Quantitative Precipitation Forecast
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579 FOUS30 KWBC 160800 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Sun Jun 16 2024 Day 1 Valid 12Z Sun Jun 16 2024 - 12Z Mon Jun 17 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY, ALONG WITH PARTS OF THE CENTRAL GULF COAST... ...Summary... Relatively minor adjustments were made to the inherited Slight Risk areas across the Northern Plains-Upper MS Valley and over the Central Gulf Coast region. We did expand the Marginal Risk along the Gulf Coast, which now includes the Upper TX Coast along with much of the FL Gulf Coast. ...Northern Plains to the U.P. of Michigan... Elevated convection along/ahead of the surface warm front late Saturday night had resulted in some prolific rainfall totals, including portions of northeast NE and south-central to southeast MN. Part of the reshaping of the Slight Risk was with this in mind, i.e. with the notable increase in 0-40cm relative soil moisture/soil moisture percentiles and subsequent lowering of the FFG over these areas. The other reason was with the model trends, particularly the high-res CAMs. The outlook areas here will see a break in the more organized, heavier rainfall today as the warm front pushes northeast and is followed by flat shortwave ridging aloft. Overnight, 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 the quasi-stationary boundary 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 1.5-2.0/hr rainfall rates where convection can train prior to the best deep- layer forcing shifting east-northeast. The Slight and surrounding Marginal Risk areas do have support from the CSU ERO first-guess fields (UFVS-verified), though as with yesterday`s Day 2 ERO, will keep the Slight Risk out of the Sandhills. ...Central Gulf Coast... Broad, weak upper level difluence will affect much of the Gulf Coast downstream of the weak upper level trough over eastern TX. 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), along with western portions of the FL Peninsula where the deep- layer easterly flow will push the Gulf-East Coast Sea Breeze boundary a bit farther west than what would otherwise would be. PWs between 2-2.25" along with ML CAPEs between 1500-2500 J/Kg during the day will result in fairly prolific short-term rainfall rates, likely 2+ inches in an hour or less, based on the latest HREF neighborhood probabilities. 0-6km bulk shear values generally 20 kts or less will favor more pulse type convection and thus more short- lived intense rainfall rates, however based on the deterministic CAMs and HREF probabilities, isolated 3-5" totals within a couple of hours can be expected, again especially (perhaps a bit more scattered) along the Central Gulf Coast region where the Slight Risk is noted. ...Tennessee Valley to the Southern Appalachians and Parts of the Southeast... Mainly pulse-variety convection (with 0-6km bulk shear aob 20 kts) along the western periphery of the expanding upper ridge may lead to localized instances of flash flooding across this area today. This as MLCAPE values climb between at least 1000-2000 J/Kg by the afternoon, while at the same time TPWs reach ~1.75". High-res CAMs show spotty 1.5-2.0+ inch/hr rainfall rates during peak diurnal heating this afternoon, then continuing (though becoming more isolated) into the evening hours. Most of the CAMs show isolated QPF totals of 3-5+ inches, which despite the relatively dry soils initially (relatively high FFGs), could nevertheless lead to localized runoff issues, especially over urban areas or more sloped terrain. 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 NORTHERN MINNESOTA AND FAR NORTHEASTERN SOUTH DAKOTA... ...Upper Midwest... Little has changed with the overall synoptic setup with this morning update. 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 central MN on Monday morning. A stalled out cold front turned stationary front will be draped across the region. 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, likely over southwest/central Minnesota Monday morning. Additional lift will be provided in the right entrance region of a 120 kt jet over southern Manitoba and western Ontario. 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 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. It`s possible the heaviest rain will concentrate over west-central Minnesota, while the heaviest rain will continue drifting northward with the front over the Arrowhead. This may concentrate the heaviest storm total rainfall across western Minnesota, with somewhat lighter but still significant amounts over northeastern MN. There remains some uncertainty as to where the "pivot point" will be, though the general consensus is west central MN near the ND/SD/MN tripoint. The Moderate Risk was trimmed out of most of the Arrowhead of Minnesota as it appears both a fair bit of rain associated with this event will occur after 12Z Tuesday, and will be more progressive to the north, resulting in lower rainfall totals than further west. The Moderate was expanded a row of counties further west in northeastern SD with this update. Consistent signal for storm formation in this area before moving northeast suggests that Moderate level impacts are probable into far northeastern SD as well. ...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 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... ...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