Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed Apr 26 2023 16:00:00 FOUS30 KWBC 262011 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 411 PM 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 ACROSS PARTS OF THE CENTRAL GULF COAST... ....Central Gulf Coast... Strong thunderstorms are expected to be ongoing at the start of the period, with a line of thunderstorms likely to be across southern/southeast Louisiana around 12Z Thursday. The latest hi-res guidance and HREF show a strong signal that this line of convection should track along the Central Gulf Coast through daytime hours, likely reaching the eastern Florida Panhandle late in the afternoon. This line should be fairly progressive with limited duration of the heaviest rainfall at any one location, but based on the environmental ingredients showing fairly anomalous moisture and an axis of deeper instability along the coast, hourly totals may reach or exceed 2" in spots. With some potential for leading convection ahead of the main line and cell mergers/boundary interactions contributing to some repeating rounds, isolated totals of 3-5" appear possible (12Z HREF neighborhood probs of 3" exceeds 60-70% with an isolated signal for 5"), which could lead to scattered instances of flash flooding. The Slight Risk was generally unchanged from the previous cycle. ....Ohio and Tennessee Valleys... An increasingly negative titled shortwave trough will open up across the Tennessee and Ohio Valleys over the course of the period while low pressure tracks from northeast Texas to southern Indiana through the period. The height falls and right entrance region of a jet streak positioned over Ohio/PA will provide sufficient forcing for ascent with the southwesterly flow bringing northeast higher moisture up into Kentucky. Strong isentropic lift overrunning the advancing warm front and then convection developing along the cold front later in the day could result in a few rounds of showers and thunderstorms. With the mean flow oriented nearly parallel to the expected storm motions, some repeating rounds or training will be possible. The lack of deep instability is likely to limit the intensity of the rainfall, as shown by the latest guidance and lack of higher neighborhood probabilities for 1" hourly totals in the 12Z HREF. Nonetheless, a few stripes of localized heavier rainfall greater than 1-2" appear possible, particularly over south-central Kentucky into middle Tennessee which may lead to isolated instances of flash flooding. Taylor Day 3 Valid 12Z Fri Apr 28 2023 - 12Z Sat Apr 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE MID ATLANTIC REGION AND TEXAS... ....Mid-Atlantic... 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). 12Z guidance continues to show anomalous low level flow and moisture transport though the lack of deeper or more sustained instability may end up limiting the intensity of the rainfall such that the main issues/concerns for flash flooding will be for urban areas or other more susceptible locations. ....Texas... A positive to neutral tilted shortwave trough will track from the Central Rockies to Texas through the forecast period. Height falls and a strengthening upper jet will provide the large scale forcing for ascent across the region while at the surface, a warm front is expected to lift through central Texas, positioning itself across central/northern Texas during the day. Ample moisture will be lifted northward over that boundary and ahead of an approaching cold front from the west, with precipitable water values expected to exceed 1.25-1.5" thanks to increasing low level flow reaching 30-40 kts. With daytime peak heating providing ample instability, convection is expected to develop and track east/southeast into the better instability pool, eventually reaching South Texas late in the period. The combination of higher moisture and instability along with the favorable dynamics should produce scattered to numerous thunderstorms capable of producing locally heavy rainfall. 12Z forecast guidance shows potential for 1-2" areal totals with some signal for locally higher amounts. Some of this rainfall may fall over areas expected to receive heavy rainfall today/tonight (northern Texas Wed-Wed Night) or already experiencing wetter soils (like South Texas) so this additional rainfall may lead to isolated instances of flash flooding and a Marginal Risk was added for this update cycle. Taylor $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .