 12z HREF members hit this area with 3"+ amounts as well. Thus see no reason to change from the going Marginal Risk area. ...Southwest U.S.... Diurnal instability should led to monsoonal thunderstorms are again expected on Friday. Model forecast lapse rates of 7-8 deg c/km combine with areas of terrain aided lift to produce showers/storms. The models do not show particularly heavy rain, but several burn scars exist in the area. Cells that develop over the burn scars will be capable of producing flash flooding. Deep layer moisture of 1 to 2 standard deviations above climatology will expand northward out of AZ into UT as southerly flow deepens around the west side of the ridge center. Therefore the Marginal Risk area extends from AZ/western NM northward into Utah. Chenard Day 3 Valid 12Z Sat Jul 21 2018 - 12Z Sun Jul 22 2018 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE EAST... ...Great Lakes into the Southeast... A broad Marginal Risk extends from the Great Lakes into portions of the Southeast. This area is along/ahead of the longwave trough axis and attendant cold front. Plentiful instability and moisture is expected along this corridor. The best overlap of instability and shear again ends up on the southern extent of this area. Thus the most organized activity will likely be here, but again some questions as to storm motions potentially limiting the flash flood threat. Some chance that 850 mb flow becomes a bit more favorable for some backbuilding of cells b this time, which could locally increase the flash flood risk. Thus some chance we may need to embed a Slight Risk within the Marginal at some point, but for now will stick with the Marginal. Instability is less further north, suggesting cell intensity will be less. However still plenty of moisture, and as you go north the front becomes nearly stationary. This persistent low level convergence axis may focus enough cells to pose a localized flash flood risk...even if the overall storm intensity as not as high as further south. ...Long Island, Northern New Jersey and southern New England... The potential low off the coast could bring a heavy rainfall threat into portions of NY into southern new England by Saturday night. Still quite a bit of uncertainty with the track and intensity of this potential low. The ECMWF remains east of the GFS/NAM solutions. For now generally favored a compromise solution...which is pretty close to the 12z GEFS mean. Low/mid level WAA and PWATS will be plentiful for heavy rainfall ahead of this low. Main limiting factor will be instability, although could see just enough move ashore to produce some locally intense rates, dependent on the low track. Given the uncertainty with both the intensity/track of the low, and the degree of instability present, will just maintain a Marginal Risk for now. Will need to monitor over the coming days, as could envision the need for a Slight Risk depending on how things trend. Chenard Day 1 threat area: http://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: http://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: http://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$