Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 24 2023 00:38:41 FOUS30 KWBC 240038 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 735 PM EST Mon Jan 23 2023 Day 1 Valid 01Z Tue Jan 24 2023 - 12Z Tue Jan 24 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 2 Valid 12Z Tue Jan 24 2023 - 12Z Wed Jan 25 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY... ....2030Z Update... For the most part, the guidance remains in excellent agreement regarding the evolution of the event expected on Tuesday. The only change of significance was to trim the Marginal back in Alabama. Many of the CAMs now cover the entire event through Tuesday night, and most of them agree that northern AL is really getting the "leftover" convection that is much stronger further south and west in the Slight Risk area. All of the convection is also expected into northern AL after sunset, so daytime heating (such as it is this time of year) will not be a contributing factor to the meager instability. Otherwise, QPF amounts across much of the Slight Risk area has come down, particularly in northern areas of northeast TX and northern LA, however there still may be enough instability and enough of the CAMs suggest convection will still be robust in these areas, so opted to leave these the same despite the lower total QPF. Ultimately strong storms exceeding 1 hr FFGs appear most likely than any longer term FFGs being exceeded. The overall convective line, which will be the when the strongest storms and heaviest rainfall occur, will be moving quickly. It's the combination of that heavy rain following behind the prefrontal convection that is expected to cause any widely scattered flash flooding. Wegman ....Previous Discussion... The latest guidance has come into much better agreement regarding the evolution of an upper-level low over the Southern Plains on Tuesday. A potent upper trough and favorable jet dynamics should support a fast moving surface low pressure system moving from southern Texas through the central Gulf Coast Tuesday morning through early Wednesday. The upper trough will tap into a reservoir of Gulf moisture (PWATs between 1-2in) and send it streaming into southeastern Texas and the central Gulf Coast on Tuesday. A line of heavy rainfall and thunderstorms are expected to initiate somewhere in southeastern Texas and spread eastward along the Gulf coast where it will encounter some decent instability with MUCAPE hovering between 500-1000J/Kg along the immediate coastlines of Texas and Louisiana. A southerly low level jet at around 60kts will provide ample shearing for these fast moving storms. Rainfall rates between 0.75-1.5in/hr over saturated soils and the presence of low FFG will amplify flash flooding concerns for the area encompassed by the Slight Risk. The 00z Canadian regional and ECMWF hourly QPF were favored since they appeared to capture the magnitude and timing of the rainfall reasonably well. First Guess ERO fields and a number of local offices supported the risk areas drawn for this day 2 period. Kebede Day 3 Valid 12Z Wed Jan 25 2023 - 12Z Thu Jan 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... ....2030Z Update... ....FL Panhandle and southern Georgia... No changes were made with this update. The total QPF has dropped a small bit, but with saturated soils from antecedent rainfall, the Marginal threat remains in effect and the previous discussion for this area remains valid. ....New Jersey into New England... A low-confidence forecast for sure, but in coordination with OKX/New York City, NY, and BOX/Boston, MA forecast offices, went ahead and removed the Marginal threat for this time period. There was a small, but still notable eastward shift in the axis of heaviest rainfall, along with a decrease in the highest amounts. It appears likely that the precipitation will at least begin as snow, particularly in the previous western ends of the Marginal in the western suburbs of NYC through central CT and south central MA. However, as mentioned in the previous discussion below, a changeover to rain when the majority of the storms precipitation will occur is still expected. Overall, only minor, nuisance flooding is possible, as these areas remain fairly dry in terms of soil moisture analyses, and SREF MUCAPE values barely get to 10-20 J/kg at the very tail end of the event early Thursday morning. Thus, think this lack of instability here will also mitigate any flash flood potential since rainfall rates are unlikely to peak very high. Wegman ....Previous Discussion... The upper level trough responsible for heavy rainfall in the Lower Mississippi Valley on Tuesday will begin phasing with a northern stream feature and spread precipitation into the Eastern third of the country on Wednesday. Guidance appears to highlight two areas of concern. Moisture continues streaming into the central Gulf Coast and Southeast early Tuesday beneath a potent upper jet and phasing upper trough. Rainfall moving through the Florida panhandle will likely be enhanced by a strong cold front succeeding it. Instability will be shallow in northern Florida and Georgia, but a 50-60kt low level jet out of the south will supply modest moisture, on the magnitude of 1-1.75in PWATs, to flow into the region. That being said, soils will be relatively dry and Flash Flood Guidance fairly high so any flash flooding threats should remain marginal. The heavy rainfall threat should subside over the Southeast by early Thursday evening. Precipitation will begin as snowfall in the Northeast before eventually transitioning to rainfall. The surface low will reintensify as it lifts through the Mid-Atlantic and Northeast. 60-70kt 850mb winds out of the south will supply anomalous PWATs of 1-1.25in, representing 3-4 standard deviations above normal, into the Northeast coast early Thursday morning. Rain rates between 0.1-0.25 won't be too high, but moisture advection running almost parallel to the surface cold front may cause isolated instances of training and flash flooding especially over urban areas. The marginal risk area was drawn to southern New Jersey because soil moisture to the south of there was notably higher than soils to the north. The heavy rainfall threat over the marginal risk area should conclude by the end of the day 3 period. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!45vgBpAbxtTVcYFGPIOgEnC2fWCl8HuKuizxOhVSFNe1= _dMaaLU0CJ-T9JEqaopfx_qr3u4o9WgcLpOmUFOqIRzY6FQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!45vgBpAbxtTVcYFGPIOgEnC2fWCl8HuKuizxOhVSFNe1= _dMaaLU0CJ-T9JEqaopfx_qr3u4o9WgcLpOmUFOqLl_GEII$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!45vgBpAbxtTVcYFGPIOgEnC2fWCl8HuKuizxOhVSFNe1= _dMaaLU0CJ-T9JEqaopfx_qr3u4o9WgcLpOmUFOqX7oCR_k$=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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