Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
901 FOUS30 KWBC 221956 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 356 PM EDT Sat Jun 22 2024 Day 1 Valid 16Z Sat Jun 22 2024 - 12Z Sun Jun 23 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST/UPPER GREAT LAKES... ...Upper Midwest into the Upper Lakes... Early morning activity continues to progress eastward across Wisconsin and the Upper Great Lakes associated with a wave of low pressure along a gradually lifting warm front. Rain rates have remained tame so far this morning and generally under an inch per hour, but with increasing instability due to daytime heating a few embedded thunderstorms should contain at least 1-2" per hour rates across the northern mitten of Michigan this afternoon. PW values expected to remain much above average in the vicinity of this west to east front extending westward to northern Iowa, 2 to 3 standard deviations above the mean, supporting the likelihood of heavy totals spreading from the U.P. of Michigan/northern L.P. west southwestward into southern Wisconsin/northern Illinois and northeast Iowa. In the wake of the initial band of potentially training convection, additional convection may form along and north of the stationary front in response to additional upstream height falls forecast to push east across South Dakota, southern Minnesota and into Wisconsin. There is potential for overlap of the early day 1 convection with the late afternoon convection from from far northeast Iowa into southern Wisconsin. Across this overlap area, the Excessive Rainfall Outlook remains a Moderate Risk. This risk area was shifted to the south and west with this update to incorporate latest CAMs further south development of afternoon convection. This corresponds to where the latest HREF neighborhood probabilities are modest (40-60%) for 3"+ totals day 1, with these probabilities also extending southward into northern Illinois as convective progresses southward overnight. ...Southern New England and the Interior Northeast... Another round of organized convection possible Saturday afternoon along and to the south of the stationary frontal boundary forecast to stretch in a west northwest to east southeast direction from northern NY state into southern New England. There is potential for overlap of where heavy rains have fallen over the past 24 hours with new rains this afternoon. The Slight Risk was maintained and stretched a little westward to incorporate latest CAMs potential for numerous rounds of convection lingering into the early overnight period. PW values nearing 2" (in the 90th climatological percentile) and surface dew points in the upper 60s and low 70s when combined with SBCAPE of 1500-2000 J/kg will support the potential for maximum rainfall rates up to 3" per hour in localized spots. This matches well with where the new 12z HREF neighborhood probabilities show high chances(60-80%) for 2"+ amounts in 6-hours (ending 00z tonight). Low probabilities (10-20%) of 3"+ amounts within an hour (ending 22z) exists across northwestern CT. Scattered convection is also forecast elsewhere due to convergent flow and increasing instability south of the main frontal boundary, with thunderstorms potentially containing efficient rainfall rates from northern PA to NJ. This prompted a subtle expansion of the Marginal across parts of PA. ...Southern Georgia into the Florida Peninsula... Only changes made were to extend the Marginal Risk to incorporate more of western Florida, where 12z HREF probabilities highlight 50% chances for over 5" of total rainfall. The weak surface low along the GA coast is forecast to linger over the region with maybe some gradual northwestward motion, accompanied by 2 to 2.25"+ PW values. Slow moving cells near the center and diurnal convection southward into northern to central FL will support locally heavy totals. HREF neighborhood probabilities are high for 1", 2" and 3"+ totals in the marginal risk area, supporting isolated runoff issues. A few CAMs do highlight the potential for 5"+ totals in very localized areas just to the south of the low pressure center, but would have to occur in an urban area for any elevated impacts. ...Far South Texas... Models are consistent in showing the next surge of high PW values, to the north of low pressure moving east to west across the southwest Gulf of Mexico/Bay of Campeche, moving into South Texas day 1. Model consensus continues to be for the heaviest precip to remain across northeast Mexico, with far South Texas on the northern edge of this precip. A Marginal Risk was maintained from the previous issuance given potential for locally heavy rainfall amounts across regions that received heavy rains recently from Alberto. ...Portions of the Southwest... Latest models continue to show the upper-level trough over California into the Southern Great Basin weakening day 1 as upper ridging expands westward from the central to eastern U.S.. An area of anomalously high precipitable water values will, however, expand farther to the north and west, encompassing much more of southern and central Arizona and continuing in New Mexico, with values 2 to 4+ standard deviations above the mean. There may likely be a