Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 04 2023 00:36:13 FOUS30 KWBC 040036 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 835 PM EDT Thu Aug 03 2023 Day 1 Valid 01Z Fri Aug 04 2023 - 12Z Fri Aug 04 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL NEAR THE CONFLUENCE OF THE MISSOURI, OHIO, & MISSISSIPPI RIVERS... ....Mid Mississippi Valley, Lower Ohio Valley, Tennessee Valley, Southern Appalachians... The setup remains favorable for heavy rainfall and flash flooding Thursday night into Friday morning as a shortwave entering northwest MO which is currently fostering some shower activity moves through within the northwest flow regime while 850 hPa inflow ramps up. The gradient in the ML and MU CAPE is aligned ahead of the disturbance, with values of 2000-8000 J/kg.=20 Effective bulk shear is 30-35 kts, which is sufficient for convective organization. PWs ~2.25" within the convective environment will support peak hourly rainfall rates of 3" wherever non-traditional cell training (at a significant angle to the low- to mean wind) can manifest, which would be double the 3-hourly Flash Flood Guidance values. The 18z HREF guidance had a good signal for 3" and a non-zero signal for 5" -- amounts usually associated with a Slight Risk environment. However, 7 day rainfall departures are 600%+ of normal -- 10"+ -- in spots, and a small portion of the MO Bootheel has Flash Flood Guidance values near zero. Given the above factors, an upgrade to a Moderate Risk appeared prudent, if not the conservative move, and was coordinated organically and quickly with the Springfield MO, St. Louis MO, Paducah KY, and Memphis TN forecast offices.=20 Progression of a separate convective Risk areas in the Midwest, southern Mid-Atlantic, and Southeast were modified/constricted based on radar reflectivity trends and deal with both current convection and convection which may enter the region from the Midwest later tonight. ....Eastern portions of the Great Basin into Wyoming and parts of the Central Plains... An upper level low lifting overtop dampening upper ridge combined with favorable right entrance jet dynamics will support a broad area of forcing for ascent from eastern portions of the Great Basin across much of WY and into the Plains. Moisture anomalies will remain above normal, nearly 2-2.5 standard deviations above normal, particularly across northern UT-eastern ID into northern WY and western SD. Convection is expected to develop initially over portions of the Great Basin before moving eastward through WY/MT and pushing into the Plains later in the period where it'll interact with the greater instability/moisture. Localized rain rates 0.5-1"/hr will be possible across ID for a little while longer while further east rain rates up to 1.5-2.0"/hr will be possible across portions of eastern WY and western SD which the guidance migrates closer to the upper level center near the WY/MY/SD/ND border region by Friday morning, where a 20-30% chance of 3" amounts was depicted by the 18z HREF guidance. Further south into northern Kansas and southern Nebraska, localized rain rates and totals may be just as intense. This along with the wet antecedent conditions may produce more scattered instances of flash flooding, and as such the inherited Slight Risk was general maintained, with changed made based on the 18z HREF guidance.=20 ....Interior Northeast... Broad forcing for ascent associated with an approaching shortwave energy embedded within the larger troughing will overspread the region while at the surface, a cold front begins to advance from the west. Ahead of that, southerly flow will bring modest moisture characterized by precipitable water values between 1-1.25". MU CAPE values have been fairly static in the 500-1500 J/kg range, which is expected to continue scattered to numerous showers and thunderstorms this evening and overnight into Friday morning which could produce localized intense rain rates in excess of 1"/hr and may lead to isolated instances of flash flooding across portions of the interior Northeast. Roth Day 2 Valid 12Z Fri Aug 04 2023 - 12Z Sat Aug 05 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN HIGH PLAINS... ....Portions of the Northern High Plains... With the benefit of full HREF guidance and CAMs for the Day 2/Fri period, the signal for heavy rainfall has continued to increase across the MT/ND/SD junction region of the northern High Plains. 100 year HREF ARIs for 24 hour rainfall are over 60 percent across much of the Moderate Risk area, and there's a 60% chance of 3 inches of rain in the HREF 40 km neighborhood probabilities for this region, 35% chance for 5 inches, and 15% chance of 8 inches. These values are near record territory for this region of the country. Further, there is ongoing heavy rain across this area, which will continue to lower flash flooding thresholds. An ensemble of the CAMs shows that convection will develop once again during peak heating across the Moderate Risk area, but will continue into Thursday night as more stratiform rain that very slowly sags southward towards the Black Hills. This pattern will favor more widespread flash flooding since the rainfall is expected to persist much longer and over a larger area than previous days. The Moderate Risk upgrade was coordinated with UNR/Rapid City, SD; ABR/Aberdeen, SD; BIS/Bismarck, ND; GGW/Glasgow, MT; and BYZ/Billings, MT forecast offices. ....Northeast... In contrast to the northern High Plains, the Slight Risk for much of New England was downgraded to a Marginal with this update. Once again with help from CAMs guidance and HREF trends, the ensemble suggested very fast moving storms will be swinging southeastward across New England Friday afternoon. PWATs across this area have come down from over 1.75 inches along the coast yesterday to around 1.5 inches, which is