Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 17 2023 17:01:42 FOUS30 KWBC 171701 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 100 PM EDT Thu Aug 17 2023 Day 1 Valid 16Z Thu Aug 17 2023 - 12Z Fri Aug 18 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE WEST, NORTHEAST, & FLORIDA... ....16z Update... No changes to the previous D1 forecast. Current satellite and radar trends put the forecast on track through the next 12 hrs. Will be keeping an eye on local maximums across S FL between MIA up to PBI as HREF Probs place highest 40km neighborhood probs >1" across the aforementioned corridor. Convective initiation across the SW US will create isolated pockets of heavier rainfall, but nothing to deviate from previous forecast. Kleebauer/Bann ....Southwest, Southern Great Basin, Northern Sierra...=20 The closed low off the central California coast will begin to progress back toward the east southeast and toward the south-central California=20 coast. A broadly upper difluent flow pattern will continue to the east of this closed low and to the west of the upper high centered from the Rockies into the Plains. PW values in this broadly difluent upper flow pattern will remain above average, ranging from 1 to 4 standard deviations above the mean. This will support another day of widely scattered convection, isolated heavy totals and isolated runoff issues, especially across any burn scar areas or slot canyons. A broad marginal risk was drawn to encompass where models are showing some precip potential and is essentially unchanged from continuity. =20 ....Florida...=20 An axis of tropical PW values, 1.75-2.25" will remain across much of the Florida Peninsula. Convergence in the west-southwest low-level flow in obvious in the guidance, in the range of 10-15 kt, which would favor the western coast during the usual overnight maximum (06-15z) and closer to the Gold Coast during afternoon heating as the East Coast sea breeze would likely not make progress westward. Enough instability should exist for hourly rain totals to approach 3", which would be most problematic in urban areas. =20 ....Northeast...=20 Increasingly negatively tilted height falls will be pushing through the Great Lakes and into the Northern Mid Atlantic States. While there is likely to be an organized band of frontal/pre-frontal precip moving through the lakes, it should be very progressive, keeping amounts from being very heavy. Due to the progressive nature of the convection, removed the Marginal Risk from the eastern Mitt of MI. However, across the Northeast, some afternoon thunderstorm is expected to percolate ahead of a second batch slated to move through Thursday night into Friday morning. Maintained the Marginal Risk here. It's possible that some localized Slight Risk impacts occur in the southern portion of the risk area due to a wet seven day period across southeast NY and northeast PA, but none of the guidance is overly wet in this area, so left the risk as Marginal. =20 Roth Day 2 Valid 12Z Fri Aug 18 2023 - 12Z Sat Aug 19 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST ARIZONA INTO SOUTHWEST UTAH...=20 =20 Northeast... A Marginal Risk remains across the region, which was slightly expanded from continuity. Heavy rain may be ongoing at 12z Friday (a continuation of the previous day's rainfall)...with another round possible during the day Friday as destabilization occurs ahead of the approaching trough/closed low. The timing of the trough and how it interacts with the moisture/instability streaming up the coast will play a role in the magnitude of the eventual flash flood risk. As the guidance amounts still appear modest, on the order of 1-2", some portions of the area have been wet this past week. Can't rule out an upgrade to Slight Risk at some point should the signal in the guidance increase. =20 Portions of the West... A Slight risk remains across northwest AZ into southwest UT. Seeing a notable uptick in mid/upper level forcing by this time as the closed low off the CA coast shifts eastward and the upper jet west of the area intensifies. The precipitable water values in the monsoon surge up the Gulf of California are increasing this period with values perhaps breaching 2" by early Saturday morning and ~1000 J/KG of ML CAPE is expected during Friday afternoon and early evening. Enough moisture and instability around to support heavy rainfall rates, up to 2" where cells merge, backbuild, or manage to train. This would be a problem over the susceptible areas of northwest AZ into southwest UT.=20 =20 Florida... A Marginal risk over portions of central/southern FL. The moisture plume from the polar front gets entangled with an incoming easterly wave that is moving through the Bahamas. Flow should be becoming more easterly, though remaining light, so there is concern about slow moving cells, and with the incoming easterly wave, increasing amounts of effective bulk shear which could lead to some loose organization. The thinking is that outflow boundaries will shift activity enough to prevent overwhelming amounts, but hourly rain totals to 2.5" with local amounts to 5" cannot be ruled out. Since Central FL in particular has been so dry lately, kept the risk level as Marginal. =20 Roth Day 3 Valid 12Z Sat Aug 19 2023 - 12Z Sun Aug 20 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF THE SOUTHWEST... Portions of the West... An upper level low temporarily stuck near the central CA coast, the approach of Hilary from the south, a deep southerly flow across the Great Basin keeps precipitable water anomalies up to 4 sigmas above the mean to mid to late August from the Desert Southeast northward into Montana. For areas from Las Vegas southward, the 1.75-2.25"+ precipitable water values will be approaching all-time records, so there will be moisture to spare.=20 While there is concern that cloudiness from Hilary will present some problems in building up instability across portions of the Southwest and Great Basin, the appearance of a low-level jet out of the Gulf of CA right at the end of the period early Sunday morning right near the apex of the monsoonal surge is of significant concern and could lead to convection during the late night of Saturday into early Sunday morning at an atypical time.=20 Some of the guidance shows local amounts of 5", though overlap of where that falls is minimal. The most likely spot would be the eastern slopes of the southern portions of the Peninsular Ranges within Southern CA...a Moderate Risk was contemplated for this area. However, the synoptic pattern is in flux by the end of day 3, with Hilary's movement complicated by several factors -- the upper low over CA thinking of escaping to the north, lured by a potentially sharpening trough across portions of WA. Hilary itself may be interacting with the landmass of Baja California and potentially experiencing some vertical wind shear. Considering the complication in the forecast scenario, have kept the risk level below Moderate for now, but this may change in later updates. Florida... An easterly wave is expected to move through the Keys and southern FL into the central Peninsula by Sunday morning. Inflow at 850 hPa is expected to increase to 15-20 kts, which should increase the effective bulk shear enough to lead to greater convective organization than seen on Friday. Probably due to the progression of the feature, amounts weren't overly high in the guidance.=20 However, should any convective training or mergers occur, 3" an hour totals cannot be ruled out, which would be most problematic in urban areas. Since the guidance QPF was so modest, left the risk level as Marginal.=20 Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_GrbZ56YSahOfz_dtkVb10sCeykR1MBx9v47mfFYx7oM= dg4UQN1D0-E_eMKJxvbzPTji2i8BqsgulD6BvfZP1Ph5iLE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_GrbZ56YSahOfz_dtkVb10sCeykR1MBx9v47mfFYx7oM= dg4UQN1D0-E_eMKJxvbzPTji2i8BqsgulD6BvfZPdfw9NMQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_GrbZ56YSahOfz_dtkVb10sCeykR1MBx9v47mfFYx7oM= dg4UQN1D0-E_eMKJxvbzPTji2i8BqsgulD6BvfZPztU3ncM$=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) .