Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 24 2024 00:58:39 FOUS30 KWBC 240058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 858 PM EDT Fri Aug 23 2024 Day 1 Valid 01Z Sat Aug 24 2024 - 12Z Sat Aug 24 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR CORNERS REGION/INTERMOUNTAIN WEST, THE GEORGIA COAST AND FAR NORTH PORTION OF THE FLORIDA COAST, AND A PORTION OF THE CENTRAL PLAINS... ....Four Corners...=20 1.25" PW advecting over eastern Arizona, southeast Utah and just into western Colorado and New Mexico on 20-30kt SSW flow with continued instability warrants maintenance of the Marginal Risk for overnight. Please see MPD 0924 for further information on this activity. ....Mid-Missouri River Valley... Organized activity currently over northeast CO should be maintained by a strong low level jet as it moves east around the high=20 centered over Texas overnight. This track along the KS/Neb border should be progressive enough to not warrant an ERO until the far northeastern part of KS/northwestern MO into southeast Neb where guidance continues to join the 12Z ARW2 in having additional development late tonight as the orientation of the activity gets more perpendicular to the low level inflow. The 18Z NAMnest and recent HRRRs suggest heavy rain generally north from the KC metro to the IA border after 09Z tonight. In coordination with WFOs EAX, TOP, and OAX, a small Marginal is added for the overnight as a lead into the larger Marginal for after 12Z (Day 2).=20 ....Georgia Coast into far northwest Florida...=20 Moist onshore flow along and north of a stationary front that is set up just north of Jacksonville should continue to be a focus=20 for periodically heavy activity into or through the overnight hours. The 2" PW with this onshore flow of 10-15kt is only about one standard deviation above normal, but ample MUCAPE of 1500-2000 J/kg should allow for repeating activity into the GA coast and Jacksonville metro. Recent HRRRs continue to suggest areas of 2-4" rainfall overnight which added to areas that received 1.5-2" over the past day warrants continuation of the Marginal Risk.=20 Jackson Day 2 Valid 12Z Sat Aug 24 2024 - 12Z Sun Aug 25 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST...MIDWEST...AND SOUTH FLORIDA... ....Southwest... Another round of daytime convection is expected in the Southwest again on Saturday, although the primary threat area should be relatively narrow, focused in a plume of deeper moisture along the periphery of an advancing deep trough. The Marginal Risk was generally maintained in the same area as the previous outlook issuance, albeit slightly narrowed, from SE AZ into NW NM and SW CO. Localized heavy rainfall will be possible, although the HREF mostly caps this at 1.5 inches or lower for the entire Day 2 period. Mid-level flow will be much stronger than usual for this time of year, with the 700-500mb wind speeds above the 99th percentile for late August near the Four Corners region. This should lead to faster storm motions than are typical in monsoon season. Nevertheless, around an inch of rain falling in a short period of time may still lead to flash flooding, and the Marginal Risk was maintained. ....Midwest... A complicated forecast exists for Saturday and Saturday Night for the Midwest, with several conditional threat areas. Models are generally in good agreement in showing several rounds of elevated convection along the periphery of a pronounced elevated mixed layer, with 700mb temperatures approaching record highs in the sounding archive and ERA reanalysis dataset. One round seems to be focused primarily in the 12-18Z time frame in or near MO, and the other looks to be focused after 00Z further to the north, with a considerable degree of uncertainty on location specifics. Per coordination with the WFOs in the region, the Marginal Risk was concentrated in areas where the signal in the 12Z HREF for flash flood guidance exceedance and 3 inches of rain in 3 hours is stronger and more consistent. This would be primarily in MO for the Saturday morning round, and in E IA and NW IL for the Saturday night round. However, a couple HREF members do show localized extreme rain events (maxima in excess of 5 inches) further to the north and northwest in MN, IA, and SW WI, and even as far as E SD. This cannot be ruled out, as anomalously high moisture (PWs above the 90th percentile near 2 inches; above the 95th percentile in S MN) and sufficient elevated instability will be present along the entire periphery of the EML. The threat is conditional upon organized convection developing, and that is highly uncertain. Multiple hi-res models also show much of the northern sections of the region remaining almost entirely dry. However, the risk should be assumed to be non- zero in the areas described above, and above 5 percent in the Marginal Risk areas. Further updates and expansion of the Marginal Risk is possible. ....South Florida.. Incoming deep moisture with roots from the Caribbean will make headway into South Florida on Saturday with area precipitable water around the 90th percentile, which is well above 2 inches. The increasing moisture will be in proximity to a quasi-stationary front to the north...with a sharp delineation point of where the moisture pooling will reside. A Marginal risk area was maintained across the Southern FL Peninsula. Lamers Day 3 Valid 12Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW MEXICO AND COLORADO...AND THE MIDWEST... ....New Mexico and Colorado... As the western periphery of the Southern Plains upper level high begins to erode, the heavy rain threat area should expand eastward on Sunday into more of New Mexico and Colorado. Scattered daytime convection should develop in a plume of deeper moisture and moderate instability, and the thunderstorms will be capable of producing flash flooding within complex terrain and especially around any burn scars. Hourly rain rates may locally exceed 1 inch. The Marginal Risk was mostly maintained in the same areas. ....Midwest... The latest guidance shows a fairly strong signal for organized convection and an associated heavy rainfall threat from SE IA into W IL and NE MO at the beginning of the period, and this is likely to be a continuation from the Day 2 period. A Marginal Risk was introduced in these areas where ensembles were most confident in heavy rainfall and close to where the CSU machine learning ERO guidance had the highest probabilities (although it seemed slightly offset from the hi-res guidance, and greater weight was given to HREF members for now). However, there is a considerable degree of mesoscale uncertainty both from the overall setup across the region, and the lead time -- around 42 hours. Hi-res models seem relatively consistent on showing localized extreme rainfall events in excess of 5 inches somewhere along the periphery of a pronounced EML in the Midwest, but they vary widely on placement. Therefore, the Marginal Risk should be viewed as the most likely placement for heavy rain on Sunday morning, but future updates are possible. Lamers Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!44OrRJGoyh3z11Axa0Ow0zyGafcSOOSNH7u6YyT8dbK1= qoRJV5gwYhI4Ntv9lHB2axGVdQkrNjvtlZzlStPJ76DE150$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!44OrRJGoyh3z11Axa0Ow0zyGafcSOOSNH7u6YyT8dbK1= qoRJV5gwYhI4Ntv9lHB2axGVdQkrNjvtlZzlStPJ13RWXo8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!44OrRJGoyh3z11Axa0Ow0zyGafcSOOSNH7u6YyT8dbK1= qoRJV5gwYhI4Ntv9lHB2axGVdQkrNjvtlZzlStPJS5zr39c$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .