Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 12 2023 08:30:02 FOUS30 KWBC 120829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 AM EDT Tue Sep 12 2023 Day 1 Valid 12Z Tue Sep 12 2023 - 12Z Wed Sep 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF CENTRAL AND SOUTHERN NEW MEXICO INTO EASTERN ARIZONA... ....Southwest and Four Corners area to the southern High Plains... 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). ....Mid Atlantic into the Northeast... Amplifying upper trough over the Upper Midwest-western Great Lakes will pinwheel east into the OH Valley and eastern Great Lakes by Wednesday morning. The bulk of the pre-trough/pre-frontal activity across the Mid Atlantic and Northeast corridor will likely hold off until later this evening/overnight per the latest CAM simulated reflectivity forecasts. The guidance continues to show an uptick in pre-trough/pre-frontal forcing on Wednesday (during the Day 2 ERO period) as shortwave energy entering the trough base leads to a more negative tilt. However, both the deterministic and probabilistic QPF guidance supports a Marginal Risk over a good portion of the northern Mid Atlantic into much of NY west of the Hudson Valley. The latest CAMs still show some spread with the heaviest rainfall, while the relative narrow/limited areal coverage of these heavy rainfall axes support a more isolated (Marginal) flash flood risk. At this point, again with the 00Z guidance in mind, the least amount of spread in the high-res guidance (and highest HREF exceedance probabilities) extend across northeast MD into southeast PA. Within this corridor, the 00Z HREF indicates the highest probabilities of 12hr QPF exceeding 3.00" between 00-12Z Wednesday (40-60%). 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... During the Day 3 Excessive Rainfall Outlook period (Thursday-Thursday night) there should be a broad area of rain and thunderstorm potential from the central half of the Rockies into the southern Plains, with one area of shortwave energy over the Great Basin and some weak impulses flowing into the Plains. Easterly low level flow directed over the southern High Plains into a potential surface trough may also support convection. A Marginal Risk area encompasses this overall region that could see locally heavy rainfall. There is one notable change from yesterday's Day 4 ERO: there is now only one Slight Risk (western OK and northern TX) that reflects the best combination of guidance signals/parameters. . Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7NxiaGHCzFK1jfCRzrAMfdLY1Gk5UVWNo1j8Nik1rdNU= SOuEoI8yFF9hHiyq4-Kt0WycLcgTz2Wr-uGUnwufOEkVKTE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7NxiaGHCzFK1jfCRzrAMfdLY1Gk5UVWNo1j8Nik1rdNU= SOuEoI8yFF9hHiyq4-Kt0WycLcgTz2Wr-uGUnwufeFm2R0k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7NxiaGHCzFK1jfCRzrAMfdLY1Gk5UVWNo1j8Nik1rdNU= SOuEoI8yFF9hHiyq4-Kt0WycLcgTz2Wr-uGUnwufSeS23ZQ$=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) .