Quantitative Precipitation Forecast
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350 FOUS30 KWBC 180111 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 911 PM EDT Mon Jun 17 2024 Day 1 Valid 01Z Tue Jun 18 2024 - 12Z Tue Jun 18 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY... ...01Z Update... For the 01Z Update, one of the bigger adjustments was to remove the Slight Risk from the Gulf Coast. With the loss of daytime heating, the potential for heavy rainfall rates and widespread runoff concerns should remain low until at least tomorrow morning with the arrival of a surge of deeper associated with Potential Tropical Cyclone One. However, a Marginal Risk was maintained to reflect the potential for isolated concerns through the remainder of the evening and the overnight. Elsewhere, made minor adjustments based on current observations and analysis. This includes maintaining a Slight Risk across portions of the northern Plains and the upper Mississippi Valley. Unfortunately, the hi-res guidance does not appear to have a very good handle on the current activity, as reflected by the difference in the guidance and radar reflectivity across portions of the area. However, despite the uncertainty, the environment appears to remain favorable for continued storm development and increasing rainfall rates. The threat for heavy amounts appears most likely along a warm front lifting into eastern South Dakota and southern Minnesota as storms begin to train along the boundary this evening. Refer to WPC MPD #0456 for additional details concerning the evening into the overnight threat for heavy rain and flash flooding across this area. Pereira ...Previous Discussion... ...Summary... Per collaboration with the WFOs, notable adjustments to the inherited ERO include dropping the Moderate Risk, which had covered much of central MN and western WI in yesterday`s Day 2 ERO. There is just too much spread in the guidance, including the most recent CAMs, to support more than a Slight Risk. The transitory nature of the warm front lifting north has a lot to do with this, limiting the threat of persistent frontal (west to east) training. Otherwise, have expanded the Marginal Risk areas across the Lower MS Valley and over the Appalachians, Ohio Valley, and parts of the Northeast based on the latest guidance trends. ...Eastern Portions of the Northern Plains into the Upper Midwest... Stationary surface front across the Central Plains early this morning will lift north as a warm front today and tonight as the longwave trough drops into the Western U.S., while the broad upper ridge builds across the eastern half of the CONUS. Elevated convection early in the period along/north of the eventual warm front will lift gradually northward today, with a higher likelihood of least some cell training through early this morning given the uptick in low-level southerly flow (nocturnal LLJ). During the day the areas of heavy, potentially excessive rainfall will become more spotty with the daytime (differential) heating and weakening low- level inflow. By tonight, the upper flow will become more meridional, with the upper level jet axis more SSW-NNE oriented vs. W-E early today. This will shift and confine the best upper level forcing (upper divergence and low-level frontogenesis) farther W-NW, closer to the surface low track and associated cold front. So while the broad Slight Risk encompasses the areas of heavy rainfall today along and north of the warm front, the focus for heavy rainfall later tonight will shift to the eastern Dakotas and NW-NC Minnesota, within the region of aforementioned favorable forcing. All totaled, the anomalous deep-layer moisture (TPW anomalies ~2 standard deviations above normal) along with other favorable thermodynamic and kinematic parameters (MUCAPEs increasing to 1500-2500 J/Kg) will favor 1.5 to 2.0+ inch/hr rainfall rates underneath the strongest cells and/or where the convection trains. ...Western to Central Gulf Coast... 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 on the western periphery is quite subtle/transient, with small-scale convectively- aided mid-level vortices within the COL or deformation zone riding northward on the western edge of the ridge. The Slight Risk continues to be aligned along the coastal regions, where the expectation for forcing is that local Gulf breezes, outflow boundaries, and cold pools will provide localized forcing for heavy convection. However given the low 0-6km bulk shear (aob 20 kts), expect predominately pulse-type convection which would become outflow dominated rather quickly. Still, given the plentiful moisture and instability still available, any storms that form will be capable of heavy rainfall rates to the tune of 2 to 3 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. Within the Slight Risk area, most of the high-res CAMs (including the ARWs and 06Z HRRR) indicate isolated pockets of 5.00+ inches of 24hr QPF. ...