lot of clouds in this high PW axis, limiting instability potential. However, there will still likely be at least scattered convection beginning over higher terrain across southern to central Arizona into southwest NM. HREF neighborhood probabilities are showing scattered 40-60% chances for 1"+ totals, from the Mogollon Rim southward into southeast AZ, and lower chances extending into southeastern UT with this region depicted in the marginal risk area where isolated runoff issues are possible. Snell/Oravec Day 2 Valid 12Z Sun Jun 23 2024 - 12Z Mon Jun 24 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN NEW ENGLAND... ..Northeastern US... The latest model suite is in fairly good agreement on the eastward push of a well defined surface low across far southeast Canada into far northern NY state and northern New England. The plume of anomalous PW values ahead of this low will continue to lie across much of the Northeast day 2 with values 2+ standard deviations above the mean. Strong isentropic lift ahead of this low in the anomalous PW plume will support potential for an axis of heavy rains from far northern NY state, southern QB and across northern New England. As CAMs come into range the expected QPF amounts have come up in parts of New England where more organized convection is likely, which prompted the Slight Risk to be extended southward into the Portland metro and much of New Hampshire. 12z HREF neighborhood probabilities are also modest (40-60%) in this region for at least 2" of rain in 6-hours (ending 00z Sun night). Storm motions for the remainder of New England and the Northeast are expected to be too fast for widespread flash flooding concerns even with the potential for organized cells containing rainfall rates up to 2" per hour. The Marginal Risk was mostly maintained with this update outside of an extension southward along the I-95 corridor to far northeast MD. Convection is likely to be fairly progressive within the MRGL given the expected fast movement of the front, but with PW values above average along and ahead of the front as well as sufficient daytime heating and instability in place, locally heavy totals and isolated runoff issues possible. ...Portions of the Southwest... The axis of much above average PW values, as high 5 standard deviations above the mean across southern Arizona and southwest California (per the 12z GEFS), will persist across much of the Southwest day 2. Another round of scattered convection likely over much of Arizona into portions of New Mexico. Expanded the Marginal Risk into southern California given the amount of anomalous moisture entering the region and scattered qpf now showing up in the CAMs. There remains low confidence in where any isolated runoff issues may occur, but low 12z HREF neighborhood probabilities (10-30%) for 2" of rain have started to increase across central and southern Arizona. ...Far South Texas... PW values 2-2.5" and sustained east southeasterly low level flow is expected to persist for most of the upcoming day 2 period across northeast Mexico into South Texas. Similar to the day 1 period, model consensus is for the heaviest totals to be across northeast Mexico with far South Texas on the northern edge of this precip. Given no big changes overall to the pattern, the marginal risk was maintained over far South Texas for isolated runoff issues. Snell/Oravec Day 3 Valid 12Z Mon Jun 24 2024 - 12Z Tue Jun 25 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MISSISSIPPI VALLEY, UPPER GREAT LAKES AND SOUTHWEST... ...Upper Mississippi Valley/Upper Great Lakes... The surface frontal boundary oriented northwest to southeast from the Northern Plains into the Mid Missouri Valley at the start of day 3 will begin to push back to the northeast toward the Upper Mississippi Valley/Upper Lakes region. Strengthening south southwesterly low level flow into this front will raise PW values to 1.5-2" along and to the northeast of the front. Shortwave energy pushing eastward across the Northern Plains toward the Upper Mississippi Valley will help strengthen isentropic lift over this front and support for overrunning convection. There is not a lot of agreement on placement of any heavy precip totals, but overall some potential given the favorable MCS set up. A broad marginal risk was maintained and trimmed somewhat on the western edge to match model qpf uncertainty and 90th percentile NBM higher amounts. ...Southwest... No significant changes expected day 3 to the large scale pattern across the Southwest. PW values will remain highly anomalous from southern California into the Southwest with a continuation of weak forcing under the upper ridge axis stretching from the Southern/Central Plains into the Southwest. Models continue to show potential for widely scattered diurnal convection, with continued low confidence with locations of any isolated heavy totals and isolated runoff issues. This update expanded the Marginal Risk to match the updated day 2 outline, which encompasses the broad isolated convection possible, sensitive terrain and burn scars, as well as the reasonable worse case scenario depicted by 90th percentile NBM. Snell/Oravec 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