Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 20 2023 00:52:22 FOUS30 KWBC 200052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 851 PM EDT Fri May 19 2023 Day 1 Valid 01Z Sat May 20 2023 - 12Z Sat May 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF EASTERN OKLAHOMA, WESTERN ARKANSAS, NORTHEAST TEXAS, AND SOUTH-CENTRAL TEXAS... 0100 UTC Update -- Mainly cosmetic updates made to the Day 1 ERO, based largely on the observational trends (composite radar/satellite/mesoanalysis), CAM trends (including HRRR), and the latest (18Z) HREF exceedance probabilities. -Hurley ....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 NEW ENGLAND, THE RIO GRANDE VALLEY IN SOUTHWEST TEXAS, THE MID-SOUTH, AND THE FOUR CORNERS REGION... ....New England... Today's guidance has trended wetter across portions of Northeast, most notably areas from the NYC metro into coastal New England. The setup features a potent 500mb low over the Great Lakes Saturday morning with strong 250-500mb divergence out ahead of the 500mb low. In addition to the conveyor belt of 850mb moisture flux with origins near the Gulf Coast streaming northeast, a surface low near the VA Capes will be direct its own shield of Atlantic moisture northward. As the slug of moisture along the coast races north, the integrated vapor transport increases to >750 kg/m/s along the southern New England coast by 18Z Saturday. These values are near the 99.5 climatological percentile according to NAEFS, indicating an impressive surge of moisture over the region. The slug of moisture from the Mid-Atlantic coast was responsible for exceptionally saturated atmospheric profiles, and just about all guidance agrees that relative humidity (RH) within the low-mid levels of the column have a good chance to be >90%, as well as seeing warm cloud layers as deep as 10,000ft. The two primary reasons there was not a Slight Risk introduced with this forecast cycle was due to a combination of meager instability aloft and parched soils. Most guidance had MUCAPE values struggling to get above 250 J/kg (with the higher values confined to the southern New England coast). Antecedent soil moisture conditions are quite dry as just about the entire Marginal Risk area features 0-40cm soil moisture percentiles below the 20th percentile (as low as 2-5th climatological percentile in northern New England). With limited instability and such dry soils, it will have to take training segments of efficient convection to prompt a flash flood threat. The area does contain some large metro areas, however, and thus contains a greater concentration of hydrophobic surfaces (NYC, Hartford, Providence, Boston most notably) for efficient rainfall processes to potentially cause some cases of urbanized flooding. Street flooding cannot be ruled out in these more hydrophobic settings and poor drainage areas. ....Rio Grande Valley... Latest guidance has come in faster with the passage of the cold front, which has prompted the Marginal Risk area to shrink some compared to the previous forecast. Still, 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. Due to the faster frontal passage, MUCAPE will be around 1,000 J/kg, which is still enough to support excessive rainfall rates. Dew points will be in the low 70s and warm cloud layers as deep as 10,000 feet Saturday afternoon. 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 slow-moving storm motions could lead to localized rainfall totals surpassing 3" on Saturday, which given the saturated soils in place, would support a growing flash flood threat. Have maintained the Marginal Risk for this forecast cycle. ....Mid-South... A passing shortwave tracking across the Tennessee Valley will coincide with a strengthening 250mb jet overhead. PWs will hover around 1.75" at their peak while a 20-25 knot 850mb jet introduces added moisture flux into the region. The cold front will provide the strong source of lift at the surface to spark scattered thunderstorms while 30-35 knots of effective bulk shear helps to maintain these storms' structures. Instability will be highest in central AL where MUCAPE will be as high as 1,500 J/kg Saturday afternoon. Storms should be fairly progressive as they traverse the Mid-South, but soils remain quite saturated thanks to 200-400% of normal rainfall in northern and central AL and into central GA over the last 7 days. Made some tweaks to the Marginal Risk footprint to account for latest QPF. ....Four Corners Region... The Four Corners region will continue to contend meandering upper level disturbances and PWs that are in the 90th climatological percentile on Saturday. Around 500 J/kg of MUCAPE will be present and the latest 12Z HREF does contain some 20-40% probabilities for 1-hr QPF to exceed 1", particularly in northern AZ. Soils throughout the region also remain rather sensitive as well, thanks to 400-600% of normal rainfall amounts in some areas over the past 7-days. Showers and thunderstorms that do form will likely be slow-moving to stationary along the higher terrain. Given these factors, the Marginal Risk remains in place as flash flooding will be possible in areas with overly saturated soils, in burn scars, slot canyons, and locally urbanized towns. Mullinax Day 3 Valid 12Z Sun May 21 2023 - 12Z Mon May 22 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE SOUTH-CENTRAL ROCKIES & HIGH PLAINS, THE NORTHERN ROCKIES, & IN THE SOUTHEAST... ....20Z Update... ....Northern Rockies... An upper trough approaching the Pacific Northwest will induce large scale vertical motion as far inland as the northern Rockies. As height falls ensue during the afternoon hours, surface based heating combined with a surge in anomalous PWs (90-97.5 climatological percentile in the Marginal Risk area) will ignite scattered heavy showers and thunderstorms. Soils are sensitive in these areas with NASA SPoRT-LIS showing 80-95% soil moisture percentiles in the 0-10cm and 0-40cm soil layers. Have hoisted a Marginal Risk in the northern Rockies to account for complex terrain, burn scars, and poor drainage areas that could contend with rapid runoff. Elsewhere, the only significant tweak was to extend the Marginal Risk from the south-central High Plains into the northern NM and southern CO Rockies. This region has dealt with repeated rounds of daily showers and thunderstorms and Sunday appears to be another one of those active days. PWs remain at or above the 90th climatological percentile according to NAEFS and storm motions around 5 knots with hourly rainfall rates up to 1"/hr could prompt areas of flash flooding in complex terrain and in poor drainage areas. The previous forecast reasoning from overnight for the south-central High Plains and the Southeast remains on track. Mullinax ---Previous Discussion--- ....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!81KfTtYiu4tqX3zTQbB89n57i5LjrBOOZfodeFX-W5V6= 9fqmFEuzCDw3wTXKBGYA_uNY60jsth_yWaOCRV-X8p36nDI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!81KfTtYiu4tqX3zTQbB89n57i5LjrBOOZfodeFX-W5V6= 9fqmFEuzCDw3wTXKBGYA_uNY60jsth_yWaOCRV-XIYSeL8s$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!81KfTtYiu4tqX3zTQbB89n57i5LjrBOOZfodeFX-W5V6= 9fqmFEuzCDw3wTXKBGYA_uNY60jsth_yWaOCRV-XyBpE2mI$=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) .