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 1/Monday-Monday night 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. ...Lower Great Lakes-Ohio Valley to the Appalachians and Parts of the Northeast... 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 2-4+ inches, which despite the relatively dry soils initially in some areas (relatively high FFGs), could nevertheless lead to localized runoff issues, especially over urban areas or more sloped terrain. Hurley Day 2 Valid 12Z Tue Jun 18 2024 - 12Z Wed Jun 19 2024 ...THERE`S A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE UPPER TEXAS COAST... ...20Z Update... Some minor adjustments were made to the previous MDT risk across the Texas and LA coastal plain. The primary change was an incremental shift to the west based on the mean QPF field relaying the maximum further south along the TX coast with a core centered between Rockport and Galveston. This correlates well with the recent ML output with the 24-hour precip forecast noted north of CRP and just south of HGX. The setup is still pointing at a significant tropical moisture advection regime with the center of circulation passing south of the International Border, but plenty close enough for a prominent stream of elevated, deep moisture advection along a steady easterly flow off the Gulf of Mexico. Forecast PWATs are expected to reach above 2.5" for a large area encompassing southwestern LA down through Deep South TX with the plume maxima located along the central TX coast. GFS/ECMWF output are particularly bullish with deterministic forecasts indicating PWATs just above 2.9" at peak, a standard deviation closing in on +4 compared to June climatology. This makes sense considering an IVT pulse exceeding 1000 kg/ms protruding the TX coast by the end of the period within the global model consensus currently, a value typically reserved for the deeper tropical connections involved in any given setup in the month of June. All this will lead to significant rainfall on the order of 2-4" areal average along the TX coast from the Deep South TX area up through the Upper Texas coast to the northeast of Houston. QPF maximum between 5-8" is anticipated within the hardest hit areas, much of the heaviest precip falling in the 12 hour span from 00-12z Wednesday. General rates between 2-3"/hr at peak are very likely within that time frame of interest with 1-2"/hr prior to the main IVT pulse moving onshore. This would allow for localized flooding early in the period transitioning to more scattered to widespread flash flood concerns overnight into Wednesday, especially in the urban corridors from Beaumont down to Corpus Christi. A MDT risk was maintained given the current signals. Across the Central Plains, the setup for widespread heavy rain remains steadfast from previous forecast issuance with the convergence signal along the stalled front draped across KS/NE up through the Upper Mississippi Valley leading to locally significant rainfall in the above corridor. The area of concern continues to be the Central Plains from southwestern KS up through eastern NE and western IA within the Corn Belt with current QPF footprint indicating a vast area of greater than 2" precip totals with local amounts exceeding 5" in the hardest hit areas. Considering the strong deep moist convergence pattern in the region with mean flow aligned parallel to the boundary, training signals will be prominent and will likely be the primary cause for the excessive totals breaching 4" in any given areas. 12z HREF probability fields depict a relatively stout reflection for at least 3" with a widespread area of 30-50% probabilities for the targeted 3" marker with bullseyes of up to 70% located across west-central KS up to the NE border. The 5" probabilities are still fairly significant as well with an areal average of 20-40% located over the same areas mentioned above, and low-end probabilities of at least 8" centered over southwest KS. The threat within that zone remains on the higher-end of SLGT risk with the potential for a targeted MDT pending the expected convective evolution and relevant probability signals prior to the event. We will continue to monitor to see if an upgrade is necessary in future packages. Kleebauer ...Previous Discussion... ...Central Plains... A cold front pushing east across the northern Plains is expected to stall out over the central Plains Tuesday and Tuesday night. Surface high pressure over Montana will dive southeastward into the Dakotas Tuesday night. The high will provide a north to northeasterly flow of cooler and drier air into the front. Northeasterly flow at 850 mb will increase to 30-40 kt Tuesday night. Meanwhile...broad southerly flow of deep Gulf moisture will stream northward across all of the southern Plains and into the central Plains and the front. The southerly LLJ will peak at 45-55 kts across southern Kansas Tuesday evening. The two highly contrasting air