Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 17 2024 20:27:54 FOUS30 KWBC 172027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 PM EDT Sat Aug 17 2024 Day 1 Valid 16Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHWEST INCLUDING THE SLOT CANYON REGION OF UTAH AND PORTIONS OF THE MID ATLANTIC... 16Z Update... Few changes needed. Biggest potential for excessive rainfall looks to be in the Southwest US where deep moisture remained in place with shortwave energy traversing an area with 1000 to 2000 J per kg of instability expected to be in place around time of max afternoon heating. Saw little reason to modify the Slight risk area in the Mid Atlantic...especially given the antecedent conditions in Pennsylvania since the passage of Debby. The surrounding Marginal Risk as well as the Marginal risk area in the Northwest US were largely left unchanged and reflects little change in the forecast reasoning. Bann ....Southwest U.S... An active convective day is in store for portions of the Southwest U.S. as the deep monsoon moisture surges northward through the region. PWs are forecast to reach nearly +2 to +3 standard deviations above normal, with values as high as 1.25" into Idaho and locally 1.5"+ across Arizona and southern Utah. Aloft, shortwave energy will be moving through the flow, providing greater forcing for ascent. Finally, with daytime heating and favorable lapse rates, upwards of 1500-2000 J/kg of instability is expected to develop. All of this points to robust thunderstorms capable of producing intense rain rates (localized up to 1" in an hour) over the more sensitive regions of the Southwest U.S. including the Slot Canyon region of Utah. HREF neighborhood probabilities are moderate (50%) for localized 2" totals over southern Utah and even show a slight signal (15-20%) for 3" totals. No significant changes were needed to the Slight Risk area and the rest of the risk areas were adjusted minimally to the latest guidance. The most vulnerable locations will be the slot canyons, normally dry washes, and areas near recent burn scars for potentially localized significant flash flooding. ....Eastern Great Lakes, Mid-Atlantic, and into the Mid-Mississippi Valley... The slow moving upper trough will continue to make a progression to the east today, primarily affecting areas from the eastern Great Lakes through the central Appalachians and Mid Atlantic region. Fairly decent forcing for ascent provided by the approaching height falls and upper divergence, which will provide the best support for convection across the Mid Atlantic later today. Further west, convection along the Appalachians/Ohio Valley will be tied to the advancing cold front. Moisture profiles are sufficient but not overly impressive and instability is expected to be enough to produce scattered to numerous showers and thunderstorms, though storm organization is generally expected to be weak and overall progressive as well. This should limit the scope of any flash flooding concerns. With that said, there are signals for some pockets of higher rain totals, one of which is across the Mid Atlantic where some of the 00Z hi-res guidance paints some hefty QPF totals, although spotty in places. This drives the HREF probs for totals for 3" above 50 percent across northern VA through eastern/east-central PA. Confidence in this is lower with the various scenarios is conditional based on how earlier/morning activity plays out. However, given the run to run continuity of the HREF signals and potential for at least a couple rounds of heavier rains, a Slight Risk for mainly urban flash flooding was introduced from portions of MD (near Washington, DC) through eastern PA. Farther south along the stalled boundary, some training of convection is possible as the mean flow becomes more parallel to the expected storm motions, particularly across portions of the Tennessee Valley southward. Despite the decent HREF signal for localized/pockets of higher rain totals, generally high FFG and drier soils should keep the flash flood threat at the Marginal level. ....Pacific Northwest... An impressive upper level trough will amplify into a closed low off the northern CA coast, dropping 500mb heights to less than -3 sigma this afternoon. Downstream of this low, pinched flow will transition into broad divergence over the Pacific and Interior Northwest, surging moisture and ascent into the region. PWs are progged to reach above 1 inch, and possibly above 1.25" in parts of the Cascades, reaching nearly +3 sigma from climatology. As the upper low pivots slowly northeast, this moisture fetch will be persistent into the region, and IVT values peak around 350 kg/m/s, reflective of a rare but weak August AR into WA/OR/northern CA. During this time, ascent increases within the RRQ of a strengthening upper jet streak, and mesoscale upslope ascent occurs into the Cascades. This will result in widespread showers and even scattered thunderstorms in response to MUCAPE reaching 500-1000 J/kg. The HREF neighborhood probabilities for 1"/hr reach as high as 30-40%, so despite generally rapid storm motions, brief torrential rainfall is likely, with total rainfall being enhanced by repeating rounds of cells. August ARs usually do not result in anything more than isolated flood/flash flood instances, but the MRGL risk was extended along the Cascades into northern CA, and rapid runoff/instances of flash flooding are possible, with impactful flash flooding, including debris flows, possible across recent burn scars. ....South Florida... Another day of enhanced coverage of convection is expected across South Florida and the Keys in the vicinity of the lingering surface trough draped across the area. This feature combined with fairly anomalous moisture should lead to another day of scattered to numerous showers and thunderstorms, some of which will be capable of producing intense rain rates (2-3" hourly totals) and potentially produce a quick 2-4"+ over some areas. The latest guidance points to the greatest focus area to be south of the most urbanized corridor but nonetheless, some isolated/localized flooding concerns will be possible and the Marginal Risk remains in place. Taylor Day 2 Valid 12Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID ATLANTIC... 