Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 01 2023 00:37:41 FOUS30 KWBC 010037 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 836 PM EDT Wed May 31 2023 Day 1 Valid 01Z Thu Jun 01 2023 - 12Z Thu Jun 01 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN HIGH PLAINS... 01Z Update: ....Florida Peninsula... Deeper convection has been confined mainly to the western areas of the Florida Peninsula this afternoon and early evening with a relative lull over southeast Florida. The latest hi-res guidance including the last several runs of the HRRR suggest that over the next several hours there should be a gradual waning of the deeper convection across the region. The 18Z HREF neighborhood probabilities show the greatest risk of any hourly totals exceeding 1-2" to be for the next few hours then those chances diminish considerably overnight. As a result, felt the Slight Risk could be removed for this update, keeping the broad Marginal in place for isolated/localized instances of flash flooding for the more vulnerable/susceptible locations. ....Central to Southern High Plains... Complex and somewhat uncertain forecast for the overnight period with two main areas of focus. For the Central Plains, remaining weakness in the mid-levels continues to generate scattered, loosely organized convection across eastern Kansas into northwest Missouri. While fairly scattered now, the hi-res guidance suggests some intensification and organization overnight with a subtle increase in the low level flow (850 mb winds increase to 25 kts) and the higher PW axis remaining in place. The Slight Risk was adjusted some, aligning up to the greatest probabilities in the 18Z HREF for rainfall amounts through 12Z exceeding 2-3" and this is near/over areas that have recently seen heavier rainfall resulting in wetter antecedent conditions and lower FFGs. Further south, mid-level shortwave energy coming out of New Mexico combined with fairly anomalous moisture and sufficient instability has led to complex of deep convection moving into the TX Panhandle. This activity is expected to continue through the rest of the evening and into portions of the overnight period, with additional development in its wake, leading to a few repeating rounds of heavier convection. There is the potential for hourly totals 1-2" early on and totals through 12Z up to 3" locally and the Moderate Risk area stands the greatest chance for these higher end totals based on the 18Z HREF probabilistic. ....West into the Northern Rockies and Plains... Localized flash flooding will remain possible through the evening hours across portions of the West through the Northern Rockies and Northern Plains as mid-level low moves eastward into Arizona and the axis of greater lift and slightly higher moisture extends from Nevada to northern North Dakota. Pockets of hourly totals 0.5-1" will be possible at times through about 04-05Z then begin to wane with the loss of daytime heating. The Marginal Risk was adjusted minimally to reflect the latest radar trends and hi-res guidance. Taylor 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... 2030Z Update... A review of the 12Z guidance did not suggest a need for any large-scale changes for now. Some smaller adjustments were made based on the most recent guidance. This included extending both the Slight and Marginal risk areas farther southeast from the Texas Panhandle into Northwest Texas. This was based on the HREF guidance indication of greater coverage and higher probabilities for heavier amounts across that region. Previous Discussion... ....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. Pereira/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... 2030Z Update... Similar to Day 2, did not see reason to make any large-scale adjustments for now. Still believe an upgrade to a Moderate Risk centered in the vicinity of the Texas Panhandle/Northwest Texas may be needed at some point. However, in addition to the typical Day 3 uncertainty, a number of models suggest the heaviest rains may set up farther east than those expected during days 1 and 2, and over areas with relatively drier antecedent conditions. Previous Discussion... ....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. Pereira/Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6Ks4B7-P_S5qIJ7fmfySMKd7rbK5jmpgreWuBopGrMAQ= V4XJis-hkgqRkyZKjyE2iuyUMWJoEGaQgK0Styzi6bji9bM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6Ks4B7-P_S5qIJ7fmfySMKd7rbK5jmpgreWuBopGrMAQ= V4XJis-hkgqRkyZKjyE2iuyUMWJoEGaQgK0StyzijcVUI-8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6Ks4B7-P_S5qIJ7fmfySMKd7rbK5jmpgreWuBopGrMAQ= V4XJis-hkgqRkyZKjyE2iuyUMWJoEGaQgK0StyzinJZ1myo$=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) .