Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 02 2023 20:29:25 FOUS30 KWBC 022029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Sun Jul 2 2023 Day 1 Valid 16Z Sun Jul 2 2023 - 12Z Mon Jul 3 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEASTERN UNITED STATES... ....Portions of the Ohio Valley Eastward to portions of the Pennsylvania/New York/New England.../ Was able to maintain decent amount of continuity for the period...especially from the Ohio Valley to the Appalachians and the Northeast U.S.. An upper low initially over the Mid-Mississippi Valley will continue to move eastward over the Midwest and allow for slow-moving storms early. The Slight Risk area was maintained without too many changes given the support from the SREF/GEFS and the HREF neighborhood probabilities. The one change in pulling the western boundary of the Slight westward was largely done in response to late night/early morning radar imagery suggesting somewhat more training potential over northern Ohio as opposed to areas just to the south. PWs in the Northeast are forecast to be as high as +2 to +3 standard deviations above the climatological average, when combined with forcing along the frontal boundary will likely lead to scattered areas of flash flooding. Farther to the south...the Marginal risk area was again extended a bit farther south over northern AL to account for slow-moving activity along and ahead of an approaching cold front. ....High Plains Adjacent to the Southern Rockies... Introduced a Marginal Risk again today as the numerical guidance develops another round of widely scattered to perhaps scattered coverage of late day convection in proximity to a lee side trough. Rainfall amounts look to be modest...but enough instability could form die to upwards of an inch precipitable water within a region of relatively steep mid level lapse rates to produce brief downpours. The areas most prone to any flooding would be run off from recent burn scars. Bann Day 2 Valid 12Z Mon Jul 3 2023 - 12Z Tue Jul 4 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST UNITED STATES, UPPER MIDWEST/NORTHERN PLAINS, DEEP SOUTH, AND SOUTH FLORIDA... ....Pennsylvania/New York Into New England... Models still linger sufficient moisture to keep modest precipitation amounts across the region as a system pulls away from the U.S. and heads out to sea. Additional rainfall amounts after the start of the outlook period at 12Z Monday are not expected to be overly impressive, but wet antecedent conditions leading up to this time period could still yield areas with isolated flooding or problems in regions of poor drainage.=20 Therefore, the Marginal risk area in this part of the country was maintained with an extension southward to include the Chesapeake Bay Region where the 12Z model guidance has trended upward some. ....Montana/Wyoming Eastward to Northern Minnesota... Models indicate a cold front will drop south across the Northern Plains on Monday and Monday night as the parent surface low pressure remains north of the border. This front should provide a focus for showers and thunderstorms from south-central Montana and northern Wyoming eastward across South Dakota/Nebraska and into the Upper Midwest. Moisture pooling along and ahead of the front combined with divergence aloft from an upper level jet will create an environment favorable for storms to produce locally heavy rainfall, although highest precipitable water values only look to be modestly above 1.25 inches which may limit rates and any associated excessive rainfall potential. No major changes were needed with this forecast update. ....Mid-South... A lingering frontal boundary extending from the Mid-Atlantic to the Lower Tennessee Valley, pooling 1.75-2.00 inch PWs combined with a subtle shortwave ejecting across the central/southern Plains should allow for slow-moving storms to develop across the ArkLaMiss and extending into central/northern Alabama. Minimal low-to-mid level flow is likely to limit storm motion and could result in localized flash flooding concerns and rainfall amounts up to 2-3" within a relatively short period of time. ....South Florida... Similar to Day 1, a Marginal Risk area was added to portions of South Florida to account for slow moving cells that develop in response to sea breeze convergence, with the potential for 1-2 inches of rainfall to fall within a 30-minute time period for affected areas. The best coverage for convection is likely to be within the 18Z-00Z time period. Bann/Hamrick Day 3 Valid 12Z Tue Jul 4 2023 - 12Z Wed Jul 5 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PLAINS TUESDAY... ....Upper Midwest Across the Plains into Portions of Wyoming and Montana... The front which helps focus showers and thunderstorms on Monday will continue to be the focus for additional showers and thunderstorms on Tuesday. Models disagreed on placement of where some heavier rainfall amounts may occur but there was a definite move to show some heavy to excessive rainfall amounts somewhere on Tuesday and Tuesday night. The GFS ramps up precipitable water values to nearly 1.75 inches from eastern Nebraska into southwest Minnesota by 05/00Z immediately ahead of a maximum in moisture flux convergence values along/ahead of the front. The placement of the QPF max by the GFS is not wildly different than the 12km NAM...although the NAM was far more aggressive in forming a 4 inch bulls-eye farther south and west in Nebraska...while the ECMWF tended to be slower than either the NAM or GFS. Given the presence of the front and increasingly difluent flow/divergence aloft due to the approach of a mid/upper level wave...placed a Slight Risk area over eastern Nebraska into western Iowa. The 01/12Z and 02/00Z GEFS spaghetti plots were consistent in showing 2 and 3 inch rainfall contours here in the 24-hours ending at 05/12Z while SREF guidance was just a bit slower. Additional shifts in placement and coverage are expected in later outlooks. 20Z Update: The greatest prospects for flash flooding potential are expected to be across portions of eastern Nebraska and into western Iowa and southern Minnesota, where the strongest 12Z model signals for QPF maxima are expected to reside, so a Slight Risk remains in effect for these areas. The existing Marginal Risk was trimmed back across portions of central South Dakota and north-central Nebraska, but was extended farther south over eastern Colorado and northeast New Mexico where scattered storms with high rainfall rates over areas with heavy rainfall over the past week may result in some flooding issues.=20 ....New England... There should still be enough moisture lingering across the region that can be tapped by a second synoptic scale disturbance approaching from eastern Canada. The 12Z guidance suggested another 0.5 to 1.5 inches of additional rainfall is possible where low/mid level flow will interact with the terrain, and the existing Marginal Risk area has been maintained. ....Southern Mississippi Valley/Tennessee Valley Eastward to Carolina Coast... The Marginal Risk area has been maintained along/near a lingering frontal boundary extending from the southern Mid-Atlantic to the Lower Tennessee Valley with 1.75-2.00 inch PWs and subtle shortwave energy ejecting into the central/southern Plains to allow for another round of slow-moving storms to develop, which could result in isolated instances of flooding. However, the northern portion of the existing Marginal has been trimmed back to account for lower QPF trends in the 12Z guidance, and it is possible that further reductions in the overall extent of the Marginal Risk could be made with future forecasts. Bann/Hamrick Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9bQ0LYP1AFY00hZsy2U2J0yfTsApxzrMX5Q7d1FI478p= hUvPvFyoXowed8DGXLvxkqgkUnmAD-iOEOG_tZQFgCgFUtA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9bQ0LYP1AFY00hZsy2U2J0yfTsApxzrMX5Q7d1FI478p= hUvPvFyoXowed8DGXLvxkqgkUnmAD-iOEOG_tZQFmji8Cqc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9bQ0LYP1AFY00hZsy2U2J0yfTsApxzrMX5Q7d1FI478p= hUvPvFyoXowed8DGXLvxkqgkUnmAD-iOEOG_tZQFgmfB6i0$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .