Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 03 2024 20:28:31 FOUS30 KWBC 032027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 327 PM EST Wed Jan 03 2024 Day 1 Valid 16Z Wed Jan 03 2024 - 12Z Thu Jan 04 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 2 Valid 12Z Thu Jan 04 2024 - 12Z Fri Jan 05 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 3 Valid 12Z Fri Jan 05 2024 - 12Z Sat Jan 06 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE CENTRAL GULF COAST AND PORTIONS OF THE SOUTHEAST AS WELL AS FOR THE OLYMPIC MOUNTAINS OF WASHINGTON STATE... ....2030Z Excessive Rainfall Discussion... No changes needed across the Gulf Coast states and the Southeast. The models and WPC deterministic QPF still focuses the heaviest rainfall along and near the Gulf where instability should be greatest and more supportive of downpours. The amount of instability drops off inland...but maintained the area without significant change given the uncertainty of the exact storm track which has implications in how far northward the instability gets drawn. Adjusted the Marginal risk area in the Olympic Mountains to minimize the overlap of areas with excessive rainfall potential and the areas of heavy snowfall as shown by the WPC Winter Weather Desk. Bann ....Gulf Coast and Southeast... A strong and fast-moving upper level shortwave will track northeastward from Texas to the Southeast by Saturday morning. A south to north jet streak is ahead of the wave, with a developing west to east jet streak over the Gulf Coast behind the shortwave. The result will be an increasingly favorable divergence pattern aloft associated with the shortwave itself, which will support deepening of a surface low that will track across the Southeast along with the shortwave. The low will take advantage of increasing Gulf moisture to result in a line of showers and storms that will from from the low center south to the Gulf. The storms will be capable of heavy rainfall, as PWATs increase to as high as 1.75 inches. However, there will be a few factors working against flooding, including the fast forward speed of the storms, which will limit how much rain any one area sees; high FFGs, which will be over 3 inches per hour along the Gulf coast, and over 2.5 inches per hour for inland areas included in the Marginal; and soils all across the Marginal Risk area are either below or well below normal, with much of Mississippi at under 2% of normal, so the rivers and streams are eager for some beneficial rainfall. The result will be any isolated flash flooding most likely occurring in flood-sensitive urban and poor-drainage areas. Only minor updates to the Marginal Risk area were made with this update, including trimming portions of east TX and northwestern LA out of the Marginal as the storms will still be forming and moving quickly Friday morning, and adding several rows of counties into southwestern GA owing also to the faster speed of the parent low, which will be in an increasingly favorable environment which will strengthen the forcing supporting any storms Friday night. ....Olympic Mountains of Washington State... A strong front and attendant shortwave will dig southeastward out of the North Pacific and into coastal Washington Friday night. The period of heaviest rainfall into the Olympic Mountains is likely to be after 06Z Saturday, but due to the strong forcing, rainfall rates of up to an inch per hour are possible, resulting in a widespread 2 to 3 inches for the storm total. Heavy rainfall rates may result in isolated flash flooding in the flood-sensitive valleys draining the west facing slopes of the Olympic Mountains. In coordination with the SEW/Seattle, WA forecast office, a Marginal Risk area was introduced to highlight the Friday night time period. The heavy rains will quickly move down the coast and the Cascades on Saturday, so the heaviest rains will be short-duration, which in turn will limit flash flooding. Any heavy rainfall will be south of the Olympics by 12Z Saturday. Soils are around average for this time of year, but based on NASA Sport relative soil moisture guidance is between 65-70% saturated. Heavy rainfall should quickly saturate the soils in this region. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_sQZ3CRAi2q-5nvhnnC0T5I07as7ideo1kkYhJbGvIWs= MbWPrlWw13vcR_q89Ph01_4ejQsnFfiz7V6s-qNLlXzwY5w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_sQZ3CRAi2q-5nvhnnC0T5I07as7ideo1kkYhJbGvIWs= MbWPrlWw13vcR_q89Ph01_4ejQsnFfiz7V6s-qNLAwYKJ0s$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_sQZ3CRAi2q-5nvhnnC0T5I07as7ideo1kkYhJbGvIWs= MbWPrlWw13vcR_q89Ph01_4ejQsnFfiz7V6s-qNLjHgxbVk$=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) .