Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 12 2023 16:04:17 FOUS30 KWBC 121604 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1203 PM EDT Tue Sep 12 2023 Day 1 Valid 16Z Tue Sep 12 2023 - 12Z Wed Sep 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NEW MEXICO INTO EASTERN ARIZONA... ....Southwest to the southern High Plains... Expanded Slight Risk a bit north in New Mexico based on 12Z HREF CAMs consensus for areas with lower flash flood guidance. Marginal Risk expanded a bit north into Colorado based on 12Z HREF CAMs consensus. Active, quasi-zonal southern stream with embedded shortwave energy traversing w-e across the area will continue to support numerous showers and thunderstorms, particularly during the peak heating hours this afternoon. Meanwhile, a subtle (70-90kt) yet persistent upper jet streak along the OK-TX Panhandles into the northern Red River Valley will favor a boost in deep-layer ascent through this evening across east-central AZ into central and southern portions of NM (including the Albuquerque area). This is where we have delineated the Slight Risk in the Day 1 ERO, and while instability will remain marginal given the degree of cloud cover (MUCAPEs generally under 1000 J/Kg), PW and 850-700 moisture flux anomalies will be fairly robust within the Slight Risk area (around 2 standard deviations above normal). The latest (00Z) HREF shows probabilities of QPF exceeding 2.00" peaking between 30-50% within the Slight Risk area during the Day 1 period. Elsewhere, the concentration of heavier rainfall is expected to be more isolated-scattered per the model consensus, and as such the flash flood threat is expected to be more limited/localized (i.e. Marginal Risk). ....Carolinas, Appalachians, into the Northeast... Expanded Marginal Risk through south-central Appalachians and central North Carolina Amplifying upper trough over the Upper Midwest/western Great Lakes will pinwheel east into the OH Valley and eastern Great Lakes by 12Z Wednesday. Pre-trough/pre-frontal activity has developed over the upper Ohio Valley which will continue to expand east through tonight. The front is fairly progressive, and the PW anomaly peaks at about one standard deviation above normal by 06Z tonight, but SWly deep layer mean flow is parallel to this boundary so there is a threat for some repeating, locally heavy activity, warranting the Marginal Risk expansion, especially where there is lower FFG from recent rains. This activity further intensifies tomorrow over the Northeast where there is a Slight Risk. Jackson/Hurley Day 2 Valid 12Z Wed Sep 13 2023 - 12Z Thu Sep 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND NORTHEAST... ....Southwest and Four Corners Region into the Southern Plains... Slight/Marginal ERO area on Day 2 shifts a bit farther east by Day 3, as the next upper shortwave drops across southern CA and the western Great Basin. Compared to Monday's Day 3 ERO, the Slight Risk area was pulled a bit farther south and west across northern TX into far eastern NM. This was based largely on the guidance (QPF) trends (decent model QPF consensus), and with the anticipation that much of the soil will continue to see sufficient moistening today. ....Northern Mid Atlantic into Eastern New York and New England... The aforementioned upper level trough, continuing to take on a more negative tilt with time, will pivot across the northern Mid Atlantic and Northeast Regions late Wed-Wed night. Strengthening deep-layer forcing (Qs convergence) ahead of the trough will likely get a boost from a 80-100kt upper jet streak on the eastern flank per the models (enhanced Qn convergence and low-level FGEN). This would support more widespread, organized convection on Wed ahead of the advancing cold front. Many CAMs indicate spotty 12hr QPF totals of 3-5" within the Slight Risk area during the 12 hour period from 12Z Wed to 00Z Thu, though at this point there continues to be some areal spread in the guidance. The Slight and Marginal Risk alignment correspond to the latest deterministic/probabilistic QPF and guidance trends. Expect TPW values to peak aoa 1.75" prior to FROPA, and considering the degree of deep-layer forcing and recent heavy rainfall (lower FFG/increasing soil moisture percentiles), the Slight Risk noted from yesterday's Day 3 ERO would appear to be on track with only minor changes made (slight westward shift). Hurley Day 3 Valid 12Z Thu Sep 14 2023 - 12Z Fri Sep 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHERN PLAINS... ....Central Rockies and High Plains into the Southern Plains... Western U..S trough shifts eastward during Day 3 (Thu-Thu night), into the Central Rockies and 4-Corners Region. Embedded shortwave energy north of the W-E subtropical stream will traverse the much of the Southern Plains on either side of the Red River Valley. Both the Slight and Marginal Risk areas in the Day 2 ERO shift a little eastward on Day 3, though based on the guidance trends, have removed the Slight Risk area over WY that was in yesterday's Day 4 ERO. As is typical for a Day 3 forecast, there is some degree of spread in the guidance with respect to the heaviest rainfall footprint, however based on the probability-matched mean of 00Z global guidance, the more enhanced ERO risk (i.e. Slight) for now would appear to be across the OK/TX Panhandles across the Red River Valley and into North TX. This is where the increased low-level southerly flow is leading to a modest uptick in 805-700 moisture flux (1.5 to 2.0 standard deviations above normal) with PWs peaking between 1.75 and 2.00". Given some overlap with the wet weather during Day 1 and especially Day 2, expect the FFG and soil moisture percentiles to lower/increase respectively, which should boost the probabilities of exceeding FFG values even if deep-layer instability is limited somewhat by the extensive cloud cover and widespread precip coverage. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-aG-PqHVO4ARf1_IKjAV7YUJm7SOQhxauQ88udKTp4QN= XPNtA5c69miKdGbK0luuotU1ieqqgk2hz5nuCy6QbeioQRE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-aG-PqHVO4ARf1_IKjAV7YUJm7SOQhxauQ88udKTp4QN= XPNtA5c69miKdGbK0luuotU1ieqqgk2hz5nuCy6QbRKKhw8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-aG-PqHVO4ARf1_IKjAV7YUJm7SOQhxauQ88udKTp4QN= XPNtA5c69miKdGbK0luuotU1ieqqgk2hz5nuCy6QnD6r7ug$=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) .