a pretty substantive decrease. Further, most of New England looks to be only seeing one round of storms, with perhaps the possibility of a couple rounds of storms over southern NH. Finally, antecedent conditions have recovered quite a bit from the rainfall over the past few weeks, and it's been drier than normal over this region for the last week based on AHPS imagery. The Slight Risk was dropped in coordination with CAR/Caribou, ME; GYX/Gray, ME; BOX/Norton, MA; ALY/Albany, NY, and OKX/Upton, NY forecast offices. Wegman ....Previous Discussion... ....Northern Rockies to Central Plains... Fairly potent shortwave energy lifting through the Great Basin is expected to become closed off at 700-500 mb across eastern Montana and the western Dakotas by early Saturday. This will wrap very anomalous moisture into the region, especially across southeast Montana and far northeastern WY into central-western South Dakota where PWs of 1.3 to 1.5"+ are expected, which is over 3 standard deviations above normal. As low pressure develops and moves from WY to SD, a very favorable frontogenetical band of precipitation is expected which could produce a longer duration moderate to heavy rainfall event. The signal for 1.5 to 3"+ looks pretty strong, especially with hints of coupled upper jet streaks, such that the Slight Risk was maintained across parts of the northern High Plains (south-central and southeast MT into north-central and northeast WY) into southwest ND, western and central SD, and northern portions of NE. Per the QPF spread in the guidance, there continues to be some uncertainty in how quickly convection progresses southeast into the Plains but the Marginal Risk was generally maintained through eastern Nebraska and western Iowa. ....Southeast... Anomalous moisture (PWs > 2") combined with favorable forcing for ascent associated with shortwave energy moving through the region will keep an active/unsettled pattern in place and the threat of heavy rainfall and flash flooding. The notable difference however from yesterday's Day 2 ERO was a southward shift in the Marginal Risk area, based on the model consensus of a noticeable southward trend in the location of the surface boundary and thus the deep-layer instability/moisture pooling ahead of the fronts. ....Northeast... Longwave troughing becomes increasingly neutrally tilted during the period with the trough axis centered through Upstate New York late Friday afternoon and evening. This will allow for a longer period of southerly flow to bring anomalous moisture northward across the Northeast, particularly across the coastal regions of southern New England through coastal Maine with the latest guidance pointing toward a narrow axis of 1.5"+ PW setting up. Depending on the timing of the advancing cold front, enough destabilization will be possible during the afternoon/evening to produce scattered to numerous showers and thunderstorms across the region before quickly moving east (00Z ECMWF shows mixed-layer CAPEs between 1000-1500 J/KG). The modest moisture and instability and favorable forcing will support localized intense rain rates (1-1.5"/hr) with some localized totals near 2" possible. The broad Marginal Risk was maintained, but so too was the Slight Risk across eastern NY into much of New England given the combination of higher exceedance probabilities (e.g. more widespread HREF probs aoa 50% of QPF>2" in the 12hr period ending 00Z Sat), along with the wet antecedent soils. Hurley Day 3 Valid 12Z Sat Aug 05 2023 - 12Z Sun Aug 06 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS INTO PARTS OF THE UPPER MIDWEST... ....2030Z Update... No big changes were made to the Slight Risk area across the northern Plains and upper Midwest. The signal in the guidance for over 1 inch of rain has expanded westward along the MT/WY border in response to the same slow-moving vertically stacked low that will cause the flooding in the area on Day 2/Fri. Further, the rainfall signal has expanded northward into central ND and portions of northern MN. There certainly is room for additional upgrades in this area in the coming days if the pattern continues to favor slow-moving storms in a widespread anomalous moisture plume. Wegman ....Previous Discussion... ....Northern Rockies and Plains into the Midwest... Aforementioned vigorous mid/upper level shortwave, along with the eastward progression of coupled upper level jet streaks (right-entrance region of the jet streak near the U.S.-Canadian border and left-exit region of the upper jet axis nudging into the central High Plains) will maintain a favorable synoptic setup for more organized areas of heavy rainfall. This as the 850-700 mb moisture flux anomalies remain 3 to 4 standard deviations above normal. Much stronger deep-layer instability south of the best frontogenetical forcing will likely result in organized convection (and heavy rainfall footprint) sliding farther south of the current ensemble mean of global guidance, and as such have altered the Slight Risk accordingly -- not only across the Dakotas (mainly SD), but also downstream into southern MN, northeast NE, and northern IA. The UFVS-verified version of the CSU first-guess field supports this shift with the Slight Risk area, as the 00Z non-CAM guidance already hints at areas of 2-4"+ max totals. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5SKYWIb-k1B9PjTgd2lIX1tBRnzqoP2ARZAZWf-kD1kd= WxyRNQqxwVSGc4ZWVlctMNbisc2JCEB1dnuug6EP2iQkeoE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5SKYWIb-k1B9PjTgd2lIX1tBRnzqoP2ARZAZWf-kD1kd= WxyRNQqxwVSGc4ZWVlctMNbisc2JCEB1dnuug6EPtcLSWj0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5SKYWIb-k1B9PjTgd2lIX1tBRnzqoP2ARZAZWf-kD1kd= WxyRNQqxwVSGc4ZWVlctMNbisc2JCEB1dnuug6EPU0safB4$=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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