Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
134 FOUS30 KWBC 191915 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 315 PM EDT Thu Sep 19 2024 Day 1 Valid 16Z Thu Sep 19 2024 - 12Z Fri Sep 20 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN COAST RANGES OF CALIFORNIA, UPPER MIDWEST, AND THE URBAN CORRIDOR OF SOUTHEAST FLORIDA... ...16Z Update... No changes were made to the inherited Marginal Risk areas in California, Wisconsin, and Florida with this update. In California, the Southern Coast Ranges continue to be the focus for rainfall capable of flash flooding as light to moderate rainfall rates fall over the many burn scars in the area, which could result in localized flash flooding. A deep, almost cutoff positively-tilted upper low will continue drifting southeast down the coast. Where forcing is maximized along the Southern Coast Ranges, locally heavy rain may occur. The forcing would be maximized by a combination of upslope/terrain influences and the upper level forcing. In Wisconsin, the Marginal remains intact largely due to some modest agreement in the CAMs that 2 rounds of strong storms may move across the region back-to-back, which may cause isolated flash flooding in flood-prone and urban areas. In Florida, the stalled out front and PWATs above 2 inches will continue to support slow-moving convection capable of 2+ inch per hour rates. At high tide, poor drainage off the urban areas will briefly and locally increase the flooding threat. A couple areas were considered for Marginals...the Blue Ridge to the Great Smokies of western Virginia and North Carolina will have a non-zero flash flooding threat due to terrain influences and lee-side low development, as well as for the coast of southeast North Carolina where the sea breeze may interact with predominantly offshore flow to result in locally heavy rainfall on saturated soils from the recent passage of Potential Tropical Cyclone 8. Ultimately both these areas were passed over for a Marginal largely due to enough unfavorable meteorology (meager moisture primarily), and some storm motion allowing any potentially heavy rain cores to move away from their respective formation regions. Wegman ...Previous Discussion... ...California... A fairly stout closed upper low over the Pacific will be the driving force behind increased forcing and region moisture advection into the southern half of California this period. Recent wildfires have brought about a plethora of burn scars within the terrain of the Southern Coast Ranges that have become increasingly susceptible to flash flooding prospects due to hydrophobic soils allowing for consistent run off capabilities. The pattern evolution supports a strong mid and upper forcing regime to be co-located within the Central and Southern CA coasts, as well as the immediate terrain to the east of the coastal plain. Latest 00z HREF mean QPF distribution pinpoints a regional maxima within the confines of the Southern Coast Range, intersecting an area with multiple burn scar locations that will likely exhibit flash flood effects thanks to the current antecedent soil environment. There`s a consensus within both the mean QPF and probability fields for >1" of rainfall in-of the aforementioned area with even some prospects for locally >2" as noted within the neighborhood probabilities for D1. With coordination from earlier updates by the local WFOs involved prompting the MRGL risk, and with the signal remaining prolific enough from the latest guidance, the MRGL risk was maintained to project continuity. ...Upper Midwest... An increasingly favorable pattern will invoke a period of scattered to widespread thunderstorm activity capable of producing locally enhanced rainfall cores leading to 1-2"/hr rain rates in the strongest convective cores. Steepening lapse rates, increasing bulk shear, and modest surface to mid-level buoyancy will create an environment conducive for not only thunderstorm initiation, but prolonged updrafts capable of maintaining significant storm structure within a core of elevated PWATs advected ahead of a mean trough to the west. Latest HREF EAS signals for >1" are pretty high (60-80%) across much of Northwest WI, an area where most CAMs are pinpointing as the primary area of focus for this afternoon`s and evening`s convective prospects. Neighborhood probs are a bit less emphatic than what normally constitutes a higher impact potential, but still fairly significant nonetheless for potential totals ranging between 2-4" over portions of Eastern MN into Northwest WI. The antecedent conditions present within the zone of highest probabilities is borderline unfavorable for flooding with NASA SPoRT indicating soil moisture percentiles closer to 40-50% over the area. Considering the favorable environment and SPC outlining the area with a Slight Risk for severe weather, believe the setup is good enough for a targeted MRGL within the core of the best probabilities based off the latest hi-res ensemble. ...Southeast Florida... A meandering frontal boundary in-of South FL will become a focal point for elevated convective coverage, especially just inland of the urban metro of Miami up to West Palm Beach when sea breeze initiation becomes tied within the frontal positioning later this afternoon. The steering pattern is favorable for convective development to drift back over the urban metro, settling within a modest PWAT anomaly situated over the area. The core of convection will likely drop rates between 1-3"/hr with intra-hour rates >3" more than plausible, a factor that typically causes flash flood concerns within the urban corridor. HREF blended mean QPF has a bullseye situated within the Fort Lauderdale/Pompano Beach area with a small secondary max near Miami. This signal is fairly classic for a lower category risk, especially when you add the steering pattern leading to favorable motions back over the cities along the Southeast FL coast. The previous forecast was maintained with only a very minor adjustment along the western periphery of the risk area. Kleebauer Day 2 Valid 12Z Fri Sep 20 2024 - 12Z Sat Sep 21 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE URBAN CORRIDOR OF SOUTHEAST FLORIDA, NORTHEAST NEW MEXICO, AND THE DESERT SOUTHWEST... ...1930Z Update... No significant changes were made to the ERO risk areas for the Day 2/Friday time period. ...Southwest... The upper low moving down the coast today will move inland over the Desert Southwest Friday. Associated moisture will accompany the low into the usually dry areas of southeast California, southern Nevada, and northwestern Arizona. The storms in this area will be associated with the instability maximum at the center of the upper low where upper level temperatures will be coldest. As is the nature of flow near the center of lows, the steering will be chaotic and highly dependent on where the exact center of the upper low will be at any given point in time Friday afternoon. With any more persistent showers and storms, expect isolated flash flooding with any burn scars, dry washes, and other flood- prone areas. PWATs between 0.5 and 0.75 inches in these areas will peak at about 1.5 sigma above normal for this time of year. ...New Mexico... Broad upper level diffluence ahead of the positively tilted low over the Desert Southwest will cause a somewhat narrow 50-100 mile wide plume of training showers and possibly embedded thunderstorms to develop over eastern New Mexico and into portions of the far western Texas and Oklahoma Panhandles. Given the sensitivities of some of these areas to prolonged rainfall, the Marginal risk remains in place, especially for any burn scars or flood prone areas. ...South Florida... A final day of heavy rainfall potential is expected in South Florida again on Friday. Once again the primary risk of flash flooding will be in the urban corridor from West Palm Beach south through Homestead. Slow-moving and persistent thunderstorms on the leading edge of an advancing cold front will be the primary drivers of any heavy rainfall. The timing of high tide with the storms will also play a big role in any potential isolated flash flooding due to much poorer drainage into the ocean during high tide. The front will clear South Florida by Saturday turning the predominant flow more northeasterly. This will cut down on the PWATs some, while also greatly limiting, though not eliminating the coverage of storms by Saturday. Thus, the Marginal flash flooding threat should end for now with the conclusion of Friday afternoon`s storms. Wegman ...Previous Discussion... ...Southeast Florida... A secondary period of impact from active sea breeze convection and propagation into the urban corridor is forecast to transpire on Friday afternoon. The signal is not as robust as the previous period, but any heavy rainfall prior would precede this potential event with lower FFG signals leading to a bit more sensitivity to flash flood prospects. The ensemble mean QPF is a touch less aggressive as stated, so it will remain a period to monitor to see if the threat trends higher once we move closer to the range of the CAMs. For now, maintained general continuity as the signal persists for a flash flood threat in-of the urban corridor extending from Miami up through West Palm Beach. ...New Mexico... The prolific closed upper circulation out across the Southwest will begin pressing eastward with an increased upper forcing pattern thanks to a broad diffluent axis downstream of the mean trough/upper low. Most of the latest global deterministic eject a rapidly moving shortwave out of Chihuahua, traversing over the Eastern half of NM by the afternoon and evening hours Friday. A tongue of modest instability based off the mean MUCAPE forecast from all the deterministic is forecast to bisect that eastern side of the state with the best focus aligned from the Sacramentos up through the western side of the Caprock and into the High Plains of Northeastern NM. Flash flooding threats will likely be tied to urban areas and some of the remnant burn scars located within the Sacramento Mountains, however these setups are notoriously tricky, especially with the propensity for convection