Quantitative Precipitation Forecast
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386 FOUS30 KWBC 170027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 827 PM EDT Sun Jun 16 2024 Day 1 Valid 01Z Mon Jun 17 2024 - 12Z Mon Jun 17 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY, & PARTS OF THE CENTRAL GULF COAST... ...Northern Plains to Michigan... Elevated convection late Saturday night resulted in some prolific rainfall totals, including portions of northeast NE and south- central to southeast MN, making the region even more sensitive to additional rainfall. Overnight tonight into Monday morning, as an upper level jet streak traverses the Dakotas and southern Manitoba- Ontario, upper level divergence and low-level frontogenesis in its right- entrance region will generate new convection which should have no trouble growing upscale given the favorable dynamical, thermodynamic, and kinematic environment. Strengthening S-SW low- level inflow into an increasingly stationary front will likely equal if not exceed the magnitude of the mean 850-300 mb flow, while becoming aligned with the mean flow by late evening/toward midnight. This will allow for an enhanced risk for cell training along/near the surface stationary boundary across northeast NE, southeast SD, southern MN, and parts of western WI -- areas where the guidance shows TPWs peaking aoa 1.75" overnight. This along with elevated MUCAPEs of at least 1000-2000 J/Kg will likely lead to up to 2.5" an hour rain totals and local amounts to 5" where convection can train prior to the best deep- layer forcing shifting east- northeast. Changes to the risk areas were minimal on this update, with a minor nudge to the south in some locations. ...Central Gulf Coast... Broad, weak upper level difluence east of a retrograding upper level trough/shearline will affect portions of the Gulf Coast overnight. Scattered weak/subtle mid- level impulses along the western periphery of the upper ridge will lead to a rather spotty, disorganized convective footprint, though the latest CAM guidance/trends favor better clustering along the central Gulf Coast (south-central to southeast LA). PWs between 2-2.25" along with ML CAPEs between 1500-2500 J/Kg could result in fairly prolific short-term rainfall rates very late tonight into early Monday morning, up to 3" in an hour. The effective bulk shear has shown some increase as of late, and could attain 25+ kts which would lead to greater cell organization. Isolated 3-5" totals within a couple of hours can be expected (perhaps a bit more scattered) along the Central Gulf Coast region where the Slight Risk is noted. ...Tennessee Valley to the Southern Appalachians... A variety of pulse and convection with some organization is occurring along the western periphery of the expanding upper ridge which may lead to localized instances of flash flooding across this area for a few more hours. Precipitable water values are ~1.75". Spotty 3" totals are possible, though such amounts should become ever more isolated from here on out as CIN is beginning to set in. Roth/Hurley Day 2 Valid 12Z Mon Jun 17 2024 - 12Z Tue Jun 18 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN MINNESOTA AND FAR NORTHEASTERN SOUTH DAKOTA... ...20Z Update... Some shifts to the MDT risk were generated this forecast update and are reflected within the "Upper Midwest" sub-heading below. An additional MRGL risk was also added across the Blue Ridge and southern Appalachians within portions of WV/VA/NC/KY/TN. More on this located in the "Southern Appalachians" sub-section below... Elsewhere, continuity remains across the Gulf coast and across MT. Heavy rains from the tropical moisture advection off the Gulf will lead to scattered instances of very heavy rainfall with hourly rates pushing 2-3"/hr within areas under the current SLGT risk. PWAT anomalies between 2-3 standard deviations above normal are forecast to protrude into the southern LA coastal plain by the second half of the period marking the beginning of the true tropical moisture transport that will lead into the greater impacts on D3. There has been little change in this forecast with the greatest flash flooding focus within urban zones and within any cells that produce rates greater than 3"/hr given the FFG`s located across the outlined area. Over into MT, scattered thunderstorms will generate periods of locally heavy rainfall outside the higher terrain with the highest chance located just to the east of the northern Rockies in western MT. Totals will generally be 1-2" at max with rates between 0.5-1"/hr, which would be enough to satisfy some localized flooding prospects across any urban areas and complex terrain. There was no need for any significant adjustments to the forecast based on the recent trends. Kleebauer ...Upper Midwest... The primary upper level evolution has not changed much since the previous forecast issuance, however there are some differences in the location of the areal QPF max due to some adjustments in the surface reflection across the Northern Plains and Upper Midwest. An unusually strong low level jet (LLJ) and broad southerly flow off the Gulf will continue to advect abundant moisture and instability into the Northern Plains and Upper Midwest through the day