Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Fri May 19 2023 15:05:00 FOUS30 KWBC 191552 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1151 AM EDT Fri May 19 2023 Day 1 Valid 16Z Fri May 19 2023 - 12Z Sat May 20 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR EASTERN OKLAHOMA INTO WESTERN ARKANSAS & SOUTH CENTRAL TX... ....Eastern Oklahoma into Western Arkansas & Northeast Texas... An elongated 850mb low tracking across northern Oklahoma continues to feed anomalous moisture north and directly into an outflow boundary over eastern OK and an approaching cold front from the north. The 12Z sounding out of Norman showed a classic skinny CAPE sounding (MUCAPE just under 1,000 J/kg) with low-mid level averaged RH values approaching 90%. Warm cloud layer depth was also roughly 10,000ft, providing a sufficient depth of the cloud to generate efficient warm rainfall processes. PWs will be highest over eastern OK and into central AR (PWs >1.75") with regenerating thunderstorms expected to form in these areas this afternoon ahead of the approaching cold front to the north. CAM guidance has also caught on more to the boundary propagating south from southeast OK into northeast TX. It is here where daytime heating and steady southwesterly 850mb moisture flux will provide the lift and moisture necessary for robust thunderstorms that could produce 2-3"/hr rainfall rates. The areas has seen below normal rainfall over the last 7 days, but there is also a greater concentration of hydrophobic surfaces closer to the Dallas/Fort Worth area and helps to cancel out recent dryness. Chose to expand the Slight Risk south to include more of northeast Texas this forecast cycle. ....South Central Texas... A surface trough over South Central Texas will be the trigger for developing thunderstorms this afternoon and evening, while the divergent left-exit region of a 250mb jet streak over northern Canada provides added vertical ascent at the top of the atmosphere overhead tonight. PWs will rise from roughly 1.5" this afternoon (1.25" farther west towards the Big Bend) to as high as 1.75" north of an including the San Antonio/Austin area overnight. NAEFS after 00Z has these PW values reaching as high as the 97.5% climatological percentile. MLCAPE will range between 2,000-3,000 J/kg and warm cloud depths as deep as 10,000ft as well. 850-300mb winds will also primarily be out of the west, which is likely to be oriented quasi-parallel to the trough. Given the ample instability, synoptically favored set up aloft, and possibility for backbuilding/training cells along the trough this evening, have upgraded portions of south central Texas to a Slight Risk with poor drainage areas and the more urbanized communities most susceptible to flash flooding. ....Coastal North Carolina... The Wilmington area woke up to a tough situation as a mesocyclone formed to the north of a stalled frontal boundary and produced hourly rainfall rates above 2"/hr. The sounding out of Morehead City, NC was remarkably saturated with 1.72" PWs and low-mid level averaged RH values of 96%. This similar profile exists near the Wilmington area, but SPC mesoanalysis shows 250-500 J/kg of "skinny" MUCAPE along the NC coast. Now that cells are moving north, they are beginning to get caught up in a more progressive 10-15 knot steering flow within the 850-300mb layer. This will help to limit residency time of these cells, but the anomalous moisture source will remain in place. As the front lifts north today and into this evening, so will the same favorable environment tracking north towards towns like Jacksonville, New Bern, and then to the Outer Banks. Given the history of this atmospheric setup producing having a history of generating 2-3"/hr rainfall rates, a Marginal Risk remains in place for much of coastal North Carolina. The Slight Risk that was issued over the Wilmington area has been dropped now that rainfall rates have diminished. While there are not as many larger cities with a greater number of hydrophobic surfaces in its path, low lying areas that drain poorly could see instances of street flooding and rapid rises in nearby creeks. ....Southern/Central Rockies into Central Arizona... Low level easterly flow upsloping into the northern High Plains and the southern Rockies is delivering an increase of 850-700mb moisture into southern CO and the lower Four Corners states. 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 90th 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 spots). Scattered convection may induce areas of flash flooding, most notably along complex terrain, burn scars, slot canyons and dry washes as hourly rates locally exceed 1 inch/hour within the more intense cells. 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 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .