Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 31 2023 16:16:05 FOUS30 KWBC 311615 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1215 PM EDT Wed May 31 2023 Day 1 Valid 16Z Wed May 31 2023 - 12Z Thu Jun 01 2023 ....THERE IS A MDT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN HIGH PLAINS... ....Central and Southern High Plains... 16Z Update... A Moderate Risk area was added for a small portion of the southern High Plains centered over the Texas Panhandle, where the consensus of the 12Z hi-res guidance shows the threat for multiple rounds of storms moving across the region, resulting in additional heavy amounts across an area where soils are already moist and FFGs are low. The added Moderate Risk area highlights an area where the 12Z HREF indicates that localized amounts of 3+ inches are likely. Previous Discussion... The notable change from yesterday's Day 2 ERO was the considerable expansion of the Slight Risk area east-northeast across southeast CO, southern to eastern KS, and parts of western MO. This is due in part to the recent heavy rains (including Tuesday night), but also given the next batch of more widespread convection later Wed afternoon and overnight. Compact mid/upper low migrating across AZ-southern UT will lead to a fairly substantial increase in upper level difluence and divergence over the southern High Plains by afternoon. Convection is expected to fire along/ahead of the dry line in eastern NM this afternoon, then become better organized/grow upscale this evening and overnight upon lifting east-northeast. This as the mid/upper low continue to track eastward, which will shift the most favorable region of difluence aloft (north of ~100 kt subtropical jet) into the TX/OK Panhandles into southern KS. Meanwhile, expect the upper divergence to become significantly enhanced locally ahead of the developing mesoscale-convective vort lobe. Above normal PWs (around the 90-95th percentile) along with robust deep-layer instability later this afternoon (mixed-layer CAPEs 1500-2500 J/kg will foster hourly or sub-hourly rainfall rates between 1.5-2.5" into the evening hours. This will support a more enhanced flash flood threat within the Slight Risk area, especially across eastern NM into the TX/OK Panhandles where it has been quite wet during past week (QPE 500-600%+ of normal). This generally aligns with the SPC Slight Risk for sever, as well as the CSU machine-learning first guess fields. Farther north-northeast, across the eastern half of the Slight Risk area, FFG values are currently higher, but expect them to drop during this morning with any lingering convection late Tue night. ....West into the Northern Rockies and Plains... The localized risk for excessive rainfall looks to continue across portions of northern CA into northwest NV today, also extending farther northeastward across ID/MT and into ND as well. This is primarily due to the aforementioned mid-upper low over SoCal beginning to lift northeastward through the Four Corners region (with another shortwave trough in the northern stream digging on its approach into the PacNW). Axis of sufficient instability, above normal PWs (as high as ~0.9" and around the 90th percentile per the NAEFS) and generally weak flow aloft will combine with afternoon heating to support another day of isolated to scattered showers and storms. 00Z models continue to show considerable spread with regard to the QPF, though there are a couple areas where there is better clustering with the heavier totals. One is over southwest MT, while the other over southwest ID. These will be areas to watch with subsequent updates to the Day 1 ERO; at this point however, given the spread in the guidance and lack of run-to-run continuity with the heavier QPF, will maintain the Marginal Risk inherited from yesterday's Day 2 ERO. ....Southern half of the Florida Peninsula... 16Z Update... Added a small Slight Risk area that covers the coastal urbanized areas of Palm Beach, Broward, and Miami-Dade counties, as well as areas farther west. The HREF continues to show significant probabilities for rainfall amounts exceeding 3 inches, with the potential for amounts above 5 inches in some areas. Given the antecedent conditions and the urban areas vulnerability to flooding, it was decided to upgrade to a Slight Risk for this area. Previous Discussion... Weak mid/upper trough will remain nearly stationary over the eastern GOMEX during day 1 (Wed-Wed night), with 500 mb heights within the trough averaging ~2 standard deviations below normal. This synoptic setup will favor persistent broad-scale, weak deep-layer QG forcing east over the FL Peninsula through the period, which combined with the high (above normal) PWs for late May (1.75-2.00") and sufficient deep-layer instability (mixed-layer CAPEs 1000-2000 J/Kg), will favor intense hourly to sub-hourly rainfall rates of 2 to 2.5" underneath the strongest convective cores. The pattern is supportive of a Marginal Risk for the Day 1 ERO, as the CAMs show numerous areas of QPF between 1-3+ inches this afternoon and evening as the upper level forcing coincides with peak diurnal heating. In fact, the 00Z HREF indicated widespread probabilities of 50-70+ percent of 24hr QPF exceeding 3" within the Marginal Risk area, while probs for >5" averaged between 20-40 percent. Pereira/Hurley Day 2 Valid 12Z Thu Jun 01 2023 - 12Z Fri Jun 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN-CENTRAL PLAINS AND MUCH OF EAST-CENTRAL MONTANA INTO NORTHEAST WYOMING... ....Central and Southern Plains... Based on the guidance trends and likely wet antecedent soils, have upgraded a portion of the inherited Day 3 Marginal Risk area to a Slight Risk in the new Day 2 ERO (from 12Z Thu to 12Z Fri). This includes a small portion of northeast NM, across the