Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 02 2023 15:54:53 FOUS30 KWBC 021554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1119 AM EDT Sun Jul 2 2023 Day 1 Valid 12Z Sun Jul 2 2023 - 12Z Mon Jul 3 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE NORTHERN OHIO VALLEY TO NEW ENGLAND... ....Portions of the Ohio Valley Eastward to portions of the Pennsylvania/New York/New England.../ 16Z Update: The existing Slight Risk area was extended eastward to include southeastern Maine with a stronger QPF axis now in better agreement among the latest CAM guidance. There was also a westward extension to include the greater Chicago area and also including nearly all of Indiana and into portions of north-central Kentucky. This is in response to ongoing slow moving convection from northern Illinois into southern Michigan, and lower flash flood guidance over much of southern Indiana and surrounding areas with the expectation of new convection developing later today.=20 There was also a slight southward expansion of the Slight Risk to cover most of northern New Jersey where considerably lower flash flood guidance exists and an improved model signal for locally heavy rainfall there later today. 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 some instances of flash flooding. Farther to the south, the Marginal risk area was again extended a bit farther south over northern AL and northern GA to account for slow-moving activity along and ahead of an approaching cold front. ....High Plains Adjacent to the Southern Rockies... The Marginal Risk remains in place today as the latest model guidance suite develops another round of mainly scattered coverage of late day convection in proximity to a lee side trough.=20 Rainfall amounts in most cases look to be modest, but there should be enough instability and low level moisture within a region of relatively steep mid level lapse rates to produce some localized downpours. The existing Marginal Risk area was extended southeastward to include portions of the Texas Hill Country where a stronger signal has developed in the HREF exceedance probabilities for flash flooding, and the same also holds true across portions of eastern Colorado and western Kansas where the Marginal Risk area was extended northward. The areas most prone to any flooding in New Mexico would be run off from recent burn scars. ....South Florida... Sea breeze initiated convection is already developing across coastal portions of South Florida late this morning, and this is forecast to gradually increase in coverage through the afternoon hours. Cell motions are expected to be quite slow owing to weak steering flow aloft, and new cell growth is expected to propagate along sea breeze boundaries and new outflow boundaries. HREF probabilities of flash flood guidance are elevated up to 40 percent across mainly interior portions of South Florida, with the greatest prospects in the 18Z-00Z time period. Therefore, a Marginal Risk has been introduced for much of South Florida through 12Z Monday. Bann/Hamrick Day 2 Valid 12Z Mon Jul 03 2023 - 12Z Tue Jul 04 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST UNITED STATES, UPPER MIDWEST/NORTHERN PLAINS, AND MID-SOUTH... ....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 blockbuster in nature, but wet antecedent conditions could still yield areas with isolated flooding or problems in regions of poor drainage. So the Marginal risk area in this part of the country was maintained with few modifications. Farther south...storms should exhibit enough forward motion to limit any flash flooding concerns behind a cold front. ....Montana/Wyoming Eastward to Northern Minnesota... Models show another cold front will be pushed south of the international border on Monday and Monday night as surface low pressure remains north of the border. This front should provide a focus for showers and thunderstorms from eastern Montana and northeast Wyoming into the eastern Dakotas and northern Minnesota. Moisture pooling along and ahead of the front combined with divergence aloft from an upper jet may set the stage 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.=20 ....Mid-South... A lingering frontal boundary extending from the Mid-Atlantic to the Lower Tennessee Valley pooling 1.75-2.0" PWs combined with a subtle shortwave ejecting into the central/southern Plains should allow for slow-moving storms to develop between northern LA/southern Arkansas and central/northern Alabama. Minimal low-to-mid level flow are likely to limit storm motion and could allow for localized flash flooding concerns and rainfall amounts up to 2-3" within a short period of time. Snell/Bann Day 3 Valid 12Z Tue Jul 04 2023 - 12Z Wed Jul 05 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. ....New England... Still enough moisture will be lingering across the region that can be tapped by a second synoptic scale wave approaching from eastern Canada. The SREF suggested another inch or two of additional rainfall is possible where low/mid level flow will interact with the terrain...although spread in the numerical guidance is large and overall confidence in amounts is below average. Felt a Marginal was still needed given antecedent conditions. ....Southern Mississippi Valley/Tennessee Valley Eastward to Carolina Coast... Maintained the Marginal Risk area from yesterday's Day 4 outlook along/near lingering frontal boundary extending from the southern Mid-Atlantic to the Lower Tennessee Valley with 1.75-2.0" 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 instances of flooding. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5Fa8vrLbXdAVjHQQMBhOatcQvqNFLZ67a8B4LR9KEC51= 7WGsb1R0zpkzwsvyNz-jEds-IbU5kSgwEZI2InnXqwgKL_M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5Fa8vrLbXdAVjHQQMBhOatcQvqNFLZ67a8B4LR9KEC51= 7WGsb1R0zpkzwsvyNz-jEds-IbU5kSgwEZI2InnXUYzTOvI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5Fa8vrLbXdAVjHQQMBhOatcQvqNFLZ67a8B4LR9KEC51= 7WGsb1R0zpkzwsvyNz-jEds-IbU5kSgwEZI2InnX4nIip_c$=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) .