Monday and into Monday night. Heavy rainfall is likely to be ongoing across southern portions of the Slight and Moderate risk areas from eastern SD into southwestern and south-central MN on Monday morning. A stalled out cold front turned stationary front will be draped across the region with the current forecast positioning located south a line from Sioux Falls through Minneapolis to north of Green Bay. The front will provide the focus for the heaviest convection as an airmass characterized by PWATs up to 1.75 inches, or 2 sigma above normal is lifted by the front. Cooler, drier air north of the front will start out the day moving into northern MN from the west. This influx of air will prevent the front from moving north very quickly to start out the day. This will allow for a more concentrated area of heavy rain with likely initiation over southern SD by late Monday morning, moving northeast along the front as the flow remains parallel to the boundary. Additional lift will be provided in the right entrance region of a 120 kt jet over southern Manitoba and western Ontario creating a scenario for widespread heavy rains extending from eastern SD through the central part of MN. On Monday afternoon, the cooler drier air north of the front will begin retreating to the north as the southerly flow south of the front begins to push the front north as a warm front. The convective coverage will diminish for a short time Monday afternoon, but with daytime heating, some heavy rain from the invigorated storms will continue across the middle of Minnesota and into northern Wisconsin. On Monday night, a low ejecting out of the Plains will rapidly lift northeast towards the ongoing convection across central and northern Minnesota, perhaps extending as far west as northeastern SD and southeastern ND. At the same time, a strong shortwave trough will move into the western Dakotas. The shortwave will support the intensifying low as it moves into the eastern Dakotas. The low will concentrate the convection along its warm front as the supporting LLJ intensifies above 50 kts. This will result in a reintensification of the heavy rain mover much of northern Minnesota. By the end of the period, a widespread area of 1-3" will be forecast from eastern SD and south-central MN up through northern MN with a maxima located across central and east-central MN where 3-6" with locally as high as 10" are plausible if there is considerable training in any given location within the confines of the warm front. The upper threshold of the setup near 10" is supported by the latest 12z HREF neighborhood probability fields where a bullseye of 20-25% for at least 8" exists north of the Minneapolis-St Paul metro. General probabilities for totals between 1-3" are well supported via the hi-resolution ensemble with a swath of 60-80% probabilities for at least 2" across all of central MN and over 90% for the same area for at least 1" within 40km of any point. The reason for the shift was due to the location of the warm front which is well-defined within the theta-E fields off model guidance. Trends have shown the higher theta-E`s to be situated a bit south of yesterday leading to the QPF maximum being displaced about 50-75 miles further south than 24 hours ago. This is beginning to correlate more with the latest ML guidance which has actually trended a bit north and is more in-line with ensemble QPF footprint and the relevant probability fields. As a result, the MDT risk was expanded on the southern edge to include the Minneapolis-St Paul metro area and the southern suburbs to account for some potential the setup could shift a bit more south. The northern edge was consolidated along with the northeastern extent into the Arrowhead with the mainly the area around DLH remaining to reflect the converging signal near the area. The probability of locally significant flooding is still very much in the cards considering the setup, antecedent conditions, and anomalous deep layer moisture being pulled poleward into the region. Wegman/Kleebauer ...Previous Discussion... ...Central Gulf Coast... Guidance across the region for the Monday and Monday night period has come down quite a bit from previous forecasts. A plume of impressive Caribbean and Gulf moisture characterized by PWATs as high as 2.25 inches is still expected to move into the central Gulf Coast, but forcing for heavy rain has become much less. Thus, the rainfall across mostly southern Louisiana, but also the Mississippi and Upper Texas Gulf Coasts has become much more spotty and less well defined. Given this downward trend, the Slight Risk in the area has been shrunk to focus on coastal regions...where the expectation for forcing is that local sea breezes, outflow boundaries, and cold pools will provide localized forcing for heavy convection. However, the convection now looks to not be as widespread. Given the plentiful moisture and instability still available...any storms that form will be capable of heavy rainfall rates to 2 inches per hour, which could still cause widely scattered instances of flash flooding. This would especially be the case should these heavy rainfall rates occur over an urban or flood sensitive area. ...Montana and Central Idaho... The comma-head region of a developing