Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jul 01 2023 19:21:04 FOUS30 KWBC 011920 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 320 PM EDT Sat Jul 01 2023 Day 1 Valid 16Z Sat Jul 01 2023 - 12Z Sun Jul 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID MISSISSIPPI INTO THE LOWER OHIO VALLEY AND UPSTATE NEW YORK... ....Mid-Mississippi Valley to Ohio Valley... Overall, minor changes were made to the Day 1 ERO update with the 16z issuance. The Slight Risk was expanded into northeast OH and western PA to account for a combination of convection along a lake breeze fighting southwesterly surface flow, as well as additional activity overnight along a lifting warm front. The Marginal Risk was also expanded farther west into Nebraska to account for slow-moving storms underneath an upper low. Elsewhere, convection capable of producing heavy to potentially excessive rainfall will be forced by broad-scale ascent ahead of a trough approaching the Mid-Mississippi Valley later today with additional convection forming along and ahead of a cold front approaching the area. An upper ridge over the central Gulf Coast will allow for storm motions to round the ridge, remaining mostly east-west oriented before incorporation more southerly forward motion near the Appalachians and Southeast. With forecast soundings showing favorable thermal and moisture profiles, as well as precipitable water values in excess of 1.75 inches across the area, convection should be fairly efficient rainfall producers that could result in instances of flash flooding due to rainfall rates between up to 1-2"/hr under the most active storm. A limiting factor for flash flooding today will be storm motions as 850-300 mb mean winds remain between 30-40 kts and the widely scattered nature of developing thunderstorms, however activity could train as the flow aloft runs parallel to a well-defined instability gradient. Scattered flash flooding is possible within the broad Slight Risk and more details about specific location/impacts may not be determined until after convective initiation. The greatest concern is over parts of IL, southern IN, and northern KY, which has lower flash flood guidance due to multiple rounds of showers and thunderstorms during the past couple of days and this morning. This region also is expected to have over 2000 J/kg of MUCAPE and along the instability gradient where training storms are most likely. 12z HREF neighborhood probabilities highlight these parts of IL and IN as having a 50-60% chances for greater than 2" in 3 hours ending 03z tonight. Between eastern NE and southern MI, storms will be slower to move and nearly stationary along a frontal boundary. However, limited instability may contain the flash flood risk somewhat and keep the impacts more localized, but still spread out across a broad region. ....Upstate New York... Maintained the Slight risk area over northern New York with only minor adjustments given the 1 to 2 inch QPF expected and the recent wet spell. This region is also within an environment with unidirectional shear and PWs over 1.5". This creates the threat for training storms and scattered flash flooding around the Adirondacks and far northern Champlain Valley. 12 HREF neighborhood probabilities for at least 2" within 6 hours is as high as 70% by 03z across far northeast NY. ....Texas Big Bend to the Rolling Plains and Cap Rock... The models continue to advertise that a mid-level disturbance and associated moisture will be moving out of northern Mexico and enter the U.S. later today/tonight. Precipitable water values between 1.3 and 1.7 inches, which is nearly 2 standard deviations above climatology for this time of year, look to be in place by the time the energy ejects out over the high plains adjacent to the southern Rockies. This environment could result in localized downpours and associated isolated flash flooding. The inherited Marginal Risk was expanded a tad east and north to account for latest hires guidance and radar trends. Snell/Bann Day 2 Valid 12Z Sun Jul 02 2023 - 12Z Mon Jul 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEASTERN UNITED STATES... ....20z Update... Subtle changes were made to the ERO areas extending from the Ohio Valley and Appalachians to the Northeast. The Marginal Risk was expanded to the west into Illinois and south towards northern Alabama. An upper low over the Midwest to continue to allow for slow-moving storms early in the forecast period and is supported by 40-60% 12z HREF probabilities for greater than 2" in 6-hours. The Marginal Risk was also extended further southwest over northern AL to account for slow-moving activity along and ahead of an approaching cold front. Additionally, the Slight Risk area was maintained and reoriented on the western edge as storms should exhibit 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. Snell ....Previous Discussion... Even though the deterministic QPF amounts from WPC and from at least some of the global models were pretty modest...both the SREF and GEFS ensembles suggest that heavier rainfall amounts are possible which could result excessive rainfall from parts of the eastern Great Lakes/eastern Ohio Valley northeastward into parts of New England. Model spread is pretty large...but the combination of deep moisture (as shown by precipitable water values of approaching 1.75 inches/approaching 2.5 standard anomalies greater than climatology) and a developing area of low pressure and increasing isentropic have the potential to focus and enhance rainfall rates across the region. The GEFS solution favored a corridor of higher amounts across upstate New York into northern New England which has had some locally heavy rainfall in the past week to ten days...and a second area of 1 to 3 inch amounts farther south and east where the synoptic/mesoscale forcing is not as strong but the instability is greater. Was swayed to introduce the Slight risk area by the combination of synoptic and mesoscale forcing overlapping the footprint of deeper moisture...even though the SREF was not nearly as focused with its higher-end amounts. A Marginal risk area was extended as far south and west as the Tennessee Valley where the dynamics may not be as strong as they will be in New England but the instability could still support localized downpours. Bann Day 3 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... Maintained a Marginal risk area with only minor adjustments as a system pulls away from the U.S. and heads out to sea. Models still linger sufficient moisture to keep modesy precipitation across the region. 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. Storms occuring to the south into the Mid-Atlantic 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 have enough support from the upper level winds to provide a focus for showers and thunderstorms from eastern Montana and northeast Wyoming on Monday which extends eastward as far as northern Minnesota by late Monday night. 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. One limiting factor may be that precipitable water values should only be in the 1.25 to 1.5 inch range and the storms may be progressive enough to prevent more than isolated problems with run-off from places with poor drainage. ....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 AR and central/northern AL. 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 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7h1XLioVzQkpPo7r4hautZFMz91r8rTpaVbzxZmezhil= iOCQ-fh_7lTT064J9omBivgyCkQ4BOa0Dh4vfVK4VZWypK4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7h1XLioVzQkpPo7r4hautZFMz91r8rTpaVbzxZmezhil= iOCQ-fh_7lTT064J9omBivgyCkQ4BOa0Dh4vfVK4St40mH8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7h1XLioVzQkpPo7r4hautZFMz91r8rTpaVbzxZmezhil= iOCQ-fh_7lTT064J9omBivgyCkQ4BOa0Dh4vfVK4unZ2E2M$=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) .