masses will slam into each other at the front into north central Kansas and south central/southeast Nebraska. The storms will track northeastward with the deep moisture and convergence into northeastern Nebraska...an area that has been very hard hit with rain in recent days, resulting in very low FFGs. Thus, despite the lesser convergence over northeast Nebraska, the risk is higher due to more favorable antecedent conditions. Corfidi Vectors south of the front are out of the north at 5-10 kts. This will strongly favor backbuilding and training storms since they will be anti-parallel (opposite) the prevailing south to southwesterly flow of moisture into the front. The two air masses colliding head on into each other near the KS/NE border will prevent much if any movement of the front...allowing the storms to repeatedly impact the same areas as plentiful atmospheric moisture replaces that lost from rainfall. Further north, rainfall across northern MN will be ongoing through the morning from the overnight rainfall event. While some convective enhancement is expected due to an abundance of moisture availability for the convection, as PWATs will be around 1.75 inches. The warm front the storms will be tracking along will be moving north quickly enough that any flash flooding should continue with the rain after 12Z Tuesday. ...Upper Texas Coast... No significant changes were made to the ERO Risk areas. A plume of deep tropical moisture originating in the Caribbean will track northwest across the Gulf of Mexico with a developing tropical low in the western Gulf. The southeasterly flow around the low will begin to spread convection with deep tropical moisture and potential for high rainfall rates into the immediate coast through the day Tuesday. However, the better forcing and thus, more widespread heavy rainfall likely holds off at the coast until Tuesday night. There has been a small westward shift, as has been common with this rainfall event. Thus, a bit less storm total rainfall is expected into southwest Louisiana...with a bit more now expected into the middle and lower Texas coasts. This should overall limit the flooding potential a bit as southern Louisiana has been much wetter in recent weeks than south Texas. With continued westward shifts in the guidance it`s likely the Moderate Risk area can be shifted accordingly out of Louisiana and perhaps expanded a little along the Texas Gulf Coast. Regardless...most of the heavy rainfall event in Texas will be on Day 3/Wednesday except for the upper Texas coast. Wegman Day 3 Valid 12Z Wed Jun 19 2024 - 12Z Thu Jun 20 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE MIDDLE TEXAS COAST INTO PORTIONS OF SOUTH-CENTRAL TEXAS... ...20Z Update... Previous MDT risk issuance was maintained with confidence growing for at least a high-end MDT risk located somewhere over south- central TX with the impending tropical moisture flux. Totals based off the latest blend and ensemble bias corrected QPF indicate upwards of 5-8" with locally higher in a large area encompassing interior Deep South TX up through Central TX, including parts of eastern Hill country and the I-35 corridor south of Waco. There`s still some spread in the overall deterministic output with the consensus indicating a maximum generally focused over south-central TX, including the Austin/San Antonio corridor. Ensemble forecast QPF is moving towards very similar output with the ECENS and GEFS pinpointing the maxima within the currently outlined MDT risk over central TX back to the TX coast near and south of Houston. If we get a general agreement within the next succession of updates, there is some opportunity for targeted upgrade, if the current QPF presentation holds firm. Stay tuned for future updates.... Across the Central Plains, remnant convection from the previous period will linger across areas of KS/NE/IA with the current QPF maximum focused over eastern NE into IA. There`s still some discrepancy on the exact location of the where the heaviest QPF will occur with the blend generally modest with widespread 0.5-1" totals centered over the current SLGT risk in place. A lot of the potential is contingent on the convective evolution the period prior. Once there is better consensus, the focus could shift a bit from current location, but the ensemble and ML output currently situates the heaviest precip within the SLGT inherited. Only minor adjustments were made to the risk area based on the areal extent of the heavier precip within the NBM and ensemble bias correction in conjunction with where the heavier precip is expected the prior period. Kleebauer ...Previous Discussion... ...Southern and Central Texas... Tropical moisture associated with a strengthening low over the western Gulf will continue pushing west and inland across much of Texas on Wednesday. The moisture plume will have a direct connection to the Caribbean and moving over the entirety of the Gulf will mean the air mass will have incredible amounts of moisture associated with it. Multiple pieces of guidance suggest PWATs could approach 3 inches and will be broadly above 2.5 inches across all of south Texas Wednesday and Wednesday night. The low will provide the forcing adding lift to the atmosphere. The result will be a prolonged period of very heavy rain across much of south Texas. Embedded convection within the broader precipitation shield will be capable of rainfall rates to 3 inches per hour. This level of rainfall should easily exceed FFGs in all but the most flood resistant portions of Texas. Outside of the heaviest rainfall rates, much of the rainfall should consistently exceed an inch per hour rates given the tremendous amount of atmospheric moisture available for the storms to efficiently convert via warm rain processes to heavy rainfall. The heaviest rainfall totals will be along the coast, with the Gulf supplying abundant moisture and instability to the rain. As the precipitation shield presses inland, amounts will gradually come down, but storm total rainfall is likely to be in the 2 to 4 inch range well up the Rio Grande Valley. Meanwhile near the coast, storm total rainfall into Thursday of 6 to 10 inches is expected, with locally higher amounts where training convection adds to the rainfall total. Fortunately, much of south Texas has been very dry of late, so there will be plenty of room for the rivers to handle the deluge of moisture. The problem will be that the rain will likely be falling so heavily locally that there will be local exceedance of FFG despite the antecedent dry conditions. If the soils had been wet prior to this event, then it`s very possible that a High Risk area would`ve been needed. Fortunately, High Risk level impacts are not yet expected, at least on a widespread basis. There remains considerable uncertainty regarding the surrounding Slight (and Marginal) risk areas. Starting in the east...the guidance has been trending west with time, so amounts may continue to come down around the LA/TX border and points east of Houston. Into central and north central Texas, there is about a 50/50 split in the guidance as to whether significant tropical moisture and rainfall tracks north into this region, or continues pressing west into Mexico and the Rio Grande Valley. Guidance the past several days has trended on the west side, but in deference to the guidance suggesting the heavy rain moves north with time, the Slight Risk has been expanded to the DFW Metroplex with this update. That said, it`s very low confidence in this area and a southward shift in the guidance would have this area downgraded. Meanwhile, further west, rainfall amounts in the 2 to 4 inch range is still a very significant rainfall event for the Rio Grande Valley to as far west as the Big Bend. The Slight risk area was nudged west to the Amistad Reservoir with this update, and continued westward shifts may require additional westward expansions with future updates. Of course, this would also imply faster motion of the rainfall, which in turn would subsequently begin to reduce total rainfall and therefore the flood threat, so it`s unlikely the risk areas will need to be expanded much going forward, at least for Slight and higher. ...Central Plains into the Upper Midwest... In coordination with GID/Hastings, NE and OAX/Omaha, NE forecast offices, a Slight Risk area was introduced with this update for much of eastern Nebraska. Heavy rainfall from the Day 2 period will continue into Day 3/Wednesday, albeit most certainly on a downward trend in intensity Wednesday morning. Unfortunately, with continued moisture advection into the area and a small area of high pressure to the north preventing much northward motion of the convection for at least the daylight hours...scattered convection is expected across eastern Nebraska throughout the period. The storms will trend toward moving north with time Wednesday night as the front forcing the rainfall begins to also move north with the retreat of the high. Regardless...forcing for convection will be greatly reduced in this area on Wednesday as compared with Tuesday, so a significant portion of the confidence in Slight level impacts from flash flooding is contingent on heavy rain having occurred on Day 2/Tuesday. Thus, adjustments to the Slight Risk are likely once the rainfall pattern from Tuesday becomes more clear. Further east, a cold front moving across Minnesota and Wisconsin from Day 2/Tuesday will continue into the morning hours of Wednesday. The front will likely stall somewhere near the IA/MN border and southern WI, becoming a warm front with the strengthening moist southerly flow into the front Wednesday night. This will support training storms across the area, though they should be generally widely scattered in coverage. Thus, the inherited Marginal risk for this area remains largely unchanged. 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