Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 25 2023 08:31:26 FOUS30 KWBC 250831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Fri Aug 25 2023 Day 1 Valid 12Z Fri Aug 25 2023 - 12Z Sat Aug 26 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS THE CENTRAL ROCKIES AND ADJACENT HIGH PLAINS... ....Southwest to the central Rockies and Adjacent High Plains... Energy that had once been Harold will continue to make its way around the periphery of building mid-level heights over the Plains states...with a surface reflection being a low center to the east of the Central Rockies and the presence of anomalously deep moisture setting the stage for a potentially impactful event.=20 Precipitable water values of 1.75 inches over eastern and central Kansas/Nebraska gets pulled westward by low level easterly flow feeding into the area of low pressure by this evening...which helps push precipitable water values over northeast Colorado and nearby southeast Wyoming to 3 standardized anomalies greater than climatology. The 25/00Z HREF's highest probabilities of the 24hr QPF exceeding 2 inches in 24 hrs peaks over 50 pct where flow over a northwest-southeast front aids lift as well as an second area farther south given proximity to better instability (1000-1500 J/Kg surface-based CAPE per the HREF). By later in the period...the surface low begins to move east and a line of precipitation to slowly press southward. Local precip maxima from deterministic models and signals from the 25/00Z HREF EAS probabilities show a potential for locally heavy rainfall with enhanced rates of up to 1"/hr and totals near 1-2". This area was already covered in Thursday's Day 2 ERO and needed little change. Afternoon and early evening convection is expected in and near the higher terrain around the 4-corners region which poses a low-end risk of excessive rainfall given favorable moisture profiles from the area soundings. ....Northeast and Mid Atlantic and into parts of the Ohio Valley/Southern Lakes.., Area low pressure will track to the northeast from the Great Lakes area on Friday with increased mid-level difluence out ahead of a stronger shortwave trough digging eastward out of Ontario. First round of rainfall will be on-going at the start of the Day 1 period at 12Z today...which eventually exitd off Southern New England. A second round of convection will develop near a warm front pushing northward along the coast with a trailing cold front pressing east along the base of the surface low. General instability axis will preside within the warm sector from the Mid Atlantic up into New England with PW anomalies a solid 1-2 standard deviations above normal, especially across New England where the heavy precipitation signature is most prominent. Any thunderstorm will be capable of producing locally heavy rainfall with rates up to 1"/hr. A secondary area of focus back into eastern KY extending back into parts of Indiana with diurnally driven convective initiation in a region of moist confluent flow. Precipitable water values will climb during the afternoon to values between 1.75 and 2 inches along this corridor which should support local downpours...especially where terrain can help provide focus for the flow.=20 Bann Day 2 Valid 12Z Sat Aug 26 2023 - 12Z Sun Aug 27 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS THE SOUTHERN-CENTRAL ROCKIES AND CENTRAL PLAINS EASTWARD INTO PARTS OF THE LOWER MISSOURI AND MID MISSISSIPPI VALLEYS AS WELL AS FAR EASTERN MAINE... Locally heavy rainfall is expected across parts of the southern and central Rockies as shortwave energy ejects eastward and out over the adjacent Plains into the Mid-Mississippi Valley later today/tonight. This channel is where the numerical guidance places highest precipitable water values and it is where model QPF is highest. There was a slight southward shift in the placement of the area based on the latest model runs but overall the difference reflected little shift in forecast reasoning.=20 Deterministic and probabilistic QPFs have came up across parts of New Mexico amd eastern Arizona on Thursday. Those increased amounts persisted in the 25/00Z model runs...so few changes needed there. Also introduced a Marginal Risk area over portions of the eastern Tennessee Valley as , and as such have expanded the Marginal Risk over that region. The models show weak shortwave energy settling in from the north accompanied by precipitable waters exceeding 2 inches late in the period. Given the difuse nature of the shortwave energy...overall confidence is below average. On the other hand...this appears to be the beginning of an expanding area of rain over parts of the area farther east on Day 3. Finally...with models showing the back edge of rainfall lingering over the far eastern bit of downeast Maine...and a couple of the ensemble members showing potential for amounts approaching in inch above what is forecast on Day 1...introduced a Marginal risk there. Bann Day 3 Valid 12Z Sun Aug 27 2023 - 12Z Mon Aug 28 2023 ....THERE ARE MARGINAL RISK AREAS EXTENDING FROM PARTS OF THE WESTERN HIGH PLAINS AND SOUTHERN ROCKES EASTWARD TO THE CAROLINAS... ....Western High Plains and Higher Terrain... Weak northerly flow with a broader 500 mb ridge will allow minor shortwave troughs to slowly propagate southward from Colorado into New Mexico. Those shortwaves will be able to tap precipitable water values of 0.8 to 1.2 inches along their path...with lower values being in Arizona. Thinking is that at least some of the resulting late day and evening convection will be able to produce local downpours that results in run off or local flooding...especially over recent burn scars..normally dry washes and arroyos. ....Central Plains... A shortwave dropping southeast from the western high plains will help focus storms along a cold front that produce local downpours as it moves into a region with precipitable water values at or slightly above 2 inches. Timing is uncertain...but think the GFS idea of development during the late afternoon and persisting into the evening is reasonable.=20 ....Carolinas... Models agree on an expanding area of rainfall over parts of the Carolinas Sunday into early Monday...in part due to an amplifying mid- and upper trough over the eastern United States. As it advances southward...it helps develop low pressure along a cold front over the Southeast U.S.. An airmass with precipitable water values generally above 2 inches will be in place...leading to forecast soundings favorable for heavy rainfall rates. Given antecedent conditions...think most places should be able to handle the forecast rainfall...although rainfall intensity may still result in local run off or flooding problems so maintained a Marginal for the time being...and may be a primer for bigger rainfall concerns beyond the end of Day 3.=20 Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!63zsD4gcGXLHnfZ48R2TyMsQY3hOupNbQbpqZ8YkwJCY= ULM_yYuxN05k3bnQKAJ7EHBFaV4QutBCSmhqKmAV9_5T81Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!63zsD4gcGXLHnfZ48R2TyMsQY3hOupNbQbpqZ8YkwJCY= ULM_yYuxN05k3bnQKAJ7EHBFaV4QutBCSmhqKmAVrM1M_2Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!63zsD4gcGXLHnfZ48R2TyMsQY3hOupNbQbpqZ8YkwJCY= ULM_yYuxN05k3bnQKAJ7EHBFaV4QutBCSmhqKmAVl64GhtU$=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. 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