Quantitative Precipitation Forecast
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647 FOUS30 KWBC 122008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Wed Jun 12 2024 Day 1 Valid 16Z Wed Jun 12 2024 - 12Z Thu Jun 13 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN FLORIDA AND CENTRAL TEXAS... 19z Update: We upgraded to a Moderate risk across portions of southern FL. Flash flooding is ongoing and expected to continue into the evening hours. Given the amount of rain that has already fallen, and what is expected to fall over the next 6 hours...at least locally significant flooding impacts are expected. Eventually we expect this axis of heavy rain to start shifting south and become a bit more progressive. However this will likely take at least a few more hours, as current radar trends do not show much of a southward push quite yet. It appears likely that some areas will see an additional 5"+ through the evening hours. The 17z HRRR was too aggressive in moving activity south, so it is likely not the best QPF guidance in the near term. Chenard ...16Z Update... Area SLGT risks across FL and TX remain as the ongoing heavy rain threat in the respective locations continues. Full regional updates in each sub-section below... ...Southern Florida... A well-defined convergence pattern is currently analyzed off the western coast of FL extending inland through the south-central part of the Peninsula. Widespread heavy rainfall of 1-4" has been seen through this morning with a couple maxima focused near Lake Okeechobee, Port St. Lucie, and Fort Myers. Additional bouts of heavy rainfall are anticipated with the coastal areas between Sarasota down to Naples as one area of focus, as well as across the Peninsula between West Palm down to Miami as the seabreeze coupled with the advancing convergence zone drifting south will enhance local rainfall prospects just away from the coast. Recent trends within the 12z HREF and associated CAMs indicate some pretty robust maxima upwards of 9-12" for the period within those urban corridors, a similar output from what we saw yesterday with a couple of "jackpot" zones that ended up materializing within the convergence pattern. The same premise today with those coastal zones the most likely to see flash flooding later this afternoon and evening with heavy rain likely extending inland through the Everglades. Additional pockets of enhanced rainfall are plausible within weak mid-level perturbations moving through the flow with adjacent areas surrounding the primary focal points having a shot at local flash flood concerns, especially any urban corridors. The previous MRGL and SLGT risks were maintained in their prior locations with a highlight to those aforementioned areas for a higher-end SLGT risk that could be upgraded to a targeted upgrade pending radar evolution. ...Texas... Across TX, a slow moving shortwave is wreaking havoc across central portions of the state with the area from I-35 south of Waco through the eastern Hill country bordering the FWD/EWX/HGX forecast areas as the primed target this morning. Expectation is for a continued threat for locally heavy rainfall as the meager forward propagation speed of the convective cluster within an axis of higher PWATs (~2" via recent Mesoanalysis) will allow for periods of heavy rain within more robust cores embedded in the main precip shield. Satellite imagery shows the convection as fairly low-topped with the warm cloud layer generally shallow compared to past events. Regardless, outflow generation from the storms has provided a surface convergence point over a sufficiently moist axis along with enough general mid-level ascent to create a pocket of enhanced thunderstorms. Given the impacts this morning, carrying through the past few hours, have opted to maintain the SLGT risk through the afternoon with an expectation for it to dwindle later in the period. Elsewhere, scattered convection is forecast across portions of east TX back into portions of the Hill country with locally heavy rainfall plausible in more active convective cores. Generally looking at 1-2"/hr at peak intensity within those more pulse variety thunderstorms, so the prospects for flooding are more MRGL with the best threats occurring over more urban areas. ...Upper Mississippi Valley... Current radar/satellite composite indicate the passage of a progressively moving shortwave across portions of the northern Midwest with a wave of moderate rainfall progressing through southern MN. This will quickly scoot off to the east with some modest totals located generally south of the MSP metro, but a low- end chance for flash flooding on its current propagation. Another shortwave is analyzed up across ND with a forward motion forecasted to take into the northern Midwest across central and northern MN, eventually ejecting eastward into WI by the end of the period. Low pressure over southern Manitoba with a cold front extending along the tail end of the surface reflection will be the focal point for a line of convection developing across northern MN with sights downstream on the MN Arrowhead through the northern 2/3`s of the state. Latest HREF neighborhood probabilities have ticked upwards for at least 1" within the corridor extending from MSP up into DLH (70-90%) with >2" now sufficiently into the 30% range with a bullseye just south of MSP of 50% thanks to the two-part impact from this mornings activity and later on this evening. The progressive nature of any convective development along and ahead of the cold front will limit the potential for flash flooding for most, but isolated instances of heavier rainfall into any urbanized settings will prompt a higher risk for flash flooding. Thus, have maintained the MRGL risk from prior forecast with an expansion to the north to account for the latest trends in hi-res guidance and ML output of the corridor of heaviest precip with the cold front aligned over northern MN into adjacent WI. Kleebauer Day 2 Valid 12Z Thu Jun 13 2024 - 12Z Fri Jun 14 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN FLORIDA... ...20Z Update... Updates can be found in the sub-sections below.... ...Florida... A persistent convergence zone will bisect the southern Florida Peninsula with locally heavy rainfall continuing over much of the area south of the line of Tampa over to Port St Lucie and points south. Totals in the periods prior will be running between 3-12" for an areal average across the above corridor with some maxes approaching 15" in the areas that have been hit the hardest over successive days. Typically this area has a climatologically high FFG for the 1/3/6 hr windows, but because of the significant rainfall the past 48 hrs, antecedent conditions are more favorable for flood concerns, especially the southeast and southwest urban corridors of the state. Latest probability fields continue to yield some impressive percentiles