Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jun 30 2023 20:35:10 FOUS30 KWBC 302035 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 433 PM EDT Fri Jun 30 2023 Day 1 Valid 16Z Fri Jun 30 2023 - 12Z Sat Jul 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS... ....16Z Update... Main adjustment for this update was to extend the current SLGT Risk over the Central Plains northwestward to the Front Range/Black Hills over northeastern Colorado, southeastern Wyoming, southwestern South Dakota, and western Nebraska. Although storms that are expected to develop off the higher terrain look to progress eastward at a moderate pace, and atmospheric conditions are not quite as robust compared to yesterday with lower CAPE, values at or a bit above 1000 J/KG along with post-frontal surface dewpoints into the mid-50s will remain sufficient for heavy rain. Ensemble probabilities for rainfall totals of 1-2", locally higher, remain notable, and likely occurring in a short time frame with rain rates of 1-2" per hour possible. Local conditions also remain more sensitive relative to other areas of the Plains. A few additional areal adjustments were made to the current MRGL risk areas based on the updated 12Z hires guidance. The MRGL over portions of the northern Appalachians/Mid-Atlantic was adjusted along a north/south axis based on expected afternoon storm development along/east of the Appalachians and surface trough which will progress eastward through the evening. Seasonable moisture and forecast CAPE of 1000-1500 J/KG will lead to a few locally heavy downpours with potential rain rates upwards of 1"+ per hour. Longevity/duration of storms will likely limit any isolated flash flood threat to more urban areas or where storm motions are more transient directed by outflows. In addition, the MRGL over Texas was extended further southwest towards the Big Bend region given model guidance indicating the potential for some slow-moving afternoon thunderstorms that could produce a quick 1-2" of rain. Otherwise, some locally higher risks may exist within the current MRGL risk area (specifically portions of the Texas Panhandle and the Middle Mississippi Valley) where expected more organized convection could result in wider areal coverage of heavy rainfall totals. However, given predictability of location/coverage of convection as well as less sensitive antecedent conditions compared to areas within the SLGT risk, a MRGL is sufficient for now. Putnam ....Previous Discussion... ....Central High Plains to the Appalachians/Northeast... Introduced a Slight Risk of excessive rainfall over portions of the Central Plains within a broader Marginal Risk area that spans from the Rockies Front Range and adjacent High Plains eastward into the Tennessee Valley. An upper trough moving out of the West is expected to trigger additional storms west from near the Rockies Front Range across the Plains. The thermal and moisture profiles suggest that convection should be efficient rainfall producers and fairly modest forward speed suggest that flooding may be a concern given some of the rainfall observed on Thursday.=20 Farther east...mid-level energy will continue to move across the top of the ridge centered over the lower Mississippi Valley and the Southeast, likely to help initiate additional rounds of showers and storms from the Mid-Mississippi Valley into portions of the Ohio Valley as embedded shortwave troughs interact with an axis of deeper moisture. A Marginal Risk was maintained to highlight an increasing threat for heavy rains to spread back into portions of the Northeast that were recently impacted by heavy amounts. Mid-level energy and deeper moisture will be directed northeast into the region ahead of a low centered north of the Great Lakes and usher in renewed PWs near 1.75 inches. Given limited confidence...tried to be more targeted in the areal coverage of the Marginal but still covers at least some potential for moderate to heavy amounts to begin to develop across some parts of Pennsylvania and New York that were recently impacted with heavy amounts and where FFGs remain relatively low. Introduced a Marginal Risk area over portions of South Florida given precipitable water values approaching 2 inches...deep layer easterly flow and the presence of a weakening boundary to the north. The HREF focused fairly high probabilities of 3-hour QPF exceeding Flash Flood Guidance. Bann Day 2 Valid 12Z 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... ....2030Z Update... A SLGT risk was introduced for a portion of Upstate New York/far northwest Vermont where incoming 12Z hi-res guidance has a strong signal for 1-3" rain totals, locally higher, with matching high ensemble probabilities. A warm front is forecast to lift northward through Upstate New York and into Canada ushering in a