to become anchored along the instability gradient favored in the terrain and adjacent Caprock. Some of the deterministic output is pretty reasonable for isolated flash flood prospects, especially when you add the increasingly favorable upper pattern. A small MRGL risk was added to encompass the areas above with the threat lying on the lower-end of the risk threshold. ...Desert Southwest... Our deep closed upper reflection will migrate slowly to the east with the circulation likely crossing Southern CA into the interior Desert Southwest as we move towards the end of the period. As of now, the current QPF output is not exactly what would constitute a favorable flash flood scenario. However, considering the expected pattern evolution and the advection of more moist, buoyant air northward ahead of the upper low, you have to acknowledge the prospects of a more scattered pulse convective pattern over an area that is much more conducive for flash flood potential. Recent ECMWF AIFS ML output was actually more aggressive than the recent deterministic in the spatial coverage of precip, and even for the magnitude of the expected setup. This tends to signal a red flag for the deterministic not being aggressive enough with such a strong mid and upper pattern. A low-end MRGL was placed over portions of Southern CA/NV into Western AZ to outline the favored area for scattered convective development with modest storm cores capable of locally heavy rainfall. Kleebauer Day 3 Valid 12Z Sat Sep 21 2024 - 12Z Sun Sep 22 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE CENTRAL ROCKIES AND CENTRAL PLAINS... ...1930Z Update... Only a few minor adjustments to the inherited Marginal across the central Rockies and Plains were made. The upper level low over the Southwest will track northeastward towards the Plains on Saturday. As usual this will help spur on a strengthening LLJ across the southern Plains, with warm and humid air from the Gulf running into a cooler air mass over the Plains. Expect the greatest coverage of heavy rainfall to develop mostly overnight Saturday night, as the strengthening LLJ interacts with an area of surface high pressure over the northern Plains. This will force strong frontogenesis in between, leading to a prolonged period of heavy rain and training thunderstorms. Thus, while the meteorology would support at least a Slight Risk, the tradeoff is that most of this area has been bone dry in terms of precipitation, and with the drought conditions in place, it will take a lot of rainfall to overcome the dry conditions and empty creekbeds to result in flooding. However, given that the surface low will be forming along the front, and the high to the north countering the low and the LLJ from the south will strongly favor training convection, it seems probable that somewhere in the Marginal Risk area the high bar will be met and exceeded for flooding. Thus, it becomes where the most likely area of training convection will set up, which for the moment appears to generally follow the KS/NE border from northwest KS to southeast NE. Fortunately much of this area is rural, so urban concerns appear to be minimized. It appears likely a Slight risk will be needed across the Plains but for now there is limited certainty as to how all these various ingredients will interact to reach that threshold. The Marginal risk was expanded southwest to include northeast NM and the TX and OK Panhandles partially due to expected lowered FFGs from the rainfall on Day 2/Friday. A bit of southeast KS was removed due to somewhat better certainty that most of the rain will be northwest of there, and a small bit of the southern Sand Hills was removed due to very high FFGs. Wegman ...Previous Discussion... The broad upper trough, closed low evolution over the West will translate eastward with an evolving setup capable of widespread convection across the Rockies through the Central Plains, especially as the trough axis picks up forward speed and slides into the Central Rockies during the back half of the forecast period. A combination of favorable ascent and increased moisture advection thanks to a developing LLJ pattern east of the Front Range will lead to widespread thunderstorm activity with several heavy cores likely to initiate and produce locally enhanced rainfall. The most favorable locations for flash flooding are likely to be across the Colorado Front Range and the Central Plains between Southern NE and Northern KS, a reflection with the current ensemble fields and bias corrected QPF footprint. There will likely be an upgrade somewhere within the broad MRGL risk encompassing the Central Rockies into the Plains, but the finer details are still another succession of runs away from pinpointing specifics with regards to rainfall totals and the upper threshold of the potential. Ensemble bias corrected QPF depicts a widespread 1-2" across the above area, a testament at range on what could transpire. This will be a period to monitor closely for upgrades considering the evolution. Kleebauer 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