for totals exceeding 5" with a span of 40-60% extending from Sarasota down through Naples on the southwest coast, and 30-45% located along the southeastern corner of the state from West Palm down to Miami proper. Considering the repeated nature of the heavy precip, expectation is for more flash flooding to arise late Thursday morning through the afternoon as diurnal destabilization within the convergent axis provides another round of significant rainfall for those that have seen a prolific amount already. There is some uncertainty on specifics on where the heaviest cores will occur and this setup is trending very delicate given the unfavorable antecedent conditions. As a result, have maintained a SLGT risk with wording for a higher-end SLGT and the potential for targeted upgrades pending the radar evolution. ...Midwest... Shortwave over the Northern Plains will pivot around the northern periphery of the mid-level ridge located across the Southwest U.S with sights on the central Midwest by Thursday afternoon. At the surface, a cold front will be pressing south-southeast over portions of the Central Plains through the Mid-Mississippi Valley. The coupling of the two distinct features in the midst of a prolific thermodynamic regime located over the above area (MLCAPE exceeding 4000 J/kg) will help initiate a robust line of convection with cores capable of locally enhanced rainfall as they move southeast with the forecast steering flow. Models are hinting at the corridor from Chicago to points southwest being in the cross-hairs of the heaviest precip threat with the HREF probability fields outlining the areas with respectable probabilities for exceeding 1" (70-90%) and 2" (45-60%) over the course of the period. There is a significant drop off once you move past 3" probabilities which is typically favored for the higher risk forecasts, however the ambient environment is likely conducive for locally significant rainfall in the strongest convective cores. There is a prospect for backbuilding if cells become anchored to the front, and this is documented within multiple CAMs at the end of their respective runs. Issue is, the location of where these occur are different with some favoring Chicago down into the IL/IN border while others are much further west into northeast MO to west-central IL. The MRGL risk was maintained to allow for more consensus with guidance as they get inside 24 hrs of the event. Expect a targeted SLGT risk somewhere in the middle of the MRGL risk currently with the best chances lying along the expected frontal placement by tomorrow evening. Kleebauer Day 3 Valid 12Z Fri Jun 14 2024 - 12Z Sat Jun 15 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN FLORIDA... ...20Z Update... There were only small adjustments based on the latest ensemble forecast QPF across the respective risk areas. The SLGT risk across FL is the headline currently with the ongoing heavy rain threat continuing through the end of the week into the early part of the weekend. Sheared out convergence zone across South Florida will plague the region for at least one more period with locally heavy rainfall anticipated south of the Sarasota to West Palm Beach line. PWAT indices are expected to slowly degrade as the pattern initiates the expected shift away from what we`ve seen over the course of the week. Still, with the prior rainfall footprint leading to much lower FFG`s across the region, a risk of flash flooding will be heightened despite some improvement in the environment. Totals of 2-4" with locally higher are forecast across parts of South FL leading to the SLGT risk continuation. The Northeast U.S will see the approach of a cold front moving through Ontario/Quebec, reaching the Northeastern U.S during peak diurnal destabilization. Scattered showers and storms capable of isolated heavy rain instances will occur from PA up through New England and points east, especially into Southeast New England where guidance maintains the highest precip forecast (1-2") aligning well with the recent ML output from both the Graphcast and AIFS from the ECMWF. The progressive nature of the convection will limit the threat to marginal territories, although the urbanization factors in-of the Lower Hudson up through Southern New England could produce some locally enhanced risks if a storm overperforms. The MRGL risk remains, but was extended south into northeastern PA to account for the latest trends in guidance. There are little changes with regards to the heavy rain potential in the plains as a strong upper level disturbance exits northeast out of the Four Corners with a strengthening low in the lee of the Rockies. Scattered heavy rain signatures with some isolated totals exceeding 2" are forecast currently, but would not be surprised if a small SLGT risk ended up being introduced in future runs as we introduce CAMs output and associated HREF probability fields. The MRGL will suffice for the time-being. Kleebauer ...Previous Discussion... ...South Florida... The frontal boundary will begin to sink south as a cold front through central Florida during this period. Meanwhile tropical moisture will persist across the state while now moving up the eastern Seaboard. Guidance suggests this period to be less wet than the last few days however the areal averages for southern and central Florida are less than 2 inches. Given the multi-day heavy rain event the threat for excessive rainfall and flooding concerns will remain elevated for this period, therefore a Slight Risk continues for South Florida and a Marginal Risk for central Florida. ...Rockies and Plains... Anomalous moisture over much of the Rockies and adjacent Plains will migrate eastward with time as a low pressure system exits into the High Plains. Some higher QPF is expected near the higher terrain of northeast New Mexico and eastern Colorado. Much Nebraska and the surrounding area will have be near or north of the warm front and could easily pick up 1 to 3 inches of rainfall. A vast majority of this region has 3-hr FFG near 1.5/2 inches however there are very isolated locations as low as 0.25 inch. A Marginal Risk is in effect from northeast New Mexico to Iowa/Minnesota/South Dakota. ...New England... The shortwave and associated cold front will sweep through the Great Lakes region and lift through the Northeast during this period. Showers and thunderstorms are expected to spread much of the Northeast however focusing the higher QPF values across southern New England where tropical moisture will be advecting northward into the region. Guidance does show variance on the location and the max value but consensus seems to favor southern New Hampshire/Maine where FFG is as low as 1.5/2 inches at this time. Isolated instances of flooding may arise. A Marginal Risk spans from southeast New York to Downeast Maine. Campbell 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