moist airmass with precipitable water values > 1.25 inches, or 1.5 standard deviations above the mean. A shortwave aloft ahead of a deeper trough to the west, positioned under the left exit region of a 250 mb jet, will help to initiate convection along and to the south of the Canadian border. Anticipated slow motions and repeated development will lead to the risk for heavy rainfall. Ground conditions are also more susceptible due to recent rains in the region. Additional convection is likely further southwest but may be of a more scattered nature, limiting the areal coverage of heavy rainfall, so a MRGL risk remains sufficient for now. Otherwise, additional areal adjustments were made to the SLGT risk area covering portions of the Ohio and Tennessee Valleys. While uncertainty remains, the current expectation is for two areas of organized convection to be favored. One ahead of an approaching trough to the west in a region of large-scale ascent over the Middle Mississippi Valley, forecast to track to the east-northeast. Another area of convection is expected to develop to the southwest of this system and track southeast into the Tennessee Valley, potential aided/enhanced by convective outflows from the northern system and ahead of an approaching cold front. Previously noted ample moisture will lead to heavy rainfall rates of 1-2" per hour, with the potential for some storm motions relatively parallel to the mean flow leading to convective training, both contributing to the risk for flash flooding. Putnam ....Previous Discussion... ....Central Plains to the Northeast... Models continue to present a good signal for widespread moderate to heavy rainfall developing ahead of a well-defined trough moving from the central Plains into the mid Mississippi Valley. Deep southwesterly flow ahead of the trough is expected to draw ample moisture (precipitable water values of 1.5-2 inches) into a region of large-scale ascent. The models suggest an environment favorable for redeveloping storms and west-east training, increasing the threat for heavy accumulations. The main change to yesterdays Day 3 outlook was to expand the Slight Risk northward where a few of the operational models got some support from the 2+ inch amounts from the SREF and GEFS. This included some areas that may be impacted by heavy amounts in Days 1 and 2.=20 Meanwhile, downstream energy is likely to contribute to at least locally heavy amounts and isolated flash flooding concerns across the Ohio Valley into the central to southern Appalachians.=20 Farther to the north, southwesterly flow ahead of a slow-moving trough over the Great Lakes will continue to channel deeper moisture into the Northeast, fueling the potential for the recurrence of heavy rains and flooding concerns for portions of the area. Bann Day 3 Valid 12Z Sun Jul 02 2023 - 12Z Mon Jul 03 2023 ....There is a Marginal Risk of Excessive Rainfall Over Portions of the Eastern United States... ....2030Z Update... Only some minor areal adjustments were made to the inherited MRGL risk for this update based on the new 12Z model guidance. An additional surge of moisture northeastward ahead of the approaching upper-level trough over the Northeast will usher in even higher precipitable water values than Saturday, with forecast values as much as 2 standard deviations above normal. As previously noted, broad large-scale forcing will likely lead to widespread areas of scattered convection, including some potentially organized, but with disagreement in the guidance on where this will occur. This is confirmed by generally low ensemble probabilities for any amounts over an 1". However, it seems likely given the coverage and environment that at least one area will eventually be pinpointed for a locally increased risk (Slight Risk) of heavy rainfall and the potential for flash flooding. Putnam ....Previous Discussion... A trough making its way eastward from the Tennessee Valley eastward will tap deeper moisture over the eastern United States to produce showers and thunderstorms on Sunday and Sunday night.=20 There was a large spread of model solutions as to where the heavier rainfall amounts will occur within a broader shield of rainfall. Upper level difluence/divergence ahead of an upper level trough and some frontogenetical forcing in an atmosphere with precipitable water values of 1.75 to 2+ inch will support local 2+ rainfall amounts over the Northeast U.S...where flash flood guidance has been suppressed by locally heavy rainfall during the preceding week. Farther south...the synoptic scale forcing looks to be weaker but the instability could also produce flooding as a result of some intense downpours. The model spread really limits confidence and precluded the issuance of a Slight risk area at this point. 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