Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 09 2023 01:08:08 FOUS30 KWBC 090108 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 807 PM EST Wed Feb 08 2023 Day 1 Valid 01Z Thu Feb 09 2023 - 12Z Thu Feb 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF NORTH CENTRAL ARKANSAS INTO FAR SOUTHWEST ILLINOIS... A Slight risk was maintained across portions of north central AR into far southwest IL. An uptick in rainfall intensity over areas with saturated or saturating soil conditions may lead to some additional flash flood concerns over the next several hours across this area. The mid level trough over OK and TX is beginning to take on more of a negative tilt as it pushes eastward this evening. This is resulting in an increase in the mid level forcing and an uptick in low level moisture transport across the lower MS Valley. The areas with heaviest rainfall over the past 24 hours, and thus locations with saturated soil conditions, across central and northern AR are north of the warm front and thus displaced from the better instability. With that said, the increase in forcing should be enough to result in some uptick in convective coverage/intensity across central and northeast AR over the next couple hours. Recent HRRR runs support hourly rainfall up to 1" or so from portions of central AR into far southeast MO and southern IL...with total additional rainfall of 1-2" in spots. This rainfall may be enough to result in some mainly localized flash flooding over the next several hours. The more robust convection is developing south of the warm front across portions of southern AR, LA and MS. The good news is that this is south of the areas hardest hit with heavy rainfall over the past 24 hours and thus there is higher FFG. Most of this area should pick up 1-2" of rain into tonight, although localized swaths of 3-5" seem probable where some short duration training can materialize. Given the better instability across this area hourly rainfall should locally approach or exceed 2", thus will only need some short term training to start getting totals over 3". Given the higher FFG across this region still only expecting localized instances of flash flooding to result from this rainfall...thus a Marginal risk should suffice. Chenard Day 2 Valid 12Z Thu Feb 09 2023 - 12Z Fri Feb 10 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE WESTERN FLORIDA PANHANDLE, SOUTHERN ALABAMA, AND WESTERN GEORGIA... A Marginal Risk of excessive rainfall has been maintained for the western FL Panhandle, southern AL, and portions of western GA for the Day 2 ERO update, mainly due to a stalling cold front moving into the region. High tropospheric moisture levels are expected ahead of the front, as indicated by HREF mean precipitable water values of 1.3-1.6 inches (well above the 90th percentile per TLH sounding climatology). The presence of sufficient instability (MUCAPE of 250-750 J/kg, per the HREF) will increase the potential for heavy rainfall, with rainfall rates locally approaching 2"/hr at times. The possibility of repeating rounds of heavy rainfall is also expected, increasing the threat of excessive runoff potential (with ECENS probabilities for 2" exceedance as high as 15-25%). However, antecedent soil conditions in the region are quite dry, with NASA SPoRT 0-40 cm soil moisture values indicated to be as low as the 10-20th percentile. This should significantly limit the areal extent of any flash flooding, with poor drainage/urban areas the most at risk of realizing any isolated flooding impacts. A more impressive surge of moisture is expected by the very end of the period near 12z on Friday as an approaching upper-level low diving into the Lower Mississippi Valley funnels moisture along a strengthening mid-level convergence axis extending into the Southeast. This potential will be monitored for any potential upgrade (most likely needed for the Day 3 time frame), but could bleed into the Day 2 period. Churchill/Snell Day 3 Valid 12Z Fri Feb 10 2023 - 12Z Sat Feb 11 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE WESTERN FLORIDA PANHANDLE, SOUTHERN ALABAMA, AND WESTERN GEORGIA... A stalling frontal zone is expected to bring a significant axis of tropospheric moisture (precipitable water values of 1.25-1.75 inches, or near the 95th percentile climatologically) to the central FL Panhandle, south-central GA, SC, and eastern NC on Friday and into Friday night. Despite the presence of these anomalously high PWATs, large-scale lift will remain fairly limited with minimal instability expected across the region (with the GFS indicating MU CAPE of generally less than 500 J/kg outside of southern GA and northern FL). Thus a fairly narrow Marginal Risk area was maintained, with rainfall totals generally expected to range from 1-3 inches across the area (with localized higher amounts possible due to the potential for training cells across the stalling frontal zone). Given the relatively dry antecedent conditions and limited upside potential for rainfall to exceed the already high FFG, any instances of flash flooding are expected to remain isolated and primarily confined to areas of poor drainage. The upper-level low associated with the upcoming moisture plume has undergone changes recently among the global guidance, largely slowing the system. More shifts in guidance are likely, which increases the uncertainty for this time period and limits the desire for any potential upgrades at the moment. Churchill/Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7Kft3B20SIed_7a-vyzM2HbzKiMBx-_roeriY_HqJ5_e= 6HvkWjgXUWsomoeKK_Q9GCezZFCXlX7MdljDXBHHpEEs24w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7Kft3B20SIed_7a-vyzM2HbzKiMBx-_roeriY_HqJ5_e= 6HvkWjgXUWsomoeKK_Q9GCezZFCXlX7MdljDXBHH4HGD8Yk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7Kft3B20SIed_7a-vyzM2HbzKiMBx-_roeriY_HqJ5_e= 6HvkWjgXUWsomoeKK_Q9GCezZFCXlX7MdljDXBHHfBqlMC4$=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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