2030Z Excessive Rainfall Discussion... Made a couple of adjustments to the western/northern periphery of=20 the Marginal risk area in the West and to the Slight risk area=20=20 based on latest model guidance with respect to the location of=20 deepest moisture but the changes were minor.=20 Previous Discussion... ....Mid Atlantic and Northeast... A cold front will advance eastward into the region as the main upper trough slowly advances through the Great Lakes. This will bring another day of scattered to numerous showers and thunderstorms to much of the region. In the low level southwesterly flow ahead of the front, with enhanced training likely on S-N oriented mean flow parallel to the from from near Washington, D.C. through Upstate NY. Here, PWs will be nearly +2 sigma according to NAEFS, which will be acted upon by the robust synoptic lift to deepen convection. Rainfall rates of 1-2"/hr are likely, which through training could produce locally more than 3" of rainfall based on the latest ensemble probabilities. Where this training occurs across urban areas or sensitive soils, scattered instances of flash flooding are possible. The recent CSU FG fields depict an enhanced risk for flash flooding along I-95 and into the Tri-State area, leading to higher confidence in the Slight Risk. Further south, the Marginal Risk extends into the Piedmont and coastal Carolinas where the combination of heavy rainfall over more sensitive soils due to recent heavy rains may result in isolated instances of flash flooding. ....Southwest and Intermountain West... Impressive and deep monsoonal moisture will continue to stream around the western periphery of the monsoon ridge and combine with daytime instability to produce scattered to widespread showers and thunderstorms from AZ as far north as WY and ID. Convection will be capable of producing hourly totals up to 0.75" at times, particularly across portions of Utah where the better instability and highest PW anomalies are forecast. It's possible a Slight Risk may be needed in future updates across portions of northern AZ into southern UT but there is some uncertainty on cloud cover potentially limiting the available instability and thus the rain rates may end up lower. ....South Florida... Another day of scattered, slow-moving thunderstorms are expected across the region where rain rates may approach 2-3"/hr at times. This is due to the lingering surface trough in the vicinity focusing the higher PWs. Given the potential for this to occur over the more urban areas, the inherited Marginal Risk was minimally changed to highlight the continued isolated flash flood risk. Taylor Day 3 Valid 12Z Mon Aug 19 2024 - 12Z Tue Aug 20 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHEAST U.S. AND SOUTHWEST INTO THE INTERMOUNTAIN WEST... 2030Z Excessive Rainfall Discussion... Main focus for isolated flash flooding/run off problems will be=20 over the Southwest again as moisture wraps around the edge of an upper level ridge...with a second focus over the Northeast US=20 where additional showers and thunderstorms may produce more rain as the synoptic scale trough over the eastern US continues to=20 amplify. Bann ....Previous Discussion... ....Northeast... The upper trough will make a final pass across the Mid Atlantic to Northeast on Monday with the cold front expected to move across the coastal areas during the late morning into the evening hours. With peak heating and sufficient destabilization, vigorous convective elements are expected to develop again, mainly from the coastal Mid Atlantic through much of eastern New York and portions of New England. Storm motions should be fairly progressive, but a few storms could repeat/train due to merging/colliding cells and storm totals could approach 1-2" in spots leading to flash flooding. A possible upgrade to a Slight Risk might be needed across portions of the area if trends for higher QPF and rain rates materializes, particularly over the more sensitive areas of western New England and eastern New York. ....Southwest/Intermountain West... By Monday, the bulk of the anomalous moisture is expected to have lifted northward through the Four Corners region and into the Intermountain West/Rockies. With another shortwave trough embedded in the flow moving through, this should be sufficient to produce another day of isolated to scattered thunderstorms, most likely over portions of northern Utah, Colorado, and into western WY, far eastern Idaho. Flash flooding will be possible, particularly for the most sensitive locations like areas near recent burn scars. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5VW3qvXe8a0a6ZKGkGcPRpa3s9QKqSjvecTXobVa7Kh5= 1gzEQfFhZlmGut1_eyVhn2rUS9r9VIZgeoxP1JxTem6jSHU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5VW3qvXe8a0a6ZKGkGcPRpa3s9QKqSjvecTXobVa7Kh5= 1gzEQfFhZlmGut1_eyVhn2rUS9r9VIZgeoxP1JxTPksGNRA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5VW3qvXe8a0a6ZKGkGcPRpa3s9QKqSjvecTXobVa7Kh5= 1gzEQfFhZlmGut1_eyVhn2rUS9r9VIZgeoxP1JxTDyqIXrs$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .