Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 26 2023 15:58:07 FOUS30 KWBC 261558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1157 AM EDT Wed Apr 26 2023 Day 1 Valid 16Z Wed Apr 26 2023 - 12Z Thu Apr 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF NORTHERN TEXAS, SOUTHERN OKLAHOMA, SOUTHERN ARKANSAS, AND NORTHERN LOUISIANA... ....Southern Plains to ArkLaTex Region... Still expecting rounds of heavy rainfall through tonight across portions of the Southern Plains eastward into the ArkLaTex region as the mid/upper level shortwave moves eastward. The area remains in the favorable area of large scale forcing with a surging low level moisture transport overrunning the stationary boundary draped across central/northern Texas. A couple rounds of thunderstorms are expected through this afternoon across northern Texas which then is expected to congeal into a more progressive line of storms this evening/tonight as it moves east/southeast. The 12Z HREF continues to highlight the greatest potential for heavy rainfall over northern Texas where hourly totals of 1-2" appear most likely and also has a slight signal for 3-4"+ totals. Further north into central Oklahoma and parts of Arkansas, the lack of deeper instability will limit the intensity of the rainfall with the 1-hr QPF neighborhood probabilities for 1" very low. As a result, the main changes to the ERO for this update was to trim back the northern extent of the risk areas across OK/AR and western TN while continuing the highlight the greatest threat for heavy rainfall and scattered instances of flash flooding across northern Texas. ....Southeast Florida... A Marginal Risk was introduced this forecast update as deep convection is expected to develop this afternoon/evening along colliding sea/land breeze interactions and in the vicinity of a surface trough and lingering surface front across the Peninsula. With anomalous moisture pooled across the region and daytime peak heating contributing to sufficient mixed layer instability, multiple clusters of deep convection are expected to develop and then move southeast along the central/eastern portions of the Peninsula. The 12Z HREF trended higher with its neighborhood probabilities of 2-3" hourly totals and the 6-hr probabilities of exceeding 3" are now up to 30-50% in/near the urbanized corridor. Isolated 3-5"+ totals will be possible. Given the possibility of this intense rainfall falling over the urbanized areas, a Marginal Risk was introduced for isolated instances of flash flooding. Taylor Day 2 Valid 12Z Thu Apr 27 2023 - 12Z Fri Apr 28 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PARTS OF EASTERN LOUISIANA INTO SOUTHERN MISSISSIPPI-ALABAMA AND THE FLORIDA PANHANDLE... The closed low ejecting out of the Colorado Rockies begins to open up as it tracks through the Southern-Central Plains and Mid-MS Valley Thursday-Thursday night, with multiple embedded vort lobes helping to provide large scale forcing for ascent across the outlook area. Deep layer southwesterly flow will bring up anomalous moisture, with 850-700 mb moisture flux anomalies between 2-2.5 standard deviations above normal per the GEFS and SREF. As the primary surface low lifts through the Ohio Valley, another area of low pressure is expected to form off the North Carolina coast. Overall, the convection that is expected to develop should be more on the progressive side, helping to limit the intense rain rates and overall duration. For this reason, the risk area remains capped at Marginal over much of the outlook area, where broadly he system will become increasingly progressive with generally lower duration potential for heavy rainfall. However, the exception will be along the Gulf Coast from the New Orleans area east to near Tallahassee, where higher available PW (1.7 to 1.8") along with greater deep-layer instability (MUCAPEs 1000-2000 J/Kg) will allow for more intense short-term rainfall rates on Thursday, i.e. 2-3"/hr, with multiple rounds of convection along/south of the surface warm front and ahead of the approaching cold front. Within the Slight Risk area, the 00Z HREF indicates a broad area of 60-90+ percent probabilities that 24hr QPF will exceed 3", with a smaller area of 40-70% probabilities of exceeding 5". Hurley Day 3 Valid 12Z Fri Apr 28 2023 - 12Z Sat Apr 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE MID ATLANTIC REGION... The aforementioned shortwave trough on day 2 will lift slowly into the OH Valley on Friday, flattening somewhat as it does in response to a more vigorous shortwave and trough amplification (height falls) over the Four Corners-southern Rockies-southern High Plains regions. Nevertheless, deep-layer moisture will be abundant across the mid Atlantic region ahead of the Miller-B type surface evolution (inland low tracking across OH Valley-eastern Great Lakes with another low forming along the Mid Atlantic Coast). Both the GEFS and SREF show a broad ribbon of anomalous 850 mb S-SW flow (between 3-3.5 standard deviations above normal) and 850 mb moisture flux (2-2.5 standard deviations above normal). Despite the strong low-level moisture transport, deep-layer instability, even elevated, is such that intense short-term rainfall rates are expected to be more localized/isolated than otherwise. Therefore, at least for now will maintain the Marginal Risk inherited from yesterday's experimental Day 4 ERO. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4zbk0L_9X_cF65VMa9IiKANvtConPHbowbVBR-rPfzvu= Pz0lNOW8y8SIinS2T1JbGUa7nA7x0vdpvaCreqql35xqQww$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4zbk0L_9X_cF65VMa9IiKANvtConPHbowbVBR-rPfzvu= Pz0lNOW8y8SIinS2T1JbGUa7nA7x0vdpvaCreqql2JLX7KY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4zbk0L_9X_cF65VMa9IiKANvtConPHbowbVBR-rPfzvu= Pz0lNOW8y8SIinS2T1JbGUa7nA7x0vdpvaCreqqlW70LFDU$=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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