Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 18 2023 01:01:49 FOUS30 KWBC 180101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Thu Aug 17 2023 Day 1 Valid 01Z Fri Aug 18 2023 - 12Z Fri Aug 18 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHEAST PENNSYLVANIA AND EASTERN NEW YORK... ....Northeast...=20 0100 UTC Update -- Upgraded a portion of the Marginal Risk in the Day 1 ERO to Slight from parts of NE PA into eastern NY, including BGM and UCA, based largely on the latest high-res guidance trends. Increasingly negatively-tilted mid-upper level trough approaching from the Great Lakes will lead to an uptick in upper level difluence/divergence and deep-layer ascent overnight, especially after midnight. The degree of amplification in the upper flow and attendant slow w-e progressing surface front will raise the probability of training convection late overnight per the high-res simulated reflectivity data. The Slight was drawn to include areas where current FFGs are running 1" or less within an hour (1.5" or less within 3 hours)...supportive by the maximum 18Z HREF 40km neighborhood probabilities between 8-12Z. ....Southwest, Southern Great Basin, Northern Sierra...=20 0100 UTC Update -- Made few changes to the current Day 1 ERO, based on the latest observational/mesoanalysis and HRRR trends.The closed low off the central California coast will begin to progress back toward the east southeast and toward the south-central California 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.=20 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. =20 Hurley/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... ....2030 UTC Excessive Rainfall Discussion Update... Made a sublte westward expansion of the Slight Risk area in Utah given the uncertainty as shown by the solutions offered by the deterministic model QPF...and reinforced by the spread in the ensemble solutions. Overall...that represented a small adjustment within the plume of moisture streaming across Arizona and curving round the periphery of a broad upper high over the Southern Plains. No change was needed across the Northeast U.S. or Florida. Bann ....0830 Excessive Rainfall Discussion... =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... ....2030 UTC Excessive Rainfall Discussion Update... The afternoon suite of deterministic and ensemble guidance strongly supported the placement of the Slight Risk over California into Nevada as moisture streams north of the international border...especially during the latter part of the period. Precipitable water values at or above 2 inches are still forecast...which could result in localized downpours (especially with the appearance of a low level jet out of the Gulf of California late in the period). There was no upgrade to a Moderate given the continued spread in possible solutions...and with more certainty in the Moderate impacts occurring beyond the end of the Day 3 period. Deterministic QPF was still on the low-side farther north in the Northern Rockies...although the anomalies start to approach 2 standard deviations above climatology for this time of year suggests that there could be some increase in rainfall amounts/rates. Florida looked to be well handled by the on-going excessive rainfall outlook with little in the way of changes needed there. Bann ....0830 UTC Excessive Rainfall Discussion... 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!-Jj6b2fS2iqS3FAvxW8I1Hj6vthWVBi2nQdpt9KM-Qn0= FLu1-oYa9z57aaPYFNi6qNBg-ouvTFYIGYGuy6PPHt2qj8M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-Jj6b2fS2iqS3FAvxW8I1Hj6vthWVBi2nQdpt9KM-Qn0= FLu1-oYa9z57aaPYFNi6qNBg-ouvTFYIGYGuy6PPdBAGhNI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-Jj6b2fS2iqS3FAvxW8I1Hj6vthWVBi2nQdpt9KM-Qn0= FLu1-oYa9z57aaPYFNi6qNBg-ouvTFYIGYGuy6PP973D0Bs$=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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