Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 19 2023 08:31:34 FOUS30 KWBC 190831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Fri May 19 2023 Day 1 Valid 12Z Fri May 19 2023 - 12Z Sat May 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN OKLAHOMA INTO WESTERN ARKANSAS... ....Eastern Oklahoma into Western Arkansas... The previous Marginal Risk area has been upgraded to a Slight Risk with this morning's update. A southwesterly low level jet (LLJ) will develop ahead of a southward moving cold front moving down the Plains. This LLJ will increase the available atmospheric moisture with PWATs approaching 1.75 inches, which in addition to MUCAPE values exceeding 2,500 J/kg will provide more than ample moisture and instability for multiple rounds of storms to develop this afternoon through this evening across the Slight Risk area. Corfidi Vectors are very weak, generally 5 kt out of the northwest. This will support training storms as several rounds of storms develop along the front, then push southeast as new storms form behind it. This area has average to above average soil moisture for this time of year. Finally, the Ozarks in the region will further increase flash flooding potential as the rainfall drains into the locally narrow river valleys that criss-cross the region. ....Southern/Central Rockies into Central Arizona... No significant changes noted with the Marginal Risk area here. As with previous days, anomalously high atmospheric moisture in this region, plenty of instability, and a weak upslope flow will all support scattered convection that could locally cause flash flooding, especially along complex terrain, burn scars, slot canyons and dry washes as hourly rates locally exceed 1 inch/hour. Low level easterly flow upsloping into the northern High Plains and the southern Rockies is set to deliver an increase of 850-700mb moisture into southern CO and northern NM. Meanwhile, the upper low over northern Baja will provide sufficient vertical ascent for developing showers and thunderstorms along the mountain ranges. PWs will reach as high as 0.75" and according to NAEFS, PWS throughout the affected regions will easily top the 80th climatological percentile. Storm motions will also be rather slow as 850-300mb wind speeds generally hover around 5 knots or less. Soils throughout the at-risk region are still quite sensitive (0-40cm soil moisture percentiles above the 90th percentile in some cases). ....Coastal North Carolina... Some of the North Carolina beaches; from Cape Fear to the Outer Banks, could see rainfall totals of 4-5" in some spots on Friday as low pressure along the front just off the Southeast coast lifts north. However, storm motions are expected to be more progressive and the beaches there will likely be able to handle the rainfall amounts in the forecasts. No Marginal Risk in place, although some localized street ponding and nuisance flooding cannot be fully ruled out. The primary threat for heavy rains will be over the Outer Banks and the swamps of far eastern NC, where flash flooding risk is negligible. Wegman/Mullinax Day 2 Valid 12Z Sat May 20 2023 - 12Z Sun May 21 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE RIO GRANDE VALLEY IN SOUTHWEST TEXAS, IN CENTRAL ALABAMA, AND THE FOUR CORNERS REGION... ....Rio Grande Valley... The trailing end of the cold front will slow down substantially as it tracks through Rio Grande Valley. 1000-500mb mean relative humidity values look to range between 80-90% while PWs could top 1.75" from Eagle Pass on south to Laredo ahead of the front. In terms of instability, MLCAPE could range between 1,500-2,000 J/kg and warm cloud layer depths hover around 9,000ft. Soils also remain quite sensitive after much of the at-risk region has seen anywhere from 400-600% of normal rainfall over the last week. The Rio Grande Valley will be closely monitored as slowing storm motions could lead to localized rainfall totals topping 3" on Saturday, which given the saturated soils in place, would support a growing flash flood threat. That said, there is not a clear enough signal in model guidance to hoist a Slight Risk at this time. Have maintained the Marginal Risk for this forecast cycle. While most of the guidance continues to highlight this region for heavy rain at times, especially as compared with the surrounding region, rainfall amounts areally have not changed overly much, so the previous thinking that a Marginal Risk sufficiently