low over Wyoming will impact much of Montana and Central Idaho throughout the Day 2/Monday period. The low will form in the divergence region between 2 different jet streaks...one to the south over Wyoming, and a second over southern Saskatchewan. Unseasonably cold air over western MT and ID will clash with moisture tracking westward to the north of the low from the excessive rainfall over Minnesota to support the rain (and snow at higher elevations) across the region. Upslope flow may enhance total precipitation in the mountains in and around Great Falls, MT. Cooling from the upslope should support a good amount of that precipitation falling as snow at the higher elevations, which should cut any resultant flash flooding at the lower elevations to just isolated instances (due to lack of runoff from the higher elevations). While the amounts of up to an inch of liquid equivalent is a wetter rainfall event for this area, they should generally stay below FFGs, keeping any flash flooding confined to flood-prone areas. No major changes were made, with the portion of the Marginal in the mountains of western MT and ID emphasizing isolated valley flash flooding, as snow is expected at the higher elevations. Wegman ...Southern Appalachians... Significant differential heating within the terrain will occur under the strengthening mid-level ridge centered over the southern Mid Atlantic. Seasonal PWATs between 1.25-1.5" along with sufficient surface based instability around 2000-2500 J/kg SBCAPE will help initiate pulse variety convection with very little motion due to the steering pattern aloft. The combination of complex terrain and very slow to nearly stationary cell motion will create pockets of localized flash flooding concerns within the mountains and adjacent valleys. Probability for flash flooding is within the lower end of MRGL thresholds which was enough to warrant the addition of a MRGL extending down the Appalachian front from northeast WV down through much of western NC and eastern TN. Kleebauer Day 3 Valid 12Z Tue Jun 18 2024 - 12Z Wed Jun 19 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE UPPER TEXAS COAST AND SOUTHWESTERN LOUISIANA... ...20Z Update... Little to no change was necessary for the previous MDT risk area across portions of the central and western Gulf coast as the overall ensemble QPF footprint and magnitudes remained very similar from the previous forecast. Deterministic output has some variance with the location of the heaviest precip and associated SLP signature in the western Gulf that has allowed for a large coverage of at least a SLGT risk at the immediate coast. There is more consensus with the heaviest rainfall being positioned within the TX Gulf coast in the ensemble output as well as the ML output off the latest Graphcast and AIFS. Those specific ML outputs are slightly more southwest down the TX coast, so would not be surprised in some minor adjustments based on that proposition as ML guidance tends to do well with SLP tracks in the short and medium range. Didn`t want to deviate too much, however with the current ensemble blend and NBM probabilities still highlighting that upper TX coast. As a result, maintained continuity for heavy rains to impact the outlined area with totals pushing upwards of 3" with locally much higher based on the tropical rainfall efficiency and deep moist convection expected. Over the central plains, the signal for heavy rain late-Tuesday afternoon and evening has increased with deterministic signals now suggesting localized areas exceeding 5" as plausible considering the strong convergence pattern and mean flow running parallel to the frontal boundary in place. The pattern is a textbook way to get a long axis of training convection with a widespread QPF footprint that would produce an areal extent of heavy precip totals. Latest 24-hour mean QPF off the NBM is up near 2" over an area extending from northern KS through eastern NE. The 75th percentile, upper quartile is closer to 3" in the same location which is a pretty aggressive signature at these leads without the addition of the CAMs. The current SLGT risk remains and will be worded as a high- end SLGT for the time being, but there is a potential for this to be upgraded in future periods considering the growing consensus. Kleebauer ...Previous Forecast... ...Upper Texas Coast and Southwest Louisiana... A Moderate Risk upgrade was introduced with this update in coordination with LCH/Lake Charles, LA and HGX/Houston, TX forecast offices. The northern side of a developing tropical low in the western Gulf is expected to begin impacting the Upper Texas Coast and southwest Louisiana on Tuesday. The north side of the low will continue the broader southeasterly flow from the Caribbean across the Gulf and into the Upper Texas and southwest Louisiana Gulf Coasts. PWATs with this air mass are very impressive, at times approaching 3 inches. Unlike on Day 2/Monday, by Tuesday the low itself and any associated upper level energy will provide the forcing for convection, so there`s higher confidence on more widespread heavy rainfall and resultant high storm total rainfall amounts. Where confidence remains lacking is where the forcing tracks...particularly how far north it gets. This will play a