TX/OK Panhandles, and into north-central OK and south-central KS. Another active day of convection looks likely, with some additional lift being provided by the northward moving aforementioned mid-upper low and associated vort max. While convection overall should be less organized than on Day 2, the 00Z guidance exhibits somewhat better clustering with respect to the heavier QPF footprint on Day 2. Meanwhile, the latest CSU First-Guess fields show an expanded Slight Risk area compared to recent runs, affording greater forecast confidence in hoisting a targeted Slight Risk area. ....Northern/Central Rockies and Plains... Things start to get interesting going into Thursday across the Northern Rockies and High Plains, as a remnant mid-upper low and associated vorticity maxima eject northward from the Four Corners region. This should provide broad ascent via DPVA, along with modest diffluence aloft and sufficient instability. While we are not quite in the range of CAM guidance at this juncture, the GEFS does indicate non-zero probabilities (5-10%) for 2" and 3" exceedance over central MT and the ECMWF EFI shows values over 0.7 (a fairly unusual event forecast from the ECMWF EPS). The GEFS also indicates PWATs increasing to as high as 1.25" (+3.0 SD, or approaching the 95th percentile climatologically). Have maintained the inherited Slight Risk and this area will continue to be fine-tuned with subsequent updates as more CAM guidance comes into range through the D2 and eventually D1 periods. Farther south into the Central Rockies and into the Great Basin, any flash flooding is expected to be localized with much less organized convection. ....Central and Southern Florida... Also maintained the Marginal Risk for much of central/southern FL going into Thursday, with an expansion across the FL Peninsula as there is potential for the surface low to become more coherent and track toward this area. Aloft, a very slow-moving upper-trough over the Gulf will finally make its way directly overhead the Florida Peninsula with the strength/track of its surface reflection still uncertain (some 12Z CAM guidance was quite aggressive). This should act to enhance typical diurnal convective activity across the region, with strong mid-level lapse rates allowing for more intense updrafts with tropical-like rainfall efficiency (given PWATs ~2.0" and wet bulb zero heights of ~15K feet). Any localized flooding impacts are most likely across urbanized metro areas with poor drainage. Hurley/Fracasso Day 3 Valid 12Z Fri Jun 02 2023 - 12Z Sat Jun 03 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF MONTANA-WYOMING AS WELL AS THE SOUTHERN-CENTRAL PLAINS... ....Central and Southern Plains... Overall pattern again shows little change given the degree of mid-upper level blocking. One notable difference is that by Day 3 (Fri-Fri night), the models show the main southern stream shortwave (north of the weakening subtropical jet streak) dropping farther south across the southern Plains -- i.e. south of the Red River across North-Central TX. This will result in a southward expansion of the more widespread, organized convection and heavier areal-average rainfall totals. Only minimal changes were made to the Slight and Marginal Risk areas inherited from yesterday's Day 4 ERO. Given the overlap of Slight Risk areas on Days 1 through 3 across parts of the TX-OK Panhandles into north-central OK and south-central KS, an upgrade to a Moderate Risk may be needed at some point somewhere within this region given the amount and coverage of heavy rainfall from the previous day(s). At this point, all current CSU First-Guess fields depict at most a Slight Risk.=20 ....Northern/Central Rockies and Plains... Stagnant, blocky upper level pattern also supports an excessive rainfall risk on Day 3 similar to that on Day 2, though the guidance trends would likely keep more of eastern MT out of the more elevated risk (i.e. Slight) on day 3 compared to Day 2. ....Central and Southern Florida... Models show the mid-upper trough across the eastern GOMEX flattening a little upon traversing the FL Peninsula and Cuba on Day 3. This would likely lead to a weakening trend with the low-level inflow and moisture transport/flux convergence, although continued favorable thermodynamic parameters (deep-layer moisture and instability) along with slow storm motions will likely maintain an isolated/localized flash flood threat, especially over urban areas. Therefore, will maintain a Marginal Risk across Central and South FL. ....Northern New England... 'Breaking' northern stream shortwave will drop south across Quebec, east of the closed mid-upper high, and reach far northern New England by the end of the forecast period (late Fri night). Given the amplification aloft, strong along-stream upper divergence will pivot across northern New England during Day 3, which along with the uptick in deep-layer instability (mixed layer CAPEs climbing between 1000-1500 J/Kg with the steepening 700-500 mb lapse rates coinciding with the diurnal heating Fri afternoon and evening), will lead to pockets of 1-3+" rainfall totals within a 3-6 hour period across northern New England. This may lead to localized short-term runoff issues, and as such have maintained the Marginal Risk area inherited from yesterday's Day 4 ERO. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_TK5hGINahMaFHQZtzIHsVL5XiZEHT_iMpLivaDNyXqI= NiKCwfnKycE22w5jZV_ykudPGI2w0RMjoQE3mH2zfeKLUeA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_TK5hGINahMaFHQZtzIHsVL5XiZEHT_iMpLivaDNyXqI= NiKCwfnKycE22w5jZV_ykudPGI2w0RMjoQE3mH2zn4jfi6M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_TK5hGINahMaFHQZtzIHsVL5XiZEHT_iMpLivaDNyXqI= NiKCwfnKycE22w5jZV_ykudPGI2w0RMjoQE3mH2zJOFhQhg$=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) .