Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 12 2022 15:58:49 FOUS30 KWBC 121558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1058 AM EST Mon Dec 12 2022 Day 1 Valid 16Z Mon Dec 12 2022 - 12Z Tue Dec 13 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS... 16Z Update... Except for pulling the eastern boundary of the Marginal Risk area eastward a bit in deference to a couple of NMM members of the 09Z SREF...saw little reason to adjust the existing ERO. Also noted that the Marginal still encompasses the 1+ inch contours from the 06Z GEFS. Timing still looks to be on track. Bann Strong height falls will be pushing eastward into the Central to Southern Plains day 1, helping to strengthen the low level southerly flow across much of the mid section of the nation. The resultant 850-700 mb moisture flux values are forecast to be very anomalous across much of the Southern to Central Plains with values 3 to 5+ standard deviations above the mean. Strong frontal convergence and favorable upper difluence in this increasing moisture flux axis will support organized convective development late in the day 1 period, mostly after 0300 utc Tuesday across potions of the Southern to Central Plains, with this line then pushing quickly downstream. The marginal risk area was drawn in the model consensus qpf axis and where the latest HREF neighborhood probabilities are highest for 1"+ totals this period. 1 hour HREF probabilities are fairly high for .50"+ amounts but drop off significantly for amounts over an inch. This is reflective of the likely progressive nature of potential convective line and warrants a continuation of only a marginal risk. With respect to changes to the previous outlook for this period, the eastern end was trimmed to the west by approximately 100 miles to reflect the above mention model qpf and probability axes. Oravec Day 2 Valid 12Z Tue Dec 13 2022 - 12Z Wed Dec 14 2022 .....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY... The developing organized convection over portions of the Southern to Central Plains at the end of the day 1 time period should continue to push eastward day 2 across eastern portions of the Southern Plains into the Lower Mississippi Valley. The overall large scale flow will remain very conducive for an organized line of frontal/pre-frontal convection moving eastward across these areas with strong frontal convergence and defined upper difluence in the axis of above average PW values along and ahead of the eastward moving cold front. Low level south southwesterly flow will remain strong ahead of the associated front off the western to central Gulf, resulting in anomalous 850-700 mb moisture flux values...3 to 4+ standard deviations above the mean...moving east from the Plains into the Mississippi Valley. Model consensus is for widespread heavy precip values, especially from eastern portions of the Southern Plains into the Lower Mississippi Valley where instability will be the greatest. The progressive nature of the front and associated convection will continue to be a detriment for very heavy totals, but the overall favorable large scale dynamics will support some localized totals of 2-4", warranting a continuation of the slight risk from previous forecasts. With respect to changes to the previous outlook, the northern edge of the marginal risk was trimmed south by approximately 150-190 miles after collaboration with WFO ILX. The marginal and slight risk areas were also expanded westward into far eastern Texas given the latest consensus on the timing of the organized convective line. Oravec Day 3 Valid 12Z Wed Dec 14 2022 - 12Z Thu Dec 15 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL GULF COAST INTO THE SOUTHERN APPALACHIANS... A broad slight risk area was maintained for the upcoming day 3 period from the Central Gulf coast, northeastward into the Southern Appalachians. The day 2 heavy precip event stretching from the eastern portions of the Southern Plains into the Lower to Mid Mississippi Valley will continue its eastward march during day 3, stretching from the Central Gulf coast into the Southern Appalachians. The overall favorable large scale pattern for organized frontal and pre-frontal convection will continue during day 3 with strong frontal convergence and well defined upper difluence continuing in the axis of above average PW values and anomalous 850-700 mb moisture flux coming off the central to eastern Gulf of Mexico ahead of the associated eastward moving cold front. We contemplated upgrading to a moderate risk area for portions of far northeast Mississippi, northern Alabama into northern Georgia where recent heavy rains have increased stream flows and lowered flash flood guidance values. Our concerns are that models may be too far to the north with their respective max precip areas. The greatest instability for the day 3 time period is expected farther to the south and closer to the immediate Gulf coast. This may support the heaviest precip falling farther to the south than models suggest. This would then be across areas that have not had as much precip recently and subsequently have lower stream flow and higher flash flood guidance values. For these reasons, no changes were made with the risk level, keeping it at slight for the time being Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4gYxFIMI9YXs9sQV7qnOPg2yIlFmrawq1o7NyuHYIs0h= RAIRUGH6lMqiM56-cFJt3msaNpF71dgeRk5gab7BUt4JUXQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4gYxFIMI9YXs9sQV7qnOPg2yIlFmrawq1o7NyuHYIs0h= RAIRUGH6lMqiM56-cFJt3msaNpF71dgeRk5gab7BfxoXGVM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4gYxFIMI9YXs9sQV7qnOPg2yIlFmrawq1o7NyuHYIs0h= RAIRUGH6lMqiM56-cFJt3msaNpF71dgeRk5gab7BLPvQmQk$=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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