critical role as to where the heaviest rainfall amounts will be. For now there appears to be fair consensus on heavy rainfall impacting the Gulf Coast from as far east as Pecan Island/Vermilion Bay, LA to as far west as Corpus Christi/Brownsville, TX. There`s better agreement highlighting the section of coast from Cameron, LA to Matagorda, TX. This includes Lake Charles, LA and Houston, TX. Since the forcing is highly concentrated, there`s likely to be a sharp gradient on the northern end as to who sees the heaviest precipitation. It appears likely that areas along a Leesville, LA west to College Station, TX line and points north will remain with either no or light, unimpactful rain. Those details too will need to be hashed out with better guidance agreement in the coming days. For now, therefore, the Moderate Risk largely stays along and south of the I-10 corridor. The heaviest rain will slowly track westward with time, so the back edge of the rain should move west along the Louisiana coast with time. Houston looks to get periods of heavy rain throughout Tuesday and Tuesday night as the storms track westward in waves. The heaviest rain rates are likely Tuesday night from Houston south along the coast as the strongest forcing associated with a front north of the low moves ashore from the Gulf into mainland Texas. PWATs approaching 3 inches would support highly efficient warm rain processes given the very high freezing levels associated with this fully tropical air mass. Any storms that track across this region will be capable of 3 inch per hour rain rates that would quickly overwhelm all but the sandiest/most flood resistant soils in a hurry. Thus, despite prior dry weather and dry soils from Houston south along the coast, these prodigious rainfall rates would easily overwhelm those soils, leading to local flash flooding. Urban and flood prone areas would be particularly susceptible. Interests in and around Houston should monitor forecast updates closely. The surrounding Slight Risk was shifted west with guidance trends, and includes nearly the entire Texas Gulf coast now, including into Brownsville in deep south Texas. Any rain that far south is likely to start just prior to the 12Z Wed cutoff and continue into D4/Wed. There remains significant uncertainty as to how far north the core of heaviest rain, and latitudinal adjustments are likely with future updates. ...Central Plains to Upper Midwest... Broad southerly flow continues across much of the nation`s heartland at the start of the period at 7am Tuesday morning. An unusually moist air mass characterized by PWATs around 1.75 inches will continue streaming north up the entire Mississippi River Valley. Rain across northern Minnesota will be ongoing at the start of the period with a stalled out/slow moving front. A surface low developing along this front should help to keep convection ongoing into the Arrowhead through the morning. Instability between 1,000 and 2,000 J/kg will ensure convection will be sustained through the period. The Slight Risk was expanded north ot the Canadian border and the Arrowhead to cover the likelihood of rain continuing from Day 2 period into Tuesday morning. Meanwhile, a burst of cooler, drier air associated with the cool air mass will track southward on gusty northerly winds over the High Plains of eastern MT, the western Dakotas and Nebraska, and eastern Colorado. This air mass will create a new frontal gradient between the drier, cooler air to the west and the hot and humid air mass across the eastern Plains and upper Midwest. An intensifying low over northern MN and eastern Ontario through Tuesday morning will push a cold front across the Dakotas and Minnesota. This will push the convection in the form of a line of storms from Minnesota south and east into Wisconsin and Iowa. The storms will likely form over hard hit areas of Minnesota, so additional flash flooding is likely Tuesday afternoon before it moves into a bit less hard-hit portions of WI. Given the eventual progression of the storms contrasted with the antecedent conditions, a Slight remains across much of Minnesota and western Wisconsin. Further south, convection across Kansas and Nebraska is likely to largely hold off until the early evening, and then continue through Tuesday night. A slow-moving almost stalled front in that area is expected as the southerly flow of hot and humid air south of the front causes the surge of drier air north of the front to stall, resulting in a clashing of air masses of southerly flow to the south and northerly flow becoming weak to the north. With ample moisture running into the front, the storms that form are likely to backbuild and train as they move northeastward along the front. Corfidi Vectors are southwesterly, parallel to the front, which will support training convection. Expect a swath of storm total rainfall through 12Z Wednesday of 1-2 inches from north central Kansas to extreme western Iowa, with locally higher embedded totals likely. The Slight Risk area was expanded southwestward to cover all of southwestern Kansas with this update, due to increasing numbers of the guidance suggesting storms will form and train into this part of Kansas as well. Wegman 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