covers the rainfall threat in this region remains valid. ....Central Alabama... A trough tracking eastward across the Great Lakes will spread upper level shortwaves across the Tennessee Valley through the day Saturday. Ample atmospheric moisture is expected as PWATs approach 1.75 inches, as a westerly LLJ to 20 kt advects the Gulf moisture along and ahead of a rather strong cold front. The front will provide plenty of forcing. Expect a round of storms to move over northwestern AL in the morning, which will maintain themselves as they move to the southeastern part of the state. Behind this first round, a second round of storms will develop with the actual front itself. Both rounds have the potential to produce 2+ inch/hour rainfall rates, though they're more likely with the second round. The expectation of 2 separate rounds of storms is why this portion of Alabama was upgraded to a Marginal with this update. ....Four Corners Region... Highly anomalous levels of atmospheric moisture will remain in place across the Four Corners Region. Various small upper level disturbances will be traversing the region on Saturday, which when adding around 500 J/kg of MUCAPE, resulted in several pieces of guidance showing maximum rainfall values in the mountains exceeding 0.5 inches, with 10 year ARI exceedance probabilities in the 50-60% range. Expect scattered showers and thunderstorms to develop from Arizona through Colorado, which each will be slow-moving or stationary if tied to the nearby mountains. This will support localized flash flooding, especially where those storms move over burn scars, slot canyons, and locally urbanized towns. ....Coastal New England... A widespread 1-2 inches of rain, with locally higher amounts are expected across coastal New England on Saturday to the west of a strong surface low moving into the Gulf of Maine. Despite the urbanized areas that are likely to be impacted, including Boston, Providence, and Hartford, the lack of any instability expected in this region should mean that all of the rain falls as stratiform rain, which will minimize the heaviest rainfall rates. Further, this area has been quite dry lately with below average soil moisture, so most of this rain is likely to be beneficial to the region. Of course, any local poor drainage spots could result in ponding water. No risk upgrades were introduced with this update but this region will continue to be monitored. Wegman/Mullinax Day 3 Valid 12Z Sun May 21 2023 - 12Z Mon May 22 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL for the OKLAHOMA AND TEXAS PANHANDLES INTO SOUTHWEST KANSAS AND FOR SOUTHEAST GEORGIA AND COASTAL SOUTH CAROLINA... ....Oklahoma and Texas Panhandles... A series of shortwaves will move across the Panhandles on Sunday. Some weak moisture advection will develop with 15-20 kt 850 mb winds helping advect a bit of Gulf moisture across the Panhandles. MUCAPE values will be between 1,000 and 1,500 J/kg, which in addition to weak Corfidi Vectors at 5-10 kt out of the northwest, should support slow moving thunderstorms moving across the Panhandles. Given the more widespread flash flooding that occurred just a few hours ago earlier tonight in the northern TX Panhandle, the potential for additional slow moving storms raises the threat in this area to a Marginal Risk. ....Coastal Southeast... A stalled out front will be parked along the Gulf Coast Sunday, with the front providing the forcing, and plenty of instability and atmospheric moisture along and ahead of the front. Weak and highly variable steering flow will favor slow-moving and training thunderstorms, which is being pointed out in much of the guidance by a local maximum of rainfall in the Marginal Risk area. This area was particularly hard hit by recent rains last evening, and so soils in this area are quite saturated and will be unlikely to be able to handle heavy rain from the scattered storms, even if the level of organization of those storms is uncertain. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Nt-6H32lp9tN7SgwhhlPQn3CH7kypGgl9Yg_6B3nCLu= MSVBOXNQbKw8K3tkqJ00_No24cXqxThZJd8pwuL1XkdFeG0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Nt-6H32lp9tN7SgwhhlPQn3CH7kypGgl9Yg_6B3nCLu= MSVBOXNQbKw8K3tkqJ00_No24cXqxThZJd8pwuL1VhnnvWU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Nt-6H32lp9tN7SgwhhlPQn3CH7kypGgl9Yg_6B3nCLu= MSVBOXNQbKw8K3tkqJ00_No24cXqxThZJd8pwuL1dTPcmSo$=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) .